With the arrival of RV Polarstern in Bremerhaven, the reporting here in the MOSAiC app ends today. We thank you very much for the continuously high interest in the expedition. The extremely positive feedback on the app has motivated us daily in our work. We were especially impressed when you drew our attention to technical problems faster than we could register them ourselves. This showed us how attentively the expedition was followed and how intensively the app was used. Today we can be particularly proud that we actually managed to post news from the Central Arctic every day - 389 times in total! Thanks to everyone who contributed to this. Stay healthy and stay true to polar research!

The MOSAiC expedition is going to end tomorrow – after more than a year of magnificent science, experiencing the beautiful nature of the Arctic, meeting unique people from so many different countries and cultures… We have completely mixed feelings regarding our homecoming: we are proud of having finished the field work after fantastic but also exhausting months and are looking forward to meet our families and friends. But leaving our small Polarstern cosmos also means to arrive back to “real life” - a world dominated by the Corona pandemic and social distancing. We are going to deal with this challenge as we have dealt with all the others we experienced. And our arrival is only a first step of the project, which will be followed by years of analysis, processing, discussions, and publications to better understand and explain climate change and its impacts on pour Planet Earth.

Two more days until the research vessel Polarstern will dock in its home port of Bremerhaven – time for a brief summary: more than 80 institutes from 20 nations have contributed to the success of the MOSAiC expedition. Seven icebreakers and research vessels were deployed. Polarstern drifted 3400 kilometres in a zigzag course, covering a linear distance of 1923 kilometres. It was up to 1500 kilometres away from the nearest settlement. The team spent 300 days researching on the first MOSAiC ice floe, another 30 days on ice floe 2.0. On 56 days, there were strong winds of over 13.9 metres per second. The lowest temperature fell to minus 42.3 degrees Celsius, and wind-chill temperatures below minus 65 degrees Celsius.

Shortly before our arrival we want to say thank you another time: to the people who make our ship running, the Polarstern crew! We are grateful for your perpetual engagement, kindness and the friendship we experienced on board. Captain Thomas Wunderlich and the 38 people working on the bridge, in the machine, on deck, in the galley and the mess rooms, with the electronics and in the hospital – it is a petty that we don’t have enough space to name all of you. We certainly also think of Captain Stefan Schwarze and the personnel of the former legs. Some crew members being on board now have spent – due to the traveling and quarantine times – the longest time of the year away from home since MOSAiC started. Thank you for providing us with a second home!

We spent most of the last days with sorting scientific instruments and other equipment, storing it into containers, preparing the related cargo lists and labels, and caring for our frozen samples. This left some space for creativity, so our photographer and educator Lianna Nixon (Team Media) did arts and crafts: Inspired by the memories of the last weeks’ work of Team Ocean, she painted the deployment of the vertical microstructure profiler VMP and found a special place to present her arts. Fittingly her model Jacob Allerholt was one of the deployers of the oceanographic instrument – but also happily provided his beard as exhibition space for other drawings.

The Polarstern is coming back to Bremerhaven! On October 12 you can join the return via different livestreams on AWI.de or MOSAiC social media channels. As there won’t be a public event, the Hansa and other ships will receive the Polarstern and accompany her to the port. One livestream (held in German) will be streamed from about 7:45 a.m. on the board of the Hansa. Via the MOSAiC Instagram account you can follow from about 8:15 a.m. a live video directly from board of the Polarstern.

There are many people who are involved in MOSAiC to ensure that our work on the ice could be done with efficiency. One of the teams that worked really hard to ensure that our fieldwork could be successfully executed is the Logistics team. Today we want to thank all Logistics team members for supporting field and cargo work, provide logistical support for many infrastructures on the ice and contribute to bear guarding. Without Logistics, we would have not been able to conduct the many tasks on the ice and ship for the MOSAiC mission – thank you!

In addition to the regular packing of all equipment, we also had to sort all the ice cores taken on the five MOSAiC legs in order to be able to deliver them to the different projects and destinations and to organize storage in Bremerhaven. Over all, we took more than 1000 sea ice cores, and some 430 cores (or core sections) are to be shipped home now, which we needed to sort by type or project into 70 different isolation boxes. The main topics of these cores are: Archive cores for storage for whatever might come up; replicates / backup cores to do additional measurements or be backup for existing cores; cores for texture analysis; cores for optical measurements in the lab; cores for specific parameters by BGC (e.g. CH4); cores for sediment and micro-plastic analysis.

We are leaving our last logistic waypoint Svalbard on Sunday at 16:00 Central European Summer Time after successfully finishing our cargo operations in the Kongsfjord in the last two days: the instruments on the bow crane and on top of the bow containers are uninstalled and the crane itself is laid down. In the meantime, our Team Logistics leader Verena Mohaupt has been in Ny-Ålesund to hand over scientific instruments, gears like safety equipment, ski and pulka as well as ammunition and weapons to AWIPEV station, where they can be used now. Accompanied by nautical officer Felix Kentges and Åshild Rye from Team Logistics, she brought back samples to be shipped home. Additionally, our co expedition leader Katja Metfies operated an “online shop” for Svalbard souvenirs, which were also delivered by Verena – whose good connections to the world’s northernmost research village from overwintering as station leader here made this possible!

The gangway hut has been standing on the working deck for eleven months, now the time came to deinstall it. The decks crew had their home inside the hut to register people leaving and entering the ship at ice stations, seldomly without joking with or at least waving at the scientists going onto the floe. Now the crew stored the hut away - as they already did with the ice gangway itself the day before - and cleaned the deck where it used to be positioned. This is another obvious sign that work on the ice floe has ended and we are cleaning up.

Another „Last“: Yesterday we launched the last radio sonde, of which the data will be integrated into the MOSAiC Atmos dataset. A lot of people assembled at the helideck next to the balloon filling station and signed it. Team Atmos celebrated balloon launch number 1574 – including many extra launches during intensive observation periods meaning to work at times like 3:00 AM for example to gather data during our passage through the marginal ice zone. Weather technician Andreas Raeke from the German Weather Service (DWD) honored Sandro Dahlke and Helene Angot for the highest balloon on leg 5 which ascended to more than 36 kilometers. Andreas and his DWD colleague Julia Wenzel will keep on launching for the standard Polarstern weather station.

Yesterday we had our last ice station: Transects with atmospheric, ice and snow measurements, some coring, flux chambers, new ice sampling and a so-called ice challenge. Organized by Marcel Nicolaus and Julia Regnery from Team Ice everybody was asked to show her or his skills after nearly six weeks since we started working on our first floe: drilling the ice (by hand, not with an electric drill, what would have been for beginners), measuring its thickness as well as snow depth, and weighing out exactly 500 grams of snow. What sounds quite easy becomes a challenge when time pressure is added to find out the best competitor. This turned out to be Jan Rohde (congratulations!) who is part of Team Ice and therefore experienced with these tasks. But all other participants had at least as much fun as Jan had.

On our way through the marginal ice zone we encountered a beautiful ice type: pancake ice. It emerges during an early stage of new ice formation from frazil ice, which consists of ice needles that stick together to grease ice. In this phase, swell is needed to let pancakes grow, as otherwise the ice would just form an even pattern and cover the ocean as so-called Nilas. But the swell makes the newly forming ice pieces colliding and theses collisions lead to more or less circular plates with usually padded edges. They can grow up to several metres in size, until they consolidate into a continuous sheet of new ice during the further freezing process.

We got a guided tour of the Polarstern's engine room and were fascinated to see the Behind the Scenes where our crew works hard to keep the vessel moving. The four main engines, the auxiliary engines, the generators, the shafts: everything is really impressive and thanks to the care of the engineers and motormen, the partially nearly 40-year-old machines run reliably. Depending on the nautical or scientific demands, engines are turned on and off to deliver enough power, but save as much fuel as possible. Additionally, the machine personnel is responsible for the water system and the electric power supply, which was even more important in winter, when the main engines could be stopped as Polarstern was moored to the ice floe and didn’t need to manoeuvre actively.

Next to the underway ice stations we conduct en route measurements with the instruments installed on board Polarstern. Team Atmos uses them to investigate the energy and gas fluxes in the marginal ice zone. The data will be combined with water column measurements of Team Ocean, which they take with a deep CTD cast. Once we are out of the ice, we will travel along the ice edge, passing it again to have a second transit through the marginal ice zone. Our last scientific waypoint is an ice floe, where position buoys had been installed during the MOSAiC flight campaign. We will hopefully have the time to work on the ice there again, to combine current data with the data from the flight campaign. Then we are going to head to Svalbard (through the marginal ice zone again) to exchange some material with AWIPEV station and use the Kongsfjord to do cargo operations before we will ultimately leave Arctic waters and steam to Bremerhaven.

We have a small bar on board called the Zillertal. Last Thursday it was renamed to Chillertal and we had a pub quiz there. Organized by a team around Alison Webb (who already entertained us with online pub quizzes during the single quarantine at the beginning of our common journey) there were several rounds of questions about our vessel, the expedition, but also classical themes like music or sports. The winner after nearly three hours of hard competition was Team Drizzle, whose name was inspired by our weathercaster Julia Wenzel, who riddled together with Jan Rohde, Jacob Allerholt, Oula Niemelä and Steffen Graupner – congratulations!

Many factors influenced our departure date from the floe. We needed to calculate for example for icebreaking if we faced difficult conditions, in order not to miss our arrival date. It figured out, that we made really good progress, bringing us some spare days, which we are able to use for underway stations. That means: On Thursday we were back on the ice again! Some members of Team ICE headed out for a long transect with thickness and property measurements. Teams BGC, Eco and Atmos combined forces for gas chamber measurements and ice coring, whereas Team Ocean went for sampling a lead. The biggest group consisted of a Remote Sensing instrument, followed by an Atmos sledge measuring energy balances and the snow team investigating the ice surface. A great combined effort to gather as many data on our way as we could.

MOSAiC leader Markus Rex obtained the SeaDevCon Maritime Award 2020 yesterday. He says: “I am honoured to receive this award on behalf of the whole MOSAiC team. By producing a solid scientific basis for political decisions on climate protection we together contribute to the protection of the Arctic Ocean with our joint research. The award is endowed with the sum of 5000€. I want to use this amount to educate the young generation about the changes we observe in the Arctic and on how relevant these changes are for them. When we return from the expedition, we all will be ambassadors from the Arctic, we have fantastic experiences to share, we can inspire people and we can motivate them to become active to protect the fascinating world we have seen. I want to encourage all activities to reach out to the public and in particular to reach out to the young generation.” That’s why the prize money will be spent on travel costs for young scientists doing outreach and education.

During the last week at the floe, the ice opened at port side next to Polarstern, resulting in an area of open water. This was an opportunity for a different kind of sampling for Team Atmos and Team Ocean: They deployed the flux chamber measuring gas fluxes between the (on this occasion) open ocean and atmosphere as well as the VMP (vertical microstructure profiler) Upriser for physical oceanography. To reach different features (e.g. ice edge, open water) they took a zodiac to get there: a varied trip away from Polarstern.

A standard atmospheric time series on board is the launch of weather balloons with radio sonde. They collect data on temperature, humidity, wind direction and wind speed as well as the height and distance from the starting point. They ascend up to more than 35 kilometres sending their data back to the vessel. Usually the soundings are conducted every six hours, but at certain events Team Atmo increases the frequency - as last night, when we experienced a storm with wind speeds over 8 Beaufort. The balloons are also a nice projection surface to send wishes back home, e.g. if we miss someone’s birthday.

Yesterday was the calculated day for the first sunset we were to experience here in the Arctic during Leg 5. At our latitude the sunset is a process that takes several hours. In the evening, people assembled outside on the various decks of Polarstern. But there were low clouds just above the horizon, which prevented us from seeing the sun sinking beneath it. Nonetheless, it was a beautiful evening with the red coloured clouds being nearly more interesting than a clear sky would have been. And there are more sunsets to come the next days!

Yesterday was a special day: It’s the first anniversary of the MOSAiC expedition start on 20 September in Tromsø, and the drift with the floe 2.0 ended. After two days of dismantling the camp we said good-bye to “our” floe with a small celebration on Sunday afternoon and took the chance for a last group photo on the ice. At 17:45 the gangway was lifted for the last time and 45 minutes later we started our trip home towards the South. Unlike Fridtjof Nansen in 1894, we are not going to overwinter in the Arctic for a second time, but will be back in Bremerhaven on 12 October.

The ROV team is interested in the optical properties under the ice due to the ongoing changes of the surface while refreezing. Therefore, the scientists repeatedly dived along certain optics grids over the last weeks to record specific light spectra under the ice to see the changes in this exciting phase. Another task is towing under-ice nets to collect biological samples for Team Eco. So after the successful optical measurements, several net hauls at 10 metres depth as well as directly under the ice are on the working plan. Last week, there was a nice “catch of the day”: four jellyfish (ctenophores) of two different species called Mertensia ovum and Beroe cucumis.

Verena Mohaupt (Team Logistics) drives the Argo into Polarstern's big wetlab. It is a hard job to manoeuvre the tracked vehicle on a centimetre scale as it is rather constructed for pulling heavy loads. Team Logistics is our good example: They started sorting and rearranging cargo already in the beginning of the week to be able to support others during the last days on the floe to bring instruments and infrastructure back on board. By Sunday everything must be back on the vessel, as we are going to leave the floe and start our return trip back to Bremerhaven.

We are delighted to announce that the communication team of the MOSAiC expedition yesterday received the BdKom Award 2020 in the category "Team of the Year". This prize is awarded annually by the German Bundesverband der Kommunikatoren, in four categories. The "Team of the Year" award recognizes the joint and professional communication performance of a department and the concept of its communication project. And, of course, any concept is only as strong as the entire team behind it, because without a team nothing works - as was repeatedly demonstrated during the MOSAiC expedition. For this reason, the communication team would like to take this opportunity to thank all the other participants of the MOSAiC expedition from various departments and fields, who contributed to the realisation of this exceptional expedition and who so magnificently supported the accompanying communication.

One of the scientists who stays outside the most time is Ruzica Dadic (Team Ice). She studies ice and snow surface structures and is absolutely enthusiastic about the development she finds within a few days: “The temperature and moisture differences between the air and the snow surface, along with particular wind conditions, can lead to different types of surface deposition on already beautiful snow surfaces. After our first clear night following many foggy days, we stepped off the ship and found beautiful tiny cups that formed overnight. The newly formed cups at the surface got buried in new snow relatively quickly. A few mornings after the surface was covered in plate-like surface hoar crystals (picture: micro computer tomography scan), and they kept growing during the day. That didn't last long either (we don't want to get bored after all), because the moisture in the air increased, the winds picked up slightly and the surface hoar plates from the day before became all frosted over. And then the rain came...”

Laboratory electrician Olaf Hüttebräucker fills liquid nitrogen (LN) which is used to preserve all kinds of samples for later analysis in the home institutes. The liquid has a temperature of -196 °C, and evaporates under normal ambient pressure with impressive mist. We can produce LN on board by fractional distillation of compressed air. It is possible to generate up to 20 litres a day, which are then transferred from the processor to a so-called Dewar container. The Dewar is the source for the scientists for getting the smaller portions they need, which amount to several litres a day.

We have deployed two iBOB (ice-tethered Bio Optical Buoy) units during the last days. Two wooden boxes with the buoys were flown to the floe as helicopter sling load. Then we needed to drill big holes through the ice to deploy the underwater units. This went quite smoothly for the first buoy, but making the hole for the second one, we experienced thick ice produced by rafting, which made us deciding to readjust it. Finally, we managed to place everything on and under the ice. The buoys are composed of several sensors measuring e.g. irradiance in the air and under the ice, chlorophyll in the water, as well as they record zooplankton and fish. They will stay on the ice after our departure and send these valuable data back home.

Although it might look like this, Mario Hoppmann (Team Ocean) is not pulling the tethered balloon Miss Piggy. Indeed, he is pulling a 100-metre long line with a CTD and a sensor (and a bottom weight), which measures nitrate in the water. It is permanently positioned at roughly 15 metres and everybody is invited to do a workout while passing along the way: Lowering the sensor to 100 metres and lifting it to the surface slowly to record a nitrate profile of the water column. This can be done by pulling it up while standing next to the hole (workout for arm muscles) or by holding the line and walking away from the hole (whole body workout).

We were able to enjoy wonderful sunlight this week, which came along with an interesting appearance: a so-called halo phenomenon. Rings around the sun are caused by refraction and reflexion of the light by ice crystals, which are swirled up from the ice surface. This phenomenon can seldomly be observed at home with cirrus clouds causing it. Here in the Arctic our views are undisturbed – and we enjoy it to be outside and watch the fantastic nature!

We use drones for scientific measurements: Roberta Pirazzini and Henna-Reetta Hannula (Team Ice) operate a small drone at a height of 30 metres to do photo mapping of the floe surface. They also use this big Spectra drone at 5, 10, and 30 metres to measure surface broadband and spectral albedo. They will later combine their data with surface-based albedo, and below-ice transmissivity measurements. That is why they fly over an albedo line and the ROV area, where colleagues conduct the corresponding measurements with their handheld and underwater devices.

Just before the planned 36-hour intense measuring period, a short weather window of flying conditions opened on Sunday morning. As these opportunities have become rare over the last weeks of fog, all interested parties rushed to make best use of the limited air time. One of the activities was a drone flight to shoot aerial photos, which have been stitched together and processed to serve as a new floe map. For our daily planning meetings, we have entered our stations to it enabling everybody to orientate - and avoid stepping on others measurement areas.

Thursdays are the days for water column work: the ship’s CTD with the water rosette is lowered to different depths measuring and sampling the water on its way down and up just next to the Polarstern. And it is the day for different nets: a multinet as well as ring nets with varying mesh sizes are lowered into the ocean. Team Eco collects phyto- and zooplankton to study the species compositions and abundances. Under windy conditions it is quite a challenge to operate the big nets. Not only is the wind attacking it above the surface, but we usually drift faster than on calm days, what means that we need to attach a heavy weight to the net, in order to lower it down to several hundreds of metres as straight as possible.

We enjoyed two days of relatively clear skies. It was impressive to see the sun constantly circling around us at about two fingers wide above the horizon. Noon and midnight blend and these terms lose their meaning at our latitude of nearly 89 °N. Fridtjof Nansen’s Fram expedition was farther south (at 81° 4' N) when he stated in his diary on 9 September 1894 that he has already experienced the first sunsets. At the North Pole Polar Day will transition into Polar Night with a single sunset around the 23 September. Depending on our drift until then it will be similar for us with perhaps very few extremely shallow crossings of the sun across the horizon during this transition.

Seaman Andreas Baecker prepares lifebuoys, which arrived with the last supply. They are delivered in neutral red and white and need to be marked with the vessel's name and home port. There is a template, which he needs to fix on the lifebuoy with tape before he dabs colour on it. The buoys themselves are sorted out after a certain time to ensure their functioning is intact after they have been outside in rough weather conditions. And we also got two nice used ones: when Polarstern met the supply vessels Sonne and Maria S. Merian, we received theirs as souvenirs, which now decorate the bar.

The weather forecast said that there would be a low passing our floe with stronger winds and the freezing takes place as we experience temperatures constantly below 0 °C. That means it’s time for a so-called intensive observation to study the influence on the water column: We are just in the middle of a continuous sampling and measuring period for 36 hours, mainly by Teams Ocean and Eco. We set up a plan for bear guarding and bridge watch and we managed to fill in the positions for ocean friends without any problems. These ocean friends accompany the scientists in the field to ensure they are not alone while measuring e.g. MSS profiles like Zoe Koenig does in the picture. Maybe it was an extra motivation, that Zoe promised to take care for chocolate, coffee, tea and hot chocolate supplies.

Zoe Koenig (Team Ocean) and Manuel Dall'Osto (Team BGC) conduct ocean profiles in the same lead we introduced the day before yesterday. They use the Upriser: an instrument that measures water turbulence throughout the different ocean layers from 60 metres depth to the surface. With temperatures slightly beneath 0 °C it is possible to do this in the field without any shelter. This enables us to operate devices like the vertical microstructure profiler (VMP Upriser) quiet flexible. So, we can measure the water turbulence in leads to get a better idea of how stable the freshwater lens is, which is present on top of the water column at different locations. The physical oceanographers study its behaviour during the onset of freezing we are just experiencing together with all other teams as a cross-cutting event at leads.

The summery Arctic is predominantly foggy. But as soon as the sun makes it through the low cloud layers, we enjoy the fantastic light here in the north. The time of the day is of minor importance: last week, there was sunshine around midnight and all of a sudden, a surprisingly high number of people bustled about on the weather decks. Steffen Graupner (Team Logistics) used these conditions for a drone flight to take pictures of the Polarstern. A nice ancillary effect of some remaining low clouds was a fog bow, which emerges at the opposite side of the sun just as a rainbow does (more on this phenomenon is given in the post of July 7).

Gunnar Spreen and Linda Thielke (Team ICE) install the microwave radiometer called ARIEL together with an infrared camera to the edge of a lead. On this spot, the instruments can measure open water and thin ice growth, which developed in the recently opened lead. Ariel is movable and, on other occasions, they used it on a transect together with the colleagues measuring snow and ice thickness as well as surface properties. The knowledge about these parameters will help interpreting the Ariel data. Comparable microwave radiometer data for ice thickness observations are collected from satellites (SMOS and SMAP), which measure at much bigger spacial scales (several tens of kilometres) and without the need of people being at site. But it is important to validate these data in the field.

How do you get across a lead? We have figured out resourceful ways of crossing: The drone operators Henna-Reetta Hannula and Roberta Pirazzini (both Team ICE, f.l.t.r.) use a pontoon as a raft, which is pulled by Bob Campbell (Team Eco). As we are just at the beginning of the freeze-up, the ice is quite dynamic with some leads and ridges developing during the tidal cycle. Therefore, we need to be prepared for changes of our access ways to occur or sites being displaced. But the floe with our research camp is stable without cracks or deformations and we can cope well with the ice dynamic surrounding us.

We had our first coring day on the new MOSAiC floe on Monday – and started the weekly routine measurements with this task. Scientists from teams Ice, Eco, and BGC took more than 30 ice cores near the vessel, what means much less transport ways than we had at the first floe. Ice thickness and core length accordingly differed from 1,40 and 1,70 metres. Already in the field, Eco scientist Emelia Chamberlain measures and records properties of an ice core as temperature and salinity before Daiki Nomura (Team BGC) bags it for further analysis.

Review to our transit: Team Ocean scientist Jacob Allerholt deploys an xCTD into the ocean as we sail in search of a new floe. This is very easy: all one has to do is toss it overboard from the vessel even when the ship is moving. We were launching xCTDs every half degree north on our way to the Pole. These probes give us temperature, salinity and depth of the upper 1000 m of the water column – important parameters to characterize different water layers. They only need 5-10 min to do the profile; the single challenge is that the communication cable is very sensitive, so it should not encounter any ice during that time. “We are coordinating these operations with the bridge, and so far, all the xCTDs except one reached 1000 m!” says Team Ice leader Zoe Koenig.

Following a five-month mandatory delay due to the coronavirus pandemic, yesterday the two German polar research aircraft Polar 5 and Polar 6 took off from Svalbard to conduct their first aerial survey flights over the Arctic this year. The flights, which will extend far into the Central Arctic, will support the investigation of the atmosphere and sea ice, and supplement the MOSAiC expedition’s extensive research agenda. Core research priorities include cloud formation over the Arctic Ocean and the question as to whether the sea ice observed during MOSAiC was generally thicker or thinner than in the past two decades, and how the above-average summer temperatures affected the Arctic ice cover.

Nansen reports in today’s diary from the historical Fram expedition on the successful installation of a coal-oil apparatus for heating the range, which exploded later. Lucky us who have three cooks caring for our physical well-being. They have a well-fitted galley and prepare delicious meals – and do not need to concern themselves with refueling the range as on board there are electric ones.

Another „First“ for Leg 5: We managed to cut an around 2 times 2 metre wide hole into the ice next to the ship. It allows us to deploy our CTD with water sampler as well as nets and other sampling devices with the ship’s crane. Teams Ocean, Ecosystem and Biogeochemistry are going to conduct their weekly sampling through this hole – and teamed up to make the hole into the ice which is around 1.5 metres thick. They first drilled through it and fixed the resulting blocks to the ship’s crane. The crew than lifted it out. A pump is lowered into the water, when the hole is not needed keeping it ice-free.

We had our first polar bears visit our research camp at 87° 51’ N, one of the most northern polar bear sightings the MOSAiC expedition has had! During the evening a mother and cub came from the east and walked towards the stern of our vessel. She and her cub were very curious to what we were doing, they displayed behaviours of lifting their heads and smelling the air, being cautious while approaching the ship. They then turned around and walked along the main road passing our Remote Sensing site and MET City. After inspecting our instruments and getting interested in a flag, we had to chase them away by relaying the ship's horn. This is to prevent them from chewing the flags, what might be harmful to them. The mother and cub safely continued on their way off our floe, navigating the ice north of us.

Among the first devices deployed on the floe were two flux sleds. These Automated Surface Flux Stations (ASFS) measure many atmospheric variables like temperature, relative humidity, water vapor, radiation, but also snow depth, GPS position etc. They formally used to be deployed at the so-called L(arge) Sites of the Distributed Network and have resisted the forces of nature e.g. being eaten up by ridges. But the participants of former cruise legs managed to recover them and we are really happy to have these devices starting to record the important data since we arrived at our new floe.

Team Atmos scientists Zoe Brasseur and Matt Boyer volunteer - together with many other team members - to build a road. It leads from the ship through a ridge to the Central Observatory. This road will be the main way for us to access our research sites on the floe as well as move large instruments on sledges to the sites. The road must be made as level as possible with few turns for large equipment to navigate the ridge. The first days on the floe kept us busy with construction work like this in parallel to the measurements we already started, resulting in a real highway.

We had our first full day on the floe on Saturday. It was a very busy day; some teams began doing logistical work offloading cargo from the Polarstern, building roads, and mapping out where infrastructures would be located on the floe. Others immediately began doing in-field observations such as sampling and taking profiles. Here Team Ocean cleared ice from a melt pond that had already drained into the ocean and conducted eight MSS profile measurements of the upper 380 metres! The Microstructure Sonde (MSS) profiler helps ocean scientists understand turbulent microstructure of the ocean layers.

The 4th expedition team is safely back in Bremerhaven today after nearly two weeks onboard Akademik Tryoshnikov. Within the last days they recovered instruments from different floes that were part of the distributed network. The instruments that we recovered can now be re-used on upcoming expeditions. After spending nearly 4 months closely together, the expedition ends with Russian cuisine, some residual working tasks, many scientific discussions, plenty of working out and a bit of relaxed time for the scientific team and some crew members of Polarstern. In the next days, the individuals will travel back to their homes in Europe and North America. Whether this group will come together again is uncertain, but they will always cherish these memories and the time spent together as one big shipboard family. They wish Leg 5 a continued good time on Polarstern and a successful final phase of the MOSAiC expedition.

Onboard the Polarstern, the scientists who joined for the final cruise leg and the last months of the MOSAiC field work underwent safety trainings for the ice these days. One of these safety trainings everybody must participate in is sea-ice self-rescue: we must be able to rescue ourselves if one falls into the ocean through the ice. To do so, we will carry ice picks with us and use those to grab hold of the ice and pull ourselves up onto the ice surface. Those people who participated in the safety trainings simulated this event in the Polarstern’s swimming pool with a wooden ice floe. This simulated exercise was certainly much warmer than the sea ice.

MOSAiC has a new home: the MOSAiC floe 2.0 is currently at roughly 87° 43‘ North and 104° 30‘ East, just 11 nm away from the path of the original MOSAiC floe as it drifted through this area in January 2020. Due to adverse weather conditions the helicopter could not be flown for the floe search and radar satellite data is of little help during the current season, because the floes are covered with melt ponds and are hardly visible in the data from the satellites. "We were limited to the ship's radar and to what we could see from the bridge in very foggy conditions. Fortunately, we directly passed a promising floe right at the beginning of our search, stopped the vessel and started to explore it on the ground," says Markus Rex, the expedition leader. He went with a small team on the ice to measure the thickness and get an impression of the structures of the floe. "We have an ideal piece of ice," he added after the exploration. The final parking position was defined based on the exploration and during the course of the day Captain Thomas Wunderlich manoeuvred Polarstern into her position for the rest of the expedition. The first scientific measurements have already started in parallel to the set-up of the research camp.

The Podcast “Frozen into the Arctic – with SWR1 on board the Research Vessel Polarstern” is one of three potential winners in the category “best innovation in the morning” of the German Radio Award. The nomination itself is considered to be an honour amongst all radio people, 430 submitted a proposal this year. “The nomination is just amazing! It shows that we take people in German along on our journey with innovative communication channels and that we inspire and delight as well as raise the awareness what a big threat climate change is for the Arctic. Besides our scientific work, also our expedition’s communications concept is a success. I am very happy that we – together with the SWR-team around Christiane von Wolff - were able to establish a format that obviously fascinates!” says MOSAiC leader Markus Rex.

In order to leave a lasting sign of our North Pole arrival, storekeeper Markus Plehn produced a steel plaque with the ship’s and the expedition’s name, and the date we arrived at 90 °North. MOSAiC leader Markus Rex and Polarstern Captain Thomas Wunderlich casted it over board after the reception we had on the bridge. Another ceremonial occasion during our visit at the world’s northernmost place.

After only six days of sailing north we reached the North Pole today! This year, satellite images showed that the ice cover north of Greenland to beyond 87 degrees north was surprisingly loose. That’s why we chose the most direct access towards the centre of the transpolar drift: north of Greenland farther northwards. We gathered on the bridge for a ceremonial occasion and tensely watched the compass while we approached 90 °N. Teams Ocean, Eco and BGC investigate the whole water column here and filled bottom water from the North Pole station into sample bottles as a souvenir for every expedition participant – many thanks for this great surprise!

On Sunday, we have deployed the first buoy of Leg 5 on one of the floes along our transit. Having finished the preparations of the buoy and dressed with a survival suit, Marcel Nicolaus from Team Ice and Mario Hoppman from Team Ocean climbed the mummy chair, which transported them onto the ice. Marcel checked its thickness and placed the buoy onto it. This buoy will contribute to the Global Telecommunication System (GTS) and monitor sea-ice drift and air pressure in this region of the Arctic. The plan is to position a few more buoys before reaching the new ice floe and also deploy the last ones on our way back out of the ice at our departure. Once arriving at the new floe, we plan to distribute more buoys around the floe in order to establish a small version of the 'Distributed Network' around the new central observatory.

After three weeks of no chocolate and candy bars onboard the Polarstern, the canteen has been restocked! For some scientists who stayed on board for Leg 5, this meant restocking on much needed supplies like milk and yoghurt, nut or dark chocolate, coconut bars, and candies, all of which keep scientists happy in the field when working on the ice. When we are on the ice, we expend a lot of energy and calories to keep warm and conduct our work. Snacks with sugars and fats help us keep our energy up. Chocolate also brings a lot of joy; it is a comfort food for many who are far away from home and missing friends and family.

We are on this yearlong expedition to obtain necessary data from the Central Arctic Ocean. Therefore, a really important task is to organize the data storage optimally to make it easily available. There is a so-called Sensor Web Application aiming to create a repository for managing platform, device and sensor-related metadata information. Then there is MOSAiC Central Storage (MCS): the central storage place for our raw data with an automatic back-up of the data. After each cruise leg, data are taken home and will be made available to the MOSAiC consortium via web access. Team Data provided a refresher to get familiar with the use of these important structures.

Much work for the helicopter crew: there are certain maintenance intervals for different parts of the machines. The team decided to advance everything that can be done meaningful. We want to conduct ice reconnaissance flights once we get further to the northeast: the availability of satellite data becomes rare in that region and the best way to find a suitable route will be from the air. That’s why we saw the helicopters outside the hangar for the first time during this cruise leg. The heli team needed to remove the blades for a check, what is impossible inside the narrow hangar. The helis will be prepared soon, now we think about means to arrange appropriate weather conditions.

We are making good progress on our way northwards and have travelled roughly 150 nm (about 277 kilometres) in the first 24 hours after we left our mooring position yesterday. This morning we had nearly open waters around the ship as we are in the so-called northeastern Greenland polynya: an area of open water, which is driven by offshore winds pushing the sea ice away from land. At the moment, the ice concentration north of Greenland is quite loose compared to areas further east. That's why we decided to steam more or less directly to the north, using low ice concentrations as long as possible before we turn eastwards and look for an ice floe that has properties comparable to the original MOSAiC floe.

This morning we waved a last goodbye to our colleagues from Leg 4. After the bunkering was finished during the night, there was the cast off at 9:00 am. Many people were standing outside waving goodbye and we saw the support vessel Akademik Tryoshnikov for roughly half an hour before we had our next appointment: the captain and the officers introduced themselves and procedures on board, which was followed by the obligatory safety drill. We are now steaming towards the Central Arctic to study the onset and early freezing phase of the sea ice - the last process until the whole annual cycle is covered. We are all looking forward to this final cruise leg of MOSAiC, go Leg 5!

Today is the official end of Leg 4 and the start of cruise leg no. 5! This means that the current team - "the hugging Leg" - leaves Polarstern and moves to Akademik Tryoshnikov while the new team takes over the MOSAiC science on board the German icebreaker. We had a really hardworking, passionate and supportive team from the beginning onwards and the atmosphere on board was really welcoming and caring for each other. The team accomplished an impressive amount of work always being motivated, kind, and joyful! The Leg 4 team leaves the ship happy to go home but at the same time with tears in their eyes. We want to thank every single person that contributed to the incredible effort and success of this awesome cruise leg! Thanks to all the effort you have all made: thanks to the crew, thanks to the scientists and all other supporters! Take care and a good journey home!

Monday morning was the time for our docking manoeuvre: Polarstern approached Akademik Tryoshnikov carefully and positioned her bow next to the stern of the Russian research icebreaker. Massive fenders are attached at Tryoshnikov’s side to protect both ships. Lines had been handed over and since noon we are moored side by side on high seas. Soon the hoses for the fuel were rolled out to be able to start bunkering. In parallel, first persons were transferred via mummy chair (a cage which can be lifted by crane from deck to deck carrying up to five people) to Polarstern to start scientific and ship related handovers. The bustling to and from the ships is going to characterize the following days and we hope things are running smoothly during this supply and handover.

Winds and ocean currents constantly move ice floes around. At some locations, these movements create tension that leads to ice openings, while at other locations ice floes are pushed together, creating 'ice mountains' - so-called ridges. In retrospect, the fourth MOSAiC team observed only little of those ice dynamics on the MOSAiC floe, although the ice surrounding the stable MOSAiC floe was very dynamic. This is also due to the fact that the ice concentration had decreased from mid-July onwards to such an extent that the floes could move around more freely. The ship's radar captured those movements by frequent scans of the surface of the ice in a radius of 3 nautical miles. The difference in brightness on the ship radar images shows deformed ice (bright) that consists of broken ice blocks and, undeformed, flat ice floes (dark).

After only five days of steaming the MOSAiC supply vessel, Akademik Tryoshnikov with leg 5 participants arrived near Polarstern tonight. We can see each other on the ships’ radars and are now going to have a first meeting of the captains and chief scientists to discuss cargo operations and handover. The transit North was quite calm, and we travelled with a speed of 12 to 15 knots. We used the time on board for teambuilding and last preparations for our arrival as well as recharging our batteries. Some of us were in close contact with our colleagues on board Polarstern to inform each other about the instruments to be recovered from the Distributed Network. Others are engaged to work on scientific publications before we will be in charge of all MOSAiC measurements from next week. And everybody was quite busy with the four delicious meals we were served every day: thank you Akademik Tryoshnikov crew for this - and the great atmosphere we experienced on board! The next days will be used for refueling, resupply, and handover before the final MOSAiC leg is going to start.

What are the secrets of a successful team exchange? Currently, we are busy with digitalizing our paper notes from the ice and lab work as well as packing our samples and ice cores. All types of instruments and other working gadgets are being sorted and prepared for the next team, or packed into a container to be sent home to Bremerhaven on the Tryoshnikov. Standard handover routines include cleaning our lab and office spaces and preparing everything for the exchange. Additionally, we coordinate handover meetings for the specific teams, so that we know who is going to be on which ship and that we have enough time to hand over our work procedures. This guarantees the continuity of high-quality scientific work, while the crews will manage the cargo operations on both ships. Cargo, in this case, means instruments, as well as equipment for recovery operations, and of course, fresh vegetables, fruits, and frozen goods.

After ten days of fog and dense cloud coverage, we can finally enjoy the sunshine again (since Wednesday night). This is a great opportunity to transform our normal office inside the ship to an outdoor office or to have at least the opportunity to take some breaks outside. Besides the good mood, this is also a great way to catch some vitamin D to get energized for the final phase of this teams’ ship time. Also, with a clear view not limited by fog it is much easier to spot the outstations that were built on the ice within a 50 km distance from the research camp. Our current focus of the expedition is the recovery of those stations. Based on GPS coordinates from the instruments we know where to find them. The recovery work goes according to plan. The Polarstern could already retrieve many instruments that originally should have been recovered later with the Tryoshnikov. We feel confident in retrieving even more instruments in the next days and bring them back onboard before we meet the next MOSAiC team.

The Ocean Team finally recovered its Cluster, an instrument that measures currents, turbulence, temperature, salinity and CO2 in the ocean at various depths. It went down to 75 m underneath the surface. It was previously located on an ice floe that broke off of the original MOSAiC Floe in May. For weeks the team would visit by helicopter to try to recover it manually, but there were a few obstacles like the fog, polar bears, and the weight of the instrument alone that got in the way. Within a week, the distance increased from 1/2 NM to 20 NM, making it more difficult to find. After locating the cluster, the Ocean Team with the working deck crew used cranes to bring the complicated mooring onboard the ship. The coordination between them went really well, as the instruments had to be lifted and lowered several times to get it safely onboard. The instruments are now back onboard and with them important data that tells us more about the Arctic Ocean.

For each cruise leg we have a logistics team. Our current team consists of eight people from Denmark, Norway, Sweden, Iceland, France, Czech Republic and Germany. Similar to many of the expedition participants, they are polar bear guards for the teams working on the ice. In total, we had 19 encounters during this phase of MOSAiC and every situation was solved in a good and safe way. Safety and protection of polar bears goes first. That´s why we have a bridge watch for the overview and coordination of the teams working on the ice as well as the “Lookout” at the stern of the ship. Each scientific group has its own bear guard. “When the visibility is good, we have much more time to react and retreat to the ship without any use of the flare gun to scare them away,” says Laura from the logistics team. “I am proud to support the scientists during their work on the ice, being responsible for safety."

MOSAiC is proud to have incredible female leadership on this project! Our current MOSAiC team on board comprises of 48 % of female scientists, with women holding 70% of the team leadership roles. Being a woman in the Science, Technology, Engineering, and Mathematics (STEM) industry certainly has its challenges, but their hard work, tenacity, and dedication to the work onboard and on the ice inspires us every day. As strong and passionate women in science, they are great role models to show the importance of women in STEM fields, pursuing one’s passions, and being empowered in science. “There are many challenges that come with such a large, multidisciplinary field campaign but for me, the joy of being on the ice and working with a fantastic team makes it all worth it,” says Ice Team Leader Melinda Webster.

Preparations for the upcoming crew exchange and supply are continued on board Polarstern. In the meantime, the Russian research icebreaker Akademik Tryoshnikov set sails in Bremerhaven, next destination: Arctic Ocean! Although we, the Leg 5 participants, had a great time in our hotel, we are really looking forward to starting our expedition after more than two weeks of quarantine. Big thanks go to the AWI’s logistics team that make our traveling possible despite the Corona pandemic. We will remember with how much engagement Eberhard Kohlberg and numerous colleagues cared for us. Therefore, we will not be resentful regarding his pragmatic nose swapping (‘don’t worry, there is nothing that could break and you are allowed to cry’).

As we finally left our floe, the MOSAiC team had a small farewell event and recalled the past months at the floe. In addition, the last weather balloon was started from the ship within the sea of small floes. As we said our last goodbyes to the floe, Holger Deckelmann prepared a radio sonde balloon for his routine measurements of an atmospheric profile of pressure, temperature, humidity, windspeed and -direction, that are also needed for weather forecasts by the German Weather Service. “I was so excited when the party was moved to 13:00 because I was deploying a weather balloon regularly at that time, which meant that everyone would be there to watch the last radio sonde balloon measurement of our floe.”

On Thursday, the MOSAiC floe finally broke into many pieces after we took down our research camp the day before. During Friday morning the pieces of ice (up to 50 m in diameter) drifted in all directions and the original MOSAiC floe does no longer exist. After the recovery of the MET hut on the floe, we left the last ice pieces that were once our MOSAiC floe. We said goodbye on the helideck together to the wonderful piece of ice that was our home for 10 months and has helped us better understand the Arctic. In the upcoming days, the MOSAiC Team will recover the outstations of the ice that were installed from the old central observatory from autumn 2019 in a distance up to 50 km. Thereafter we will meet the Tryoshnikov in this region and wait for the arrival of the last team of scientists who will complete the MOSAiC expedition.

After exactly 300 days of drifting with the MOSAiC floe, the international team started the dismantling of the research camp and evacuation of the floe on Wednesday. The experts’ timing was flawless: Just one day later the floe finally broke into several fragments, which will drift out to the open ocean of Fram Strait within the next few days. Momentarily, the floe fragments are five kilometres apart from the ice edge. Now the team will focus on the last piece of the puzzle in the annual cycle: the start of freezing at the end of the summer. For this phase they’ll be heading farther north, where the ice formation will soon begin.

During this cruise leg, the Atmosphere team operates the largest balloon on the MOSAiC project. The tethered balloon known as Beluga, flies up to 1000 m and measures atmospheric properties above the floe. “The vertical distributions of aerosol particles we measure help to understand the atmospheric boundary layer dynamics and cloud formation processes in the Arctic,” says Christian Pilz from TROPOS in Leipzig. The calm conditions throughout the last weeks were favourable for the measurements, since only strong winds prevent Beluga from operations. “Given the role of clouds in warming the Arctic, quantifying the energy fluxes between them and the surface is of key importance, and our balloon is a great platform to perform such measurements,” adds Michael Lonardi.

Ever heard of an ablation shield? While not complex in its design, ablation shields are very simple and highly effective in diminishing ice melt. At MOSAiC, ablation shields have mostly been built by constructing a wooden frame and stapling a white tarp onto it like a canvas. They are then strategically placed around instruments, huts, and installations where the ice is melting too quickly. The white tarp protects the areas underneath by reflecting sunlight and heat that would otherwise melt the surface. Co-cruise leader Matt Shupe has been making these throughout the cruise leg. “The ice surface around our installations will melt by a metre or more and we want to prevent these installations, like our MET tower, from falling down. So managing the ablation is essential.”

While the ice surrounding us is breaking apart, our MOSAiC floe chosen in October 2019 has an impressively stable base for our work on the ice. This floe, however,is slowly reaching the end of its life cycle as it arrives to the ice edge. Today the air temperature was 14 degrees Celsius at 300 m above the floe and the melting of the ice surface is constant. As our distance to the ice edge is continuously decreasing, we get more and more affected by the open ocean’s influence at our current location. Waves and swell on the ocean can accelerate the breaking of the floes in the marginal ice zone and with that our floe. This is the natural life cycle from the initial buildup, the growing, and the melting process until the breakup of the ice floe, which we have been able to capture the last ten months during MOSAiC.

The Arctic sea-ice extent is the lowest it’s ever been in July since the beginning of satellite observation. The sea-ice retreat is especially pronounced off the Siberian coast. Virtually from the outset, the year 2020 promised to be an anomalous one in terms of both ice thickness and ice drift. In addition, in June there was a cell of warm air that produced extremely high temperatures in Siberia and seriously impacted the sea-ice cover: in the Russian Arctic, roughly 1 million square kilometres less of the ocean is covered with ice than in the past 7 years. “Circa 40 percent more of the ocean than usual is ice-free,” explains Gunnar Spreen (Team Ice). “Introducing so much warmth into the system so early in the year has accelerated the melting of the ice. This has also been worsened by the low albedo at this time of year, when the sun sits high in the sky during the Polar Day, producing an especially pronounced feedback," adds Marcel Nicolaus (Team Ice). Albedo describes the extent to which sunlight is reflected back by a surface: ice-covered, white surfaces reflect more energy (high albedo), while dark patches of open water reflect less (low albedo).

For the past week, we have had a polar bear encounter every day! Different bears have been sighted around our floe. Some are very curious and have come to our research area to check out our scientific installations, while others keep their distance and continue on their way. The amount of bears we are seeing during this time of year does not surprise us, as we are closer to the ice edge, further south, and polar bear populations depend on sea ice and spots of open water for their food and their habitat. These sightings remind us to always be vigilant and safe while conducting science on our floe.

This week the Atmosphere team hosted scientists and crew at an 'open house' in their scientific containers so that we could learn more about their scientific goals for MOSAiC. There are five containers on the Polarstern’s bow that contain different instruments that help us to understand atmospheric processes, including particles in the atmosphere, their properties, and their formation; atmospheric radiation; cloud properties; and much more. Some of the instruments that they use to measure these are LIDARs that send out laser impulses to detect clouds and particles in the air, as well as cloud radars, particle counters, and mass spectrometers to look at trace gases that lead to particle formation. The onboard visitors enjoyed learning about what science activities are happening in these containers, as MOSAiC is about sharing knowledge so that we can be interdisciplinary in understanding the central arctic climate.

While the current MOSAiC team is hard-working on board and on the floe, around 80 participants from scientific and Polarstern crew of the fifth (and final) cruise leg prepare for departure in a hotel in Bremerhaven. Today’s dinner will be the first time, we are able to meet personally! The last six days we have been under strict room quarantine, which can be lifted after everybody has been tested negative for COVID-19 twice (see App post from 13 May to learn more about the precautionary measures). This might also be a relief for inhabitants and guests of the city of Bremerhaven, who seemed to worry about us: Some people called the hotel reception to ask if we were doing well, as several of us were leaning out of the windows to catch some sunlight. But there was no need to worry: the hotel crew did a brilliant job caring for us, thanks a lot! We are now going to start our safety briefings and trainings, to travel well prepared towards the Arctic in the beginning of August.

Miss Piggy has returned to the ice! Atmospheric Physicists Holger Deckelmann and Alexander Schulz brought the tethered balloon called “Miss Piggy” back on the floe. Alex felt it was a pioneering moment; for a more mobile setup, all equipment was put into Zarges boxes, instead of an equipment tent. Also, winch and cables had to become mobile to be prepared for potential changes in ice conditions. Holger felt very accomplished with their work, as Miss Piggy can now be easily operated and deployed and continue to conduct their science. Miss Piggy carries a fibre optics cable creating continuous profiles of temperatures in the first hundreds of metres above the surface to better understand the dynamics in the so-called atmospheric boundary layer, where all exchange processes between the atmosphere and the surface take place.

Can you guess how many airborne platforms the MOSAiC team can deploy on the ice floe? The MOSAiC team on a good weather day can have up to 5 airborne platforms on the floe: the helicopter, the fixed wing and copter drones, the tethered balloons “Beluga” and “Miss Piggy” and the weather balloons. The drones and balloons collect atmospheric data to study energy budgets, cloud properties, aerosols and atmospheric boundary layer dynamics while the helicopter may carry instruments (HELiPOD, EM-Bird, ALS Laser and cameras) that survey on a larger scale the ice surface of our floe and surrounding icescapes, as well as measuring trace gas and aerosol concentration in the atmosphere. To have these platforms in the air at the same time, the teams are constantly coordinating with each other and designate their instruments in certain areas so that they all remain at a safe distance from each other.

While the MOSAiC team collects samples and data from the ice, many scientists spend time in labs onboard the ship to conduct experiments and process the samples. ECO team members are among the most active users of lab containers on the ship. One will often find them in these containers with lights off, red headlamps, and many layers on, as the samples they process require 0 to 1 °C and minimal light exposure. Cold temperatures are maintained in the lab to mimic the Arctic environment. Lights are turned off to prevent artificial stimulation of biological activity. Microbiologist JP Balmonte processes an experiment in one of the cold container labs by filtering sub-ice water to analyse carbon and nitrogen isotope incorporation in microorganisms that are >0.2 micron in size and their gene expression patterns. We are constantly impressed by the long hours and endurance to conduct the ECO core and project-specific research.

We have comprehensive Search and Rescue plans ready for various potential accident scenarios during MOSAiC. This week the procedures were trained and tested in an exercise. The scenario staged for this exercise was a person breaking through the ice on a remote part of the floe and suffering from severe hypothermia. When the emergency call came via radio the predefined procedures were implemented: Immeadiately a rescue team was deployed to the emergency site. They resuced the victim and brought her back to the vessel safely, following the procedures for handling a case of hypothermia, which includes strictly horizontal transport with minimal movement of the victim. 35 minutes after the alarm the victim arrived at Polarstern's hospital.

Volunteering on board is part of our daily routine. Without the engagement of the MOSAiC teams ATMOS, BGC, ECO, ICE, OCEAN, Logistic, Data, and Media the whole science schedule would not be feasible. Every day we cover polar bear lookouts on the bridge and stern and bear guards on different spots on the ice. For conducting successful work on the ice, helping hands are needed for activities like launching balloon flights, assisting at ice coring or melt pond measurements, fishing long line recovery, etc. Also, logistical tasks on board or clean-up operations in labs or storages are supported by the entire MOSAiC team. We see us as one team instead of eight small groups, give and take is key for our onboard life! We are happy that this works out smoothly and that we have an appreciating and welcoming atmosphere onboard, as this is our home and daily life in the Arctic sea ice! With gratitude, thanks to all participants!

Today the current MOSAiC team onboard the Polarstern celebrates the success of the expedition so far. The current team has been on its way for 78 days and have about a month to go until the Russian icebreaker Tryoshnikov will arrive for the transfer of new scientists. At the same time, the scientists who will come onboard the Polarstern for the remainder of the expedition just arrived at a hotel in Bremerhaven where they will undergo quarantine for two weeks before the transfer. In addition to this, today is the 302nd day of the overall 389 days of the MOSAiC expedition - about three quarters of the ongoing expedition are done!

Yesterday we again had a polar bear visit our research camp. Right before the gangway was lowered for scientists to go out on the ice, a polar bear was reported on the portside of the vessel. The bear was very curious about our ship, as could be seen by its head raised in the air and standing calmly on large ice blocks. Once the bear went on exploring the camp and approaching installations which could potentially be harmful for the animal it was slowly chased out of the region for its protection.

Throughout the MOSAiC project, the MOSAiC teams give all of their energy and efforts to collect data in the field, as time on the ice is precious in light of its constant change and dynamic movements. To have successful days on the ice, it would not be possible without the great collaboration of the on-deck Polarstern crew! They give us the logistical support for all our research activities. They drive the winch for launching the CTD and nets and help when we need a hand. The crew and the MOSAiC scientists work together as a team and look out for each other. “We as the crew are always here, whether there are few or many scientists onboard, we are a family. When we collaborate in the best way we can, we achieve maximum results,” says Bosun Andreas Sedlak.

Greetings from Team ATMOSPHERE! Today we want to introduce the team's research topics in more detail: Our new ATMOS team consists of 15 scientists, and compared to earlier cruise legs, our measurements will provide an enhanced focus on the atmospheric boundary-layer. Some team members are here specifically to profile atmospheric turbulence, thermodynamic quantities, and aerosols and cloud properties from airborne instruments using unmanned aerial vehicles (UAVs) and tethered balloon systems. The other members of the ATMOS team are investigating a wide range of atmospheric processes at various scales, including atmospheric circulation and weather patterns; the boundary-layer and its aerosol properties; the phenomenon of low-level clouds; photochemical reaction cycles and halogen chemistry; the transfer of heat, momentum, and trace gases at the ocean-atmosphere-sea ice interface; and much more. There are many topics focused upon – good that the team has 15 persons of woman/man power!

How fast can one become a polar researcher? This is exactly Felix Linhardt has just experienced. As a studied measurement engineer and current PhD student of remote sensing, he was sent to the MOSAiC expedition as the third backup person after the MOSAiC roster shifted plans due to coronavirus. “With a phone call two weeks before the quarantine started, I was thrown into cold Arctic waters in the true sense of the word. It was a jumpstart to fall directly into the greatest Arctic expedition! But of course, it is also really nice to operate the instruments that I personally developed”. Equipped with newly developed and different spectrometers, he is responsible for the measurement of the melt ponds on the sea ice. He measures the depth and the backscattering radiation of the sunlight in melt ponds. Every day he is on the ice to document the steady growth of the melt ponds and learning what it means to be a true arctic researcher!

Yesterday's dynamic ice movements changed Polarstern's stable position at the floe: around midnight we got high ice pressure from the aft of the ship. The Polarstern was pushed 150 m forwards along the floe. We quickly turned off our instruments and unplugged the power cables connected between the ice to the ship. An ice floe tilted into the ruder underneath the stern of the ship. Through a pressure build-up, a hydraulic leakage formed at the helm, the leaking oil mist triggered a fire alarm on the bridge. Later that night the steering gear hydraulic pump unit was repaired. Our floe itself has not been affected much by the ice dynamics. To move back to a central position relative to the research camp Polarstern took a short loop back to our original parking spot at the floe. The maneuvering went well, as we successfully have returned to our former position at the floe. Since the evening we have power back at alle sites. Other ice work was cancelled that day.

Scientist Katy Shoemaker sorts through small organisms up to a size of 150 micron that the ECO team caught in their nets between 200 meters and the surface. These nets are deployed by attaching them on the Polarstern’s winch and deploying it into the ocean. Once the samples are collected, the ECO team runs them through a sieve and begins to pick out, categorize, and identify them. Sometimes to further observe the specimens, Katy will place one on a petri dish with some filtered saltwater and place the dish under the microscope. She then uses a camera connected to the microscope to capture detailed pictures of what she is observing. Here, she is looking at an amphipod, a small crustacean that is found in aquatic environments across the globe. There are over 9,900 amphipod species described so far around the world. The ECO team deploys these nets twice a week, collecting many samples that tell us more about the organisms that are foundational in the Arctic Ocean food web.

Months before the Polarstern takes off the plans for food supply are coordinated. To guarantee the freshness of groceries as long as possible, we strategically buy fruits and vegetables that last longer than others. The precise menu will be created later on board. On Polarstern we have several -5 °C and -20 °C cold rooms. There are specific cold storage rooms for dairy products, vegetables, and a cool dry room for noodles and flour. For MOSAiC, we have an emergency deep-freeze container in case food supply has to be shifted. We can always be sure that there is enough food on board! The next food delivery we will get from the Russian icebreaker Tryoshnikov in August. Fresh, easily perishable food like leaf lettuce and cucumber will be some of the items are served at first until they are all eaten! With lettuce and cucumber gone, for salad we currently have red and white cabbage, for fruits we have oranges and apples, and from the 10,800 eggs we consumed just about 3,800…

From today until tomorrow midday we are conducting a 24-hour research marathon! We aim to record a “snapshot” of the daily cycles in the arctic summer. We focus on the ice surface, the interface between ice/ocean and the water column down to 350 m depth. We analyse the surface of the sea ice by measurements of the microstructures in the snow, the light insolation above and underneath the ice and the characterisation of melt ponds. In the ocean we will record physical parameters (turbulence, temperature, salinity, fluorescence and oxygen), as well as organic particle flux and the biological activity of algae, zooplankton, and bacteria throughout a diurnal cycle. Our biogeochemists go into smaller scales and look into the methane production and consumption underneath the sea ice. Subsequent to this data collection, we will gain a greater understanding of the biogeochemical and physical diurnal cycles in the arctic summer.

Liyang and Bin, both participants of Leg 3, have now also finally arrived at home in Xiamen, China. This also marks the end of their almost six-month research trip, which they started in January 2020. After returning to Germany from Leg 3, the Corona pandemic prevented them from returning home for more than two weeks. Although the AWI logistics team organised several flights for them, these were cancelled one after the other. Even a 15-day sea transportation was considered, but again this option was cancelled as this sea transportation was interrupted due to Corona and will not be resumed before the end of the year. However, the AWI logistics team found a solution, and so Liyang and Bin finally got their visa from the Bremerhaven customs office and the tickets for the flight from Amsterdam airport. At the beginning of July, they finally arrived in Xiamen, China - where another 14-day hotel quarantine was awaiting them. After that Liyang may see his family again - and Bin can take care of the postponed preparations for starting his own new family: Because, originally, he had planned to get married in May. The MOSAiC team wishes the young couple all the best!

How is our drift progressing? Since we have returned to the floe, we have slowly drifted 25 nautical miles eastward in a circling pattern. During this time our drift pattern was mainly caused by the tides. Since end of June the drift has been dominated by increasing easterly winds and is taking us westwards. The drift velocities have alternated between stagnation and 0.7 knots. The current position of our floe is 75 nautical miles north from the sea ice edge. Sometime during this summer, the floe will end its life cycle at the ice edge, will break down and finally melt there.

Did you know that the Arctic summers are very foggy? The fog is formed by the transport of warm air to the north which then hits the cold sea ice surface. Here, moist air condenses above the cold ice surface and builds fog. This fog can sometimes delay our helicopter operations and thus the scientific airborne measurements. Since Polarstern is back in the ice, we had 17 days of fly weather. But nevertheless, the fog is a very interesting weather phenomenon the scientists are exploring through Albedo and continuous atmospheric measurements. Another weather event that we have often experienced in the Arctic is the fogbow. It is a form of a rainbow that builds by the light reflection in the water drops in the humid air, in this case in the fog. Though the water drops in the fog are much smaller than in the rain, the reflecting sun rays overlay one another, causing the fogbow to appear white instead of coloured. It is very pretty to see this bow in the Arctic sky!

The teams ICE and ECO accomplished their first ice coring on the ice ridge “Jaridge!” The ice ridge is made up of first and second-year ice. Looking down from top of the ice ridge to the left you see the pure white first-year ice, while when you look to the right you see second-year ice with much more shaded patterns on its surface. The Polarstern is also moored to that second-year ice as it will sustain the melting process longer than the first-year ice. The scientists at Jaridge site extracted 4 ice cores from the ridge. Additionally, they pumped and sampled water from 2 gap layers in-between thick ice layers. The ridge had a thickness of approx. 6 m with an unconsolidated bottom. Once the ice cores were drilled, they took temperature and salinity measurements. Later onboard, the cores are analyzed by the ECO team for processing such as identifying organisms in the ice ridge. These results help us improve our understanding of the impact ice ridges have in the Arctic ecosystem.

Onboard Polarstern self-care is very important. One of the ways scientists invest in self-care is through haircuts. Microbiologist JP Balmonte gives Physical Oceanographer Morven Muilwijk a haircut. “I finally feel more like myself, especially when we are working so hard during the expedition,” says Morven. These moments are really important to find time to relax and to ensure that they are caring for themselves, while the everyday work of the expedition just continues around them.

Again, we had visitors to the MOSAiC Floe! A few days ago, we spotted a young polar bear walking through our camp in the early evening. It came from the northeast and walked through the Central Observatory through Met City and paused at Ocean City. The bear was very curious and stuck its head into the ice hole and played with the GPS on top of the ocean instrument box. After fulfilling its own curiosity, it continued walking east beyond our floe. Luckily for the OCEAN Team scientists, the equipment was okay after the bear encounter! A few days later, in the early morning, a polar bear mother and her two cubs crossed outside of our camp and played with some flags. They quickly continued their way toward north. We are lucky that the timing of these encounters has been good so far as the approaching bears visited in the evenings while we were safe on board and we could watch them from afar.

In the new Met City, the Met Tower has finally been raised again! The Met Tower is perched on an ice ridge extending out into the central part of the floe. This sensor mast stands at 10 metres tall and required seven people to raise it. “After a week of hard work by many people it's really great to have the tower back in operation out here on the MOSAiC floe, continuing an important timeseries. In its new position it looks out over a variety of ice types with some amazing melt ponds forming in all directions," says Matt Shupe, Co-leader of the expedition. The mast, with its 19 sensors attached, continuously measures the physical and chemical state of the near-surface atmosphere as well as the surface properties like heat flux, temperatures, snow depth and snow particles. The tower is secured to a hut and by two ice anchors underneath the 5 m thick sea ice.

Greetings from the ECOsystem Team! We are eager to continue time series measurements, but with a special focus for this time of year: capturing the biological and ecological changes occurring in sea ice and seawater in response to the 24 hours of sunlight and the onset of melt. The main objective of our work is elucidating how ecosystem properties and processes evolve in response to these changing conditions. With constant sunlight, the food web is jump-started by the growth of sea ice algae and phytoplankton, whose activities and diversity we will investigate. As the physical and chemical conditions of both the sea ice and upper ocean rapidly change, we will adapt our sampling program to capture how these conditions impact microbial and animal communities. We also have close links to activities conducted by the Biogeochemistry, Ice, and Ocean teams, and look forward to our cross-cutting activities, including 24-hour sampling events.

Ever heard of a “pulka?” Pulkas are essential items for our daily field work on the ice. A pulka is a combination of a sled and small boat. They help us to carry our scientific gear, storage boxes for samples and ice cores, but also exchange clothes, and provisions out to the ice. Once packed, we cover and secure the load and pull the pulkas by harness or ropes behind us through the snow and ice. For the melt ponds we have to cross on our way to the different research sites, the pulkas are very practical as they float and the instruments and gear does not get wet. Crossing the melt ponds is sometimes challenging as the bottom of the ponds are often uneven and they can get up to our knees. Some of us prefer to wear the ropes around the hips, as it makes the pulka easier to pull. This gives us an idea of how a sled dog must feel - but it is fun as well!

From now on the long-range Acoustic Doppler Current Profiler (ADCP) of Team OCEAN is running! The instrument is placed underneath the sea ice and measures current velocities and direction in different layers of the ocean. Before installing, the team had to drill a large hole through the 2-metre thick ice. Then they attached two large straps from a snowmobile to the instrument and the snowmobile slowly backed up to carefully lower it into the ocean. Two metal chains were secured onto a wooden beam above the ice hole keeping the profiler instrument in a stable water depth. It went down 2 metres below the ice, and it will measure the entire water column down to approximately 800 metres in depth permanently throughout our stay at the floe. Depending on our drift position, the water depth varies between 700-880 m. The information of the ADCP is important in helping the MOSAiC scientists understand how the ocean current transports heat and nutrients throughout the Arctic Ocean.

Mondays are ice coring days for Team Ecosystem (ECO) and Team Biogeochemistry (BGC)! Here the teams use a large corer to sample a cylinder of ice from the surface to bottom of the sea ice. One of the scientists, Oliver Müller stuck his camera down the ice core hole to see what was underneath the ice. He found beautiful strings of Melosira, an algae that grows underneath the sea ice. These algae can be found in high concentrations during the spring and summer on the bottom of the ice. They are part of the base of the Arctic food web, providing nutrients to amphipods, tiny crustaceans, and higher trophic levels. “This was our first peek under the ice since returning to our ice floe and to see these beautiful strings of alga moving in the ocean currents was very exciting and motivating to get as many measurements done as possible,” says sea ice biologist Oliver.

Alexey Niubom was a participant in leg 3. When he arrived in Bremerhaven after almost five months, he just wanted to see his family in St. Petersburg as quickly as possible. But before 1 July there were no flights to Russia due to Corona - however, there were flights to Helsinki. Elena Tschertkowa-Paulenz from AWI Logistics checked with the border police in Finland, learning that from there it would be possible to cross the border into Russia by "personal means of transport". So she went to buy a used folding bike for Alexey. He took it with him on the plane to Helsinki, and there, Alexey and his bike boarded the train to Imatra. Finally, it was time to put the bike to good use. Because what would have been impossible by foot, was allowed by bike: crossing the border to Russia. And so Alexey was able to embrace his family shortly afterwards and was overjoyed that his two-year-old daughter still recognized him after such a long time.

After the ROV hut, ROV, and winch was delivered by the helicopter, the ICE team has deployed the ROV named “Beast” underneath the ice. The ROV is a remotely operated underwater vehicle that has 16 different sensors to measure ice conditions, light availability, and water column properties. Some of the optical properties it measures of the ice are transmitted radiance and the fraction of shortwave radiation in the ice, that is available at different levels of the water column. It also measures depth profiles descending to 100 m depth at a single location to get a vertical profile of light attenuation. It also has a multibeam scanning sonar for profiling the ice bottom topography, sensors that measure water properties like water pH and a CTD to measure the conductivity, temperature and depth of the ocean column. Even with the many sensors, the ROV also has accessories and can attach nets to collect zooplankton and an Acoustic Doppler Current Profiler to measure water current velocities.

Changes are happening on all scales at our research camp site! Since we arrived at the floe 9 days ago, our white snow-covered MOSAiC floe with a few blue melt ponds has changed to a much wetter landscape. If we look at a smaller scale, we can observe changes in the snow structure. The “depth hoar” snow crystals (left) that are produced by large temperature gradients in the winter, are now being replaced by rounded grains (right) as the temperature in the snow settles around 0°C. These isothermal conditions destroy the sharp edges of the snow crystal structures. This structural change in snow crystals is clearly visible, even without using a microscope.

The MOSAiC team has successfully implemented the basic infrastructure for the research camp. Now the teams are able to customize their research areas for their science and to the melting season. For this time of year, it is important that we mobilize our research sites by choosing light versions of measurement equipment and making them smaller. The customization of the research areas is highly intensive laborious work. As the ATMOS Team rebuilds Met City and Balloon Town, they install most of their equipment on the ice compared to other teams. The heavy equipment is carried by skidoos or helicopter. The ICE, ECO and BGC teams are more mobile and carry equipment with sledges every time they take ice cores or collect snow, ice and melt pond measurements. Team OCEAN builds holes for measurements in the water column. While each day is filled with a lot of hard work, the diligence of the scientists brings us closer to finally rebuilding the MOSAiC research site.

Greetings from Team OCEAN! On Leg 4, our small four-person party will continue to measure the properties of the ocean water, like salinity and temperature, and how they change between the different water layers. We expect the ice melt to create less saline surface waters and are curious to see how this fresh water input from the melting ice is distributed in the water. To do so, we have a probe called microstructure profiler that measures how strong the turbulence – our word for mixing – in the water is. Later in the season, when the growing algae beneath the ice have consumed the available nutrients in the sun-lit surface zone, our measurements are even more important: The deeper waters contain a large amount of nutrients, but only turbulence can bring them up to the sun-lit surface water where the organisms can grow. During Leg 4 we are looking forward to observing this process and putting a number on this upward nutrient flux.

Greetings from the Team ICE! We’re one of the biggest teams on this Leg – 14 in total! We’re thrilled to be arriving at a very interesting time: melt season is just beginning, and snow cover is already being replaced by melt ponds. Over the next few months, we will continue the time series measurements of the previous teams. All year long, they’ve been documenting the evolution of the sea ice and snow cover. Since the sun rose over the horizon in March, radiation from the sun has been affecting the snow and ice surfaces. Snow is already beginning to melt into ponds on the surface. These melt ponds increase how much of the sun’s energy is absorbed into the sea ice and underlying ocean and accelerate ice melt. This is a large part of what we’re focused on understanding during this Leg. Once the summer hits, the ice begins to melt quickly. We’re preparing for some very wet work over the next few months! Soon, we’ll be putting on our dry suits…

Now that the Team has surveyed the MOSAiC floe during phase 1, it is time to rebuild the basic infrastructure of the research sites – Phase 2! The basic infrastructure of the ice camp on the floe are power lines, flags and pathways so that the research sites have power and we can safely traverse the ice. The roads are especially important because they help guide us away from important areas where scientific observations and measurements are running. This also keeps our footprint on the floe as small as possible. While we begin to install infrastructure, the scientific teams have started to prepare the Phase 3. They prepare the setup of all the research sites. Our map of the floe assumes its shape more and more…

Greetings from Team Biogeochemistry! The new BGC team maintains interest in how climate-relevant gasses move between the ocean and the atmosphere. To observe these fluxes, we measure gas concentrations in seawater, sea ice, and air along with rates of consumption and production by microbial communities. For ex., in the ocean a unique subset of bacteria and archaea can regulate methane release into the atmosphere by consuming it for energy. To estimate the flux of the cloud forming and climate-cooling gas DMS in the air, we measure the production of its precursor DMSP from other microbes, primarily algae, in the water and ice. Our helicopter borne system HELiPOD measures then concentrations of carbon dioxide, methane, and ozone in the atmosphere as well as aerosol concentrations and physical data from the ice surface, to quantify fluxes of energy and trace gasses. Our Team’s goal is to characterize the cycles of elements, like Carbon and Sulphur, in the Central Arctic.

Since our return to the MOSAiC floe, there has been a lot of planning on which exact position we park Polarstern and where and how the teams rebuild their research sites on the dynamic ice floe. The team has decided upon three phases to set up the new camps. The first phase consists of the Team Leaders from the Atmosphere, Biogeochemistry, Ecosystem, Ice, and Ocean with expedition leader Markus Rex and co-lead Matt Shupe observing the conditions of the floe. To do this, a few survey teams of very experienced scientists went on the ice and mapped the ice floe and investigate the ice, snow and melt pond conditions. In one of the survey teams, scientists Julia Regnery and Amy Macfarlane drilled into the ice to measure its thickness as bear guard Delphin Ruche watched the icescape.

Today we parked at our final position at the floe! When we arrived, the sun was shining and the temperatures were mild. At the wind-protected side of the ship it felt even warm enough without a jacket. It was a warm, welcoming and exciting atmosphere on board. From the stern you could see a fogbow, a rainbow formed by fog above the ice surface. From today onwards we will set up our research camp, continue our measurements and will call this our new home for the next weeks!

We are back at the MOSAiC ice floe! It feels very good to have returned to our “home”. First impressions of the floe upon returning: “The thickest area of the floe, which we dubbed the fortress, has for the most part weathered the deformations in the spring quite well, and continues to offer a good basis for our research camp,” reports Prof Markus Rex, leader of the MOSAiC expedition.“In the summer that’s just now beginning, we’ll investigate the processes at work in the Arctic climate during the melting season in unprecedented detail,” stresses Markus Rex. These include ocean eddies, which are produced below the ice by ocean currents; how the thickness and properties of sea ice influence climate processes; the role of snow cover on sea ice; and the nature of the interplay between the sea ice, atmosphere and clouds.

The Polarstern has arrived at the MOSAiC floe! After 8 days of travel from the ice edge, the Polarstern has parked strategically next to the floe where the scientists and crew members will call home for the next months. Now it is time to explore the status of the floe and the research camp and to redeploy the equipment that was brought on board during the travel of Polarstern. Everyone will work together to complete the research camp quickly. First, the basic infrastructure with power lines and "roads" will be reestablished. Once that is installed on the ice, the remaining research equipment will be reinstalled and we are looking forward to observing the melt season. We are going to report on our floe in more detail tomorrow.

Team Ocean took advantage of our break above the Yermak Plateau (760 m water depth) and performed a Conductivity-Temperature-Depth (CTD) measurement in the open water. They used a small CTD sensor, attached to a handy fishing rod, that was lowered through the water column down to 450 metres. From these measurements they get information about the properties of the different layers in the ocean. The layering of the ocean is dependent on the density of different water masses, which changes with varying temperature and salinity. They measured a classical ocean column profile one would expect in this region: from the water surface to 40 metres depth, they detected a cold and slightly less saline well-mixed layer. Below this layer is another cold layer where the salinity of the ocean water increases with depth. Even deeper, at a depth of around 150 to 300 metres, they found a layer of much warmer and saline Atlantic Water. This Atlantic Water mass is a remainder of the Gulf Stream which brings warm water from the equator towards the north.

After 140 days on three different research vessels, we - the scientists and crew of Leg 3 - finally reached the port of Bremerhaven today in the early morning hours. We spent the last part of our journey on the German research vessels Maria S. Merian and Sonne. While the transit started with high wind speeds and waves, we could spend the following days in the sun, reading a book – or joining the well-known “bootcamp” from Polarstern to stay fit. During these 8 days at sea, however, one highlight was the first sunset after 2.5 months, last Friday. Having this said, we want to thank the crews of both ships for their warm welcome and support on board. Now, we are curious to experience the somehow changed world back home. As it is definitely different than 20 weeks ago.

Yesterday evening we had an impressive polar bear sighting! A mother and a spring cub were curiously inspecting the area around the ship while travelling north towards a sea hole. We could observe them safely onboard the ship, even when they approached our portside metres away. The team spent hours observing her and taking photos. For photographers onboard the leg, they appreciated the polar summer light as it provided good lighting for wildlife photos and a nice natural contrast between the polar bear’s fur, ice, and blues from the water. After two hours of inspecting many sides of the ship, they turned their sights back to the north and ice.

The Polarstern has travelled approximately 115 nautical miles from the ice edge towards the MOSAiC floe, with only 30 nautical miles to go. Since yesterday evening the ice stays under pressure which currently prevents the continuation of our track north. This means for us that we have to wait for the ice to release. However, this is no lost time for us: We use the time to do science and had our first ice station today. A group of scientists went out for collecting ice cores and samples from the melt ponds, as well as measuring the snow and ice thickness.

While transiting to the MOSAiC ice floe, scientists marveled at the beauty of the icescape: snow, ice, melt ponds… and algae. What? Yes! Many broken and flipped ice chunks revealed fabulous golden hues of brownish-green. Ecologists quickly speculated the identities of resident algae, while biogeochemists envisioned rapid fluxes of organic and inorganic compounds. All were united by an eagerness for spontaneous sampling. Transit science! After getting approval from the captain, the scientists sampled sea ice and spent an afternoon melting samples and viewing sea-ice algae under the microscope (A). For comparison, algae in seawater were also imaged (B). Microscopic analysis showed varying types and abundance of algal species in both habitats: a group of algae called diatoms dominated in sea ice, but dinoflagellates were prevalent in seawater. Similar concentrations of algae within and beneath the MOSAiC ice floe would warrant a grand celebration, especially for teams ECO and BGC.

As the Polarstern heads back to the MOSAiC floe, the team spends some of their downtime looking for wildlife! There is an abundance of wildlife the team has been spotting as we navigate the ice: Some melt ponds are green or brown with algae, polar cod and some seals can be seen towards the surface. Since we entered the ice, the seagulls are our steady companions who benefit from the under ice fauna that we bring up from our track. We have even spotted a few polar bear tracks. Perhaps the most exciting wildlife sighting we have had is of a minke whale, who surfaced between large pieces of ice. Some even saw the minke breach in the ice!

On board Polarstern: After saying goodbye to Maria S. Merian and Sonne we arrived at the ice edge in less than a day. Everybody on board was very excited to enter the sea ice. For some it was even the first time in their lives seeing sea ice and the wildlife of the Arctic. At the same time, the five scientific teams slowly run test measurements and prepare for the upcoming field campaign. After 40 days of waiting, it feels good to finally be able to start our engagement for MOSAiC.

On our way back to Bremerhaven: The next MOSAiC milestone is reached. We just finished another 5 exciting days. 5 days which have been used to handover all our knowledge and experiences about our MOSAiC floe of the last couple of months to the team of Leg 4. 5 days that we used to welcome the team of Leg 4 to their new home. Yesterday, the perspective finally changed. Now it was us onboard the German research vessels Maria S. Merian and Sonne watching the Leg 4 team on Polarstern. After countless goodbyes, some tears and final warm farewell words from Captain Wunderlich, the gangway between Polarstern and Maria S. Merian was lifted the last time. While the actual work starts now for the team on Polarstern, we are steaming southwards – and will start to process all our experiences of the last months. But especially, we are looking forward to our foreseen arrival in Bremerhaven.

RV Polarstern took off again, heading back to the floe! After five days, the transfer of scientists and crew is officially complete. The operation ended with smiling faces, hugs, and waving hands as the Maria S. Merian, Sonne, and Polarstern left Adventfjorden. For the team returning home after approx. 5 months, many are feeling a combination of sadness, exhaustion, and relief as they prepare to travel south. For the new MOSAiC team onboard the Polarstern an exciting phase in the ice – the melting season – has started. The scientists are motivated and looking forward to getting on the ice to continue the research in the Arctic summer.

9600 - that's the proud number of eggs eaten on board Polarstern by the participants of Leg 3. Also: 1960 kilograms of meat, 1100 kilograms of potatoes, 1040 kilograms of vegetables and 1440 litres of milk. And: The participants of Leg 3 emptied 76 glasses Nutella glasses.

While the exchange of team and ship crew is ongoing in the Isfjorden / Spitsbergen, we can take stock of Leg 3: In total, Polarstern has drifted a distance of almost 965 kilometres with the ice. During this time, there were 120 helicopter flights. There were 16 storm and 6 rainy days. Our measuring balloon, lovingly called "Miss Piggy", rose 30 times in the air. And at least 323 flags were removed from the floe. In addition, 213 ice cores were collected, which - put one behind the other - would give a length of 393 metres. Together they would weigh around 1769 kilograms.

For the first time on the MOSAiC expedition, the Polarstern Captain will be exchanged. After 260 days out at sea, Thomas Wunderlich will take over for Captain Stefan Schwarze for the remaining expedition. „Originally the Captain exchange was planned in April via airplane. “It is now a different challenge to take over the ship after Polarstern came out of the ice. We [the new team] have the advantage of being impartial [in continuing the work], but on the other hand, we have to newly establish the infrastructures on the floe. We have to be prepared for the challenges around dynamic ice such as thinner ice and building of leads. […] The challenge is now to continue with what is left behind,” said Polarstern Captain Thomas Wunderlich.

It’s done! 20 days ago, we left our MOSAiC floe. On the following transit to the ice edge, the ice did not really act in our favour. But on Tuesday, 23:10 ship’s time, we finally passed the ice edge. An emotional moment that came in the end much faster than expected as the ice edge was rather sharp. Circumstances which were already implied by the satellite images of the area. Thus, already a few minutes after passing the ice edge, we saw nothing else than a blue ocean in front of us. While the ice-covered ocean was a faithful companion for 119 days, we saw the first mountains of Svalbard on the port side since yesterday morning. During the night, we met the supply vessels Maria S. Merian and Sonne in front of the entry of the fjord, in which we now started the bunkering operations as well as the handover procedures between the ships. Now, a couple of exciting days are ahead of us in which we, from Leg 3, will recall all our memories and experiences from the time in the ice to pass them over to the different teams of Leg 4.

The ice gave us a tough time between the MOSAiC floe and ice edge – and even stopped us once in a while completely. Thus, we had to wait several times until the ice pressure released and we could continue our way southwards. However, us being on board Polarstern wanted to continue moving independent of the ice masses around us, especially as we missed our daily workouts on the floe. Therefore, several outdoor activities established on board. Already on Kapitan Dranitsyn, “Harold’s Hiking Club” (HHC) became famous: A quick walk around all outer decks of the ship – upstairs, downstairs, and back again. This tradition continues now on our way back on Polarstern. An hour later, a big group is gathering on the helicopter deck for the daily “Bootcamp”. At 15 different stations we train all parts of the body for endurance and strength – while watching the surrounding ice masses.

Despite having left the MOSAiC floe, quite a lot measurements – apart from the autonomous instruments left behind - continue on board Polarstern. One example are the nine containers of the ATMOS team on the bow and the monkeydeck. The instruments provide continuous data on the atmospheric state, trace gases, composition of aerosol particles and cloud properties. An extensive suite of instruments from various institutes is necessary to capture all relevant parameters. For the scientists and technicians responsible for these observations normal working routine continues until the handover to Leg 4. In fact, the opportunity to observe a transect from the dense multi-year ice to open water provides valuable insight into the spatial variability of the atmosphere and of chemical and microphysical processes over the Arctic sea ice cover.

Some action during “hold-up mode”: Maria S. Merian delivered three large rubber fenders to the Sonne. These fenders are buoys that are used as a buffer between two ships. When the Polarstern arrives in the fjord, the Sonne will dock next to her using the fenders to operate the exchange of cargo. The fender transaction in the Arctic waters was coordinated by the captains of Merian and Sonne. To transport the fenders to the Sonne, the Merian approached her on her port side. A crane lowered the fenders to a zodiac boat and three crewmen onboard the zodiac safely delivered them to the Sonne. The crew operating this exchange were especially secured by survival suits, giving them the necessary flotation assistance and insulation in an emergency case. The scientists safely watched the transaction and welcomed the distraction from the daily life on the vessel.

Throughout the 4 plus months of our expedition two members of the ice team have been noticed for their increasingly luxuriant beards. “For me, my beard is a technical piece of equipment—just like my ice axe and goggles,” says David Clemens-Sewall, Ph.D. candidate at Dartmouth College. “When it’s cold and sunny outside you need to keep your face warm but in a balaclava you risk fogging and icing your sunglasses. Beards are a practical and low-cost solution to this problem. I started growing mine January 17, the day I left the US. Five weeks of steady preparation aboard the Dranitsyn enabled my beard to be fully functional when we arrived at the MOSAiC floe.” While gloves, hats, and socks slowly wore down over leg 3, the beards just kept growing. “Beards do more than just keep your face warm while working in the Arctic,” adds Steven Fons, Ph.D. Candidate at the University of Maryland and NASA GSFC, “they also help you look good while doing it.”

Both the thick sea ice and the ice pressure cause ongoing tough times for the Polarstern in the high Arctic latitudes. However, when observing for several hours the same ice floe through the ships’ windows, the view is not getting tedious. Instead, the opposite is true: The ongoing temperatures above the freezing point lead to a fast and impressive change of the sea-ice surface. While a couple of days ago, we observed consistent white ice floes, these turned into increasing blue surfaces. The reason is the widespread snow melting due to the high temperatures. In addition, already snow-free areas allow for the formation of first melt ponds on the ice. These observations give clear evidence of the begin of the Arctic summer melt season in that area – one of the most exciting and important phases for the Arctic sea ice and its associated interdisciplinary processes.

During Leg 3, we set up several MOSAiC records – not only related to our scientific achievements on the ice but also the nature provided a valuable contribution. Thus, we experienced the widest temperature range from -39.5 °C up to 0 °C. We therefore used the topic of these changing temperatures over the course of our cruise leg and its numerous scientific consequences as a basic for a scientific seminar on board. Here, we learnt that, for example, the polar vortex broke down in mid-April and the atmospheric boundary layer raised during our time on board. At the same time, we observed decreasing ice growth rates due to the increasing air temperatures – even up to weak melting signals. The resulting interdisciplinary changes will cause much more discussions here on board and beyond.

On board Polarstern: Weather balloons are routinely launched four times daily by members of the ATMOS Team and German Weather Service from the helicopter deck. The measurements of temperature, humidity, pressure and wind in up to 35 km height are essential to characterize the atmospheric background for all measurements during MOSAiC. They are also vital for our daily weather briefing, since the data is sent home and contributes to an improved weather forecast in our region. During our cruise to Svalbard, we will increase the frequency of launches to eight per day once we reach proximity of the ice edge. We use the opportunity to gain more insights into the transition of the atmospheric state between ice-covered ocean and open sea water.

Yesterday, the Maria S. Merian had visitors: The Svalbard Rescue Service “Sysselmannen” deployed two Eurocopter AWSAR Super Pumas to practice their helicopter rescue operations. These helicopters are well equipped for polar conditions: they have a long flying range, de-icing equipment, and thermal cameras that allow them to fly for long ranges and during the night. They practiced approaching the Merian’s bow and had two Search and Rescue crewman drop down from cables onto the bow and redeploy back on the helicopter. Direct contact between the ship crew and rescue team was strictly avoided. This practice is important because the rescue teams must be prepared to carry out operations in extreme weather conditions and various situations. During this phase of the MOSAiC expedition the Polarstern is in range of the Svalbard Search and Rescue helicopter team, so in emergency cases they would be able to give us assistance onboard the ship in the high Arctic.

Meanwhile in the Arctic sea ice. Polarstern moves slowly but steady through the ice floes up here. Not an easy task as the ice pack is thick and really dense. To get a better overview and to find a suitable path through the frozen ocean, we use again our helicopters since a couple of days. These ice reconnaissance flights allow identifying areas which we can potentially maneuver easier through. Observations during these flights are in accordance with the daily satellite maps: Bigger lead systems are primarily east-west orientated and less dominant in north-south direction. However, once in a while we find small ice-free areas to drive through quicker – until the tides close them again. Here, only one thing helps: Waiting until the ice pressure decreases. Because patience has become one of our great strengths in recent months.

Greetings from aboard the Maria S. Merian and Sonne from Svalbard! After being on transit for one week, we arrived in Isfjorden / Spitsbergen this morning. Since we heard that Polarstern is delayed, we reduced speed from 12 to 8 knots to prevent waiting time in the fjord and save emissions. Yesterday the wind picked up somewhat, allowing the crew and scientists to watch 2-meter-high waves from the bridge and listening to the howling winds passing the ship decks. Now safe in the fjord, we are now waiting for Polarstern. She is expected to arrive around the end of this week.

On board Polarstern. Even though the surrounding sea ice is rather challenging for the progress southwards, it is still an interesting matter for one group on board: the sea-ice group. Together with many other helpers on board, the 12-person team conducts hourly ice observations on the bridge of the ship. These observations are based on an international standardized protocol. Here, we observe sea-ice concentration (How many tenths are ice-covered in the visible range around the ship?), sea-ice thickness and snow depth as well as the average floe sizes and the proportion of pressure ridges in the area. Such data are used later for the validation of satellite and model data of the ice-covered polar oceans.

Norwegian Sea: After 5 days of sailing, the Leg 4 team will arrive in Isfjorden/Spitsbergen on Monday. We have had a good transit with sunny weather and calm seas. Some of us saw dolphins and the blow from a whale from afar while underway. We have been spending our time in meetings, preparing for the transfer, and recharging for the weeks to come. We also spend our free-time with reading, knitting, sunbathing on the decks, and bonding together. We are very grateful for the amenities onboard the RV Maria S. Merian and RV Sonne such as a gym, sauna, and coffee machine. “Sonne for Maria“ that is the radio call from the Maria S. Merian to the Sonne. The two research vessels are in frequent contact with each other. They communicate about the track route and check that they are within the 5 nautical miles from each other, as this is a requirement for ships sailing together as a convoy of MOSAiC supply vessels.

On board Polarstern: Our progress through the ice is still slow. However, these conditions are not surprisingly: During spring, the Arctic sea ice cover is rather thick. Also, we are close to the entry of the export path of the central Arctic where especially the second-year ice of the area is transported southwards. A challenging task for our German icebreaker. However, life on board is still busy. Both the ship and the scientific documentations need to be prepared for the upcoming MOSAiC leg. Thus, we spent the days with sorting and cleaning the labs, last data entries and a detailed documentation of our work over the last weeks and months. Also, the surroundings of the ship are getting more active. In the beginning of the week, another polar bear visited us. In addition, many birds can be observed on the outer decks – and with a lot of luck sometimes even a seal.

There are many different Lidar systems operating during MOSAiC, all with very individual purposes. Some of them are capable of detecting the speed of reflecting particles in the air by the use of the Doppler effect. Two systems from the Universities of Trier (Germany) and Leeds (UK) are doing exactly that since the start of the expedition last September, thereby yielding continuous information on winds and turbulence in the lower atmospheric boundary layer. Due to the ever-changing ice situation during Leg 3, it was a challenge for Andreas Preusser from the ATMOS team to get them synced up nicely in order to let the Lasers intersect on designated locations over the floe. However, by now several of these 'virtual towers' could be generated. They reach up to 1000m in height, but are unfortunately invisible to the human eye.

Polarstern. We left our mosaic of floes in the late Saturday evening hours. The second-year ice surrounding us gives our old lady a hard time to break through – and slowed her progress completely down. Thus, the engines have been stopped on Monday morning and we started drifting again southwards with 0.2 knots. However, one scientist on board is happy about the forced stopping of the ship: Serdar Sakinan. The echo lot data from the ship gave evidence of fishes in the vicinity of the ship. A reason for him to unpack his long fishing lines again. And just the moment the line went into the water, he caught the first fish since December – happy Serdar! But now it’s time to continue our journey southwards with the power of the ship’s engines.

North Sea: The German research vessels RV Maria S. Merian and RV Sonne are sailing to Svalbard. Today we are passing by Stavanger/Southern Norway. Our departure yesterday went well. We put our face masks on and finally left the hotel after spending the last 17 days of quarantine. We were warmly welcomed by the crews and went through our proper safety training before leaving port. Some last interviews were made and then we waved goodbye to colleagues, families, and friends from afar. We will arrive to Svalbard on Sunday and the exchange can begin.

German Research Minister Anja Karliczek sent us a greeting message on board.

Now it is done, team 3 is leaving. The floe is cleanly tidied up and it doesn't remind us much of the MOSAiC floe we arrived at over 11 weeks ago. The floe that was selected as home floe for the MOSAiC drift over 7 months ago. A floe that has been explored and sampled from all imaginable sides in the last months better than any floe ever before. During the last days the ice around us has broken up even more. On Tuesday evening we were hit by a strong storm with wind peaks of more than 9 Bft, and the ice has lived up to the project name: A mosaic of many cracks has formed around Polarstern. Because the storm was well predicted, the important and large installations were removed from the ice floe and brought to the safety of the ship before. As the ice does not seem safe enough at the moment, it was decided to further reduce the instruments remaining on the ice during Polarstern's absence. On Thursday, our old lady's machines were turned on for this purpose. With great sensitivity, the ship was moved to a new parking position through the systems of channels in the ice. Thanks to countless helping hands on the ice and on the ship, the ice floe was then "tidied up" for 1.5 days in sunshine and temperatures around freezing point. All that remains on the ice is a variety of autonomous measuring systems, which continue to measure above, in and under the sea ice, even while Polarstern will not be here for the next three weeks. These systems will also help the next research team to find our mosaic again. On Saturday at 11 a.m. the crane brought the last hard-working helpers back on board - an emotional moment for all involved. Afterwards, an area next to the floe was washed free of ice for CTD and net work. This work was also completed in the evening hours. So our work here is done. We say goodbye and now break our way south. Soon team 4 will take over our work.

Various websites nowadays offer us the possibility to explore whole regions virtually in 3D models. „So why shouldn't this be possible for the floe of the largest Arctic expedition?“ Niels Fuchs (AWI) asked himself this question before the MOSAiC expedition and developed a method to derive such 3D models from highly resolved aerial photographs taken from the helicopter nadir camera. After aerial imaging is possible again in polar day, the time had come for Robert Ricker (AWI) to test the application method in a software on board successfully. The generated maps, so-called floemosaics, are used by Niels Fuchs and Gerit Birnbaum (AWI) to derive the spatio-temporal evolution of surface type classes on the MOSAiC floe and surrounding areas (<50km).

One main aim of the MOSAiC remote sensing program is to better understand satellite observations in terms of changing sea-ice and snow properties that can only be measured in situ. For this we use various instruments that measure with the same principles and at the same frequencies as existing and planned future satellite sensors. During Leg 3 we experienced a wide range of air temperatures, from a -39.4 °C minimum on 10 March to a 0 °C maximum on 19 April. The resulting melting and subsequent refreezing reduced the snow surface roughness and had a substantial impact on the observed microwave signatures.

Besides the already presented leisure activities on the ice, one type of sports is really unique here on board: Waterbasketball! Once or twice a week, up to 13 people walk down the stairs to the F-Deck to gather in the swimming pool of Polarstern. There, they play a weird, but highly entertaining mixture of swimming, basketball and sometimes a bit of wrestling. The matches can sometimes get really intense, but luckily no major injuries have occurred so far – maybe because our doctor joins in from time to time? After one hour in the quite cold water, shivering bodies with lots of smiling faces climb out of the pool – luckily there is the small sauna next door to warm up.

Meanwhile in Bremerhaven: The Leg IV team prepares for the team exchange to continue the expedition. The preparations for this transfer, however, are much different than what the team expected! The coronavirus has greatly impacted the progress of the expedition. In order to keep all participants safe and healthy, the team has currently undergone strict quarantine protocols with a local hotel. The participants started with individualized quarantine where they remained in their rooms for one week without any physical contact. The staff delivered food to their doors three times a day. The team was able to communicate to each other through online platforms. This made them feel more connected and positive during quarantine. The participants took two coronavirus nasal swab tests to ensure that they were healthy for the expedition. Fortunately, everyone has tested negative! Next week, the Leg IV team will go onboard two research vessels, Sonne and Maria S. Merian, to meet the Polarstern.

The Be-7 atmospheric deposition project is drawing to a close. For the past two legs of MOSAiC, members of Team Biogeochemistry have been measuring Be-7 (a naturally occurring radioactive isotope of the element beryllium) in snow, ice core and seawater samples in order to determine the rate of deposition of material from the atmosphere. Combined with our aerosol samples from which both Be-7 and concentrations of other chemical elements will be measured, the project will provide a unique dataset of the wintertime atmospheric deposition of biologically important elements, such as iron, to the central Arctic. This will be the first dataset of its kind.

One of the questions we keep asking ourselves is ‘What are the origin and sources of aerosol particles over sea ice?'. "This is important, because sea ice is a natural source to background aerosol in the remote Arctic and a vast reservoir of chemical species," says Markus Frey from the British Antarctic Survey. "These compounds are ultimately important as condensation and ice nuclei for cloud formation, and they are key players in the chemistry of the lower atmosphere," adds Julia Schmale from EPFL in Switzerland. To tackle this challenge we deploy a number of aerosol instruments across the central observatory that record data in high time resolution and continuously. This is combined with thorough program of surface snow, frost flowers and slush sampling, a collaboration between the teams ATMOS and ICE. These samples can then be analyzed in the laboratory to reveal their chemical fingerprint.

These days, our ice floe is changing as never before. The number of small cracks and leads within the central observatory increases daily. Also, the sea ice surrounding us is still highly dynamic which is also shown in the strong movement of MetCity Island – a frequently used reference point when looking out of the windows on the bridge. However, something else is attracting our attention recently: The ice floe is getting less crowded with scientific instruments. The reason is that the scientists on board start slowly to prepare to temporarily leave our MOSAiC floe. This implies not only the collection of last important measurements and samples but also to recover parts of the instruments on the floe. Thus, we make sure that not only the floe stays dynamic – but also us.

Zooplankton including copepods, are an important link in the Arctic food chain between phytoplankton and fish, seabirds, and even marine mammals. We have been doing experiments to measure copepod grazing rates on phytoplankton. To date, the phytoplankton levels have been too low to detect grazing. However, one copepod species (Metridia longa) seems to be busy eating something! Microscopic examination shows that they are full of bright, orange eggs that were produced by their larger cousin, Calanus hyperboreus. This week we did an experiment feeding Calanus eggs to some very hungry Metridia; they were very enthusiastic consumers of the eggs! Predation of Calanus eggs by Metridia could be an important source of mortality of Calanus in the spring in the Arctic.

While many of the measurements taken here on MOSAiC come from near the ice surface, they can relate to those taken hundreds of kilometres above. Steven Fons, a PhD candidate at the University of Maryland and NASA Goddard Space Flight Center, collects sea ice freeboard data that can be used to help validate NASA’s ICESat-2 satellite. ICESat-2 uses a precise laser beam to measure sea ice freeboard – the height of the ice above sea level – in order to calculate sea ice thickness around the globe. On Leg 3, scientists have collected extensive freeboard measurements both on the ice and with the helicopter laser scanner. Comparing these measurements taken on the floe to those made from ICESat-2, we can scale-up the data collected on MOSAiC to better understand the broader Arctic sea ice cover.

Clouds are important to the arctic climate and involve processes that differ from clouds at lower latitudes. "Generally, clouds consist of small liquid droplets, ice particles or a mixture of both. Even at temperatures as low as -38 °C the liquid usually forms first. When, if and how many ice particles are formed depends on many variables, like temperature, humidity, turbulence and the presence of aerosols," explains Martin Radenz. Chris Cox expands: "Clouds modify the atmospheric boundary layer and the surface energy budget. We are particularly interested in mixed-phase clouds, which contain both ice and liquid. The liquid layer has insulating and shading effects on the surface, both of which impact the growth and melt of sea ice. In late spring those liquid layers become more frequent reaching a peak by autumn.”

It is by the ocean mixed layer that oceanic heat is carried to the sea ice and atmosphere. During Leg 3, Team OCEAN produced an unprecedented time series of the late-winter changes of the mixed layer and the turbulence occurring therein. We find that the mixed layer has thickened by 80 m since the onset of the MOSAIC drift. Particularly strong changes in the ocean mixed layer have occurred during the cold period in early March. Furthermore, we observed highest levels of turbulence during the passage of storms, which most likely also contributed to the thickening of the mixed layer. Working out exactly what caused the pronounced changes throughout the MOSAIC drift will be an exciting challenge.

Due to the ice dynamics in March, our fast connection cable (fibre optic) to the meteorological city (Met City) had to be rescued and brought back to the ship. The cable network was replaced by our RadioLan system, which is a WiFi System for wide areas. Currently the system is also used in Ocean City and the Remote Sensing Site. The data team on board (Heidi Turpeinen and Frederic Tardeck, company FIELAX GmbH) is responsible for the data connection to the different 'cities' on the floe. Additionally they take care of the data infrastructure, data management, servers and backup of the scientific data. On the picture we see the two adjusting the RadioLan antenna from Ocean City.

For the Atmospheric Radiation Measurement (ARM) team, every day is full of new challenges. ARM Technical Project Manager John Bilberry says: “With 44 instruments on the ship, and 12 instruments at Met City to measure a full suite of atmospheric parameters, we stay busy with daily routine checks and maintenance.” All of the instruments run continuously, 24 hours a day 7 days a week. The suite of ARM instrumentation will ultimately help characterize the atmosphere and its interactions with the sea ice surface. Dean Greenamyer adds: “Whether it is route finding in bad weather over ridges and across leads to get to Met City, midnight radiosonde balloon launches or instrument repairs; we do whatever it takes to keep the data streams flowing.”

For the first time during Leg 3, a visit to two of the M-sites took place, namely to M3 and M5, currently located roughly 20 nautical miles (≈37 km) away from Polarstern. The M-sites are part of the distributed network and are home to autonomous buoys measuring sea ice and ocean parameters since the deployment during Leg 1. During perfectly sunny and calm conditions, Natalia Sukhikh and Torsten Kanzow from Team OCEAN flew out there by helicopter with Hannes Laubach and Delphin Ruché. At both sites we drilled a hole through the 2 m thick sea ice, so that we were able to take profiles of temperature and salinity down to a water depth of 250 m, using our extremely compact fishing rod CTD. These data are very much needed to assess the quality of the measurements of ocean buoys.

To observe the evolution of sea-ice thickness and snow depth on different ice types on our floe, the sea-ice physicists Robert Ricker and Stefanie Arndt from AWI walk regularly along fixed transect loops across the floe. Doing so, they pull a sledge with an electromagnetic measurement device to measure continuously the total sea-ice thickness, meaning sea-ice thickness plus snow depth. In addition, snow depth is measured every other step with a GPS-equipped snow probe. The results show nicely: Despite snow fall and strong snow drift over the last couple of weeks and months, snow depth increased on average by only 10 cm on second-year ice and 15 cm on first-year ice since October. However, the new snow got mainly accumulated close to the various formed ridges and on leads on our floe.

Since the end of the dark polar night, team ECO is excitedly waiting for the resumption of growth of the phytoplankton, which are single-celled algae that serve as the base of the Arctic food web. In the beginning of April, we detected a slight increase in our daily measurements of the still very low chlorophyll concentrations, which are a good general indicator of phytoplankton biomass. Looking at the data for a couple of days and in combination with the oceanographic measurements we figured out, however, that this increase was associated with a change in the water masses we are drifting over, rather than growing cells. We are now more influenced by Atlantic water masses, and the algae community reflects that with more bloom-forming diatoms species such as Thalassiosira. In the last week, we seemed to detect a further increase, and we are excited to see what the next days will bring.

Usually, we are daily on the ice to investigate the sea-ice properties and its interactions with the atmosphere and ocean. However, Wednesday and Sunday evenings are an exception. On those days, we change our routines and both scientists and crew of Polarstern meet on the ice for different leisure activities. A part of the group is going for a walk along the marked roads to visit the installations of other science teams on the ice. Other people just take their sport shoes and meet for frisbee and soccer games in the logistics’ area close to the ship on the ice. The joint active time on the ice causes not only red cheeks but also frees a lot of new energy for the upcoming tasks at the next day.

Since the beginning of MOSAiC, a camera team from the UFA Show&Factual is on board to document life and science over the course of the expedition. As media representatives, they are two exotics in the crowd of international scientists, experienced sea men and sensible instruments. Our team of Leg 3, Dieter Stürmer and Manuel Ernst, is working hard to keep their sensible filming equipment running also in these harsh conditions and therefore to follow the ongoing science on the floe on a daily basis. However, at times the camera is freezing, the filming drone is losing its way, and fingers are getting cold. But it’s definitely worth the effort. In the cutting room back home different documentaries for international audiences are produced out of hundreds of hours of filming material already now.

To better understand ocean mixing, we deploy daily our little ‘hippie’ in Ocean City. “We spent so much time with him, that I simply needed to name him!” explains Janin Schaffer from Team OCEAN. The little red-haired Hippie dives daily from the ocean surface down to 400 m depth. On his way he measures not only high-frequent ocean temperatures and salinities. Other specialized sensors reveal how turbulent the ocean is, i.e., how strong the water mixes vertically. Thus, we recorded enhanced mixing rates during a storm event with wind velocities peaking at 18 m/s that passed us last week. The wind not only caused heavy blowing snow but also drove strong vertical mixing in the upper 70 m of the ocean.

MOSAiC has experienced very persistent northerly air masses in March and April that were amongst other responsible for the fast drift towards the south. “Speaking from an atmospheric science perspective”, Julia Schmale says, “this meant that we observed very old Arctic air masses. Old means that this air has resided several weeks in the ice-covered high Arctic and is so well-mixed that trace gas and aerosol composition are very homogeneous and changes happen only very smoothly over the course of days.” The warm air mass intrusions in mid-April brought an abrupt change: The air originated from the north Atlantic where spring algal blooms emit gases to the air that are transformed into aerosol particles. “We saw first traces of methanesulfonic acid, an emission product of phytoplankton. Also clouds were very different and carried liquid droplets in contrast to the many ice crystals otherwise. This is a first glimpse to very exciting times of atmospheric chemistry and microbiology that lie ahead of us.”

During the last weeks, the bridge of Polarstern was constantly visited to get an overview on recently formed leads and ridges on our MOSAiC floe. On Wednesday this week, we experienced a new excitement: In the early morning hours, a polar bear was spotted in the central observatory. The first one since beginning of January. Here, the well-fed polar bear took a curious tour through our installations on the floe. On his way from Remote Sensing City to DroneVille the careful visitor followed even widely the marked roads. However, he seemed to be very pleased with our instruments in his “living room” – and nothing got damaged.

After successfully completing the first half of its more-than-a-year-long drift, the MOSAiC expedition suddenly faced unforeseen challenges, posed by the coronavirus pandemic. The massive restrictions on global travel hindered the third team exchange, which had been planned as an aerial transfer in early April. Since the current situation also means the international icebreakers that were originally meant to resupply the expedition are also prohibited from making any staff transfers, in the span of just a few weeks a completely new alternative plan was developed: The upcoming transfer will be completed out with the aid of the German research vessels RV Sonne and RV Maria S. Merian. As a result of the pandemic-related measures, both ships have just returned to Germany. Polarstern will meet the two vessels in calm waters off Svalbard, in order to carry out a complete personnel transfer (ca. 100 people), and an exchange of cargo and provisions. Once the process is complete, Polarstern and her new team will return to the ice and continue their expedition in the Arctic Ocean. Over the past months, Polarstern has rapidly advanced along her projected drift corridor; as a result, she is already between the North Pole and Fram Strait, i.e., fairly far to the south. For the upcoming logistical operation, this position is advantageous. Some instruments on the MOSAiC floe will continue recording autonomously until Polarstern returns, while others will be dismantled. Depending on how the drift progresses, the Ice Camp may be relocated closer to the North Pole. The possibility of tearing down and relocating the camp was always part of the planning scenarios, in the event that the ship drifted faster than anticipated. This would have only minimal effects on the expedition timeline.

Since Wednesday last week we experienced a real temperature roller coaster: Starting at about -30 °C, temperatures increased to above -2 °C before decreasing to about -20 °C – and finally rising above the freezing point close to the snow surface. With dropping temperatures these cycles took place within only one week! This warm air intrusion caused not only scientists sweating but also the snowpack on the floe. Due to the low temperatures between the warm phases, the snow went through so-called thaw-freeze cycles. These cause both a growth of the snow crystals and the formation of ice crystals within the snowpack. To observe these processes in more detail, we use computer tomography on board which allows us to get an amazing insight into the numerous snow crystals, as seen in the picture. Now, we are curious to track further changes of our growing crystals.

A new ATMOS site recently appeared close to Polarstern. It started when one of the Doppler Lidars (named "Flossy") had to be moved due to the power loss at Met City on March 11. Its new location close to Ocean City allowed for a continuation of wind-profiling on April 05. The 23-m mast (the "Newdle") was also disassembled after the storm. The BGC team offered a suitable part of one of their sampling sites and the "Newdle" was reestablished on April 14. Its meteorological measurements are important for understanding the interactions between the ice and the atmospheric boundary layer. A previously autonomous surface flux station (ASFS) and an ice-mass balance buoy were also installed nearby. Although damaged by a ridge event at L3 in February, the ASFS was repaired and retrofitted for operation on the camp's line power. Hence, it became an ASFS-CO. Together with Ocean City, the suite instruments capture both atmospheric & oceanographic processes in the most central part of our floe - just in time for some interesting weather-events in mid April.

A cruise like this one, seafarers Uwe Grundmann, Igor Hering and Tibor Fischer have never done, despite 90 years of seafaring experience between them. All three belong to the command team of Polarstern and are responsible for the safe navigation of the vessel. However, during MOSAiC, Polarstern is drifting with a floe. But the three gentlemen are on bridge watch as usual. During MOSAiC, one of their duties is to maintain a safe position of the ship on the floe. They monitor changes in the ice surrounding the vessel 24 hours a day in order that the ropes by which Polarstern is fixed to its ice anchors can be adjusted. The bridge personnel also maintain the safe position of the gangway, allowing scientists to enter or leave the floe in a safe way when needed.

There are different ways to determine the actual weekday. For example, based on the daily menu. On Thursdays and Sundays, ice cream is served as dessert. Fridays are fish days and, on Saturday, it’s soup for lunch. Knowing in addition that the sauna is open for women on Monday and Thursday, for men on Tuesday and Friday, and for everybody on all other days, solves the weekday challenge finally. However, the same is possible from the well-known scientific weekly routines: Monday is coring day, on Tuesday and Thursday the underwater vehicle is operated in ROV City, Thursdays and Fridays are water days in Ocean City for Team ECO and OCEAN, respectively, and Sunday morning is off for everybody. However, in the end, checking the calendar might be still the safest option.

ROV means Remotely Operated Vehicle and was specifically designed to the needs of AWI for remote dives under the sea ice. It measures ice topography below the sea surface and the solar radiation that penetrates through the snow and the sea ice into the upper water column. Pressure sensors ensure the determination of the exact diving depth. Next to a tent, from which the 140-kilogram heavy robot is lowered into the water through a hole in the ice, is a small control centre from which the ROV is operated and the data streams of the sensors are monitored. A 280-metre-long fiber optics cable ensures power supply and data collection. The whole area is known as ROV City. The main responsibility for ROV City is with Philipp Anhaus who is actively supported by ICE TEAM leader Stefanie Arndt (both AWI).

Dinitrogen monoxide (N2O), better-known as laughing gas, is not only a substance used in your kitchen to whip the cream, in the hospital for anesthesia, or in the engine to boost your racing car; it is also a powerful greenhouse gas on the IPCC list, which ranks the third right after CO2 and CH4. Ocean contributes about 20 percent to the natural N2O source. However, very little data is available in the Arctic Ocean, whereas the winter data is absent. An underway observation system for N2O measurement was deployed since Leg 3, maintained by Liyang Zhan from the Third Institute of Oceanography (Xiamen, China). With this instrument, high resolution N2O dataset in the surface water will be obtained, which will put one of the long missing pieces to the jigsaw puzzles of the global oceanic N2O Atlas.

Caught in the glare of Polarstern’s spotlights, a curious mother polar bear and her cub explore the MOSAiC ice camp – with this image Esther Horvath won the World Press Photo Award for an individual picture in the category “Environment”. “For me as a photographer, it is almost like taking home the Oscar in the film industry,” says the photographer of the Alfred Wegener Institute. The shot documents one of the first polar bear sightings during the MOSAiC expedition, one of the sensational moments in its first few weeks.

In order to be able to predict the possible course of the Polarstern’s drift in preparation for the campaign, satellite data were used to reconstruct the paths taken by the ice in recent years from the MOSAiC starting point. After about 180 days of drifting, it is safe to say that, up to now, the drift has actually occurred within the drift corridor based on the original data. However, a comparison of the data from previous years with the chronological course shows that the drift, since the turn of the year, has been fairly straightforward, first westwards and then southwards, without any major detours or loops. An analysis of the ship's GPS data shows that Polarstern drifted at an average speed of 0.23 knots (0.43 km/h). Consequently, the vessel reached its present position earlier than most drift scenarios have suggested. Now, the next 180 days will be exciting: a team of sea ice researchers is currently working to answer the question of how the ice around Polarstern will change over the next few months and when the ice edge might be reached. Continuously updated forecasts for the drift of the next 120 days, based on model data from several forecasting centres, can be downloaded here:

Fancy a coffee break? Using our new “Coffee Break” format on the MOSAiC Instagram channel @mosaic_expedition, participants of the MOSAiC expedition will be chatting about their experiences in the Central Arctic live. The series will kick off with photographer Esther Horvath and scientist Maria Josefa Verdugo tomorrow, 16 April, at 4 p.m. (CEST). Both were on the first leg of the expedition. We look forward to seeing you on Instagram @mosaic_expedition!

It took a while. To be exact 21 days. After the first lead opened between Polarstern and Met City on the 11th of March, there was no electrical power at the location. Cables had to be unplugged. To overcome the power loss, we operated a generator fed by a fuel drum. “Refueling” was hence added to the daily routine visit. While this solution worked nicely, the more power-intensive instrumentation had to be shut down. So the ATMOS team, together with the crew and logistics team, put in several efforts to re-establish the power line. But the ice was so dynamic that for long it was not feasible. Only recently the area has stabilized enough, and we could celebrate the reconnection on 1 April with a home-brewed coffee in the Met-City hut. Now, a week later, the line crosses four cracks and runs along a ridged lead. We keep a close eye on it, and we have an emergency phone list in place for a potential rescue mission over night. The effort is worthwhile, all instruments are operational again.

We are sending Easter Greetings to our beloved ones back home and all friends of MOSAIC. The friendly weather has brought us long hours of sun during the last days, so that despite icy temperatures we slowly start to feel that spring is approaching. How do we spend the Easter days? On Saturday evening we had a gathering on the ice floe for both scientists and crew members, and on Sunday a joint barbecue took place. We are fully aware, that activities like these represent a privilege in this extraordinary time.

German Research Minister Anja Karliczek sent us a greeting message on board.

Ludovic Bariteau, researcher at CIRES and NOAA in Boulder, Colorado, and Hans-Werner Jacobi, Research Director at the French National research Center CNRS in Grenoble, are part of the team measuring greenhouse and trace gases in the atmosphere from the vertical mast installed at the bow of Polarstern. “MOSAiC offers for the first time the possibility to perform such measurements throughout the winter period in the central Arctic," says Ludovic. Among these trace gases is ozone, which is ubiquitous in the terrestrial atmosphere and a major driver in atmospheric chemistry. “Since mid-March we encounter prolonged periods with low or even no ozone. That is exciting," reports Hans-Werner. The researchers expect that at the end of MOSAiC new conclusions concerning chemical processes in the Arctic atmosphere can be drawn from their measurements.

"We sometimes need to remind ourselves that our MOSAiC floe is floating on top of the 4000m deep ocean” are the introductory words of Janin Schaffer from Team OCEAN. But you become immediately aware of it once you enter the tent of “Ocean City” and watch Janin lowering the so-called CTD rosette with 12 big water bottles into the water. “Ocean City” is the water meeting point for the teams Ocean, BGC and ECO. It is always busy in the tent when scientists group around the CTD hole to sample water. Still, they do not just sample water for different lab studies. The true heart of the CTD are the sensors measuring e.g. temperature, salinity, dissolved oxygen, and nutrients continuously from the ocean surface down to the deep ocean.

Clouds are a key driver of the Earth’s climate. In order to develop reliable climate models our understanding of cloud formation in the remote polar atmosphere needs to be much improved. Markus Frey’s (British Antarctic Survey) research contribution within MOSAiC addresses an important gap in our understanding of Arctic clouds. Cloud formation requires water vapour and very small particles or aerosols, so-called cloud condensation nuclei or ice nucleating particles (INP), to which the water vapour can attach itself and which then grow into water drops or ice crystals. Markus Frey investigates the origin and fate of INPs above Arctic sea ice by collecting snow and ice samples as well as measuring atmospheric samples in up to 1000 metres above the ground using a tethered balloon.

The title on Andreas Winter`s overall is self-explanatory. Andreas studied electronic engineering, a professional background that would be an important requirement for anyone thinking about applying for a job as system administrator on Polarstern. And this is exactly what Andreas did, 11 years ago. Since then he has been the man for all work related to network tasks, PC and server problems, and the storage and accessibility of a huge amount of scientific data. It’s hard to believe that despite this challenging job, Andreas has found time to write a children`s book about the Polarstern, in which two cute penguins, Fritz & Pitt, educate and excite school-children about the work on Polarstern.

Talking about MOSAiC includes both the description of our MOSAiC floe and the surrounding sea-ice area. Thus, we aim to understand changes in sea-ice thickness of our floe relative to its surrounding and over the time period of the drift. End of last week we had therefore the possibility to use the EM-Bird the first time since October. The torpedo-shaped measurement device is towed underneath the helicopter and measures the sea-ice thickness by means of multi-frequency electromagnetic (EM) induction sounding. Due to perfect weather conditions, we could even go for two survey flights at the same day. During the respective 2-hour flights towards the north and south of us, we surveyed triangles with a total edge length of 160 nautical miles. Here, we observed many open leads in the North, while the sea ice seemed to be much more compacted in the South.

The moon or the Arctic? - 18m/s winds and 50m visibility due to wind-blown snow are a regular occurrence for scientists currently on board. One way to understand the effects of these storms on snow and ice conditions is to return regularly to the same site to conduct the same measurements and understand the changes which have occurred. Observations are not always made with high-tech equipment but Amy Macfarlane returned to find a two-week-old footprint in the snow – a scientific experiment itself. “After standing on a new snow dune with fresh snow, the compaction of this area means it becomes more resilient to the wind in comparison to the surrounding snow. After such strong winds the surrounding snow is redistributed leaving behind a raised footprint.” In the image you can see the wind direction from the previous storm as in the top left a small dune has gathered in the lee of Amy’s footprint.

David Clemens-Sewall is one of the most frequent visitors of our MOSAiC floe and has an enormous interest in its mass balance changes – in all weather conditions: “Today was perfect terrestrial laser scanning weather, clear skies, no wind, and a warm -29 °C. We use the laser scanner to make high resolution 3D maps of the snow surface so that we can study in detail where and how the snow changes. Because the snow cover modulates the transport of heat and momentum between the ice and the atmosphere it’s important to study it in detail. The scanner itself sits within a custom-built heater case (on top of the yellow tripod)—just like the operator it needs insulation to work in these temperatures. To collect the measurements I work on the laptop with the same software that I use to process the data back in my office. So sometimes it feels like I’m in my office—until I look up!”

"Miss Piggy" is the nickname of the balloon that measures in situ meteorological data. During MOSAiC, a dedicated base was built for Miss Piggy called "Balloon Town". The AWI team, consisting of Jürgen Graeser and Ralf Jaiser, conducts not only their own measurements, but also provides a service to other scientists, who deploy their instruments. The British Antarctic Survey samples aerosols, University of Colorado Boulder is interested in tropospheric ozone and the Institute for Tropospheric Research from Leipzig measures turbulence. Miss Piggy is a bit picky regarding her requirements for launch. Wind speeds at ground level should not exceed 20 km/h, while at the height of deployment 40 km/h is the limit for safe operation.

The man with the bow: On leg 3, Tomasz Krause is the Chief Steward on board. Smart appearance, always wearing white shirts with a black bow, properly shaved, in such a state Tomazc could cut a good figure on vessels such as Queen Elizabeth II as well. However, his heart beats for expeditions with Polarstern. Starting his career in the restaurant business, Tomasz has been a seaman now for more than 8 years. Since 2016, he has been a crew member of Polarstern and has already partaken in 10 Arctic and Antarctic expeditions. Together with his team of seven, Tomasz is not only responsible for the meal services three times a day, but also operates three nicely decorated kiosks where scientists and crew members can get everything one's heart desires. All kinds of beverages, candies, pretzel sticks as well as drugstore articles and souvenirs are on offer.

Gina Jozef is a PhD student at University of Colorado Boulder, involved in scientific drone operations during MOSAiC. “From an outsider’s perspective, it probably looks like we are playing with a toy plane all day, and maybe that’s true, but we are also collecting advanced atmospheric measurements of the boundary layer. Our drones, called DataHawks, can fly up to 1000 m high, measuring temperature, pressure, humidity, wind, and turbulence. Our scientific interest is to learn how the Arctic atmospheric boundary layer responds to changes in surface features. For example, with the many cracks opening up around the MOSAiC floe, we are able to measure the effects of leads on the downwind environment. When we fly, it takes about 10 minutes to complete some pre-flight checks, but once the plane is in the air, we sit back, relax, and let the autopilot guide the plane through the air for the 40-minute flight. We monitor it from the comfort of our lounge chairs at the new MOSAiC suburb, Droneville. It’s a pretty sweet life.”

Looking 5 weeks back, our surrounding was pitch-dark. While only in the beginning of March beautiful twilight colours dominated the sky above us, on 12 March, the sun finally rose the first time above the horizon. And now, not even 2.5 weeks later, it will not set anymore. With Polar Day having started yesterday, we can pack up our head lamps and unpack our sunglasses. This has also implications for the work on the ice. During the dark months, the visible area was determined by the light range of the headlamps and spotlights from the ship. Now, we are able to observe the endless ice desert around us with naked eyes. However, the countless reflections of the sunrays at the leads and ice blocks are not only tempting for dreaming – but also to think about new scientific questions.

Welcome back! Our leg 2 participants have arrived in Tromsø today. Three and a half weeks after the departure from the MOSAiC floe, the Russian icebreaker Kapitan Dranitsyn moored in the harbour around 7:00 am local time. After a four-day refueling stop in the ice with the Russian icebreaker Admiral Makarov in mid-March, the Kapitan Dranitsyn had to wait another week near the ice edge for suitable weather conditions to cross the Barents Sea. Wave heights had to be below five metres before the ship could make the last two days through open waters. During the long wait, the team of leg 2 was visited by a polar bear and could witness how the ice floes around them broke up in the huge swells. Today they have already passed immigration checks by the Norwegian authorities. Tomorrow, there will be a charter flight for the participants from Tromsø to Bremen, from where everybody will then travel to their home country or town, depending on the national measures against the Corona pandemic. Thank you, dear crew of the Kapitan Dranitsyn, for safe travels and great hospitality! And thank you, dear team of leg 2, for these memorable four months together in the Arctic! Big thank you also to NPI (Norwegian Polar Institute) and authorities of the City of Tromsø for support in these challenging times!

Harold Jager from the Netherlands is our Chief Pilot on board. After his pilot training with KLM he started an additional training program for helicopters in the late 1990ies. During the past 20 years he flew innumerable helicopter missions. Since 2015 he has been in close contact with the AWI, participating meanwhile in already 16 Polarstern expeditions. During the present period of MOSAiC, it is in the responsibility of Harold and his team to guarantee a smooth operation of Polarstern`s two helicopters type BK117-C1. The diverse tasks are in numerous hands. Beside Harold, we are glad to have a second pilot, Sebastian Drach from France, furthermore chief mechanic, Daniel Fuchs from Austria - being responsible for all the maintenance and technical checks – and last but not least Elena Maria Prieto Turienzo from Spain, as technical expert and flight assistant. A truly international team with a very important function for the whole expedition: Helicopter flight missions make scientific projects and measurements even possible!

This is the day Jennifer Hutchings, from Oregon State University, has been looking forward to for months. Jennifer is using buoys to monitor the ice motion around MOSAiC to better understand the interactions between ocean, ice, and atmosphere. She also supports the Sea Ice Dynamics Forecasting Experiment (SIDFEX) as the buoys she deploys capture meteorological measurements as well as scientific data about Arctic climate change. Presented with favourable flight conditions, Chief Pilot Harold Jager finally decided to determine an appropriate time slot for long deployment flights. On this day, Jennifer launched 6 new buoys in the Arctic ice pack, one of them at a distance of 80 miles from the Polarstern. During the next months, these buoys will drift with the ice southwards to arrive in the North Atlantic during summer, transmitting in total 12 months of valuable data about temperature, atmospheric pressure and GPS position.

David Clemens-Sewall is PhD student from Dartmouth College in the US and is on MOSAiC for the third and fourth Leg. Here on board, he is well known for requesting time for battery changes – a running gag. However, his work is surely not about batteries, but stress sensors: “March 21st was the equinox and to celebrate I finished changing the last stress sensor battery for this month. Stress sensors continuously measure pressures within the ice and will contribute to our understanding of ice dynamics. They are distributed throughout the MOSAiC ice floe—or should I say floes? Over the last two weeks we have observed the ice rending into smaller and smaller pieces such that our 9 stress sensors that were once on the same piece of ice are now on 6 separate pieces. Although these ice dynamics make logistics more challenging by accessing them by helicopter, understanding how and why sea ice breaks apart is important both for predicting future climate change and navigation.”

Even after two weeks, the dynamics of our floe do not calm down. Our ice floe with initially a diameter of several kilometres is getting smaller and smaller as a consequence of natural forces. Tides in the ocean and strong winds in the atmosphere are pushing the floe. At the same time and due to the same forces, the leads around us give space when the ice is relaxing again. These ongoing dynamics keep our personal tensions up: How will the floe look like tomorrow morning? Can we work on the ice? Which installations on the ice might need to be rescued? Can we use the powerline between the ship and the ice floe? Over the last days, the latter was not always the case as the ship was simply moving too much. Also, small “island states” that are not accessible by foot have formed. However, using the helicopters on board, allows to continue our measurements in these outposts with limitations. The expedition name “MOSAiC” says it all!