| Summary: | The article of record as published may be found at https://doi.org/10.5194/tc-14-2173-2020 In September 2019, the research icebreaker Po larstern started the largest multidisciplinary Arctic expedi tion to date, the MOSAiC (Multidisciplinary drifting Obser vatory for the Study of Arctic Climate) drift experiment. Be ing moored to an ice floe for a whole year, thus including the winter season, the declared goal of the expedition is to better understand and quantify relevant processes within the atmosphere–ice–ocean system that impact the sea ice mass and energy budget, ultimately leading to much improved cli mate models. Satellite observations, atmospheric reanalysis data, and readings from a nearby meteorological station in dicate that the interplay of high ice export in late winter and exceptionally high air temperatures resulted in the longest ice-free summer period since reliable instrumental records began. We show, using a Lagrangian tracking tool and a ther modynamic sea ice model, that the MOSAiC floe carrying the Central Observatory (CO) formed in a polynya event north of the New Siberian Islands at the beginning of De cember 2018. The results further indicate that sea ice in the vicinity of the CO ( < 40 km distance) was younger and 36 % thinner than the surrounding ice with potential consequences for ice dynamics and momentum and heat transfer between ocean and atmosphere. Sea ice surveys carried out on vari ous reference floes in autumn 2019 verify this gradient in ice thickness, and sediments discovered in ice cores (so-called dirty sea ice) around the CO confirm contact with shallow (grant no. 03F0777A), (63A0028B) (grant no. AO OCE3562) (grant no. 640161) grant no. AO/1-6772/11/I-AM) German Ministry for Education and Research German Aerospace Center German Minsitry for Education and Research (MOSAiC20192020) EU H2020 European Space Agency
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