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Sea ice thickness is a key parameter in the polar climate and ecosystem. Thermodynamic and dynamic processes alter the sea ice thickness. The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition provided a unique opportunity to study seasonal sea ice thickness changes of the same sea ice. We analyzed 11 large-scale (∼50 km) airborne electromagnetic sea thickness and surface roughness surveys from October 2019 to September 2020. Data from ice mass balance and position buoys provided additional information. We found that thermodynamic growth and decay dominated the seasonal cycle with a total mean sea ice thickness increase of 1.4 m (October 2019 to June 2020) and decay of 1.2 m (June 2020 to September 2020). Ice dynamics and deformation-related processes, such as thin ice formation in leads and subsequent ridging, broadened the ice thickness distribution and contributed 30% to the increase in mean thickness. These processes caused a 1-month delay between maximum thermodynamic sea ice thickness and maximum mean ice thickness. The airborne EM measurements bridged the scales from local floe-scale measurements to Arctic-wide satellite observations and model grid cells. The spatial differences in mean sea ice thickness between the Central Observatory (<10 km) of MOSAiC and the Distributed Network (<50 km) were negligible in fall and only 0.2 m in late winter, but the relative abundance of thin and thick ice varied. One unexpected outcome was the large dynamic thickening in a regime where divergence prevailed on average in the western Nansen Basin in spring. We suggest that the large dynamic thickening was due to the mobile, unconsolidated sea ice pack and periodic, sub-daily motion. We demonstrate that this Lagrangian sea ice thickness data set is well suited for validating the existing redistribution theory in sea ice models. Our comprehensive description of seasonal changes of the sea ice thickness distribution is valuable for interpreting MOSAiC time series across disciplines and can be used as a reference to advance sea ice thickness modeling. 

Abstract. In September 2019, the researchicebreaker Polarstern started the largest multidisciplinary Arctic expedition to date,the MOSAiC (Multidisciplinary drifting Observatory for the Study of ArcticClimate) drift experiment. Being moored to an ice floe for a whole year,thus including the winter season, the declared goal of the expedition is tobetter understand and quantify relevant processes within theatmosphere–ice–ocean system that impact the sea ice mass and energy budget,ultimately leading to much improved climate models. Satellite observations,atmospheric reanalysis data, and readings from a nearby meteorologicalstation indicate that the interplay of high ice export in late winter andexceptionally high air temperatures resulted in the longest ice-free summerperiod since reliable instrumental records began. We show, using aLagrangian tracking tool and a thermodynamic sea ice model, that the MOSAiCfloe carrying the Central Observatory (CO) formed in a polynya event northof the New Siberian Islands at the beginning of December 2018. The resultsfurther indicate that sea ice in the vicinity of the CO (<40 kmdistance) was younger and 36 % thinner than the surrounding ice withpotential consequences for ice dynamics and momentum and heat transferbetween ocean and atmosphere. Sea ice surveys carried out on variousreference floes in autumn 2019 verify this gradient in ice thickness, andsediments discovered in ice cores (so-called dirty sea ice) around the COconfirm contact with shallow waters in an early phase of growth, consistentwith the tracking analysis. Since less and less ice from the Siberianshelves survives its first summer (Krumpen et al., 2019), the MOSAiCexperiment provides the unique opportunity to study the role of sea ice as atransport medium for gases, macronutrients, iron, organic matter,sediments and pollutants from shelf areas to the central Arctic Ocean andbeyond. Compared to data for the past 26 years, the sea ice encountered atthe end of September 2019 can already be classified as exceptionally thin,and further predicted changes towards a seasonally ice-free ocean willlikely cut off the long-range transport of ice-rafted materials by theTranspolar Drift in the future. A reduced long-range transport of sea icewould have strong implications for the redistribution of biogeochemicalmatter in the central Arctic Ocean, with consequences for the balance ofclimate-relevant trace gases, primary production and biodiversity in theArctic Ocean. 

Year-round observations of the physical snow and ice properties and processes that govern the ice pack evolution and its interaction with the atmosphere and the ocean were conducted during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition of the research vessel Polarstern in the Arctic Ocean from October 2019 to September 2020. This work was embedded into the interdisciplinary design of the 5 MOSAiC teams, studying the atmosphere, the sea ice, the ocean, the ecosystem, and biogeochemical processes. The overall aim of the snow and sea ice observations during MOSAiC was to characterize the physical properties of the snow and ice cover comprehensively in the central Arctic over an entire annual cycle. This objective was achieved by detailed observations of physical properties and of energy and mass balance of snow and ice. By studying snow and sea ice dynamics over nested spatial scales from centimeters to tens of kilometers, the variability across scales can be considered. On-ice observations of in situ and remote sensing properties of the different surface types over all seasons will help to improve numerical process and climate models and to establish and validate novel satellite remote sensing methods; the linkages to accompanying airborne measurements, satellite observations, and results of numerical models are discussed. We found large spatial variabilities of snow metamorphism and thermal regimes impacting sea ice growth. We conclude that the highly variable snow cover needs to be considered in more detail (in observations, remote sensing, and models) to better understand snow-related feedback processes. The ice pack revealed rapid transformations and motions along the drift in all seasons. The number of coupled ice–ocean interface processes observed in detail are expected to guide upcoming research with respect to the changing Arctic sea ice.