Description: Changes in Arctic sea ice thickness are the result of complex interactions of the dynamic and variable ice cover with atmosphere and ocean. Most of the sea ice exiting the Arctic Ocean does so through Fram Strait, which is why long-term measurements of ice thickness at the end of the Transpolar Drift provide insight into the integrated signals of thermodynamic and dynamic influences along the pathways of Arctic sea ice. We present an updated summer (July–August) time series of extensive ice thickness surveys carried out at the end of the Transpolar Drift between 2001 and 2020. Overall, we see a more than 20 % thinning of modal ice thickness since 2001. A comparison of this time series with first preliminary results from the international Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) shows that the modal summer thickness of the MOSAiC floe and its wider vicinity are consistent with measurements from previous years at the end of the Transpolar Drift. By combining this unique time series with the Lagrangian sea ice tracking tool, ICETrack, and a simple thermodynamic sea ice growth model, we link the observed interannual ice thickness variability north of Fram Strait to increased drift speeds along the Transpolar Drift and the consequential variations in sea ice age. We also show that the increased influence of upward-directed ocean heat flux in the eastern marginal ice zones, termed Atlantification, is not only responsible for sea ice thinning in and around the Laptev Sea but also that the induced thickness anomalies persist beyond the Russian shelves and are potentially still measurable at the end of the Transpolar Drift after more than a year. With a tendency towards an even faster Transpolar Drift, winter sea ice growth will have less time to compensate for the impact processes, such as Atlantification, have on sea ice thickness in the eastern marginal ice zone, which will increasingly be felt in other parts of the sea-ice-covered Arctic.

Description: Fram Strait is the main exit gate for sea ice in the Arctic Ocean. Observations of changes in sea ice thickness (SIT) in this region are therefore an integration of time-varying changes along the pathways of sea ice that reaches Fram Strait. We present an extended time series of combined ground-based and airborne electromagnetic induction (EM) measurements of summer (July/August) SIT from within a selected area of interest (AOI, 81 to 86°N, 30°W to 20°E) between Svalbard and Northeastern Greenland, capturing the end of the Transpolar Drift. Measurements were taken within the framework of the regular IceBird Summer campaigns and ship-based expeditions conducted by the Alfred Wegener Institute for Polar and Marine Research between 2001 and 2020. While sea ice reaching the AOI was dominated by multi-year ice (ice older than two years) at the beginning of the time series, the fraction of second and first-year ice increased over the last decade. Mean and modal SIT decreased by about 0.5 m from 2001 to 2018. Minimum values were reached between 2016 and 2018, with 2016 showing the absolute minimum in modal SIT (approximately 1 m). Sea ice reaching the selected AOI was backtracked using the Lagrangian ice tracking tool, ICETrack. Resulting sea ice trajectories show that about 65% of the AOI-sampled ice originated from the Laptev Sea. The simple thermodynamic SIT model introduced by Thorndike (1992, T92) was utilized to model thermodynamic sea ice growth along the trajectories. The thermodynamic model generates ice thicknesses that are comparable to the modal thickness from EM measurements. T92 shows a general underestimation of AOI EM SIT for all years except 2016, when the modal AOI EM SIT is overestimated by about 0.4 m. This model overestimation was potentially connected to the increased upward ocean heat flux and more specifically a strong atlantification event in the regions of ice formation along the Russian shelves in 2015 (Polyakov, 2017).

Description: Melt ponds play a key role for the summery energy budget of the Arctic sea-ice surface. Observational data that enable an integrated understanding and improved formulation of the thermodynamic and hydrological pond system in global climate models are spatially and temporally limited. Previous studies of shallow water bathymetry of riverbeds and lakes, experimental studies above sea ice and increasing availability of high-resolution aerial sea ice imagery motivated us to investigate the possibilities to derive pond bathymetry from photogrammetric multi-view reconstruction of the summery ice surface topography. Based on dedicated flight grids and simple assumptions we were able to obtain pond depth with a mean deviation of 3.5 cm compared to manual in situ observations. The method is independent of pond color and sky conditions, which is an advantage over recently developed radiometric retrieval methods. We present the retrieval algorithm, including requirements to the data recording and survey planning, and a correction method for refraction at the air— pond interface. In addition, we show how the retrieved elevation model synergize with the initial image data to retrieve the water level of each individual pond from the visually determined pond exterior. The study points out the great potential to derive geometric and radiometric properties of the sea-ice surface emerging from the increasingly available image data recorded from UAVs or aircraft.