Seasonal Variability of Sea Ice Bulk Density During the MOSAiC Expedition: Results From Synergistic In Situ, Airborne, and Satellite Observations

Satellite-based retrievals of Arctic sea ice thickness and its thermodynamic modeling typically assume a constant sea ice bulk density (IBD), disregarding its intricate seasonal variations influenced by factors such as ice age, temperature, brine content, and air intrusion. This study investigates the seasonal variability of IBD during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, spanning from late October 2019 to April 2020. To achieve this, we integrated sea ice thickness and snow depth observations from 15 ice mass balance buoys, snow bulk density measurements from snow pits, along with high-resolution total freeboard data from airborne laser scanning (ALS) and the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) within the MOSAiC Distributed Network (DN; 50 km radius). Assuming hydrostatic equilibrium, the regionally integrated IBD was calculated for the buoy-deployed level ice floes, covering 11 sites on second-year ice (SYI) and 4 sites on first-year ice (FYI). These buoy-based IBD estimates were then compared with direct measurements obtained at the MOSAiC Main Coring Sites (MCS). Our findings provide preliminary evidence of IBD seasonality during the Arctic freezing season: (1) From late October to early December 2019 (transition phase), the IBDs derived from both MCS-FYI and regional integration increased from ~880 to 910 kg m−3, possibly due to a reduced air volume fraction in the ice; (2) From late December 2019 to late April 2020 (quasi-stable phase), all IBDs largely persisted within 900−920 kg m−3, with modal values near 910 kg m−3, consistent with historical observations for the same period. However, the IBD at MCS-SYI remained relatively stable throughout the study period due to the low porosity of the sampled SYI cores. For cross-validation, the regional-scale IBD retrievals were further upscaled to characterize mean level ice conditions across the MOSAiC DN, yielding IBD seasonal patterns consistent with those observed at the buoy and coring sites. To better represent IBD in the contemporary Arctic Ocean, we propose a time-varying proxy model and explore multiple parameterizations based on the MOSAiC synergistic observations. Furthermore, the uncertainties associated with IBD retrievals and developed parameterization schemes are also assessed.