Estimating seasonal bulk density of level sea ice using the data derived from in situ and ICESat-2 synergistic observations during MOSAiC

Abstract. Satellite retrievals of Arctic sea ice thickness typically assume fixed values of sea ice bulk density (IBD), overlooking its seasonal evolution and spatial heterogeneity, which are influenced by factors such as the age, deformation, brine, and air inclusions of the sea ice. This study investigates the seasonal variability of IBD during the Arctic freezing season from October to April, across the Distributed Network (DN) scale of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. To estimate IBD, we combined sea ice and snow observations from ice mass balance buoys, snow pits, repeated transects, and ice cores, together with high-resolution along-track freeboard data obtained from airborne laser scanning (ALS) and the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2). Assuming hydrostatic equilibrium, IBDs were determined for the level ice components of the MOSAiC ice floes, which consisted predominantly of second-year ice (SYI). Our results revealed significant seasonal variability of the IBD with two main phases during the MOSAiC freezing season at scales of DN (~ 50 km), L-sites (~ 25 km), and Main Coring Site (MCS, ~ 50 m). Throughout the freezing season, the mean IBD estimated at the DN scale (910 ± 7 kg m⁻³) was close to that of the SYI cores at the MCS (912 ± 2 kg m⁻³), highlighting the SYI-dominated regional ice properties. We also identified that sea ice freeboard, along with the ratios of ice freeboard to total freeboard or ice freeboard to thickness, are critical indicators to determine IBD at the scale of tens of kilometers. We have therefore developed parameterizations for IBD that are expected to be applicable throughout the freezing season for the SYI region, which is also the ice type that currently dominates the central Arctic Ocean. The proposed parameterizations have the potential to optimize basin-scale IBD estimation and improve satellite-derived sea ice thickness.