Impacts of Seasonal and Interannual Sea Ice Changes on Arctic Ocean Stratification

Seasonal and interannual variations in Arctic Ocean stratification significantly influence the vertical exchange of heat, salt, nutrient fluxes and the surface ice cover. On the seasonal scale, Arctic stratification is mainly influenced by ice melting/freezing processes. We used a one-dimensional (1D) coupled sea ice-ocean model to understand the effects of ice melting/freezing processes on stratification and their feedback on the ice itself. This 1D model can accurately simulate observed seasonal changes in the vertical structure of the upper Arctic Ocean. Then, we prevent the model from releasing meltwater into the ocean or maintaining a constant ice cover during the melting season, in a series of decoupling experiments, which reveal the following points: In summer, meltwater has negative feedback on ice melting by insulating a portion of the solar radiation into the Near Surface Temperature Maximum (NSTM); sea ice changes primarily manifest as the well-known albedo feedback. In winter, meltwater has minimal impact in strongly stratified regions, however, in weakly stratified regions, meltwater promotes freezing by hindering the heat upward mixing from Atlantic warm water (AWW); In regions with less ice cover, if there is no meltwater to counteract the stronger mixing due to the winter atmosphere-ocean energy exchange, the AWW can mix dramatically upwards, and even melt the ice in winter. In contrast, if there is enough ice cover to insulate the atmosphere from the ocean, strong mixing will not occur, even without meltwater. The 1D-model study demonstrates that, as Arctic sea ice diminishes and Atlantification intensifies in the future, the impact of meltwater on the ice-ocean system will become increasingly significant. For multiyear scales, we utilized CIGAR historical ocean reanalysis (1961-2022) data and extensive in situ observations from the Arctic Ocean to investigate the long-term variations in Arctic Ocean stratification. The results show a strong correlation between stratification strength and freshwater content in the Arctic Ocean. However, over the past decade, while the freshwater content in the Beaufort Sea has remained regionally stable, stratification strength has shown a decline. This suggests that, with the retreat of sea ice, atmospheric energy input is becoming increasingly significant in influencing stratification.