Processes of Stratification Breakdown and Restratification in Antarctic Coastal Polynyas

Yilang Xu, Ph.D., 2024
Weifeng (Gordon) Zhang, Advisor

Antarctic coastal polynyas are areas of persistent open water surrounded by sea ice. They are characterized by deep winter mixing due to dense water formation from sea ice production and elevated biological productivity after spring restratification. Antarctic coastal polynyas are diverse in terms of their mixing and stratification pattern, as well as the associated biological productivity. Here, we combine satellite and in situ observations, idealized numerical models, and analytical scaling to investigate the three-dimensional polynya circulation and explore the physical factors that control the winter destratification and springtime restratification in coastal polynyas. The high-resolution coupled model with ice shelf, sea ice, and ocean components qualitatively reproduces the observed coastal polynyas and sea ice fields, as evidenced by satellite measurements. In winter, strong offshore ocean currents driven by offshore katabatic winds carry some newly-formed dense water away from the polynya, weakening the destratification rate in the polynya water column. In contrast, coastal easterly winds induce onshore Ekman transport, constrain dense water outflows, and intensify vertical mixing. Moreover, an ice tongue and coastline geometry can modify sea ice and ocean circulations, thus influencing the dense water dispersal pathways and destratification in polynyas. In spring, offshore-originating sea ice meltwater primarily drives polynya restratification in the top 100 m of the water column. Even though ice shelf basal meltwater can ascend to the polynya surface, much of it is mixed over the upper 100–200 m and does not have a significant contribution to the near-surface restratification. The surface runoff from the ice shelf surface melt could potentially contribute significantly to the near-surface restratification, but its magnitude is less constrained with high uncertainty. This thesis provides a framework to study mixing and stratification dynamics in Antarctic coastal polynyas. It helps to explain their associated variabilities in dense water formation and biological productivity.