Influence of Physical Factors on Restratification of the Upper Water Column in Antarctic Coastal Polynyas

Antarctic coastal polynyas are hotspots of biological production with intensive springtime phytoplankton blooms that strongly depend on meltwater-induced restratification in the upper part of the water column. However, the fundamental physics that determine spatial inhomogeneity of the spring restratification remain unclear. Here, we investigate how different meltwaters affect springtime restratification and thus phytoplankton bloom in Antarctic coastal polynyas. A high-resolution coupled ice-shelf/sea-ice/ocean model is used to simulate an idealized coastal polynya similar to the Terra Nova Bay Polynya, Ross Sea, Antarctica. To evaluate the contribution of various meltwater sources, we conduct sensitivity simulations altering physical factors such as alongshore winds, ice shelf basal melt, and surface freshwater runoff. Our findings indicate that sea ice meltwater from offshore is the primary buoyancy source of polynya near-surface restratification, particularly in the outer-polynya region where chlorophyll concentration tends to be high. Downwelling-favorable alongshore winds can direct offshore sea ice away and prevent sea ice meltwater from entering the polynya region. Although the ice shelf basal meltwater can ascend to the polynya surface, much of it is mixed vertically over the water column and confined horizontally to a narrow coastal region, and thus does not contribute significantly to the polynya near-surface restratification. Surface runoff from ice shelf surface melt could contribute greatly to the polynya near-surface restratification. Nearby ice tongues and headlands strongly influence the restratification through modifying polynya circulation and meltwater transport pathways. Results of this study can help explain observed spatiotemporal variability in restratification and associated biological productivity in Antarctic coastal polynyas. Antarctic coastal polynyas are key habitats of regional marine ecosystems. During spring, the upper part of the polynya water column restratifies and forms a near-surface layer of low-salinity water. This process is important for springtime phytoplankton blooms, as the stable surface layer keeps phytoplankton in the well-lit region and enhances phytoplankton growth. Employing high-resolution models of idealized coastal polynyas, this work unravels the spatial variation of restratification processes in a polynya and investigates the physical factors that affect them. It shows that sea ice meltwater from offshore regions is the foremost contributor to the near-surface restratification in a polynya. Meanwhile, low-salinity water from basal melt of a neighboring floating ice shelf contributes little to the near-surface restratification in a polynya because much of the meltwater mixes vertically with ambient waters as it rises. This is in contrast to the sea ice meltwater being directly injected into the ocean surface. Freshwater runoff from the surface melt of an ice shelf is also directly injected into the polynya surface. However, due to earth rotation, it is often confined in a narrow coastal region next to the ice shelf and thus does not contribute to restratification in most of the polynya area. Biologically-important springtime near-surface restratification in Antarctic coastal polynyas varies spatially within a polynya Sea ice meltwater from regions offshore of the polynya is the primary buoyancy source of polynya near-surface restratification Ice shelf basal meltwater mixes over the water column during its ascent and contributes little to polynya near-surface restratification