Presenter: Yilang Xu (MIT-WHOI Joint Program)

Description:
Antarctic coastal polynyas are sites of strong winter mixing due to sea ice and dense water formation. Understanding the timing and magnitude of mixing in the polynya water column is vital to understanding variability in biological productivity and Antarctic Bottom Water formation in polynyas. However, the roles of local physical factors on the variability in polynya winter mixing remain largely unknown. Previous studies mostly characterize polynya mixing as a one- or two-dimensional process, and neglect the momentum transfer from winds into the ocean and/or the impacts of icescape and coastline geometry. 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 parameters that control the winter destratification timescales in Antarctic coastal polynyas. The high-resolution coupled model with ice shelf, sea ice, and ocean components qualitatively reproduces observed coastal polynyas and offshore sea ice plumes. It shows that 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, icescape or coastline geometry can modify sea ice and ocean circulations, thus influencing the dense water dispersal pathways and destratification in polynyas. Analytical scaling is developed and verified by numerical sensitivity simulations, and the analyses show that the destratification timescale is subject to the influence of offshore winds, alongshore winds, air temperature, initial ambient stratification, water depth, and the presence of ice tongues and coastal headlands. The results here will help explain the observed spatial and temporal variability in the stratification in Antarctic coastal polynyas.