| Summary: | Thesis (Ph.D.)--University of Washington, 2019 This dissertation explores key physical mechanisms that control upper ocean and sea ice variability in the Southern Ocean. The first portion of this work presents an observational analysis of wintertime upper ocean stability and pycnocline heat availability in the Antarctic sea ice zone. This analysis reveals that the southern Weddell Sea region, which features a weak upper ocean stratification and relatively strong thermocline, is preconditioned for exceptionally high rates of winter ventilation. In other open-ocean regions, such as the northern Ross Sea, the stronger winter stratification greatly limits the efficiency with which heat may be extracted from the pycnocline. The coupling between winter ice growth and upper ocean ventilation is further explored using an idealized 1D sea ice-ocean model. This model is used to simulate winter ice growth in different regions under identical surface forcing. Consistent with the observational analysis, these simulations show that the unique thermohaline structure of the Weddell Sea, specifically that near Maud Rise, facilitates a strong negative feedback to winter sea ice growth. For this region, the entrainment of heat into the mixed layer can maintain a near-constant ice thickness over much of winter. However, these simulations also reveal that this quasi-equilibrium is attained when the pycnocline is thin and supports a large vertical temperature gradient. Further experimentation demonstrates that the surface stress imparted by a powerful storm may upset this balance and lead to substantial ice melt. In simulations initialized with profiles from more strongly stratified regions, such as near the sea ice edge of the major polar gyres, the entrainment of heat into the mixed layer had weak impact on winter ice growth---even during periods of strong wind forcing. Thus, a key takeaway is that the thermodynamic coupling between winter sea ice growth and ocean ventilation has significant regional variability. This regionality ...
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