Feedbacks between ice cover, ocean stratification, and heat content in Ryder Bay, western Antarctic Peninsula

A multi-year, all-season time series of water column physical properties and sea ice conditions in Ryder Bay, at the western Antarctic Peninsula (WAP), is used to assess the effects on the ocean of varying ice cover. Reduced ice cover leads to increased mixing and heat loss in the winter. The reduction in stratification persists into the following summer, preconditioning the water column to a greater vertical extent of surface-driven mixing. This leads to an increased amount of heat from insolation being mixed down, affecting approximately the top 100m. The increased heat uptake in summer exceeds the heat lost the preceding winter, giving the initially counter-intuitive effect that enhanced winter cooling generates warmer temperatures in the following summer and autumn. This process is therefore a positive feedback on sea ice, as reduced sea ice leads to increased heat content in the ocean the following autumn. It also causes increased winter atmospheric temperatures due to the increased winter heat loss from the ocean. In the deeper part of the water column, heat and carbon stored in the Circumpolar Deep Water (CDW) layer are released by deep mixing events. At these depths, conditions are restored by advection and vertical mixing on multi-year timescales. In recent years, stronger deep mixing events in winter have led to a persistent reduction in CDW temperatures at the study site. Ocean glider data demonstrate the representativeness of these results across the wider region of Marguerite Bay, within which Ryder Bay is situated.

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