Eddy Energetics and Momentum Transfer on the Antarctic Continental Slope

The Antarctic Slope Front (ASF) almost completely encircles the Antarctic continent, separating cold shelf waters from relatively warm Circumpolar Deep Water (CDW) at mid-depth offshore. Exchanges across the ASF transport CDW toward marine-terminating glaciers, and export Antarctic Bottom Water (AABW) to the abyssal ocean. Recent studies indicate that this exchange may be modulated by mesoscale eddies, which facilitate cross-slope exchanges via stirring along isopycnals and eddy bolus transport.

We employ an eddy-resolving process model of the Antarctic continental shelf and slope to investigate the generation of eddy energy over the slope, and its resulting impact on the cross-slope transport of mass and tracers. The model enforces realistic offshore ocean stratification over idealized shelf/slope bathymetry, in order to provide a realistic representation of the water masses in a configuration that can be analyzed cleanly. We impose a westward wind stress over the continental slope and buoyancy loss on the continental shelf, consistent with the observed easterly winds and brine rejection in coastal polynyas.

Over the continental slope the upper-ocean dynamics resembles the Antarctic Circumpolar Current (ACC): The wind-driven shoaling of the pycnocline to the north is resisted by baroclinic conversion of available potential energy to eddy kinetic energy, and eddy form stress transfers the wind-input westward momentum down from the surface. The situation in the deeper isopycnal layers (separating CDW from AABW) is quite different. There the isopycnals lie parallel to the continental slope, so available potential energy is removed by both the wind-driven mean overturning and the generation of baroclinic eddies. Instead potential energy is sourced from the buoyancy loss on the continental shelf, and advected to the continental slope. The resulting eddy form stress transfers westward momentum upward from the AABW layer, resulting in a convergence of momentum at mid-depth that drives the residual shoreward transport of CDW. The baroclinic the deep isopycnals, and thus the eddy kinetic energy supplied to the CDW layer, is sensitive to all aspects of the model forcing and geometry, raising the possibility that shoreward CDW transport around Antarctica may be localized to a few favorable locations.