Eddy-Mediated Transport of Circumpolar Deep Water Across the Antarctic Shelf Break

Abstract:

The continental shelves of Antarctica produce the ocean's densest water, Antarctic Bottom Water (AABW), which ventilates over 50% of the sub-surface global ocean. The heat needed to melt marine-terminating Antarctic ice sheets and produce dense water is supplied by Circumpolar Deep Water (CDW), a relatively warm, mid-depth water mass found offshore. The onshore transport of CDW is obstructed by the Antarctic Slope Front (ASF), a westward current at the continental shelf break that almost completely encircles the continent. Relatively little is understood about the processes that control the exchange of water masses and shoreward heat transport across the ASF, due to a scarcity of observations and the prohibitive cost of simulating turbulent flows in this region. Using a process model of the ASF that resolves the mesoscale eddies at the shelf break, we show that the ASF is shaped by an interplay between the surface wind forcing, transport by mesoscale eddies, and the geometry of the continental shelf. Consequently the onshore transport of CDW and the properties of the outflowing AABW are strongly sensitive to the wind and buoyancy forcing at the ocean surface, and to the geometry of the continental shelf. The onshore mass transport of CDW occurs through an eddy thickness flux. We develop a scaling for this transport that accurately captures the strong sensitivity to forcing and geometry, which is largely controlled by the eddy kinetic energy (EKE) over the continental slope. We find that the EKE is enhanced in the CDW density classes over the continental slope, but cross-slope mixing is constrained by the strong topographic potential vorticity gradient. Our results offer an explanation for the substantial changes in the structure of the ASF around Antarctica, and provide insight into future rates of dense water production and shoreward heat transport around Antarctica.