The partition between barotropic and baroclinic motions in the Antarctic Circumpolar Current
Beatriz Peña-Molino

Authors

Peña-Molino B
Antarctic Climate & Ecosystems Cooperative Research Centre

Rintoul S
Antarctic Climate & Ecosystems Cooperative Research Centre, CSIRO

Mazloff M
Scripps Institution of Oceanography

Abstract

The Southern Ocean’s ability to store and transport heat and tracers as well as to dissipate momentum and energy are intimately related to the vertical structure of the Antarctic Circumpolar Current (ACC). Here the partition between barotropic and baroclinic flow in the ACC is investigated from a Southern Ocean state estimate. The flow is found to transition from an equal partition between the barotropic and first baroclinic modes in the deep basins, with approximately 45% of the absolute geostrophic flow projecting onto each mode, to values for the barotropic mode in excess of 60% in the vicinity of large topographic obstacles. This increase in the depth-independent component of the flow is mostly due to an increase in the bottom velocity, rather than a decrease in the vertical shear. The large-scale distribution of shear is shaped by changes in the bathymetry, with larger than average shear values around topography and smaller than average in unobstructed areas. The overall distribution of bottom velocity implies that baroclinic (defined now as bottom-referenced) transport streamlines and barotropic (bottom-derived) transport streamlines are not aligned, thus a transport by the bottom flow across the baroclinic transport streamlines occurs. And although this cross-streamline transport is not directly related to changes in the direction of the flow with depth, which are small in the fast moving waters within the ACC, it is very variable in space, and strongly dependent on the interactions between the deep flow and bathymetry, thus hard to inferred from surface and hydrographic observations alone.