Eddies and Mean Flow in the Antarctic Circumpolar Current
University of California San Diego,
9500 Gilman Dr., MC 0230,
La Jolla, CA 92037,
The Antarctic Circumpolar Current (ACC) is a multi-jet flow that encircles Antarctica, providing a Southern Ocean connection between the Atlantic, Indian, and Pacific Ocean basins. Exact repeat altimeters are highly successful at identifying eddy variability, and in the Southern Ocean they have clearly demonstrated that strong eddy variability is associated with the ACC. Altimeter observations by themselves do not distinguish the time invariant geoid from time invariant features of the dynamical ocean circulation, so efforts to study the mean flow of the ACC have proved more difficult than studying the eddies themselves.
Since the first altimetric studies, a number of strategies for studying mean circulation have been explored. One strategy is to assume that the ACC consists of meandering jets with Gaussian velocity profiles and to use an iterative process to reconstruct an estimated mean flow. This method can work well near large meandering jets but fails in places where jets do not meander and cannot capture large-scale background mean flows. A second strategy is to infer the mean sea surface height from hydrographic atlas data by determining dynamic topography relative to a known ``level of no motion''. This method can prove problematic in the Southern Ocean where recent in situ estimates have suggested that bottom velocities are as large as 2 to 4 cm/s. New satellite geoid observations (from GRACE) and new dynamic topography estimates developed from autonomous ALACE and ARGO floats now offer alternate strategies for assessing the large-scale mean flow of the ACC and its interactions with eddies, although these means offer coarser spatial resolution than meandering jet models. Thus merged sea surface height maps may ultimately need to combine aspects of a variety of different strategies. Altimeter-derived estimates of the mean ACC have shown the current to be strongly steered by topography with enhanced eddy variability in regions of strong topography. Newer data will allow these analyses to be refined to demonstrate more completely the interactions between eddies and mean flow in the ACC.