Eddy momentum flux and its contribution to the Southern Ocean momentum balance

Abstract

A LARGE amount of momentum is transferred to the Southern Ocean by strong westerly winds. Analytical and numerical models have suggested that transient eddies may be important in transporting this momentum away from the region of wind forcing, either horizontally¹or vertically downwards where it is balanced by bottom topographic drag^2–5. There are, however, few long-term in situ observations of horizontal eddy momentum flux^6–8, and no large-scale measurements of vertical eddy fluxes, to test these models. As a result, the momentum balance of the Antarctic circumpolar current (ACC) remains uncertain, and the role of eddies controversial. Here we use Geosat satellite altimeter data to resolve directional eddy kinetic energy and horizontal eddy momentum flux in the ACC on fine spatial and temporal scales. The complex spatial distribution of surface eddy momentum flux is strongly influenced by bottom topography. The horizontal eddy momentum flux tends generally to concentrate the mean flow, although some regions of divergence are observed. Our results show that the zonally averaged horizontal eddy momentum flux from transient eddies is an order of magnitude too small, and in the wrong direction to directly balance the eastward momentum input from wind^1.