Surface Eddy Momentum Flux and Velocity Variances in the Southern Ocean from Geosat Altimetry

Abstract

Satellite altimetry has previously been used to map the magnitude of the surface eddy variability of the global oceans, but the direction of the time-variable velocities have been more difficult to determine. Here, a technique is presented for resolving both magnitude and direction of residual surface geostrophic velocities at Geosat altimeter crossover points; providing a two-year time series with a temporal resolution of 17 days and horizontal resolution of around 100 km. The time series of residual velocity components are then used to determine surface eddy statistics in the Southern Ocean and to investigate the role of transient eddies in the Southern Ocean momentum balance. The surface eddy statistics from Geosat crossover points show a complex spatial distribution in the surface Reynolds stresses (u?2, v?2, u?v?). In contrast to the assumptions of isotropic variability in previous analyses of altimeter data, velocity variance ellipses are found to be distinctly anisotropic in many regions. The surface eddy statistics compare favorably with the available in situ current meter data and surface drifter data. The complex spatial distribution of surface eddy momentum flux is strongly influenced by bottom topography and the position of the mean current. On a zonal average, the horizontal divergence of eddy momentum flux from transient eddies is found to be around two orders of magnitude too small to directly balance the eastward momentum from the wind. In the Agulhas Return Current and near the Macquarie Ridge/Campbell Plateau, the Reynolds stress are significant and they act to concentrate the mean jet, in agreement with recent numerical models. However, circumpolar streamwise averages along paths parallel to the mean axis of the Antarctic Circumpolar Current show only a small net convergence.