Ocean-atmosphere interaction over Agulhas Extension meanders

Five years of Lagrangian drifter and QuikSCAT measurements were used to study persistent meso-scale ocean-atmosphere coupling over the Aqulhas Extension Current (AEC). The drifter measurements locate the semi-permanent meanders of the AEC extending from the southern tip of the African continent east into the Indian Ocean. Cold sea surface temperature (SST), as measured by the Advanced Microwave Scanning Radiometer and Advanced Very High Resolution Radiometer, and high chlorophyll, as measured by the Sea-viewing Wide Field Sensor, are located with the clockwise current. Warm SST and low chlorophyll are located with anticlockwise currents. High chlorophyll is also found poleward of the strongest zonal current. The SST and chlorophyll measurements confirm the locations of current and its meanders revealed by the drifters.

With the large-scale zonal averages filtered out, the anomalies of equivalent neutral wind vectors (ENW) provided by QuikSCAT circulate in opposite direction to the currents, confirming the postulation that spaceborne scatterometers measure stress (the shear between wind and surface current) rather than the wind vector. The wind acts through stress to spin down the current. ENW was selected as the geophysical product of scatterometer to accommodate the stability (buoyancy to shear produce turbulence) dependence of the relation between wind and stress. ENW is found to be stronger in magnitude over the warm water of the meander than over the cold part. The increase of ENW with instability is consistent with turbulence theory and more momentum is also likely to be transported down to the surface over warmer water.

Westerly wind anomalies are collocated with warm SST anomalies, and easterlies are collocated with cold SST, so that surface wind convergence is in quadrature with SST. Evaporative cooling is enhanced over areas of collocated warmer water and higher wind speed and reduced over areas of the lower SST. The contemporary positive correlation are found among SST, wind speed, and latent heat flux over the meanders with the zonal mean removed, indicating a negative feedback; this is opposite to negative correlations between SST and wind speed and correlation between SST and latent heat flux when the means are not removed. The results infer that the observed feedback is scale-dependent.