The surface thermal signature and air–sea coupling over the Agulhas rings propagating in the South Atlantic Ocean interior

Abstract

The surface signature of Agulhas rings propagating across the South Atlantic Ocean is observed based on three independent data sets: Advanced Microwave Scanning Radiometer for the Earth Observing System/Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) (TMI/AMSR-E) satellite sea surface temperature, Argo profiling floats and a merged winds product derived from scatterometer observations and reanalysis results. A persistent pattern of cold (negative) sea surface temperature (SST) anomalies in the eddy core, with warm (positive) anomalies at the boundary, is revealed. This pattern contrasts with the classical idea of a warm core anticyclone. Taking advantage of a moving reference frame corresponding to the altimetry-detected Agulhas rings, modifications of the surface winds by the ocean-induced currents and SST gradients are evaluated using satellite SST and wind observations. As obtained, the averaged stationary thermal expression and mean eddy-induced circulation are coupled to the marine atmospheric boundary layer, leading to surface wind anomalies. Consequently, an average Ekman pumping associated with these mean surface wind variations consistently emerges. This average Ekman pumping is found to explain very well the SST anomaly signatures of the detected Agulhas rings. Particularly, this mechanism seems to be the key factor determining that these anticyclonic eddies exhibit stationary imprints of cold SST anomalies near their core centers. A residual phase with the maximum sea surface height (SSH) anomaly and wind speed anomaly is found to the right of the mean wind direction, apparently maintaining a coherent stationary thermal expression coupled to the marine atmospheric boundary layer.