Observations of Coupling between surface wind stress and Sea Ssurface Temperature at mid-latitudes

Recent analysis of QuikSCAT observations of surface wind stress and TRMM Microwave Imager (TMI) observations of sea surface temperature (SST) have revealed a close ocean-atmosphere coupling in the vicinity of the equatorial cold tongue in the eastern Pacific. As first hypothesized by Wallace et al. (1989), when the southeast tradewinds blow across the cold tongue, stabilization of the atmospheric boundary layer by the underlying cold water decouples the surface winds from the winds aloft, resulting in a decrease of the surface wind stress over the cold tongue. Destabilization of the boundary layer over the warmer water to the north enhances downward vertical turbulent mixing of momentum from aloft, resulting in increased surface wind stress. This coupling is manifested most clearly as linear relations between the wind stress divergence and the downwind SST gradient and between the wind stress curl and the crosswind SST gradient.

Analysis of QuikSCAT winds in the vicinities of midlatitude SST fronts reveals the same ocean-atmosphere coupling. The TMI does not sample poleward of 38 deg latitude and the prevalence of clouds at these higher latitudes limits the utility of satellite infrared observations of SST. SST is therefore be deduced from the weekly average blended Reynolds SST fields. The effects of SST gradients on the overlying winds become apparent after filtering the QuikSCAT data to remove the large-scale background wind field. Perturbations of isotherms of SST associated with topographically induced meandering of the midlatitude jets (the Antarctic Circumpolar Current, the Kuroshio Extension and the Gulf Stream Extension) induce perturbations of the surface wind stress field. As in the eastern tropical Pacific, the perturbation wind stress divergence and curl are linearly related to the perturbation downwind and crosswind SST gradients, respectively.