It is well established from satellite observations of near-surface winds and SST from QuikSCAT and the Advanced Microwave Scanning Radiometer, as well as from numerical model simulations, that winds increase over warm water and decrease over cold water. This kind of mesoscale air-sea coupling is pronounced in all of the western boundary current systems and their extensions into the interior ocean. The meandering SST fronts associated with these current systems effectively impose quasi-stationary oceanic eddy forcing of the overlying atmosphere.

In order to understand the SST influence on the atmosphere and improve the accuracy of numerical model representations of mesoscale air-sea coupling in the regions of strong SST gradients associated with the major western boundary current systems, we have conducted sensitivity studies with the Weather Research and Forecasting (WRF) model for the Agulhas Return Current (ARC) region in the South Indian Ocean. Because it is far from land influence, the ARC region is ideally suited to studies of the SST influence on the atmosphere. We show that the resolution and accuracy of the SST boundary forcing and the accuracy of model parameterizations of vertical mixing in the marine atmospheric boundary layer (MABL) are the leading order factors in determining the accuracy of model simulations of this coupling.

The thermodynamic similarity between SST gradient forcing and topographic forcing of the near-surface wind variability suggests a possible connection between surface air-sea coupling and free tropospheric variability, analogous to the well-studied influence of topography on tropospheric winds. This hypothesis was tested with simulations made with the WRF modeling system for the ARC region. The quasi-stationary eddies of SST associated with the meandering ARC generate standing gravity waves whose origins are tied directly to the interaction of the ambient air flow with strong SST gradients. The vertical propagation of wave motions induced by the SST front is evident in the vertical structure of the horizontal divergence and vertical velocity fields. The results of the experiments suggest that the generation of frontal gravity waves and their upward propagation and associated eddy fluxes can play an important role in the coupling between the SST front, the MABL and the troposphere.