The Atmospheric Response to Surface Stress Induced by Ocean Currents

Frontal air-sea interaction is typically cast in terms of the thermal impact of ocean fronts on the atmosphere. The effect of ocean current drag on the lower troposphere is assumed to be small, but of interest as it isolates the modulation of the surface stress independent of the thermally induced modulation of the boundary layer stability and hydrostatic pressure.

We have investigated the impact of drag induced by the Kuroshio Current in the winter East China Sea on the overlying atmosphere by using a regional atmospheric model. The ocean current enhances the wind stress curl compared to the impacts of the associated sea surface temperature (SST) front alone. In addition, the stress across the current direction, generating the stress divergence, is also enhanced weakly via the atmospheric adjustment to the oceanic curl. These modifications change the linear relationships between the wind stress curl (divergence) onto SST Laplacian or onto crosswind (downwind) SST gradient, known as coupling coefficients. The influence of the current induced drag beyond the sea surface is affected by advection and weaker than expected from Ekman pumping alone, but some clear signatures are found: sea surface pressure decreases just over the current axis, and precipitation increases over the southeast side of the current corresponding to the induced wind convergence. A linear reduced gravity model successfully capture the response in the marine atmospheric boundary layer, and it is found that the response strongly depends on the spatial scale of the ocean currents.

The strength of the response depending on the spatial scale can be indicated by a transfer function, which is a kind of a coupling coefficient that is expanded into the wave-number space. We will show the global transfer functions for both the mechanical and thermal effect, as functions of sea level anomaly and SST anomaly, respectively, by using satellite observation data.