The Atmospheric Response to Surface Stress Induced by the Kuroshio Current in the East China Sea

Abstract:

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. Here, we use a regional atmospheric model to investigate the impact of drag induced by the Kuroshio Current in the winter East China Sea on the overlying atmosphere. 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 is used to explain the dynamics of the response in the marine atmospheric boundary layer. Transfer function, a kind of a coupling coefficient which is expanded in wave-number space, indicates the strength of the response as a function of the ocean current speed depending on the spatial scale.