The effect of current-induced stress perturbation on large- and meso-scale air-sea interactions

The effect of the stress modulation by the surface currents is studied using a quadratic stress law, for various physical processes in the atmosphere-ocean system. In the presence of surface currents, a stress perturbation is induced by the current structure because of a modulation of the velocity shear at the air-sea interface. In the case of a unidirectional current and a uniform wind, it is proportional to and opposing the surface current vorticity. This causes a spindown effect on the surface vorticity field at a rate proportional to the wind speed. In the steady state, or in slowly varying processes which can be treated as parametrically developing quasi-steady states, the surface layer potential vorticity modulation causes up- and down-welling patterns associated with the surface-current vorticity. These effects are analyzed, first in a 2.5-layer quasi-geostrophic (QG) f-plane system and then in a two-layer as well as in a continuously stratified QG beta-plane system. The f-plane solution shows that the damping effect is effectively limited to the surface layer. The beta-plane solution shows that the perturbation stress induced by the surface current is an important damping mechanism for the baroclinic Rossby waves. The responses of the tropical atmosphere-ocean system to the current-induced stress perturbation are explored in a simple coupled model. We find that in the absence of mean currents the current-induced stress perturbation imposes a damping effect on both sides of the air-sea interface. In the presence of a mean current, the stress perturbation created by the mean current imposes a damping effect on the mean current itself, but can enhance the perturbation flow field through momentum exchanges between the two mediated by the interface stress. The coupling strength of the atmosphere-ocean system is thus modulated by the stress perturbation. Finally, the reactionary effect of the current induced stress perturbation on the atmospheric boundary-layer flow is briefly discussed. Computations for simple current patterns and constant large-scale winds show that substantial updraft at the top of the trade wind boundary layer can be produced by the ageostrophic transport divergence induced mechanically by an intense oceanic jet.