Air‐sea boundary layer dynamics in the presence of mesoscale surface currents

Abstract

In the presence of surface currents, a shear stress at the air‐sea interface is induced by the surface currents. In the case of a unidirectional current, a quadratic stress law leads to a stress curl proportional to and opposing the surface current vorticity even with a uniform wind. 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 upwelling and downwelling patterns associated with the surface‐current vorticity. These effects are analyzed for an idealized jet current, and for a physical situation characteristic of a Gulf Stream boundary ring along the Florida Keys, where the induced transport patterns may be important for onshore transport of fish and spiny lobster larvae, as well as for onshore transport to the Florida Keys of general flotsam transported past them by the Gulf Stream. The spindown time scale (t*) for a 1.5‐layer system is H/( ρ′cdVa) for a surface jet on the deformation radius scale (where H is the thickness of the surface layer, Va the surface wind speed, ρ′ the air to water density ratio and cd the surface drag coefficient) and increases for large horizontal scales in proportion to the current width squared. For a typical wind speed of 5 m/s and a density normalized drag coefficient ρ′cd= 2 × 10−6, t* is on the order of 1 month for a 30‐m surface layer. In the more general case of a stratified interior water column, the vorticity spindown directly affects only the potential vorticity of the surface layer and generally leads to subsurface velocity and vorticity maxima for mesoscale eddies and jets.