WIND-DRIVEN SECONDARY CIRCULATION IN OCEAN MESOSCALE

A two-dimensional, numerical circulation model is used to study the re sponse of a stratified, f-plane ocean current to wind stress forcing a t the surface. Nonhydrostatic, primitive equations are integrated on a 3 m vertical and 400 m horizontal grid in a periodic domain perpendic ular to the ocean current. Initially, a geostrophically balanced curre nt [V(i)(x, z)] with a maximum Rossby number of 0.16-0.8 is maintained against horizontal and vertical diffusion by a body force. A spatiall y uniform wind is applied along and across this jet. A secondary circu lation is created as a result of the nonlinear interaction between the jet and wind-driven flow in the Ekman layer. We present results from seven numerical experiments. When the wind blows in the direction of t he jet (against the jct), a narrow upwelling (downwelling) area and br oad downwelling (upwelling) area are formed. This secondary circulatio n pattern extends well below the mixed layer. When the wind blows perp endicular to the jet, the secondary circulation does not extend below the mixed layer. The fully nonlinear secondary circulation is 50% weak er than the circulation produced by the semi-linearized calculation ar ound the basic state, V(i). Near-inertial fluctuations appear and are confined to the negative relative vorticity side of the circulation (d VBAR/dx < 0). The time-averaged vertical velocity can be as high as 1. 5 m/day with a wind stress of 1 dyne/cm2 over a jct and a maximum Ross by number of 0.16. The magnitude of the vertical circulation in this s ymmetric basic state is dependent on the Rossby number and the horizon tal and vertical mixing coefficients.