A NONLINEAR MECHANISM FOR MAINTAINING COASTALLY TRAPPED EASTERN BOUNDARY CURRENTS

Several versions of a 2 1/2-layer model are used to investigate a nonlinear mechanism for maintaining wind-driven currents trapped to an eastern ocean boundary. All the models neglect the advection terms in the momentum equations, retaining advection only in the continuity equations. Solutions are forced by a band of upwelling-favorable wind without curl. A possible solution to this system is a state of rest in which the upper layer pressure gradient everywhere balances the wind stress (the Sverdrup balance). As expected, solutions to a linearized version of the model adjust to this equilibrium state by the westward radiation of both mode-1 and mode-2 Rossby waves. In contrast, when the wind is sufficiently strong, the nonlinear solutions retain a reasonable coastal circulation (with an equatorward surface jet and a poleward undercurrent) even after 40 years of integration. The basic nonlinear process that causes this marked difference in response is the following: an onshore geostrophic flow, which compensates for offshore Ekman drift, is established by the radiation of mode-1 Rossby waves. and this current can be strong enough to reverse the propagation direction of mode-2 Rossby waves. As a consequence, the resulting eastern boundary currents are dynamically similar to the frictional, western boundary currents that occur in other models. The equilibrium state of the nonlinear solutions is presumably also a state of rest, but the diffusive adjustment occurs at such a slow rate that we have not been able to confirm this conclusion numerically.