WAVE-INDUCED ABYSSAL RECIRCULATIONS

The forcing of abyssal recirculation gyres by cross-isopycnal mixing a nd wave fluxes near the deep western boundary is investigated. A three -layer isopycnal primitive equation model is applied in a series of ex periments to an idealized basin with bottom topography. In the absence of deep western boundary current instabilities, cross-isopycnal mixin g forces a cyclonic recirculation gyre, modified by topography, which is consistent with the traditional Stommel-Arons model. Instabilities of the boundary current fundamentally alter the mean basin-scale deep flow from a cyclonic recirculation to an anticyclonic recirculation. B ottom topography plays a key role in destabilizing the mean flow. The forcing mechanism for the interior recirculation is the horizontal div ergence of momentum and potential vorticity fluxes carried by topograp hic waves that are forced by the boundary current instabilities. The s trength of the recirculation gyre is linearly proportional to the kine tic energy of the waves, which is controlled in the present model by b ottom drag, and well predicted by a simple scale analysis. This is ess entially an adiabatic process. The addition of cross-isopycnal mixing forces the large-scale interior recirculation toward the pole, partial ly into boundary currents, through linear vorticity dynamics. Vorticit y budgets reveal three dynamical regimes for the eddy-driven flows, th e western boundary current, the recirculation region, and the interior . Similarities and differences between the mean flow and recent observ ations in the Brazil Basin are discussed.