NUMERICAL-SIMULATION OF FLOW AROUND A TALL ISOLATED SEAMOUNT .2. RESONANT GENERATION OF TRAPPED WAVES

A sigma-coordinate, primitive equation ocean circulation model is used to explore the problem of the resonant generation of trapped waves ab out a tall, circular, isolated seamount by an incident oscillatory bar otropic current. The numerical solutions are used to extend prior stud ies into the fully nonlinear regime, and in particular to quantify and interpret the occurrence of residual circulation. Specific attention is also devoted to the dependence of the resonance and rectification m echanisms on stratification, forcing frequency, and choice of subgrid- scale viscous closure. Resonantly generated trapped waves of significa nt amplitude are found to occur broadly in parameter space; a precise match between the frequency of the imposed incident current and the fr equency of the trapped free wave is not necessary to produce substanti al excitation of the trapped wave. The maximum amplification factors p roduced in these numerical solutions, O(100) times the strength of the incident current, are consistent with previous studies. In the presen ce of nonlinear advection, strong residual currents are produced. The time-mean circulation about the seamount is dominated by a strong bott om-intensified, anticyclonic circulation closely trapped to the seamou nt. Maximum local time-mean current amplitudes are found to be as larg e as 37% of the magnitude of the propagating waves. In addition to the strong anticyclonic residual flow, there is a weaker secondary circul ation in the vertical-radial plane characterized by downwelling over t he top of the seamount at all depths. Maximum vertical downwelling rat es of several tens of meters per day occur at the summit of the seamou nt. The vertical mass flux implied by this systematic downwelling is b alanced by a slow radial flux of mass directed outward along the flank s of the seamount. Time-mean budgets for the radial and azimuthal comp onents of momentum show that horizontal eddy fluxes of momentum are re sponsible for transporting net radial and azimuthal momentum from the far field to the upper flanks of the seamount. There, Coriolis and pre ssure gradient forces provide the dominant balances in the radial dire ction. However, the Coriolis force and viscous effects provide the pri mary balance for the azimuthal component.