OSCILLATING FLOW OF A HOMOGENEOUS FLUID OVER AN ISOLATED TOPOGRAPHIC FEATURE

The circulation of a homogeneous fluid over an idealized, axisymmetric feature is examined. The motion is forced by a large-scale background flow that is periodic in time. The focus of the study is the effect o f topographic Rossby wave resonance on the mean flow and the movement of passively advected particles. The approach is based on integration of a nonlinear, primitive equation model. Mean flows at fixed location s and particle trajectories are then calculated from the time-varying model solutions. In agreement with earlier studies the time-mean flow around the bump [upsilon] is shown to be approximately proportional to q(2) where q is the amplitude of the time-varying, cross-isobath flow . Sensitivity studies show that q(2) and hence [upsilon] can decrease as the amplitude of the oscillating background flow increases. This is explained in physical terms by a nonlinear dependence of the effectiv e resonant frequency of the system on the amplitude of the oscillating background flow. To describe the motion of particles passively advect ed by the flow we present maps showing net particle displacement (Delt a) over one cycle of the background flow. As with [upsilon] and q it i s nor possible to parametrize simply the net displacements ira terms o f the strength and frequency of the background flow: allowance has to be made for the effective resonant frequency of the system and its dep endence on the strength of the background flow An effective diffusivit y kappa(e) is estimated from the loss rate of particles from the top o f the bump. For small net displacements kappa(e) scales approximately with <(Delta)over bar>(2) omega where the overbar denotes an average o ver the top of the bump and omega is the frequency of the background o scillation. However, even this limited parametrization depends implici tly on the effective resonant frequency of the system through its infl uence on the net drift of particles.