ON THE INFLUENCE OF BOTTOM TOPOGRAPHY ON THE AGULHAS EDDY

A series of numerical experiments with a two-layer primitive equation model is presented to study the dynamics Of Agulhas eddies. The main g oal of the paper is to examine the influence of an underwater meridion al ridge (modeled after the Walvis Ridge) on an Agulhas eddy hitting i t. First, the propagation of an eddy of the specified vertical structu re over a flat bottom is considered, varying the initial eddy horizont al scale from 40 to 120 km. Unlike small nonlinear eddies, large nonli near eddies (on the scale of Agulhas eddies) do not rapidly evolve int o a compensated state (no motion in the lower layer). Second, the infl uence of a ridge on eddies of differing vertical structures having a s pecified intensity in the upper layer and a prescribed horizontal scal e is analyzed. Significantly baroclinic eddies can cross the Walvis Ri dge, but barotropic or near-barotropic ones cannot. The evolution of e ddies crossing the ridge is compared with that of initially identical eddies moving over a flat bottom and with field observations. Eddies i n our model tend toward the compensated state, with a motionless lower layer, when they cross a steep ridge. This tendency appears largely i ndependent of the initial state of the eddy. Eddies crossing the ridge show an intensification just before the eddy center encounters the ri dge, expressed as a deepening of the thermocline depth and a heighteni ng of the sea surface elevation. This effect is large enough [O(10 cm) ] that it should be noticeable in altimeter records such as the one fr om the Topex-Poseidon satellite. The translational speed and direction of model eddies agree with observations, even in the absence of exter nally prescribed large-scale currents or friction; model eddies averag ed 4.6 km day(-1) and moved westward. The modeled eddies proved an eff ective transport for passive tracers; tracers initially located near t he center of the eddy were transported with practically no losses. The influence of the ridge leads to the substantial increase of the trans ported tracers. Model eddies show a realistic e-folding scale for ampl itude decay of 2680 km. This long scale, combined with the tracer tran sport, indicates that Agulhas eddies, which cross the Walvis Ridge, ar e capable of carrying their observed thermal and salinity anomalies fa r into the South Atlantic subtropical gyre.