On the Influence of Bottom Topography on the Agulhas Eddy

Abstract

A series of numerical experiments with a two-layer primitive equation model is presented to study the dynamics of Agulhas eddies. The main goal of the paper is to examine the influence of an underwater meridional ridge (modeled after the Walvis Ridge) on an Agulhas eddy hitting it. First, the propagation of an eddy of the specified vertical structure over a flat bottom is considered, varying the initial eddy horizontal scale from 40 to 120 km. Unlike small nonlinear eddies, large nonlinear eddies (on the scale of Agulhas eddies) do not rapidly evolve into a compensated state (no motion in the lower layer). Second, the influence of a ridge on eddies of differing vertical structures having a specified intensity in the upper layer and a prescribed horizontal scale is analyzed. Significantly baroclinic eddies can cross the Walvis Ridge, but barotropic or near-barotropic ones cannot. The evolution of eddies crossing the ridge is compared with that of initially identical eddies moving over a flat bottom and with field observations. Eddies in our model tend toward the compensated state, with a motionless lower layer, when they cross a steep ridge. This tendency appears largely independent of the initial state of the eddy. Eddies crossing the ridge, show an intensification just before the eddy center encounters the ridge, expressed as a deepening of the thermocline depth and a heightening 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 from the Topex-Poseidon satellite. The translational speed and direction of model eddies agree with observations, even in the absence of externally prescribed large-scale currents or friction; model eddies averaged 4.6 km day?1 and moved westward. The modeled eddies proved an effective transport for passive tracers; tracers initially located near the center of the eddy were transported with practically no losses. The influence of the ridge leads to the substantial increase of the transported tracers. Model eddies show a realistic e-folding scale for amplitude decay of 2680 km. This long scale, combined with the tracer transport, indicates that Agulhas eddies, which cross the Walvis Ridge, are capable of carrying their observed thermal and salinity anomalies far into the South Atlantic subtropical gyre.