The Effect of Continental Rises on the Wind-Driven Ocean Circulation

Abstract

The effect of continental rises on the wind-driven circulation is investigated by developing a modification to the Rhines and Young theory and by running a three-layer eddy-resolving quasigeostrophic numerical model. Large-scale bottom topography has an important influence on the mean ocean circulation, particularly when eddies are present. Bottom topography breaks the symmetry between the subpolar and subtropical gyres in a standard double gyre ocean. In the numerical model, the mean midlatitude jet has eddy-scale meanders in it and is deflected to the north of the zero wind-stress curl line. Without topography, there are four zonal eddy-driven gyres in the bottom layer beneath the midlatitude jet. With a continental rise present at the western boundary, these gyres become eddy-scale features except for the southernmost cyclonic gyre, which remains coherent over a large scale and extends southward along the western boundary. A rise at the southern boundary forms closed geostrophic contours and an additional strong bottom layer gyre appears. This cyclonic gyre is driven by pseudowestward stress of the eddies, and the strength of its circulation can be predicted by a modification to the Rhines and Young theory. The amplitude of the circulation depends on the location of the closed geostrophic contours relative to the eddy-rich regions in the middle layer. The topography also changes the character of the spinup of the flow. Initially, the ocean responds barotropically so that the circulation within the closed geostrophic contour region is anticyclonic. Once the upper-layer western boundary current becomes unstable, the flow in this region changes direction to cyclonic.