The translation of isolated cold eddies on a sloping bottom

Abstract

A two-layer analytical model is considered to examine the dynamics of cold isolated patches on the ocean floor. Such patches have been observed in the North Atlantic Ocean and are characterized by a bounding interface that intersects the bottom along a closed curve. They correspond, therefore, to isolated anticyclonic eddies with a lens-like cross section. The model incorporates steady movements resulting from the swirl velocity within the eddy and a topographically-induced translation. The movements are assumed to be frictionless and nondiffusive but are not restricted to be quasigeostrophic in the sense that the Rossby number is not necessarily small. For steady motions, analytical solutions are obtained using the full equations of motion in a coordinate system moving with the eddy itself. A uniformly sloping bottom causes a steady translation at 90° to the right of the downhill direction. Thus, the model predicts that an anticyclonic eddy on the ocean floor will migrate along lines of constant depth. Suprisingly, the predicted translation speed depends only on the gravitational acceleration, the density difference between the layers, the Coriolis parameter, and the bottom slope. It is independent of the intensity, size, and depth of the eddy. For the range of parameters typical for the deep ocean, the predicted translation speed is 5 to 10 cm s −1. It is estimated that isolated eddies on the ocean floor may be able to carry temperature anomalies for a few thousand kilometers away from their origin.