Depth Dependence and Mass Transport of Recirculating Midlatitude Gyres

Abstract

The northwest corners of the major ocean basins are characterized by seaward jets, flanked by tight, nonlinear gyres exhibiting closed potential vorticity contours. At deep levels, isolated from surface forcing, areas of homogeneous potential vorticity are apparent. A model is presented describing these ”recirculation“ regions extending the quasi-geostrophic layer models of Marshall and Nurser to the continuously stratified two-dimensional case. The study is diagnostic, concentrating on numerical inversions of idealized quasi-geostrophic potential vorticity distributions in a vertical, meridional section through a free inertial gyre. An iterative approach is used to find the “bowl” of the circulation. the free boundary between the deep recirculating homogenized water and the stagnant water below. It is shown that the homogenized recirculation has a finite depth penetration, possibly not extending to the ocean floor. In cases where the flow reaches the bottom, the recirculation can be divided into two regions: a “core” region, where bottom currents exist and a baroclinic “fringe” to the south. The surface intensified part of the eastward jet is recirculated in the broad, westward flowing fringe, while the component of the transport returned within the core itself is largely depth independent. The enhanced man transport of the Gulf Stream can be accounted for by the model. Its magnitude is sensitive to the upper-level potential vorticity imposed. For realistic parameters, the core carries the greater proportion of the transport. The structure of the recirculation is dependent on the value assumed for the deep homogeneous potential vorticity. If a positive deep potential vorticity anomaly is imposed, the upper-level gyre interface moves northward while a cyclonic gyre becomes dominant in the abyssal flow. If the anomaly exceeds a certain limit, solutions can no longer be found.