On the theory of strong, midlatitude wind-driven ocean circulation: I. The north pacific countercurrent as a quasi-geostrophic jet

Abstract

Abstract A characteristic feature of the circulation in the tropical ocean is that a few degrees north of the equator there exists a strong easterly surface flow, the Equatorial Countercurrent. Sverdrup (1947) correctly showed that the momentum balance in the Countercurrent is quasi-geostrophic, but the corresponding description of the vorticity balance over its entire longitudinal extent is not complete. The present analysis is based on a model of a two-layer quasi-geostrophic circulation. The observation that latitudinal changes in upper-layer depth are less than longitudinal changes is exploited to reduce the nonlinear vorticity equation to an ordinary differential equation by a class of similarity transformations. Some of the features of the circulation in the western tropical Pacific that can be modeled include the observation that north and south of the Countercurrent, there exist westward currents, somewhat broader and less intense than the Countercurrent, and that there is a monotonic decrease of zonal transport to the east. In addition, the structure of the Countercurrent is not necessarily determined by the local structure of the zonal wind field. These solutions asymptotically approach Sverdrup circulation to the north and south of the zero of the wind-stress curl.