The Influence of Longitudinal Variations in Wind Stress Curl on the Steady Ocean Circulation

Abstract

The influence of longitudinal variations in wind stress curt on the steady circulation in a rectangular ocean basin is investigated analytically in a linear barotropic model. It is shown that longitudinal variations in the interior circulation are produced by longitudinal variations in the magnitude of a wind stress curl of constant (negative) sign. If friction is small in the interior, the east-west variation in southward flow is directly proportional to that of the applied wind stress curl in accordance with the Sverdrup vorticity balance. Examples show that when the wind stress curl has a maximum in the center of the basin, the southward flow is also concentrated in the center of the basin. When the wind stress curl has a minimum in the center of the basin, the southward flow is concentrated in two regions, one on either side of the minimum curl. This variation in southward flow causes a concentration in cast-west flow along the northern and southern boundaries of the basin, which is not obvious from the Sverdrup balance. If the minimum value of the wind stress curl term is smaller than the friction term in the vorticity equation, there is northward flow in the center of the basin which is part of a closed anticyclonic gyre in the eastern half of the basin. Longitudinal variations in wind stress curl can also produce differences in the northward flow in the western boundary current compared to the case of a wind stress curl which is independent of longitude. Observations of monthly mean sea surface temperatures are presented which show a quasi-steady longitudinal variation with an east-west scale of order 1000 km, an inferred north-south scale of order 100 km, and an amplitude of order 1°C at mid-latitudes in the central and eastern North Pacific. When plotted as a function of longitude, the monthly mean temperatures at 35 and 40N show a relative minimum near the central longitude of the basin. This longitudinal temperature variation exists in a long-term annual average and it also exhibits apparent seasonal changes. It is suggested that the quasi-steady longitudinal temperature variation is a real feature of the North Pacific and that it is associated with the large-scale wind-driven ocean circulation. The longitudinal distribution in mean sea surface temperature is consistent with the circulation in the dynamical model which is produced by a qualitatively realistic longitudinal variation in mean wind stress curl.