The Wind-Driven Circulation

Abstract

In Chapter 9 we saw that the ocean comprises a warm, salty, stratified lens of fluid, the thermocline, circulating on top of a cold, fresh, relatively well mixed abyss, as sketched in the schematic, Fig.10.1. The time-mean circulation of thermocline waters is rapid relative to the rather sluggish circulation of the abyss. There are two processes driving the circulation of the ocean. 1. tangential stresses at the ocean's surface due to the prevailing wind systems which impart momentum to the ocean – the wind-driven cir-culation and 2. convection, induced by loss of buoyancy in polar latitude, due to cooling and/or salt input, causing surface waters to sink to depth, ventilating the abyss – the thermohaline circulation. This separation of the circulation into wind-driven and thermohaline com-ponents is somewhat artificial but provides a useful conceptual simplification. In this chapter we will be concerned with the circulation of the warm, salty thermocline waters sketched in Fig.10.1 that are brought in to motion by the wind. We shall see that the effects of the wind blowing over the ocean is to induce, through Ekman pumping or suction (see Section 10.2), a pattern of vertical motion indicated by the arrows on the figure. Pumping down of buoyant surface water in the subtropics and sucking up of heavier interior fluid at the pole and the equator, tilts density surfaces, as sketched in Fig.10.1 and evident in Fig.9.7, setting up a thermal wind shear and geostrophic mo-tion. The presence of jagged topography acts to damp strong mean currents 335