As we have previously noted, a major dynamic feature of the oceans is the thermocline, which separates a thin well-mixed top layer from a deep stably stratified layer beneath. The oceans have three types of large-scale motion that involve one or both of these layers, as summarized in Table 27.1.We have previously investigated the tides; in this chapter we will look into barotropic currents in the upper ocean (above the thermocline) driven by wind stress at the ocean surface and deep currents driven by subtle differences in temperature and salt content, beginning with the former.

Figure 27.1 shows the major oceanic currents in the upper ocean, consisting primarily of gyres having ﹛clockwise, counterclockwise﹜ flow in the ﹛northern, southern﹜ hemisphere. There are five main gyres, occurring in the five major ocean basins: the northern Pacific, southern Pacific, northern Atlantic, southern Atlantic and (southern) Indian Oceans. In addition there are several minor gyres and currents near the equator and at high latitudes.

The near-surface flows within an ocean basin are driven by the prevailing winds illustrated in Figure 26.1, but in a somewhat indirect manner. The prevailing winds are predominantly along lines of constant latitude, being easterly (from west to east) in the tropics and westerly (from east to west) in mid-latitudes. As we saw in § 25.5.4, these winds drive near-surface flows (Ekman transport) in the normal direction: north-south, along lines of constant longitude. These potential north-south flows have no dynamic outlet and instead the wind stresses result in deformation of the ocean surface which sets up a counterbalancing north-south pressure gradient. Near-surface flows arise due to a latitudinal variation of this dynamic balance, quantified by the curl of the wind stress, forming the great oceanic gyres illustrated in Figure 27.1. These flows are predominantly in the east-west direction in the open oceans. With the exception of the southern ocean surrounding Antarctica, these east-west flows are blocked by continents and are diverted in the north-south direction in thin boundary layers, predominantly on the western boundaries of the oceans, forming the western boundary currents.