The Observed Mean Circulation

The global pattern of mean flow at the surface of the ocean is plotted in Fig. 9.13, where the names of the major current systems are also given. The colors separate the circulation patterns into tropical (pink), subtropical (yellow), and subpolar (blue) regimes (inspired by the dynamical discussion to be developed in Chapter 10). Because the detailed patterns are difficult to discern on this global map, regional circulations in the Pacific, Atlantic, and Indian

Oceans are also shown in Figs. 9.14,9.15, and 9.16, respectively, with the annual-mean SST superimposed. Note the tiny vector on the bottom right of these plots, which represents a current of 10 cms-1; ocean currents are typically 100 times smaller than atmospheric winds. These maps are based on surface drifter data6 that have been smoothed to aid visual presentation in vector form.

Despite the richness of the structures revealed in these data, on close inspection clear patterns emerge. The flow is dominated by closed circulation patterns known as "gyres," which are particularly pronounced in the northern hemisphere, where all zonal flow is blocked by coasts. In the subtropics of the northern hemisphere there are anticyclonic gyres, known as subtropical gyres (yellow shading in Fig. 9.13, with eastward flow in middle latitudes—the North Pacific and North Atlantic Currents—and westward flow in the tropics—the North Equatorial Currents of the Pacific and Atlantic. Typical current speeds in the interior of the gyres are < 10 cm s-1 (see Fig. 9.22, top). At the western edge of these subtropical gyres, there are strong, poleward currents reaching speeds > 100cms-1, the Kuroshio in the North Pacific Ocean and the Gulf Stream in the North Atlantic Ocean, as is evident in Figs. 9.14 and 9.15. These swift currents flow northward along the coasts from the tropics, turn into the interior at about 40° latitude, and then spread eastward across the ocean basin. This east-west asymmetry is perhaps the most striking aspect of the observed current system; all intense boundary currents are on the western, rather than the eastern, boundaries of ocean gyres. The interior extensions of these boundary currents are evident in the thermal structure at the ocean surface, most obviously in the strong temperature gradients near the western boundaries of the

6Most surface drifters providing these data consist of a spherical surface float tethered to a cylindrical cloth tube hanging below with holes in it, centered at a depth of 15 m. Within the float is a radio transmitter allowing it to be located through Doppler ranging by satellite. The drifters can survive for 450 days or so. For a more complete description, see Niiler (2001).

FIGURE 9.13. Major surface currents observed in the global ocean, with the names of key current systems marked. The colors separate the circulation patterns into tropical (pink), subtropical (yellow), and subpolar (blue) regimes based on the pattern of zero wind-curl lines shown in Fig. 10.11 (see Section 10.1.3). Data courtesy of Maximenko and Niiler (personal communication, 2003).

FIGURE 9.13. Major surface currents observed in the global ocean, with the names of key current systems marked. The colors separate the circulation patterns into tropical (pink), subtropical (yellow), and subpolar (blue) regimes based on the pattern of zero wind-curl lines shown in Fig. 10.11 (see Section 10.1.3). Data courtesy of Maximenko and Niiler (personal communication, 2003).

Longitude

FIGURE 9.14. Major surface currents observed in the Pacific Ocean from surface drifters with annual-mean SST superimposed. The vector at the bottom right represents a current of 10 cms-1. Data courtesy of Maximenko and Niiler (personal communication, 2003).

Longitude

FIGURE 9.14. Major surface currents observed in the Pacific Ocean from surface drifters with annual-mean SST superimposed. The vector at the bottom right represents a current of 10 cms-1. Data courtesy of Maximenko and Niiler (personal communication, 2003).

middle-latitude oceans and in the Antarctic Circumpolar Current of the southern hemisphere.

In the polar regions of the north Pacific and Atlantic basins there are cyclonic gyres, known as subpolar gyres (blue shading in Fig. 9.13), with southward flowing western boundary currents: the Oyashio Current in the Pacific, the Labrador Current in the Atlantic. Again we observe no intense eastern boundary currents. As discussed earlier, and in more detail in Chapter 11, the northern margin of the subpolar gyres in the Atlantic Ocean—in the Labrador and Greenland Seas—are favoured sites of deep-reaching convection.

In the tropics (pink shading in Fig. 9.13) we observe strong zonal flows in each basin, running eastward just north of the equator and counter to the prevailing winds! (See, for example, Fig. 7.28, middle panel.) The Equatorial Counter Currents very evident in Fig. 9.14 have, on either side of them, strong North and South Equatorial Currents flowing westwards. Somewhat weaker meridional temperature gradients are found in the tropics than in higher latitudes, but significant zonal temperature gradients exist along the equator. The interaction between the atmosphere and ocean in the tropical Pacific, which leads to variability of the coupled system known as

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