The horizontal structure and the ocean currents

A schematic of the main horizontal ocean currents at the ocean surface is shown in figure 2.3. In most regions of the world, these currents extend a few hundred meters into the ocean, with the exception of the equatorial region. Here there is a shallow westward-flowing surface current and a more substantial eastward flow beneath (as well as narrow countercurrents on either side of the westward flow, not shown in the schematic). At first glance, the circulation seems complicated, especially as the currents interact with the geography of the continental land masses. However, if we look more closely, the circulation actually simplifies, and we can identify a number of features common to each of the major ocean basins.

In mid- and high latitudes, the most conspicuous aspects oflarge-scale circulation are the circulating gyres. Between latitudes of about 15° and 45°, in either hemisphere,

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Figure 2.3. A schematic of the main surface currents of the world's oceans. The panel at the left shows the zonally averaged zonal (i.e., east-west) surface winds.

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Figure 2.3. A schematic of the main surface currents of the world's oceans. The panel at the left shows the zonally averaged zonal (i.e., east-west) surface winds.

the circulation is dominated by the subtropical gyres. The flow in the subtropical gyres is westward on their equatorial branch and eastward in midlatitudes on their poleward branch, and it is not too difficult to imagine that this circulation is directly driven by the wind—the westerlies (i.e., wind from the west) in midlatitudes and the generally easterly (wind from the east) trade winds in low latitudes. There are subtropical gyres in both the Atlantic and Pacific oceans in both hemispheres (figure 2.3), as well as in the southern Indian Ocean. The circulation in these gyres occurs mainly in the upper few hundred meters of the ocean, weakening and eventually becoming unrecognizable in the deep abyss.

A rather conspicuous aspect of the subtropical gyres is their east-west asymmetry: the meridional component of all of these gyres is much stronger in the west of the oceans, giving rise to what are called western boundary currents. In the North Atlantic, this current is called the Gulf Stream, in the North Pacific, the Kuroshio, in the South Atlantic, the Brazil Current, in the South Pacific, the East Australia Current, and in the Indian Ocean, the Agulhas Current. The cores of these currents are often only 100 km or so wide, and the speed of the flow can reach a quite tidy 1 m/s, which is not at all sluggish for the ocean, where most currents are more like a few centimeters per second. The equatorward return flow in all the subtropical gyres is spread over a much greater longitudinal extent and so is much weaker.

In the Northern Hemisphere, poleward of the subtropical gyres, the circulation consists of subpolar gyres. Because of the converging meridians and the complicated geography of the North Atlantic and North Pacific, these gyres are not nearly as conspicuous as their subtropical counterparts, but they too are primarily wind driven, a consequence of the strong midlatitude westerly winds and the weak easterly winds at high latitudes. They also have intense western boundary currents, now flowing equatorward, and it may be useful to look ahead to see a schematic of how the ocean gyres might be if the oceans were purely rectangular in figure 4.1 in chapter 4. The reader may imagine how the circulation would be distorted, but might keep its essential structure, if the rectangle were replaced by the more complicated geometry of the real ocean basins.

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