Oceanic heat transport is mainly effected by the large-scale circulation, with some transport by mesoscale eddies, mainly in the ACC. As we discussed in chapter 4, there are two distinct aspects to this circulation: the wind-driven gyres and upper ocean circulation, and the meridional overturning circulation. Let's see how each of these transport heat.
The wind-driven gyres, especially the subtropical wind-driven gyres, are a major factor in the heat transport in both hemispheres. If we consider the North Atlantic as an example, poleward heat transport occurs because the western boundary current (the Gulf Stream in this case) brings warm water up from the tropics along the eastern seaboard of the United States, releasing heat (especially in winter) when the warm water comes into contact with the colder air coming off the cold continental land mass. Such a release of heat occurs at the western edges of all the major ocean basins in midlatitudes, for example, off the coasts of Japan, the Eastern United States, South America (south Brazil, Uruguay, and Argentina) and southeastern Australia. The poleward flow of the western boundary currents is balanced by equatorial flow in the middle of the gyres that brings cold water equator-ward, although the flow is broader and weaker so that the transport of cool water equatorward is spread over a large area. But, in any case, the consequence of warm water flowing poleward in the western boundary currents and cool water flowing equatorward in the interior means that the wind-driven gyres transport heat poleward. The transport occurs in both the Pacific and the Atlantic, and in both the Northern and Southern hemispheres. The subpolar gyres in the Northern Hemisphere also transport poleward, but they are less well defined than the subtropical gyres and cover less of the ocean so that their heat transport is somewhat weaker than that of the subtropical gyres.
We saw in chapter 4 that the meridional overturning circulation has two mechanistically distinct components: a mixing-maintained component and a wind-maintained component, and the net overturning circulation is a combination of the two. The mixing-maintained component of overturning circulation responds to the buoyancy gradient at the surface between the equator and the pole, and in today's climate that buoyancy gradient is mainly a consequence of the temperature gradient. As described in chapter 4, the buoyancy gradient leads to a circulation in which cold water at high latitudes sinks and moves equa-torward, balanced by warmer, near-surface water moving poleward. The net effect of this circulation is a poleward transport of heat that would be, in the absence of other effects, roughly equal in magnitude in the two hemispheres.
The other component of the overturning circulation is a pole-to-pole circulation, driven by the wind in the southern oceans. In the Atlantic Ocean, this component tends to dominate the purely buoyancy-driven circulation, as suggested by figure 4.8 in chapter 4. We see from this figure that there is a large northward, and so poleward, heat transport in the Northern Hemisphere because the equatorward moving water has come from high northern latitudes and is correspondingly cold, and the poleward moving water nearer the surface is warmer. In the Southern Hemisphere, however, the southward moving water is colder than the northward moving water because the cold North Atlantic Deep Water continues its path into the Southern ocean and the water moving northward near the surface is relatively warm. That is, the water moving equatorward is generally warmer than the water moving poleward! Thus, the heat transport in the Atlantic from the deep circulation is northward in both hemispheres. The wind-driven gyres, of course, transport heat poleward in both hemispheres, partly counteracting the equatorward heat transport by the deep circulation in the Southern Hemisphere, so that the net effect is that the heat transport is weak and equatorward in the South Atlantic, whereas it is strong and poleward in the North Atlantic. In the Pacific and Indian ocean basins, there is no corresponding wind-driven deep circulation that transports heat northward, and the wind-driven and buoyancy-driven circulations both act to transport heat poleward, as we can see in figure 5.3.
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