Putting it all together

Thus, to summarize, the meridional overturning circulation has two mechanistically distinct components: a component maintained by mixing and a component maintained by wind, both responding to the surface buoyancy distribution. The two can exist side by side, and the overturning circulation in the Atlantic Ocean is schematically illustrated in figure 4.8. Some of the water that sinks in the North Atlantic moves across into the Southern Hemisphere and upwells in the ACC (enabled by the wind), and some upwells and returns in the North Atlantic itself (enabled by mixing). The water that sinks in the North Atlantic (forming the North Atlantic Deep Water) does not in fact extend all the way to the bottom of the ocean because there is some even denser water beneath it—Antarctic Bottom Water, which comes from high southern latitudes and circulates through the effects of mixing.

Which component of the circulation is dominant? only careful observations can tell us, although currently it is often believed that the wind component is stronger than the mixing component in the Atlantic Ocean. The North Pacific Ocean is generally less dense than the North Atlantic because it is fresher; also it does not support a vigorous interhemispheric circulation and so partakes more weakly in the global-scale overturning circulation that is sketched in figure 2.6. Note finally that the horizontal velocities in the abyssal ocean are usually quite small, on the order of 1 mm s1, and at this speed it would take some 300

Figure 4.8. Schematic of the meridional overturning circulation, most applicable to the Atlantic Ocean (D.P. indicates the Drake Passage, the narrowest part of the ACC). The arrows indicate water flow, and dashed lines signify water crossing constant-density surfaces, made possible by mixing. The upper shaded area is the warm water sphere, including the subtropical thermocline and mixed layer, and the lower shaded region is Antarctic Bottom Water. The bulk of the unshaded region in between is North Atlantic Deep Water.

Figure 4.8. Schematic of the meridional overturning circulation, most applicable to the Atlantic Ocean (D.P. indicates the Drake Passage, the narrowest part of the ACC). The arrows indicate water flow, and dashed lines signify water crossing constant-density surfaces, made possible by mixing. The upper shaded area is the warm water sphere, including the subtropical thermocline and mixed layer, and the lower shaded region is Antarctic Bottom Water. The bulk of the unshaded region in between is North Atlantic Deep Water.

years for a parcel to move from its high-latitude source to the equator, still longer if the path were not direct. Thus, if the surface conditions change, it will take several hundred years for the deep ocean to re-equilibrate.

OCEAN CIRCULATION IN A NUTSHELL

The large-scale ocean circulation may usefully be divided into a quasi-horizontal circulation, comprising the gyres and other surface and near-surface currents, and a meridional overturning circulation. Embedded within the circulation are smaller mesoscale eddies, which actually contain the bulk of the kinetic energy of the ocean and which are analogous to atmospheric weather systems.

The ocean gyres

• The ocean gyres are primarily wind driven, responding in particular to the north-south variations of the zonal wind. The subtropical gyres lie between about 15° and 45° in both hemispheres, with the subpolar gyres poleward of that in the Northern Hemisphere.

• The wind stress has a direct effect in the uppermost few tens of meters of the ocean, where it induces an Ekman flow at right angles to the wind. This Ekman flow in turn causes the sea surface to slope and produces a geostrophic flow, which is the main component of the gyres and which extends down several hundred meters.

• The main gyres all have a strong intense current at their western boundary (e.g., the Gulf Stream in the North Atlantic, the Kuroshio in the North Pacific), which arises from the combined effects of Earth's sphericity and its rotation.

The overturning circulation

• The overturning circulation is a response to variations in surface buoyancy, in that the densest water at the surface (usually at high latitudes) sinks and moves away from the sinking region at depth.

• For the circulation to persist, the deep water must be brought up to the surface; otherwise, the abyss will fill up with the densest water available and then stagnate. Two processes bring deep water up to the surface: mixing and the wind.

(continued)

— Mixing warms the deep water at low latitudes, which may then rise through the thermocline, maintaining a circulation of sinking at high latitudes and rising at low latitudes.

— Strong westerly winds in the Antarctic circumpolar current can draw water up from the deep and induce an in-terhemispheric circulation, which is particularly strong in the Atlantic.

The other main currents

• The Antarctic Circumpolar Current is the collection of eastward flowing currents around Antarctica, which taken together form the largest sustained current system on the planet. It is a response both to wind and to the meridional temperature gradient.

• The equatorial current systems are predominantly controlled by the winds, consisting typically of a westward flowing current and eastward countercurrents and undercurrents.

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