Latitudes

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FIGURE 10.25. A schematic of the mechanism by which a large-scale sub-surface horizontal density gradient is maintained in the middle-latitude ocean. Ekman suction draws cold, dense fluid up to the surface in subpolar regions; Ekman pumping pushes warm, light fluid down in the subtropics. The resulting horizontal density gradient supports a thermal wind shear. Its baroclinic instability spawns an energetic eddy field which tends to flatten out the horizontal gradients.

FIGURE 10.25. A schematic of the mechanism by which a large-scale sub-surface horizontal density gradient is maintained in the middle-latitude ocean. Ekman suction draws cold, dense fluid up to the surface in subpolar regions; Ekman pumping pushes warm, light fluid down in the subtropics. The resulting horizontal density gradient supports a thermal wind shear. Its baroclinic instability spawns an energetic eddy field which tends to flatten out the horizontal gradients.

contrast is established across the current. In middle latitudes, then, the oceans have a thermal structure that is somewhat similar to that of the atmosphere, albeit maintained by a very different mechanism. In particular, the isopycnals slope poleward/upward, as sketched in Fig. 10.25, just as do 0 surfaces in the atmosphere. Along with horizontal density gradients there is a reservoir of available potential energy which, through the agency of baroclinic instability, can be released into kinetic energy in the form of eddies, just as in GFD Lab XI.10 Thus oceanic eddies are analogs of midlatitude atmospheric eddies and have many similar properties. It is less clear, however, just how important oceanic eddies are in the "big picture'' of the ocean circulation. They are certainly less crucial to poleward heat transport than their atmospheric counterparts. This (as discussed in Chapter 11), is because the wind-driven and thermohaline circulations are quite capable of efficient poleward heat transport even in the absence of eddies. This contrasts with the atmosphere where, in middle latitudes, the zonal flow that would exist in the absence of eddies cannot transport heat, or anything else, in the N-S direction. In this context, the crucial difference between ocean and atmosphere is the E-W confinement of the oceans by continents, permitting western boundary currents and meridional transport. Note, however, that the ACC is not blocked by coasts and the mean flow is west-east: this is one place in the ocean where eddies are known to play a central role in meridional property transport.

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