there is a stagnant region between the continental slope and the edge of the deep western boundary current. For the North Atlantic deep western boundary current, the second condition applies, although at low latitude the zero-thickness condition eventually takes over.

Avery interesting phenomenon is that, as the deep western boundary current flows toward the equator, it moves up along the continental slope. Pickart and Huang (1995) performed the calculation for a case with uniform potential vorticity mimicking the deep western boundary current in the North Atlantic Ocean, and the solution is shown in Figure 5.67. Note that at low latitude the current becomes wider and the maximum velocity becomes larger.

Pickart and Huang (1995) also discussed the case with non-uniform potential vorticity and exponential type of continental slope. The calculations were carried out in the stream-function coordinates. By using the potential vorticity profile diagnosed from data, the model simulation of the deep western boundary current has been improved (Fig. 5.68).

5.2.4 Mixing-enhanced deep circulation

Abyssal flow induced by bottom-intensified mixing over topography In the Stommel and Arons theory, it is assumed that deep circulation is driven by uniform upwelling fed by point sources of deep water that are confined to several isolated places in

Fig. 5.67 Along-stream evolution of the uniform potential vorticity current with parameters g' = 0.001, a = 0.01, transport = 5 Sv: a along-stream path of the current; the topography is indicated by the dotted lines; b cross-section of the interface revealing the change in structure from 60° N to 10° N; c along-stream trend in peak velocity of the current, where the maximum possible value is indicated by the dashed line (Pickart and Huang, 1995).

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