There is confusion on what drives the circulation of the North Atlantic Deep Water (NADW). Some numerical studies have indicated that the strength of the circulation strongly depends on the strength of the windstress over the Southern Ocean (SO). Thus a picture has emerged in which major part of NADW upwells in SO and flows northward as an Ekman flow. This is in contradiction to observations.
In this study, we analyze the thermally equilibrated states of an oceanic general circulation model with 1 degree by 1 degree horizontal resolution to investigate the fate of NADW in SO and the sensitivity of its circulation to the windstress over SO. We examine the mechanism controlling the strength of the NADW circulation.
In our model, major part of NADW flows in to the Indian and Pacific Oceans and upwells there, and only minor part upwells in the Southern Ocean. This is presumably because sinking of the deep water is successfully reproduced around Antarctica by mixing with the denser shelf waters. In this case, the sensitivity to the SO windstress is lower than previously considered: The shutting off of SO wind reduces the cross-equatorial southward transport of NADW from 17 Sv to 8.5 Sv.
The part of NADW circulation which is affected by the Southern Ocean windstress is controlled not by mechanical driving of wind as is often argued but by the surface buoyancy flux in the Southern Ocean (the ″wind-enhanced thermohaline circulation″ mechanism) and by the competition between the abyssal waters of the northern and southern origins. The SO wind creates northward Ekman transport, which enhances heat flux into the ocean. This leads to strengthening of the NADW cell. At the same time, the surface northward flow weakens the sinking of the Antarctic Bottom Water (AABW) by pushing northward the surface water around Antarctica. Since the AABW circulation becomes weaker, the NADW circulation becomes stronger. |
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