Observations Of Abyssal Ocean Circulation

It is very hard to test whether the circulation schematic, Fig. 11.16, has parallels in the ocean because the predicted mean currents are so very weak and the variability of the ocean so strong. However, one of the key predictions of Stommel's abyssal theory was that there ought to be deep western

FIGURE 11.22. Three photographs charting the evolution of dye from source (white circle) to sink (black circle) using the apparatus shown in Fig. 11.21. The shallow end of the tank is marked with the 'N' and represents polar latitudes.

boundary currents which are sufficiently strong that they can be directly observed. In a rare case of theory preceding observation, the Deep Western Boundary Current on the Atlantic continental rise south of Cape Cod was indeed subsequently confirmed by direct observation.

There is also definitive evidence of deep western boundary currents in hydrographic sections. For example, Fig. 11.23, a cross section of water properties at 30° S, shows NADW, formed by convective processes in the northern North Atlantic, flowing as a deep, relatively salty, western boundary current in to the South Atlantic. One can also detect Antarctic Bottom Water, flowing northward as a relatively fresh, but very cold body of water, right at the bottom. The entire abyssal ocean is supplied via these western boundary currents, which feed the interior circulation of the open ocean. Other regions of the world ocean are also fed from the Atlantic, but by a more circuitous route which involves the Antarctic Circumpolar Current, as roughly captured by Stommel's remarkable schematic. See also Fig. 11.28.

One very vivid depiction of the abyssal circulation is revealed by the pathways of CFC from the ocean surface in to the interior, shown in Fig. 11.24. Recall, CFCs (see Table 1.2) are man-made substances and only appeared in the 20th Century. The atmospheric concentration of CFC-11 increased dramatically in the middle part of the last century and dissolved into surface waters. They were first observed invading the ocean in the 1960s. At high latitudes (in the Labrador and Greenland Seas for example) CFC-enriched surface waters are mixed by convection to great depth—recall the deep mixed layers in the polar ocean shown in Figs. 9.10 and 11.11—and carried away in to the abyssal ocean by deep western boundary currents. A plume of CFC-11 can be seen in Fig. 11.24 extending down the western margin of the ocean indicating the presence of a strong deep western boundary current which advects the CFC-rich waters away from the source region.

Finally, lest we leave the impression that Stommel's schematic is a detailed representation of the abyssal ocean, the gentle meridional flows sketched in Fig. 11.16

FIGURE 11.23. Zonal section across 30° S from the coast of South America (as marked in the inset). Top: temperature (°C); bottom: salinity (psu). We see North Atlantic Deep Water flowing southwards at a depth of some 2 km, and Antarctic Bottom Water moving northward right at the bottom. Plotted with Ocean Data View.

FIGURE 11.23. Zonal section across 30° S from the coast of South America (as marked in the inset). Top: temperature (°C); bottom: salinity (psu). We see North Atlantic Deep Water flowing southwards at a depth of some 2 km, and Antarctic Bottom Water moving northward right at the bottom. Plotted with Ocean Data View.

are not borne out by observations. Instead, as is evident in Fig. 11.25, the mid-depth flow in the ocean appears to show a marked tendency for zonal jets of small meridional scale which connect to the western boundary currents, much as seen in GFD Lab XV, Fig. 11.22. The lateral extent of these jets and their geographical and vertical structure are still largely unknown. However, the predominantly zonal interior is consistent with the idea that there is little mixing and upwelling and so no stretching of Taylor columns, which therefore move zonally to conserve their length if not interrupted by topography.

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