Abyssal circulation

The Weddell Sea, because of its very low temperature, is the main producer of Antarctic Bottom Water. The rate of production is not well known but the best estimates are close to 38 x 10 m /s (Gordon, 1975). The bulk of the Antarctic Bottom Water has an initial average potential temperature of — 1°C. Most of the Antarctic Bottom Water flows north into the South Atlantic and through the Verna Channel in the Rio Grande Rise into the North Atlantic. It returns southward combined with the North Atlantic Deep Water. 

The abyssal circulation model of Stommel (1958) (Fig. 9-11) predicted that the deep waters flow most intensely along the western boundaries in all oceans and gradually mix into the interior during this flow. 

These intense western boundary currents have been identified but the flow patterns in the rest of the ocean are more complicated with topographic features playing an important role, as discussed earlier. 

The general abyssal circulation can be summarized rather briefly. Antarctic Bottom Water, the densest of 

Channel and remains a distinct water mass as far north as 40 N in the western North Atlantic basin. 

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In this section we briefly review what controls the density of seawater and the vertical density stratification of the ocean. Surface currents, abyssal circulation, and thermocline circulation are considered individually. 

Although the general circulation patterns are fairly well known, it is difficult to quantify the rates of the various flows. Abyssal circulation is generally quite slow and variable on short time scales. The calculation of the rate of formation of abyssal water is also fraught with uncertainty. Probably the most promising means of assigning the time dimension to oceanic processes is through the study of the distribution of radioactive chemical tracers. Difficulties associated with the interpretation of radioactive tracer distributions lie both in the models used, nonconservative interactions, and the difference between the time scale of the physical transport phenomenon and the mean life of the tracer. 

Arons A. B. and. Stommel H. (1967) On the abyssal circulation of the world ocean 111. An advection-lateral mixing model of the distribution of a tracer property in an ocean basin. Deep-Sea Res. 14, 441-457. 

Curry W. B. and Lohmann G. P. (1982) Carbon isotopic changes in benthic foraminifera from the western South Atlantic reconstructions of glacial abyssal circulation patterns. Quat. Res. 18, 218-235. 

Schnitker D. (1974) West Atlantic abyssal circulation during the past 120,000 years. Nature 248, 385-387. 

Fig. 9-11 The abyssal circulation generated by equal sources in the North Atlantic and in the Weddell Sea with uniform upwelling elswhere. Reproduced from Stommel and Arons (1958) with the permission of Pergamon Press.

Miller, K. G. Fairbanks, R. G. 1985. OUgocene to Miocene carbon isotope cycles and abyssal circulation changes. The Carbon Cycle and Atmospheric COit Natural Variations Archean to Present. American Geophysical Union, Washington DC, 32, 469-486... 

Lonsdale, P., 1976. Abyssal circulation of the southeastern Pacific and some geological implications. Journal of Geophysical Research, 81 1163-1176. 

The cadmium/calcium ratio is a proxy for the nutrient (phosphate) content of sea water that reflects abyssal circulation patterns. Carbon isotope ratios also reflect deep-ocean circulation and the strength of organic matter fluxes to the seafloor. 

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