THE MERIDIONAL OvERTuRNING circulation is an oceanic system driven by physical forces related to temperature, salinity, and momentum. The circulation is a generally closed system with an important role in redistributing heat on a global scale. Regions of intense heat loss at high latitudes produce cold (dense) water that sinks to the oceans' depths. This abyssal water spreads toward the equator, while the sinking water is replenished by warmer water flowing poleward near to the ocean surface. The upper level waters link the circulation, fed in regions of upwelling from the oceans' depths. Although other areas of subduction exist, especially in the Waddell Sea in Antarctic waters, a key source of the cold abyssal water is in the North Atlantic Ocean. It is the Atlantic Meridional Overturning Circulation (AMOC), which has garnered the largest amount of attention from researchers.
The usually stratified ocean surface layers in the North Atlantic may become disturbed by a weather system, allowing the particularly cold surface water in this region to sink, adding to the North Atlantic deep water, driving the circulation. In the return flow, the heat flux into the North Atlantic carried by the North Atlantic Drift (a part of the meridional overturning circulation, as opposed to the wind-driven Gulf Stream) makes a substantial contribution to keeping the region warmer than at comparable latitudes in the North Pacific. Because the circulation involves the movement of surface waters to abyssal depths and vice versa, it also plays a key role in the redistribution of dissolved carbon dioxide and inorganic carbon in oceanic waters. The role of this in modulating or enhancing climate change is an area of active investigation.
The accumulation of evidence from studies of past climates provides one of the most persuasive links between the meridional overturning circulation and climate change. Studies of deep sea sediments, glacial ice cores, varves, and peat sediments show that, as the North Atlantic region emerged from the Pleistocene glacial period, the general warming process was punctuated by periods of intense cold (such as the Heinrich, and the Younger Dryas cold events). The transition between events was very rapid. In the case of the Younger Dryas, temperatures may have declined by 9-18 degrees F (5-10 degrees C) in as little as 10 years. Although there are alternative hypotheses, the most compelling explanation for these oscillations in climate is a disruption in the North Atlantic circulation. Evidence suggests that the circulation was entirely, or almost completely eliminated during these events. The periods of a strengthening meridional overturning circulation also experienced the most rapid warming. Variations in the meridional overturning circulation in the North Atlantic have been significant in driving abrupt climate change.
The meridional overturning circulation exhibits considerable variability. In recent years, the upper layers of the North Atlantic appear to be warming, and the fluxes associated with the meridional overturning circulation appear to be diminishing. It is not yet clear if the observations are part of a process of natural variability or represent a longer-term trend. However, a majority of computer models suggest a general weakening of the circulation during the coming century as a result of climate change.
In a warming world, any melting of the Greenland Ice Sheet could enhance ocean layering, thereby impeding the processes of convection and reducing the warming flow of the North Atlantic Drift. Increased precipitation in the region could produce a similar effect. In models, if the meridional overturning circulation is shut off, results indicate a significant cooling of several degrees C in the surrounding continental areas. However, indications are that the region has been warming. Also, a complete shutdown of the circulation in the North Atlantic in the coming century is unlikely.
sEE also: Atlantic Ocean; North Atlantic Oscillation; Thermohaline Circulation.
BIBLIOGRAPHY. H.L. Bryden, H.R. Longworth, and S.A. Cunningham, "Slowing of the Atlantic Meridional Overturning Circulation at 25° N," Nature (v.438, 2005); A.L. Gordon, "Inter-Ocean Exchange of Thermocline Water," Journal of Geophysical Research (v.91, 1986); Gerold Siedler, John Church, and John Gould, eds., Ocean Circulation and Climate: Observing and Modelling the Global Ocean (Academic, 2001).
MlCHAL J. BARDECKI Ryerson University
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