Atlantic Meridional Overturning Circulation

Atlantic Meridional Overturning Circulation (AMOC), combination of surface and deep ocean currents in the Atlantic Ocean that conveys warm surface water northward and cold deep water southward while also circulating nutrients. It is the Atlantic component of thermohaline circulation, and, as such, it plays a substantial part in regulating heat near Earth’s surface, which affects global and regional temperature and precipitation patterns (see also climate). Mathematical models suggest that the AMOC has been undergoing a gradual decline in strength—which is attributed to decreases in salinity at the ocean’s surface due to increased inputs of fresh water—since the late 19th century, whereas historical studies using sea surface temperature (SST) data suggest that this decline began during the middle of the 20th century.

Process and pathways

Surface currents of the AMOC, such as the Gulf Stream, the North Atlantic Current, and the equatorial currents, are driven largely by wind patterns, whereas the deep ocean currents depend on differences in water temperature, salinity, and water density (see also density current). The flow rate of the AMOC averages about 18 Sverdrups (Sv; 1 Sv = 1 million cubic meters [about 35.3 million cubic feet]) per second, but it can vary between 12 and 25 Sv.

In the North Atlantic, warm surface water flows northward through the Gulf Stream along the coast of eastern North America toward northwestern Europe. As this current continues northward, the water cools and some evaporates, which leaves water at the surface denser and more saline by the time it reaches the Arctic Ocean. Cold, dense water sinks in the Arctic to join the North Atlantic Deep Water (NADW), a deep ocean current that moves southward. Water from this current rises to the surface in the Southern Ocean, and it warms as it is exposed to sunlight. Some of this heated water is cooled by ice near Antarctica and sinks once again to travel in the Antarctic Bottom Water (AABW) current, a deep ocean current that transports water to the Indian and Pacific oceans. Rising water from the NADW that emerges in areas free of sea ice joins surface currents that circulate back to the North Atlantic.

The AMOC governs a wide range of environmental processes, the most significant being the distribution of heat in the Atlantic, and, through other components of thermohaline circulation, around the world. The effect of this movement of heat is most pronounced in northwestern Europe. The relatively warm waters of the eastern North Atlantic help to warm that region more than the average temperatures that occur in other areas at similar latitudes. Meanwhile, prevailing westerly winds carry relatively warm air and the moisture it entrains deep into the continent. The AMOC also moves nutrients to various regions throughout the Atlantic basin, which promotes primary productivity that supports marine ecosystems and adjacent terrestrial ecosystems.

Weakening and slowdown

Climate modeling and sea surface temperature (SST) measurements have indicated a decrease in the strength of the AMOC since 1870; indeed, some models show a 1.7-Sv decline per century, and the results of SST measurements show a 0.46-Sv decline per decade since 1950. Experts ascribe the weakening of the AMOC to freshwater inputs from melting glaciers, particularly those in Greenland, combined with Earth’s natural climate variability. As global warming continues, Greenland’s glaciers will continue to melt, adding fresh water to the AMOC. This slows the AMOC by reducing the salinity (and thus the density) of surface water, inhibiting its ability to sink, which weakens the mechanism that drives the current.

Some scientists report that these measurements portend disastrous regional and global climatic effects. They suggest that the weakened currents will accentuate temperature differences between the Equator and the poles, with reduced amounts of warm and cold water in the North Atlantic circulating northward and southward, respectively. Scientists note that such a development would spawn several disruptive effects, including a rise in the average temperatures of continents in the Southern Hemisphere, a dramatic cooling of Europe, a relocation of the rain belts in southern Africa (which could increase drought frequency and duration over a large part of that continent), a change in the timing of the Indian monsoon, an acceleration in the melting of Antarctic ice sheets, and a reduction in the ocean’s ability to absorb carbon dioxide from the atmosphere.

In a more extreme prediction of the AMOC’s weakening, some research suggests that the AMOC will collapse entirely by mid-century. This modeling projection relies on the assumption that the pace of global warming will continue, which will cause the Greenland Ice Sheet to melt and flood the northern part of the AMOC with fresh water. The last time the AMOC shut down fully is thought to have been about 14,500 years ago as Earth’s climate warmed from the last glacial period (see last glacial maximum).

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Other researchers, however, have raised doubts about whether the prospect of a complete shutdown is realistic. They argue that the AMOC remains largely stable and that dire predictions from others rely on extrapolations from oversimplified models. Other modeling studies note that, while freshwater inputs from Greenland’s melting ice sheet could destabilize the AMOC, this phenomenon could be offset by the accelerated melting of the West Antarctic Ice Sheet (WAIS). These studies posit that adding fresh water from the WAIS would cool the Southern Ocean, lowering the depth of the ocean’s pycnocline (the boundary separating two liquid layers of different densities). This change could strengthen the AMOC by increasing the density gradient between water at the surface and water at depth in the North Atlantic. Still, the consensus among experts remains that the AMOC will continue to weaken to some extent with the addition of fresh water from melting glaciers.