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Concept
Atlantic meridional overturning circulation
Résumé
The Atlantic meridional overturning circulation (AMOC) is part of a global thermohaline circulation in the oceans and is the zonally integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters. These "limbs" are linked by regions of overturning in the Nordic and Labrador Seas and the Southern Ocean, although the extent of overturning in the Labrador Sea is disputed. The AMOC is an important component of the Earth's climate system, and is a result of both atmospheric and thermohaline drivers.
Climate change has the potential to weaken the AMOC through increases in ocean heat content and elevated freshwater flows from the melting ice sheets. Oceanographic reconstructions generally suggest that the AMOC is already weaker than it was before the Industrial Revolution, although there is a robust debate over the role of climate change versus the circulation's century-scale and millennial-scale variability. Climate models consistently project that the AMOC would weaken further over the 21st century, which would affect average temperature over areas like Scandinavia and Britain that are warmed by the North Atlantic drift, as well as accelerate sea level rise around North America and reduce primary production in the North Atlantic.
Severe weakening of the AMOC has the potential to cause an outright collapse of the circulation, which would not be easily reversible and thus constitute one of the tipping points in the climate system. A shutdown would have far greater impacts than a slowdown on both the marine and some terrestrial ecosystems: it would lower the average temperature and precipitation in Europe, slashing the region's agricultural output, and may have a substantial effect on extreme weather events.
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