Thermohaline Circulation / AMOC (Atlantic Meridional Overturning Circulation)
In the Atlantic Ocean, there is a net flow of water northwards in the surface layers in the Gulf Stream and its extension, the North Atlantic Current. This brings warm, salty water from the tropics to the high northern latitudes. At high latitudes, the ocean releases heat to the atmosphere, making the surface waters cooler, and sea ice is formed, making the surface waters saltier. Both the temperature decrease and the salinity increase make the water denser. This allows the surface waters to sink and return southwards in the deep ocean. This deep water finally returns to the surface, closing the circulation loop; it is thought that one way this happens is through upwelling in the Southern Ocean around Antarctica. The AMOC transports around 20 million cubic metres of water per second, and it transports around 1PW of heat northwards in the Atlantic basin, contributing to the mild climate of Western Europe.
What would happen if the AMOC collapsed?
Model studies suggest that a collapse of the AMOC could lead to a reduction in surface air temperature of around 1 to 3°C in the North Atlantic region and surrounding land masses, but with local cooling of up to 8°C in areas of increased sea ice. A smaller cooling effect would be expected throughout the northern hemisphere, with a slight warming in the southern hemisphere after a few decades. Several studies suggest that there would be a change in precipitation patterns over the tropics, associated with a southward shift of the inter-tropical convergence zone, which could also affect the intensity of the El Nino Southern Oscillation (ENSO) in the Pacific. A collapse of the AMOC might also lead to an intensification of the North Atlantic storm track, with stronger winds over Europe. Over a period of years to decades, there would be regional changes in sea level, with a sea level rise in the North Atlantic of up to 80cm. Studies also suggest there could be impacts on the carbon cycle and on soil moisture and primary productivity of the terrestrial vegetation.