The Atlantic meridional overturning circulation (AMOC), an ocean current system that carries warm water from the tropics into the North Atlantic and transports cold water from the northern to the southern hemisphere, is an important mechanism for the regulation of Earth’s climate.
The system has collapsed in the past due to natural factors. The most recent shutdown played a key role in the last deglaciation. The system is now threatened by global warming.
“An investigation of marine sediments collected between Canada and Greenland led to the discovery that, in the past, glaciers covering the territories that now correspond to Canada and the northern United States released colossal numbers of icebergs into the Atlantic owing to ocean surface warming in the region,” said study author Cristiano Mazur Chiessi.
The massive volume of fresh water added by the melting of icebergs altered the composition of the ocean at high latitudes of the northern hemisphere. This has a huge impact on the global climate because the region between Greenland and Canada is a very sensitive part of AMOC.
“This gigantic conveyor belt carries lighter, warmer surface water from the South Atlantic to the North Atlantic. At high latitudes of the North Atlantic, this surface water releases heat into the cold atmosphere, becoming heavier and sinking down the water column. The deeper, colder and denser water then flows southward again until it reaches the vicinity of Antarctica, where it returns to the surface, forced by an intense upwelling. On the surface, it warms up, loses density and completes the circulation,” said Chiessi.
By discovering that North Atlantic subsurface warming at high latitudes preceded massive release of icebergs from the US and Canada into the Atlantic, the researchers were able to establish the sequence of events responsible for the collapse of AMOC.
“The process begins with an apparently insignificant weakening of AMOC, which causes subsurface warming at high latitudes of the North Atlantic. This warming melts the glaciers’ sea snouts, moving the glaciers rapidly seaward and releasing colossal armadas of icebergs. As the icebergs melt, surface water salinity decreases in the region. The surface water isn’t dense enough to sink, and AMOC collapses,” said Chiessi.
https://phys.org/news/2022-10-scientists-mechanism-collapse-great-atlantic.html