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article imageGlobal ocean circulation appears to be collapsing due to warming

By Karen Graham     Aug 7, 2017 in Environment
Researchers have found a correlation between melting Arctic sea ice and changes in the planet's largest water circulation system that could lead to the collapse of the Atlantic Meridional Overturning Circulation (AMOC).
In January this year, Digital Journal reported on a study funded by the National Science Foundation, the US Department of Energy and the Ministry of Science and Technology of the People’s Republic of China that found the possibility of a collapse of the AMOC due to global warming had been underestimated.
Now, the new study, conducted by researchers at Yale University and the University of Southampton has found that warming may be playing an active role in altering the AMOC, backing up the January research.
"Conventional thinking has been that if ocean circulation weakens, reducing the transport of heat from low to high latitudes, then it should lead to sea ice growth. But we have found another, overlooked, mechanism by which sea ice actively affects AMOC on multidecadal time scales," said professor Alexey Fedorov, a climate scientist at the Yale Department of Geology and Geophysics and co-author of a study detailing the findings in the journal Nature Climate Change.
Topographic map of the Nordic Seas and subpolar basins with schematic circulation of surface current...
Topographic map of the Nordic Seas and subpolar basins with schematic circulation of surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic meridional overturning circulation. Colors of curves indicate approximate temperatures.
. Curry, Woods Hole Oceanographic Institution/Science/USGCRP.
Refresher course on the AMOC
The Atlantic Meridional Overturning Circulation (AMOC) can be likened to a climate conveyor belt. The AMOC is a deep-sea system that circulates warm salty water from the South Atlantic Ocean and the Gulf of Mexico to the North Atlantic, where because of its density, it sinks and cools.
The water is then pushed southward in a clockwise motion along the abyss of the Atlantic Ocean, where the circulation pattern allows the UK and parts of Europe to enjoy a relatively moderate climate. The whole process actually acts to distribute ocean water on a global scale.
For millennia, the AMOC has been the Earth's most efficient mechanism for the distribution of heat from the Tropics to the Northern Latitudes. And of course, the most famous part of the AMOC is the Gulf Stream. The Gulf Stream is responsible for bringing warm tropical water to the coasts of Europe.
The North Atlantic's "warming hole" is a key player
There is a cold patch in the North Atlantic that has mystified scientists for a long time. The warming hole (WH) refers to an area in the subpolar North Atlantic, which has cooled in recent years and is projected to continue to warm less rapidly than the global mean.
Observed trend in temperature from 1900 to 2012; yellow to red indicates warming  while shades of bl...
Observed trend in temperature from 1900 to 2012; yellow to red indicates warming, while shades of blue indicate cooling. Gray indicates areas for which there are no data. There are substantial regional variations in trends across the planet, though the overall trend is warming.
However, in climate model projections, the warming hole has already been associated with a weakening of the Atlantic meridional overturning circulation (AMOC), and this is likely to lead to somewhat different projected climate impacts in nearby Europe and North America in the coming years.
The presence of the warming hole has now led climate scientists to believe it could be the reason behind a slowdown of global ocean water circulation. Likely, the cold zone is due to melting ice in the Arctic and Greenland. This action would leave a "cold cap" of fresh water over the North Atlantic, keeping the warmer AMOC current water from sinking. This, in turn, would slow down the AMOC, effectively hindering global ocean circulation.
"In our experiments, we saw a potential loss of 30% to 50% of AMOC's strength due to Arctic sea ice loss. That is a significant amount, and it would accelerate the collapse of AMOC if it were to occur," Fedorov said.
Basically, what we are seeing is an increase in the melting of Arctic sea ice and according to the study, from all indications, we will see a continued weakening of the global ocean circulation system.
More about AMOC, Global warming, ocean circulation, warming hole, Sea ice