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North Atlantic circulation: why it is declining, what are the implications?
THE NORTH Atlantic Ocean circulation, known officially as the Atlantic Meridional Overturning Circulation (AMOC), is losing its stability, a study published last week in Nature Climate Change has noted. The Intergovernmental Panel on Climate Change (IPCC), too, has noted in its latest Assessment Report, released on Monday, that AMOC will decline over the 21st century.
The author of the paper, Niklas Boers, said in a release: “The findings support the assessment that the AMOC decline is not just a fluctuation or a linear response to increasing temperatures but likely means the approaching of a critical threshold beyond which the circulation system could collapse.”
What is AMOC, and why is it important?
It is a large system of ocean currents. It is the Atlantic branch of the ocean conveyor belt or Thermohaline circulation (THC) and distributes heat and nutrients throughout the world’s ocean basins. AMOC carries warm surface waters from the tropics towards the Northern Hemisphere, where it cools and sinks. It then returns to the tropics and then to the South Atlantic as a bottom current. From there it is distributed to all ocean basins via the Antarctic circumpolar current.
What happens if it collapses?
The Gulf Stream, a part of the AMOC, is a warm current responsible for mild climate at the eastern coast of North America as well as Europe. Without a proper AMOC and Gulf Stream, Europe will be very cold.
Modeling studies have shown that an AMOC shutdown would cool the northern hemisphere and decrease rainfall over Europe. It can also have an effect on El Nino.
A 2016 paper in Science Advances noted: “AMOC collapse brings about large, markedly different climate responses: a prominent cooling over the northern North Atlantic and neighboring areas, sea ice increases over the Greenland-Iceland Norwegian seas and to the south of Greenland, and a significant southward rain belt migration over the tropical Atlantic.”
Has the AMOC weakened before?
“AMOC and THC strength has always been fluctuating, mainly if you look at the late Pleistocene time period (last 1 million years). The extreme glacial stages have seen weaker circulation and slowdown in AMOC, while the glacial terminations have shown a stronger AMOC and circulation,” said Nirmal B, a Ph.D. scholar from Geoscience Research Lab, VIT Chennai, who has been studying Atlantic Paleoclimate.
“But the changes we experience in the last 100-200 years are anthropogenic. And these abrupt changes are destabilizing the AMOC, which could collapse the system,” he said.
In February, researchers found that AMOC is at its weakest in over a millennium. They studied the evolution of AMOC over the last 1,600 years. That paper was published in Nature Geoscience.
Why is it slowing down?
Climate models have long predicted that global warming can cause a weakening of the major ocean systems of the world.
Last month, researchers noted that a part of the Arctic’s ice called “Last Ice Area” has also melted. The freshwater from the melting ice reduces the salinity and density. of the water. Now, the water is unable to sink as it used to and this weakens the AMOC flow.
A 2019 study suggested that the Indian Ocean may also be influencing the flow of AMOC. The researchers said: “As the Indian Ocean warms faster and faster it generates additional precipitation. With so much precipitation in the Indian Ocean, there will be less precipitation in the Atlantic Ocean, leading to higher salinity in the waters of the tropical portion of the Atlantic. This saltier water in the Atlantic, as it comes north via AMOC, will get cold much quicker than usual and sink faster.”
“This would act as a jump start for AMOC, intensifying the circulation,” author Alexey Fedorov said in a release. “on the other hand, we don’t how long this enhanced Indian Ocean warming will continue other plans warming especially the Pacific, catches with the Indian Ocean, the advantage for AMOC will stop.”
CONTENT BY PRASHANT SINGH
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