Context: A study published in Nature Climate Change notes that the Atlantic Meridional Overturning Circulation (AMOC) is losing its stability
What is AMOC?
The AMOC 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 AMOC collapses?
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.
Modelling studies have shown that an AMOC shutdown would cool the northern hemisphere and decrease rainfall over Europe.
It can also have an effect on the El Nino.
There will be 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
Previous models overestimated the AMOC’s stability as it did not look at freshwater influence. Freshwater from melting Greenland ice sheets and the Arctic region can make circulation weaker as it is not as dense as saltwater and doesn’t sink to the bottom.
Has the AMOC weakened before?
AMOC and Thermo-haline Circulation strength has always been fluctuating, mainly if we 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.
But the changes we experience in the last 100-200 years are anthropogenic, and these abrupt changes are destabilising the AMOC, which could collapse the system
In February, researchers noted that AMOC is at its weakest in over a millennium.
The study results suggest that AMOC has been relatively stable until the late 19th century. With the end of the little ice age in about 1850, the ocean currents began to decline, with a second, more drastic decline following since the mid-20th century
Why is the AMOC slowing down?
Climate models have long predicted that global warming can cause a weakening of the major ocean systems of the world.
Melting of Arctic: Researchers noted that a part of the Arctic’s ice called “Last Ice Area” has 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 weakens the AMOC flow.
Indian Ocean Warming: 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
If other tropical oceans’ warming, especially the Pacific, catches up with the Indian Ocean, the advantage for AMOC will stop.
Conclusion
If we continue to drive global warming, the Gulf Stream System will weaken further – by 34 to 45 percent by 2100 according to the latest generation of climate models
We urgently need to reconcile our models with the presented observational evidence to assess how far or how close the AMOC really is to its critical threshold.
