Atlantic Meridional Overturning Circulation (AMOC)

In Context 

 A study published in Nature Climate Change has noted that Atlantic Meridional Overturning Circulation (AMOC) is losing its stability. 

What is Atlantic Meridional Overturning Circulation (AMOC)?

• The AMOC is a large system of ocean currents. 
• It is the Atlantic branch of the ocean conveyor belt or Thermohaline circulation (THC).
  • THC is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes.
• AMOC distributes heat and nutrients throughout the world’s ocean basins.
  • It carries warm surface waters from the tropics towards the Northern Hemisphere, where it cools and sinks. 

Why is the AMOC slowing down?

• Global warming 
  • Climate models have long predicted that global warming can cause a weakening of the major ocean systems of the world.
    • Recently, 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.  All these changes make the ocean water lighter and so reduce the sinking in the ‘conveyor belt’, leading to a weaker AMOC. 
    • If we continue to drive global warming, the Gulf Stream System will weaken further – by 34 to 45 per cent by 2100.
      • This could bring us dangerously close to the tipping point at which the flow becomes unstable.

• Role of Indian Ocean

• A 2019 study suggested that the Indian Ocean may also be helping the slowing down of AMOC. 
• As the Indian Ocean warms faster, it generates additional precipitation and 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

Implications of AMOC collapsing 

• 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 El Nino.
• AMOC collapse brings about large, markedly different climate responses:
  • A prominent cooling over the northern North Atlantic and neighbouring areas,
  • Sea ice increases over the Greenland-Iceland-Norwegian seas and to the south of Greenland. 
  • A significant southward rain-belt migration over the tropical Atlantic.

Has the AMOC weakened before?

• AMOC and THC strength has always been fluctuating, it is noted by looking 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
  • These past fluctuations are known by studying paleoclimate proxies such as sea surface temperatures (SST), salinity, and isotope signatures from single-celled organisms called foraminifera. 
  • But the changes that have been experienced in the last 100-200 years are anthropogenic, and these abrupt changes are destabilising the AMOC, which could collapse the system.
• In February 2021, researchers noted that AMOC is at its weakest in over a millennium.
  • The team studied the evolution of AMOC over the past 1600 years which suggested 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.

Way Forward 

• 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.

Source: IE