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Editorials In-Depth, 26 March

Atlantic Meridional Overturning Circulation

General Studies- I (Important Geophysical phenomena)

In the recent decades tropical Indian Ocean has experienced a rapid increase in ocean warming with an average rise in Sea Surface Temperature (SST) of about 1oC over the period of 1951-2015 at a rate of 0.15oC/decade. 

In addition, a recent study by the Indian Institute of Tropical Meteorology (IITM), an autonomous institute under the Ministry of Earth Sciences has investigated the marine heatwaves. 

  • The study shows that the western Indian Ocean had a total of 66 Marine Heat Wave (MHW) events while the Bay of Bengal had 94 events during 1982–2018.
  • The western Indian Ocean region experienced a four-fold rise in marine heatwaves events (increasing at a rate of 1.5 events per decade).
  • The north Bay of Bengal experience a two-to-three fold rise (at a rate of 0.5 events per decade). 

Heating up the Indian oceans and AMOC:

According to a new study, while greenhouse warming caused by human activity is heating up the Indian oceans, it is likely to boost a key system of currents in the Atlantic Ocean that plays a key role in determining the weather across the world.

What is the Atlantic Meridional Overturning Circulation?

Atlantic meridional overturning circulation (AMOC) — which is sometimes referred to as the “Atlantic conveyor belt” — is one of the Earth’s largest water circulation systems.

  • In Atlantic meridional overturning circulation (AMOC) system ocean currents move warm, salty water from the tropics to regions further north, such as western Europe and sends colder water south.
  • It aids in distributing heat and energy around the earth, as the warm water it carries releases heat into the atmosphere, and in absorbing and storing atmospheric carbon.  

How does the AMOC work?

The AMOC is a large system of ocean currents, like a conveyor belt, driven by differences in temperature and salt content – the water’s density.

  • As warm water flows northwards it cools and some evaporation occurs, which increases the amount of salt. 
  • Low temperature and a high salt content make the water denser, and this dense water sinks deep into the ocean. The cold, dense water slowly spreads southwards, several kilometres below the surface. 
  • Eventually, it gets pulled back to the surface and warms in a process called “upwelling” and the circulation is complete.
  • This global process makes sure that the world’s oceans are continually mixed, and that heat and energy are distributed around the earth. This, in turn, contributes to the climate we experience today.

Effect of climate change on the AMOC:

  • Climate models suggest that the AMOC will weaken over the 21st Century as greenhouse gases increase. 
  • This is because as the atmosphere warms, the surface ocean beneath it retains more of its heat. Meanwhile increases in rainfall and ice melt mean it gets fresher too.
  • All these changes make the ocean water lighter and so reduce the sinking in the ‘conveyor belt’, leading to a weaker AMOC. 
  • So the AMOC is very likely to weaken, but it’s considered very unlikely that large, rapid changes in the AMOC, as seen in past times, will happen in the 21st Century.
  • The effect of a weaker AMOC is included when making projections of future climate change for the UK. 
  • A weaker AMOC will bring less warm water northwards, and this will partly offset the warming effect of the greenhouse gases over western Europe. 
  • For the gradual weakening that is likely over the 21st Century, the overall effect is still a warming.

Effect of rising temperatures in the Indian Ocean:

For thousands of years, AMOC has remained stable, but since the past 15 years, it has been weakening — a development that could have dramatic consequences for Europe and other parts of the Atlantic rim, according to the study published in the journal Nature Climate Change.

  • The rising temperatures in the Indian Ocean can help boost the AMOC and delay slow down. 
  • Warming in the Indian Ocean generates additional precipitation, which, in turn, draws more air from other parts of the world, including the Atlantic. 
  • The higher level of precipitation in the Indian Ocean will reduce precipitation in the Atlantic and increase salinity in the waters. 
  • This saline water in the Atlantic, as it comes north via AMOC, will get cold much quicker than usual and sink faster, acting “as a jump start for AMOC, intensifying the circulation”.

Raising Concerns: 

  • However, scientists don’t know for how long this enhanced warming in Indian Ocean will continue. “If other tropical oceans’ warming, especially the Pacific’s, catches up with the Indian Ocean, the advantage for AMOC will stop.
  • Moreover, it isn’t clear whether slowdown of AMOC is caused by global warming alone or it is a short-term anomaly related to natural ocean variability.
  • AMOC last witnessed a slowdown 15,000 to 17,000 years ago. It caused “harsh winters in Europe, with more storms or a drier Sahel in Africa due to the downward shift of the tropical rain belt.”
  • The mere possibility that the AMOC could collapse should be a strong reason for concern in an era when human activity is forcing significant changes to the Earth’s systems.

Source: Down to Earth

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