Climate Change Impact on India and World – International Reports, Key Observations, etc.

Atlantic Meridional Overturning Circulation (AMOC)


From UPSC perspective, the following things are important :

Prelims level: Atlantic Meridional Overturning Circulation (AMOC)

Mains level: Assessing Climate Change impact

According to the IPCC’s Report (AR6), 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.
  • 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.

What happens if AMOC collapses?

  • Colder Europe: Without a proper AMOC and Gulf Stream, Europe will be very cold.
  • Rainfall decline: Modelling studies have shown that an AMOC shutdown would cool the northern hemisphere and decrease rainfall over Europe.
  • El-Nino trigger: It can also have an effect on the El Nino.
  • Cooling of Atlantic: AMOC collapse could bring about large, markedly different climate responses: a prominent cooling over the northern North Atlantic and neighboring areas.
  • Weaker thermohaline: Freshwater from melting Greenland ice sheets and the Arctic region can make circulation weaker as it is not as dense as salt water and doesn’t sink to the bottom.

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.
  • 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.
  • 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 weakens the AMOC flow.

Influence of Indian Ocean

  • Another study suggested that the Indian Ocean may also be helping the slowing down of AMOC.
  • 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 acts as a jump start for AMOC, intensifying the circulation.

Now try this:

Q.With reference to Ocean Mean Temperature (OMT), which of the following statements is/are correct? (CSP 2020)

  1. OMT is measured up to a depth of 26ºC isotherm which is 129 meters in the south-western Indian Ocean during January-March.
  2. OMT collected during January-March can be used in assessing whether the amount of rainfall in monsoon will be less or more than a certain long-term mean.

Select the correct answer using the code given below:

(a) 1 only

(b) 2 only

(c) Both 1 and 2

(d) Neither 1 nor 2

Post your answers here:
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Back2Basics:  Ocean Currents

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