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Atlantic Meridional Overturning Circulation (AMOC)

  • AMOC is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters that are part of the thermohaline circulation.

Thermohaline Circulation

  • Winds drive ocean currents in the upper 100 meters of the ocean’s surface.
  • However, ocean currents also flow thousands of meters below the surface.
  • These deep-ocean currents are driven by differences in the water’s density, which is controlled by temperature (thermo) and salinity (haline).
  • This process is known as thermohaline circulation.
  • The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt.
  • Ocean bottom relief greatly influences thermohaline circulation.

Thermohaline Circulation

Thermohaline Circulation (Wikipedia)

How Atlantic Meridional Overturning Circulation (AMOC) works?

  • As warm water flows northwards in the Atlantic, it cools, while evaporation increases its salt content.
  • Low temperature and a high salt content raise the density of the water, causing it to sink deep into the ocean.
  • The cold, dense water deep below slowly spreads southward.
  • Eventually, it gets pulled back to the surface and warms again, and the circulation is complete.

Atlantic Meridional Overturning Circulation (AMOC)

Image Credits: Wikipedia

Importance of Atlantic Meridional Overturning Circulation (AMOC)

  • AMOC aids in distributing heat and energy around the earth (heat budget).
  • Western Europe’s climate is less harsh even in winters because of AMOC (Gulf Stream, North Atlantic Drift).
  • It acts as a carbon sink by absorbing and storing atmospheric carbon.
  • AMOC’s carbon sequestration has significant implications for evolution of anthropogenic global warming.

Cause of concern

  • AMOC has undergone exceptional weakening in the last 150 years compared to the previous 1500 years.
  • 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 reduce the sinking in the conveyor belt (weaker AMOC).
  • A weaker AMOC will bring less warm water northwards, and this will partly offset the warming effect of the greenhouse gases over western Europe.

Effects of AMOC Slowdown

  • IPCC report indicates that AMOC has already weakened.
  • Any substantial weakening of the AMOC would cause
    • further decrease in marine productivity in the North Atlantic (less sinking will lead to less mixing of water),
    • more storms in Northern Europe,
    • less Sahelian summer rainfall and South Asian summer rainfall,
    • a reduced number of tropical cyclones in the Atlantic and
    • an increase in regional sea level along the northeast coast of North America.

Rising temperatures in Indian Ocean can boost Atlantic’s ocean currents temporarily

  • For thousands of years, AMOC has remained stable, but since the past 15 years, it has been weakening.
  • However, rising temperatures in the Indian Ocean can help boost the AMOC and delay the slow down.
Indian Ocean’s role
  • The Indian Ocean is one of the fingerprints of global warming.
  • As the Indian Ocean warms faster and faster, it generates additional precipitation.
  • This draws more air from other parts of the world to the Indian Ocean, including the Atlantic.
  • With so much precipitation in the Indian Ocean, there will be less precipitation in the Atlantic Ocean.
  • Less precipitation will lead to higher salinity in the waters of the tropical portion of the Atlantic.
  • This saltier water, 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.
  • 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.

Sources: D2E | MetUK | IE

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