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 (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.

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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