Weakening of Atlantic Meridional Overturning Circulation

Context: A recent study notes that the Atlantic Meridional Overturning Circulation (AMOC) is losing its stability. Modelling studies have shown that an AMOC shutdown would cool the northern hemisphere and decrease rainfall over Europe.

• Note:  Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents which transports warm water from the tropics northwards into the North Atlantic.

Key Findings

• As the atmosphere warms due to increased greenhouse gas emissions, the surface ocean beneath retains more of heat. 
• A potential collapse of the system could have severe consequences for the world’s weather systems.
• Climate models have shown that the AMOC is at its weakest in more than a 1,000 years. However, it has not been known whether the weakening is due to a change in circulation or it is to do with the loss of stability.

About Atlantic Meridional Overturning Circulation (AMOC)

• 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 where ocean currents move warm, salty water from the tropics to regions further north, such as western Europe and sends colder water south.
• As warm water flows northwards in the Atlantic, it cools, while evaporation increases its salt content.
• Low temperature and high salt content increases 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.
• This continual mixing of the oceans and the distribution of heat and energy around the planet contributes to the global climate.
• Atlantic Meridional Overturning Current (AMOC) ensures the oceans are continually mixed, and heat and energy are distributed around Earth.

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.