ATLANTIC MERIDIONAL OVERTURNING CIRCULATION

 

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.  It carries the warm surface water from the tropics to  northwards into the North Atlantic. 

This ocean current system is driven by differences in temperature and salt content i.e. the water’s density. 

The tropical warm water flows northwards ,due to high temperature evaporation occurs, which increases the amount of salt.  The temperature of water drops at north. Low temperature and a high salt content make the water denser, and this dense water sinks deep into the ocean.

This cold and dense water creeps towards the south to the tropics, typically 2–4 km below the surface. Eventually, it gets pulled back to the surface and warms in a process called “upwelling” and the circulation is complete.

The global circulation process keeps on mixing the water so that the heat and energy are distributed around the earth. So a strong impact of ocean currents and circulations is seen on the climate.  

The measurements have shown that the AMOC varies from year to year, and it is likely that these variations have an impact on the weather in the Europe. 

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. 

A weaker AMOC will bring less warm water northwards, and this will partly offset the warming effect of the greenhouse gases over western Europe. 

According to the IPCC’s Report 2021, the AMOC decline is not just a fluctuation or a  response to increasing temperatures but likely means the approaching of a critical threshold beyond which the circulation system could collapse. 

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. Without a proper AMOC and Gulf Stream, Europe will be very cold.

AMOC shutdown would cool the northern hmisphere and decrease rainfall over Europe. It can also have an effect on the El Nino. 

According to a 2016 paper in ‘Science Advances’ , a weak AMOC or its collapse may cause a prominent cooling over the northern North Atlantic freezing of the Greenland-Iceland-Norwegian seas and and resultantly shifting in the rainbelt to the  south over the tropical Atlantic.

Freshwater from melting Greenland ice sheets and the Arctic region can make circulation weaker as it is not as dense as saltwater and doesn’t sink to the bottom.Now, the water is unable to sink as it used to and weakens the AMOC flow.

Researchers noted that AMOC is at its weakest in over a millennium.

According to Stefan Rahmstorf, 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.

A 2019 study suggested that the Indian Ocean warms faster and, it generates additional precipitation in Indian Ocean region. With so much precipitation in the Indian Ocean, there will be less precipitation in the tropical Atlantic Ocean. 

Less precipitation 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 speeds up and intensifies the circulation . If other tropical oceans’ warming, especially the Pacific, catches up with the Indian Ocean, the advantage for AMOC will stop. 

The continuation of global warming will weaken the gulf stream.