Atlantic Meridional Overturning Circulation
Tags: Weather, Climate, Amoc, Atlantic Meridional, Environment

The Atlantic Meridional Overturning Circulation (AMOC) is an extensive system of ocean currents that carry warm water from the tropics northwards into the North Atlantic.

The AMOC is an extensive system of ocean currents, like a conveyor belt, driven by differences in temperature and salt content — the water’s density. As warm water flows northwards it cools, and some evaporation occurs, which increases the amount of salt. Low temperature and high salt content make the water denser, and this dense water sinks deep into the ocean. The cold, dense water slowly spreads southwards, several kilometres below the surface. Eventually, it gets pulled back to the surface and warms in a process called “upwelling”, and the circulation is complete.

This global process makes sure that the world’s oceans are continually mixed, and that heat and energy are distributed around the Earth. This, in turn, contributes to the climate we experience today.

Oceanographers have measured the AMOC continuously since 2004. The measurements have shown that the AMOC varies from year to year, and these variations likely have an impact on the weather in the UK. However, it is too early to say for sure whether there are any long term trends. Before 2004 the AMOC was only measured a few times, and to go back further into the past we need to look at indirect evidence (for example from sediments on the seafloor). The indirect evidence does not always agree on the details, but it seems likely that there have been some massive, rapid changes in the AMOC in the past (for example around the end of the last ice age).

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. So the AMOC is very likely to weaken, but it is considered very unlikely that large, rapid changes in the AMOC, as seen in past times, will happen in the 21st Century.

The effect of a weaker AMOC is included when making projections of future climate change for the UK. A weaker AMOC will bring less warm water northwards, and this will partly offset the warming effect of greenhouse gases over western Europe. For the gradual weakening that is likely over the 21st Century, the overall effect is still warming.

A new study suggests a link between AMOC and the Indian ocean, AMOC is thought to be slowing down for the last 15 years, which could have drastic consequences on global climate. However, a study suggests that AMOC is getting help from the Indian ocean. While greenhouse warming caused by human activity is heating the Indian oceans, it is likely to boost a critical system of currents in the Atlantic Ocean, that plays a crucial role in determining the weather across the world, according to a new study.

Researchers from the Scripps Institution of Oceanography at the University of California-San Diego and Yale University found that rising temperatures in the Indian Ocean can help boost the AMOC and delay slow down. Warming in the Indian Ocean generates additional precipitation, which, in turn, draws more air from other parts of the world, including the Atlantic. The higher level of precipitation in the Indian Ocean will reduce precipitation in the Atlantic and increase salinity in the waters, the researchers explained. This saline water in the Atlantic, as it comes north via AMOC, will get cold much quicker than usual and sink faster, acting “as a jump start for AMOC, intensifying the circulation”, said Alexey Fedorov from Yale.

However, scientists do not know for how long this enhanced warming in the 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,” Fedorov added.

Moreover, it is not clear whether slowdown of AMOC is caused by global warming alone or it is a short-term anomaly related to natural ocean variability, said the researchers, emphasising the need to understand the importance of AMOC stability.

AMOC last witnessed a slow down 15,000 to 17,000 years ago. It caused “harsh winters in Europe, with more storms or a drier Sahel in Africa due to the downward shift of the tropical rain belt,” Fedorov said. “The mere possibility that the AMOC could collapse should be a strong reason for concern in an era when human activity is forcing significant changes to the Earth’s systems,” Fedorov added.