The Atlantic Meridional Overturning Circulation (AMOC) is a large system of ocean currents that carry warm water from the tropics northwards into the North Atlantic.
The AMOC is a large 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 a 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 been measuring the AMOC continuously since 2004. 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 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 sea floor). The indirect evidence doesn’t always agree on the details, but it seems likely that there have been some large, 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’s 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 the greenhouse gases over western Europe. For the gradual weakening that is likely over the 21st Century, the overall effect is still a warming.