The Atlantic Meridional Overturning Circulation (AMOC) is an important and currently topical component of Earth’s climate. Here, Met Éireann climate scientist and AMOC expert, Dr Tido Semmler, provides an introduction to the system, its influence on the planet, how it is likely to evolve, and how climate change may be affecting ocean currents and temperatures.
What is the AMOC?
The Atlantic Meridional Overturning Circulation (AMOC) is an ocean current system that exchanges water masses between the Southern Ocean and the Atlantic Ocean (hence the word “Meridional”) and between the surface and the deep ocean (hence the “Overturning”). It is part of the global ocean circulation system that is driven by differences in the density of the water, which depends on temperature and salinity.
At the surface, cool water masses are transported from the Southern Ocean to the Gulf of Mexico, where they are heated in the tropical and subtropical regions. These heated water masses flow along the east coast of the US towards the north-east.
The eastern branch of the system crosses the North Atlantic towards Ireland, Scotland, Norway, and the island of Spitsbergen in the Arctic Ocean; while the western branch flows towards the southern tip of Greenland. Once in the Northern Hemisphere, the water brought up by the AMOC from the tropics becomes cooler and then denser. The denser water sinks and then travels back south, where it will eventually warm and resurface again. A representation of these processes can be seen in Figure 1.
Fig 1 – (modified from R. Curry and C. Mauritzen © Woods Hole Oceanographic Institution, reproduced from Rahmstorf, 20241): Different branches of the ocean circulation in the northern North Atlantic and the Nordic Seas. Solid lines represent surface flows and dashed lines represent deep ocean flows
Why is the AMOC important for the planet and for Ireland?
The AMOC enables heat to be distributed around our planet and therefore has a major influence on regional and global weather and climate. Across the Atlantic Ocean for example, it accounts for around 90% of the total northward ocean heat transport.
Met Éireann’s AMOC expert, Dr Tido Semmler, explains: “When the heated water masses cross the North Atlantic, part of the heat is released into the atmosphere because the water is warmer than the air. The eastern branch of the system has an important influence on Irish climate, making Ireland relatively mild, particularly in winter, in comparison to places at similar northern latitude, such as parts of Canada”.
What is happening to the AMOC?
One of the main driving forces of AMOC is the marked temperature and density difference between the cold North Atlantic Ocean and the warm tropics.
Warming of North Atlantic waters, due to natural variability and human-caused climate change, has the potential to reduce the temperature difference with the tropics. This temperature-driven weakening could lead to sudden disruption, and slowing down, of major ocean current systems, such as AMOC.
Greenland’s ice sheet melt, due to human-caused global warming, has added more freshwater into the subpolar North Atlantic Ocean. The increase in freshwater input has decreased the concentration of salt, and in turn, ocean water in this region is becoming less dense. With reduced density, the sinking of these water masses and consequently the whole overturning circulation is expected to slow down.
However, at this point, there is no clear trend in AMOC observations of the last few decades, the slowing-down is not observed at the moment. There are different possible reasons for this, such as the effect of other ocean currents and the fact that oceans are slow reacting systems, and changes occurring now could take decades for their effects to be observed.
What could happen to the AMOC in the future?
According to recent research, a general AMOC slowdown is expected, but the United Nations body for assessing the science related to climate change, the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report states that “There is medium confidence that there will not be an abrupt collapse before 2100.”
Computer models that focus on forecasting AMOC changes are currently unable to capture all of the important ocean and climate processes involved – and often exclude Greenland ice melt – but they do indicate that climate change poses a real threat to the Atlantic’s transport of heat and nutrients.
In fact, according to recent research, the AMOC slowdown is expected to particularly affect the western branch of the AMOC, while the eastern branch passing Ireland, Scotland, Norway, and Spitsbergen may actually increase in intensity due to water density changes, leading to continued warming in those regions.
If you are interested to read more about the Atlantic Meridional Overturning Circulation (AMOC) and its evolution, check out our dedicated landing page.