The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that moves water around the Atlantic Ocean, transporting warm water north and cold water south. The AMOC is part of a worldwide network of major ocean currents driven by changes in salinity and ocean temperature - a process called thermohaline circulation.
The world’s oceans are crucial in regulating Earth’s climate through complex processes and interactions. Among them, the Atlantic Meridional Overturning Circulation (AMOC) stands out as a large system of currents in the Atlantic Ocean, transporting warm surface waters from the tropics to the subpolar North Atlantic. As these waters travel northward, they cool and become denser and more saline, until they finally sink to the deep ocean, returning southward as cold deep waters and completing the overturning cycle – a cycle that can take 1,000 years.
The AMOC plays a central role in regulating Earth’s climate, especially in the northern hemisphere. As the warm surface waters travel to the north, they release heat into the atmosphere, making winters milder in Europe. In fact, the Atlantic is the only ocean with a net northward heat transport. As those waters lose their heat, become denser and sink – a process known as deepwater formation – they take with them massive amounts of carbon from the atmosphere into the deep ocean, creating the main carbon sink in the northern hemisphere. As the AMOC is a large component of the global ocean circulation system (see image, right), changes in the AMOC could impact climate patterns on a global scale.
Understanding the AMOC is a challenging task due to its complexity and scale. Even though oceanographers have been observing it continuously since 2004, and new generation climate models and other technologies are being used to improve our understanding of its dynamics, fundamental issues such as what drives the AMOC and what mechanisms make it change over time are still unclear. Continuous monitoring is crucial for detecting any changes in the AMOC and predicting future climate trends.
It is expected that the strength of the AMOC will decline because of anthropogenic climate change, but the extent and duration of this decline are still highly uncertain. An increase in freshwater flux from melting ice sheets and glaciers can alter the average density and temperature differences in the North Atlantic Ocean, potentially disrupting the sinking process and consequently weakening the AMOC. Weakened circulation means less heat transport northwards, which could cool the North Atlantic region by several degrees and make the European climate significantly colder – which in turn would modify regional and global climate patterns. Towards the end of the last glacial period (around 14,500 years ago), an AMOC shutdown may have triggered a series of abrupt temperature shifts, but it is extremely unlikely that something similar would happen during the 21st century.
The AMOC has a strong influence over the Earth’s climate. New observations and climate models are helping to improve its understanding, although unsolved questions about the AMOC dynamics remain. As we navigate the challenges of climate change, it becomes even more essential to monitor and comprehend the AMOC’s behaviour, so we can better prepare for the potential impacts and work towards sustainable solutions for the future.
Spotlight article by María Jesús Rapanague
Reference: Meredith, M. (2022) Carbon storage shifts around Antarctica. Nature Communications 13. DOI: 10.1038/s41467-022-31152-3
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