A new study sheds light on the critical role that mixing of Atlantic and Arctic waters plays in maintaining the Atlantic Meridional Overturning Circulation (AMOC), which is critical for regulating Earth’s climate.

Researchers from the University of Southampton, the Indian Institute of Technology Bhubaneswar, the National Oceanography Center and Stockholm University analyzed ocean data from 1979 to 2021 to better understand how the mixing of Atlantic and Arctic waters helps maintain the AMOC.

The AMOC acts like a giant ocean conveyor belt, transporting warm water from the tropics north and cold water south, distributing heat around the planet. It helps keep northern Europe, including the UK, relatively mild compared to other regions at similar latitudes.

The study was conducted in Nature communicationfound that the lower part of the AMOC – the part of this “conveyor belt” that consists of deep, cold, dense water flowing south in the Atlantic – is composed of 72 percent Atlantic water and 28 percent Arctic water.

“When the warm water reaches the cooler regions of the North Atlantic, it releases heat into the atmosphere, becomes denser and sinks to great depths,” explains Dr Dipanjan Dey, the lead author of the paper, who conducted the research as a postdoctoral fellow at the University of Southampton.

“We found that while some of this dense water immediately flows south again, much of it flows north, where it mixes with colder, fresher Arctic water in regions such as the Denmark Strait between Iceland and Greenland. This mixing process makes the water even denser before it also flows south, contributing to the strength of the AMOC.”

The researchers estimate that mixing of Atlantic and Arctic waters is responsible for 33 percent of the transformation of warm, salty water into colder, fresher and denser water, while 67 percent is due to ocean-atmosphere interactions.

The study challenges previous assumptions that focused primarily on heat loss in specific areas without taking into account the crucial role of Atlantic-Arctic water mixing.

Models predict that the AMOC circulation could slow down as the planet warms due to climate change. A weaker, shallower AMOC circulation, such as occurred during the last ice age, has significant consequences for global climate patterns.

The new insights into the role of mixing of Atlantic and Arctic waters help us to better understand these processes.

Professor Robert Marsh, a co-author of the study from the University of Southampton, explains: “As the ocean surface warms and gains freshwater, the resulting increase in stratification (water layering) hinders this crucial mixing between Atlantic and Arctic waters. This reduced mixing weakens the AMOC by reducing the density and depth of its southward flow, potentially leading to an overall slowing of the circulation.”

“A slowdown in the AMOC circulation would have serious consequences, from significantly lower temperatures in northern Europe to sea level rise on the east coast of the United States. If it were to weaken significantly, it could lead to abrupt, dramatic and possibly irreversible changes in our planet’s climate.”

A weaker, shallower AMOC could also shorten the time carbon dioxide remains in the ocean before it is released back into the atmosphere, potentially accelerating climate change and its impacts.

“Climate models need to accurately model these water mixing processes to better predict future climate scenarios,” says Dr. Dey. “Our study highlights the complex interplay between our climate and global ocean circulation processes. We urgently need to address global warming to avoid crossing potential tipping points where circulation could slow significantly or even collapse.”