What is the Atlantic Overturning Meridional Circulation—and how can climate change slow it down?

If you stand on a beach in New Jersey in the summertime and gaze at the vast blue horizon of the Atlantic Ocean, it’s hard to imagine the physical processes that help make this climate warm and sunny. Yet a complicated series of processes—known as the Atlantic Meridional Overturning Circulation, or AMOC—help bring warmth from the equatorial to the north Atlantic. However, this delicately balanced system could be at a dangerous tipping point due to climate change.

It all begins with the density of water. Cold water is denser than warm water, and salty water is denser than fresh water. In the ocean, water with varying degrees of heat and salinity are constantly swirling around, jockeying for prime position as denser water sinks to the bottom and warmer water rises to the top. This is known as thermohaline circulation, and along with wind patterns, affects how ocean currents move. And as thermohaline circulation moves water around the ocean, it also moves heat around the ocean.

Over the past few thousand years, the AMOC, has worked like this: heat from the sun warms the surface of the ocean near the equator, and this water slowly migrates northward. In cooler, higher latitudes, the heat of this surface water is eventually lost to the atmosphere, which warms the air. This is due to the same forces that cause a mug of coffee to cool off over time—anyone who’s ever put their hand over a steaming mug knows that the air above the surface of the liquid absorbs the heat from your drink. As heat evaporates from your mug, what’s left? Cold coffee.

This same phenomenon occurs in the ocean. Once heat is lost to the atmosphere (much to the delight of New Jersey beachgoers), the remaining water is much colder than it was near the equator. This colder water is denser, and denser water sinks. In the North Atlantic, this cooler, denser water, now toward the bottom of the ocean, migrates southward—and once it reaches the equator, eventually rises to the surface to replace water that has been heated by the sun. 

As such, AMOC is an important part of Earth’s climate system since it redistributes heat from the equator to the northern hemisphere. It’s helpful to think of these currents as a conveyor belt. The upper conveyor picks up heat in the equator and drops it off in higher latitudes, like New York, while the lower conveyor transports that same water, which is now much colder, back ot the equator to pick up more heat. 

Without the AMOC, the northeast would experience cooler temperatures, and the equator would remain hot. And unfortunately, climate change could be affecting the AMOC in a way that makes a future with a cooler northeast—and more storms—more likely.

As climate change forces more precipitation to fall as rain and warming temperatures melt the ice sheets of Greenland, more freshwater is flooding the North Atlantic. This reduces the density of the surface water, meaning that less of this traditionally cold, dense water sinks to the bottom. With less water sinking to the bottom, the warm equatorial water—which has historically migrated northward to take its place—will slow down. The overall effect is less heat being transported north from the equator to North Atlantic. [cite] [cite]

So what does this mean for the average beachgoer? The last time that comparable volumes of freshwater flooded the North Atlantic was during the Younger Dryas period, approximately 10,000 years ago. This profusion of fresh water slowed down the AMOC and weakened monsoons in Africa and Asia but intensified monsoons in the Southern Hemisphere. Mesoamerica and Europe were drier, while North America received more precipitation. 

While it’s difficult to say exactly how a slowdown of the AMOC will affect any given region, one thing is certain: the climate will likely look a lot different from the way it does today, which, if you like the beach in the summertime, should cause you to take notice.

References

Boers, N. Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation. Nat. Clim. Chang. 11, 680–688 (2021). https://doi.org/10.1038/s41558-021-01097-4

Caesar, L et al. “Observed Fingerprint of a Weakening Atlantic Ocean Overturning Circulation.” Nature (London) 556.7700 (2018): 191–196. Web.

CarbonBrief. Explainer: Nine tipping points that could be triggered by climate change. CarbonBrief. February 10, 2020. https://www.carbonbrief.org/explainer-nine-tipping-points-that-could-be-triggered-by-climate-change

Potsdam Institute for Climate Impact Research (PIK). (2021, February 25). Gulf Stream System at its weakest in over a millennium. ScienceDaily. Retrieved May 6, 2022 from www.sciencedaily.com/releases/2021/02/210225113357.html

Rainsley, E., Menviel, L., Fogwill, C.J. et al. Greenland ice mass loss during the Younger Dryas driven by Atlantic Meridional Overturning Circulation feedbacks. Sci Rep 8, 11307 (2018). https://doi.org/10.1038/s41598-018-29226-8