One of the world’s most important ocean currents is actually slowing

The Atlantic Meridional Overturning Circulation (AMOC) has slowed significantly since the mid-1990s, a new study reveals. AMOC is an essential part of the systems that balance Earth’s regional climate. Without this climate, Europe could experience colder winters, while the tropics could warm up even faster. Climatologists have identified AMOC as one of the most vulnerable parts of the planetary heat balance for decades, but uncertainty remains about how much change is occurring.

Ocean currents move amounts of water that make the world’s largest rivers seem small in comparison. In doing so, they redistribute the heat and ensure that deeper waters are supplied with oxygen. The factors that cause these currents vary, with physical processes such as the Coriolis force having an important influence. However, AMOC is primarily the result of saltwater left behind when sea ice forms that sinks to the depths, leaving room for tropical waters to flow in.

Many climate models suggest that if melting ice from Greenland floods the North Atlantic Ocean with cold but very fresh water, it will sit above salt water instead of sinking. Without a boost to depth, the water will no longer flow south in the deep ocean, and the Gulf Stream will flow weaker. However, the Atlantic Ocean is a large area and tracking the movements of that much water is difficult, especially as the data becomes increasingly patchy the further back you go. Measurement efforts consistently show AMOC weakening, but disagree to what extent.

Dr. Alexey Mishonov of the University of Maryland and co-authors measured the power of AMOC in a new way. They studied temperatures and sea surface heights in the North Atlantic Ocean using extensive databases. Where possible, salinity and density measurements were also used, providing a more comprehensive picture than previous attempts.

“It is shown that although the entire North Atlantic Ocean is warming systematically, climate trajectories in different subregions of the North Atlantic reveal radically different features of regional decadal variability,” the authors write. Specifically, the subpolar regions are warming more slowly than those in the subtropics and even off the coast of Norway. Because the poles are generally warming faster due to global warming, this is an indication of reduced transport of warm water from the tropics.

The team found patterns consistent with AMOC remaining stable from 1955, when widespread measurements began, until 1994. However, AMOC has slowed since then. “There has also been a slowdown in thermohaline geostrophic circulation throughout the North Atlantic over the past decade,” the authors note.

AMOC involves so many currents and undercurrents that it is difficult to describe the changes in one figure, but the authors note a 20 percent reduction in the warm water transported by the Gulf Stream.

A diagram of the upper layer circulation of the North Atlantic Ocean.  Red – warm currents, blue – cold currents.  White boxes 1 to 5 indicate five different analysis areas where temperature, salinity, and flow rates are likely to differ significantly.

The currents in the North Atlantic Ocean are very complex, which contributes to the difficulties in measuring the changes over time.

Image credit: Mishonov et al, Frontiers in Marine Science 2024 (CC BY 4.0)

“If AMOC slows down, heat exchange will decrease, which in turn will affect the climate, making warm areas hotter and cold areas colder,” Mishonov said in a statement. Most climate changes have at least some beneficiaries, but this one will likely be bad for almost everyone affected by it.

An article published last month flagged warning signs of an impending collapse of the AMOC. Mishonov and co-authors’ work looks backward, not forward, and they explicitly state that this research is not confirmed. Still, it probably makes it more plausible.

An additional fear regarding AMOC’s failure is that oxygen depletion will cause the collapse of deepwater ecosystems. This remains a real threat, but a recent study raised the possibility that other sources of oxygenation could be stronger in a warming world, partially offsetting this aspect of AMOC’s weakening.

The possibility of AMOC’s collapse captured public attention when it was the center of the Hollywood blockbuster The day after tomorrow. Like most disaster movies, it took something real and exaggerated it to ridiculous proportions. “Of course, most climate scientists do not share these Hollywood fantasies, and no one within scientific communities believes that something similar could happen,” Mishonov said. “However, most believe that a substantial slowdown of AMOC could result in significant climate change that cannot be anticipated and predicted. Therefore, increased interest in AMOC functionality is fully justified.”

The research has been published open access in Frontiers in Marine Science.

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