AMOC Study Reveals Steady Strength of Vital Ocean Current Over Six Decades

Earth, consisting of 71% water, is influenced by the ocean and its movements. In the Atlantic Ocean, a network of interconnected currents, referred to as the Atlantic Meridional Overturning Circulation (AMOC), circulates water across the globe, powered by a blend of winds and ocean density. It not only distributes the ocean’s warmth, moisture, and nutrients but also regulates the planet’s climate and weather patterns.

As climate change persists and the atmosphere continues to heat up, numerous scientists worry that freshwater from melting polar ice sheets could severely disrupt—or even collapse—the AMOC. While a decrease in the AMOC could yield serious repercussions, a total collapse would be exceptionally disastrous.

Nevertheless, investigations concerning the long-term outlook for the AMOC are unclear. Rather than attempting to forecast the future, a team of researchers from Woods Hole Oceanographic Institution (WHOI) assessed the historical data to inform potential future scenarios.

In a recent article published in Nature Communications, the researchers concluded that the AMOC has not decreased over the past 60 years.

Authors Nicholas P. Foukal, adjunct scientist in Physical Oceanography at WHOI and assistant professor at the University of Georgia; Jens Terhaar, affiliated scientist at WHOI and senior scientist at the University of Bern; and Linus Vogt, a visiting student at WHOI during the initiation of this research, now scientist at LOCEAN, Sorbonne Université, state their findings indicate that the AMOC is presently more stable than anticipated.

“Our paper asserts that the Atlantic overturning has not diminished yet,” remarked Foukal, who conducted the research while at WHOI. “This doesn’t imply anything about its future, but it doesn’t seem as though the expected changes have taken place yet.”

Their results diverge from prior studies, particularly a 2018 publication referenced in their work, which claimed that the AMOC has reduced over the last 70 years. This earlier research depended on sea surface temperature assessments to gauge how the AMOC has transformed, but, according to Terhaar, “we’ve discovered that sea surface temperature isn’t as effective as we initially believed,” as he began leading this investigation at WHOI as a postdoctoral scientist and finalized the work in Bern.

To tackle the uncertainty, Terhaar and the team utilized new information from the Coupled Model Intercomparison Project (CMIP), which are climate-earth models developed by the World Climate Research Program. They employed 24 distinct CMIP models and discerned that the most current surface temperature data did not faithfully reconstruct the AMOC.

Moreover, the investigators explored an alternative measure: air-sea heat fluxes, which refer to the heat transfer from the ocean to the atmosphere. When the AMOC is more potent, a greater amount of heat is emitted from the ocean to the atmosphere over the North Atlantic.

The authors developed this AMOC proxy using the CMIP models and subsequently applied it to observational data. The most reliable data for surface heat fluxes over the North Atlantic stem from reanalysis products that amalgamate direct observations into a model, akin to the methodology employed in weather forecasts. The study authors concentrated on two reanalysis datasets that date back to the late 1950s to reconstruct the AMOC.

“Based on the results, the AMOC is more stable than we previously assumed,” stated Vogt. “This could imply that the AMOC is not as close to a tipping point as was previously believed.”

The article asserts that air-sea heat flux anomalies in the North Atlantic are closely connected to the AMOC and that “the decadal averaged AMOC has not weakened from 1963 to 2017.” Given that numerous processes contribute to substantial year-to-year variability in the AMOC, the air-sea heat flux and the AMOC are most strongly correlated at these timescales rather than annual averages.

“It’s nearly unanimous at this stage that the Atlantic overturning will decelerate in the future, but whether or not it will collapse remains a topic of discussion,” noted Foukal. “This research indicates that there remains time to act before we approach this potential tipping point.”

Like all proxy-based reconstructions, there are restrictions and considerations. The authors emphasize that direct measurements of air-sea heat flux over time are limited, resulting in significant uncertainty in the reanalysis products. Nonetheless, despite these constraints, Terhaar concludes, “a decline in AMOC over the last 60 years seems highly improbable.”