UNIVERSITY PARK, Pa. — Climate observers have long tied the strength of a major ocean current to water temperatures in the so-called "cold blob" in the North Atlantic Ocean. The 965,000-square-mile area south and southeast of Greenland cooled by as much 1.8 degrees Fahrenheit over the past century as other ocean waters warmed.
Some indicators signal that a sustained decline in water volumes in the current, known as the Atlantic Meridional Overturning Circulation (AMOC), is a persistent cause of the blob's lower temperatures. The current's degradation began decades ago, a shift likely influenced by climate change that could reduce oceans' absorption of carbon dioxide and heat, according to a team led by researchers in Penn State's College of Earth and Mineral Sciences . But they cautioned, the link between AMOC and surface temperatures in the blob, also called the North Atlantic warming hole, is more irregular and complex than previously thought.
As published in the journal Nature Communications , the team found that the diminishing ocean current may impact the North Atlantic climate differently over time, with disparate phases in the relationship between AMOC strength and blob temperatures, said corresponding author Laifang Li , an assistant professor in the Department of Meteorology and Atmospheric Science . The work builds on their finding in 2025 that radiation from the atmosphere helps depress water temperatures.
"With more field data and study, researchers can reduce uncertainty in models that anticipate temperatures and climate patterns in the North Atlantic," Li said, explaining that sustained ocean temperature readings via satellite and related monitoring would benefit this work. "If we can forecast better when these ocean temperatures will reach key thresholds, that will create a clearer picture of the timeframe available to develop technology and mitigation policies addressing climate change."
Climate researchers regard AMOC as a core piece of ocean circulation and heat transfer — a conveyor belt that moves warm surface water to the north and cold, more dense water to the south in the deep ocean. When the current slows down, the North Atlantic just south of Greenland and Iceland can cool as regions elsewhere warm up.
Common thinking — that a weaker AMOC drives the cold blob — implies the connection between ocean current and North Atlantic surface temperatures is constant over time, according to the researchers. But the assumption is shaky in part because AMOC observations stretch back only about two decades, limiting historical analyses, Li said.
To explore how other factors interact with water temperatures and ocean currents, the researchers made simulations that incorporated a variety of influences, such as greenhouse gases at different concentrations, and climate conditions representing past and present environments. The team analyzed simulations representing a large number of scenarios spanning decades to account for several phenomena: heat transfer within the ocean; heat exchange between the ocean and the atmosphere; and heat radiated from the air, including the effects of low clouds.
The evaluation found that AMOC's cooling effect on ocean surface water splits into three phases. In simulations where the ocean current weakened from its greatest strength, surface temperatures slipped modestly. When the current weakens from a mid-range strength, the surface temperature drop roughly tripled, with the cold blob growing to cover the subpolar basin.
But when the ocean current loses steam beginning from an already-diminished state, the cooling effect fades and the cold blob shifts northeast toward Iceland and the Norwegian Sea, the analysis showed. The researchers pointed to variable impacts of heat radiation to and from the atmosphere, as well as reflections from low clouds, as contributing factors.
Once the ocean current weakens enough, the ocean will absorb more heat from the atmosphere and likely approach its capacity to buffer climate change, said Yifei Fan , lead author on the paper and doctoral candidate in the Department of Meteorology and Atmospheric Science.
"Climate models predict the ocean current will trend toward a weaker state in the future," Fan said. "This is concerning: If we cross the threshold into a new phase of ocean current behavior, how the North Atlantic uptakes heat will diverge from the AMOC patterns we know now. It would mean more uncertainty in the ocean surface temperatures, ocean currents and effects on climate and weather."
In particular, modeling suggested AMOC will decline from its higher-strength phase today to its lowest-strength phase by the year 2100. Timing of the cold blob's most significant temperature shift could be as late as 2067, or it could have been as early as 2021, the researchers said.
Simulations also suggested earlier peaks in ocean temperature change would correspond to an earlier disappearance of the cool patch. The researchers emphasized that ocean temperatures alone don't prove ocean current activity, and that variation in those temperatures doesn't necessarily signal a collapse of AMOC. Some predictions suggest the current's prolonged demise could lead to chillier winters in North America and Europe.
"We're not saying that AMOC will vanish. We're setting a foundation to understand nuanced interactions among ocean surface temperatures, ocean currents and other environmental forces," Li said. "More study and sustained Earth system monitoring will inform more revealing assessments of changes in the coupled ocean-atmosphere system, such as the points when the ocean absorbs heat from the atmosphere and then releases that heat."
Future research should narrow predictions for the cold blob and help scientists forecast ice conditions in Greenland, which influence oceanic and atmospheric activity, according to Fan.
"By reducing the range of scenarios, we can establish higher confidence and less ambiguity in predicting North Atlantic climate change," Fan said.
A full list of collaborators may be found in the paper .
This work was funded by NASA FINESST (Future Investigators in NASA Earth and Space Science and Technology) through grant 80NSSC24K1595 ; NASA grant 80NSSC22K0997 ; and U.S. National Science Foundation grant OPP-2106228 . This content is the sole responsibility of the authors and does not necessarily reflect the views of the authors.
