This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 (2.5 million square kilometers) area south and southeast of Greenland cooled by as much as 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 that 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 time frame 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 weakened from a midrange strength, the surface temperature drop roughly tripled, with the cold blob growing to cover the subpolar basin.
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