North Atlantic 'cold blob' and ocean current link shifts across three phases

The link between a weakening Atlantic meridional overturning circulation (AMOC) and the cool patch of water south of Greenland is less straightforward than often assumed. A team led by researchers at Penn State's College of Earth and Mineral Sciences, writing in Nature Communications, used climate simulations to show that the effect of AMOC on surface temperatures divides into three distinct phases.

A cold patch with a changing cause

The so-called cold blob, or North Atlantic warming hole, covers about 2.5 million square kilometres south and southeast of Greenland. Over the past century it has cooled by as much as 1.8 degrees Fahrenheit (roughly 1 degree Celsius) while most other ocean regions warmed. Several indicators point to a sustained decline in AMOC, the system that carries warm surface water north and cold, dense water south at depth, as a persistent cause.

The usual interpretation implies that the relationship between current strength and surface temperature stays fixed over time. The authors argue that this is a shaky assumption, partly because direct AMOC observations span only about two decades, which restricts historical analysis.

How the simulations were run

To get around the short record, the team analysed a large number of simulated scenarios covering decades. These included different greenhouse gas concentrations and climate conditions representing past and present. The analysis considered heat transport within the ocean, heat exchange between ocean and atmosphere, and radiation from the air, including the influence of low clouds. The work builds on the group's 2025 finding that atmospheric radiation helps to depress water temperatures in the region.

Three phases of response

The results indicate that AMOC's cooling influence depends on the state from which the current weakens:

• When AMOC weakens from its greatest strength, surface temperatures fall only modestly.
• When it weakens from a midrange strength, the temperature drop roughly triples and the cold blob expands to cover the subpolar basin.
• When weakening begins from an already diminished state, the cooling effect fades and the cold blob moves northeast towards Iceland and the Norwegian Sea.

The researchers point to variable radiative effects, including reflection from low clouds, as contributing factors. Once the current is weak enough, they say, the ocean will take up more heat from the atmosphere and is likely to approach the limit of its capacity to buffer climate change.

Timing and uncertainty

Modelling suggested that AMOC will move from its higher-strength phase today to its lowest-strength phase by 2100. The most significant temperature shift in the cold blob could occur as late as 2067, or could already have happened as early as 2021. Earlier peaks in temperature change corresponded to an earlier disappearance of the cool patch in the simulations.

The authors stress the limits of what can be inferred. Ocean temperatures alone do not prove changes in current activity, and variation in those temperatures does not necessarily signal a collapse of AMOC. Lead author Yifei Fan, a doctoral candidate, said that crossing into a new phase of current behaviour would make North Atlantic heat uptake diverge from the patterns now familiar, adding uncertainty to forecasts of surface temperatures, currents and weather. Corresponding author Laifang Li, an assistant professor of meteorology and atmospheric science, said the study does not claim that AMOC will vanish. Some predictions link a prolonged decline of the current to colder winters in North America and Europe.

Why it matters and what comes next

The findings suggest that using the cold blob as a simple gauge of AMOC strength could mislead, since the same temperature signal may reflect different underlying states. The results come from simulations rather than new direct measurements, so their reliability depends on how well models represent processes such as low-cloud effects.

The team calls for sustained satellite and related ocean monitoring to reduce model uncertainty. Narrowing the range of plausible scenarios, Fan said, would also help forecast ice conditions around Greenland, which in turn influence ocean and atmospheric behaviour. Better forecasts of when ocean temperatures cross key thresholds would indicate how much time remains to develop mitigation technologies and policies.

The paper is Fan et al., "Regime shifts of AMOC-sea surface temperature relationship", Nature Communications (2026).

Source: North Atlantic's 'cold blob' may contain climate nuance (Phys.org, 4 November 2026).

Image: AI-generated illustration.