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A patch of the Atlantic Ocean just south of Greenland is cooling while much of the world warms. The origin of this "cold blob" has been linked to weakening ocean currents that help regulate global climate—called the Atlantic Meridional Overturning Circulation (AMOC). A team of scientists led by Penn State has found a weakening AMOC impacts not just the ocean but also the atmosphere, and that these two factors may contribute equally to the cold anomaly.

The researchers have their findings in the journal Science Advances.

"In the past century, most of the planet has warmed while the subpolar North Atlantic has been stubbornly cooling," said Pengfei Zhang, an assistant research professor in the Department of Meteorology and Atmospheric Science at Penn State and a co-author on the study. "Our findings help explain why this so-called cold blob exists and shed light on how future changes in ocean currents could ripple through the climate system."

Previous studies on the cold blob have focused on ocean currents that bring warm water to the North Atlantic. But a cooling ocean will also result in a cooler, drier atmosphere, which can further amplify the cold anomaly, the scientists said.

"We analyzed state-of-the-art climate models to quantify two pathways for how the AMOC contributes to the cold blob," said Yifei Fan, a graduate student at Penn State and lead author on the study. "And we found that the contribution from the atmosphere is comparable to that from ocean transport itself, which has never been found before."

The AMOC brings warm, salty water from the tropics to the North Atlantic, where the water becomes cooler, and therefore denser, and sinks. In a movement like an ocean conveyor belt, the cooler, deep water travels south and warm tropical surface water moves north, the scientists said.

But excess freshwater from the melting Greenland Ice Sheet entering the ocean dilutes the salty ocean water, making it less dense and less able to sink, which threatens to weaken the conveyor belt.

"There's a traditional view that as this large-scale circulation weakens, ocean heat transport will be reduced and the higher latitudes in the north Arctic will cool," Fan said. "But we found that's not the only way the AMOC could have influence. Another potential contribution is how the cold blob influences the atmosphere, specifically the coupling between the atmosphere and the ocean."

Cooler ocean surface temperatures can reduce evaporation and moisture in the atmosphere. This means, for example, there will be less water vapor, a greenhouse gas that traps heat radiating from Earth's surface.

"Reducing the greenhouse effect, to put it simply, will feed back to the surface and amplify the pre-existing cold anomaly," Fan said. "And on a longer time scale, this feedback can make the cold blob more persistent."

The researchers analyzed simulations from multiple advanced global climate models to investigate the physical processes linking the AMOC to the cold blob. They used a diagnostic tool called a partial temperature decompositional framework, which breaks down distinct influences on temperature.

The approach identified the atmospheric feedback as more important than previously realized, the scientists said.

"Usually, people think about why this cold blob occurs and their very natural, intuitive thought is to look for the oceanic contribution," said Laifang Li, assistant professor of meteorology and atmospheric science at Penn State, co-author of the study and Fan's adviser. "We ask the question, why can't the AMOC influence the cold blob through other processes? And I think that is a philosophical novelty to this study."

The scientists said a better understanding of the unique cold blob region is important because of its potential climate impacts.

"The cold blob can disturb the atmospheric jet stream and storm activities, so it has implications for extreme weather events in North America and Europe," said Li, who is also a co-hire of the Institute for Computational and Data Sciences at Penn State.

The scientists said their findings are based on climate models, which provide good—but not perfect—representations of the real world. Future research is needed to confirm the extent to which the two pathways contribute to the cold blob.