A patch of the Atlantic Ocean just south of Greenland is cooling while much of the world warms, and scientists have found that both ocean and atmospheric factors contribute equally to this "cold blob" phenomenon, Penn State University reported on June 30.

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 found that a weakening AMOC impacts not just the ocean but also the atmosphere.

"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. "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 analysed 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, denser, and sinks. In a movement like an ocean conveyor belt, the cooler, deep water travels south and warm tropical surface water moves north.

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.

Cooler ocean surface temperatures can reduce evaporation and moisture in the atmosphere, meaning less water vapour, a greenhouse gas that traps heat radiating from the planet'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.

The full report by Penn State can be found on the university’s website.