Scientists have discovered a new factor that contributes to the so-called ‘cold blob’ – an unusual area of cooling waters in the Atlantic Ocean, says Melissa Hobson.
According to a new study in the journal Science Advances, the atmosphere plays a role along with ocean conditions.
“In the past century, most of the planet has warmed while the subpolar North Atlantic has been stubbornly cooling,” according to a statement from the study’s co-author 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.”
What is the cold blob?
In the Atlantic Ocean, just south of Greenland, water temperatures are cooling rather than warming. “In contrast to global warming, the subpolar North Atlantic has experienced long-term cooling throughout the 20th century,” says the study. This spot of cooling waters has been dubbed the ‘cold blob’.
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On the other end of the spectrum is ‘The Blob’ – an area in the Pacific Ocean that experienced unusually high temperatures between 2013 and 2016, resulting in the deaths of millions of seabirds and other marine animals.
While ‘The Blob’ was caused by marine heatwaves, the ‘cold blob’ was a puzzle. Researchers had previously suggested this ocean anomaly was caused by a weakening of the Atlantic Meridional Overturning Circulation (AMOC) – a system of ocean currents that play a role in regulating the climate.
The AMOC carries warm, salty surface water to the North Atlantic from the tropics. As it cools, it sinks and this colder, deeper water flows south. The current continues in this conveyor belt-like motion: moving north, cooling, sinking, and heading south.
Melting ice sheets impact this cycle by adding more freshwater to the environment. Freshwater is less dense than salty water and sinks less easily, so the conveyor belt effect is not as strong.
When the AMOC is weaker, less warm water flows from the tropics to the poles. This could be a reason for the strange patch of cooling water.
But now scientists have used climate models to discover that something else is at play – both the atmosphere and the ocean are affected when the AMOC weakens. A combination of these two factors might explain the ‘cold blob’.
“We analysed state-of-the-art climate models to quantify two pathways for how the AMOC contributes to the cold blob,” says lead author Yifei Fan in a statement, “and we found that the contribution from the atmosphere is comparable to that from ocean transport itself, which has never been found before.”
“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,” says Fan, who is a graduate student at Penn State. “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.”
When ocean temperatures are lower, less moisture evaporates. The drier atmosphere has less water vapour (a greenhouse gas that traps heat) and so is also cooler. This amplifies the cold anomaly, according to the experts.
“Reducing the greenhouse effect, to put it simply, will feed back to the surface and amplify the pre-existing cold anomaly,” says Fan. “And on a longer time scale, this feedback can make the cold blob more persistent.”
Understanding this odd phenomenon is important because it can influence the world’s climate, says co-author Laifang Li, assistant professor of meteorology and atmospheric science at Penn State: “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.”
Top image: Atlantic sea surface temperature trend between 1900 and 2005 for the average of six observation datasets. Credit: Kai-Yuan Li/UCR
