A mysterious “warming hole” in the North Atlantic Ocean, an anomalous zone of cooling temperatures which has fascinated and puzzled scientists for the past few years, may be evidence of more troubling processes at work.
“I think the main thing about our paper that we show is that the Arctic sea ice loss can play an active role in climate change and ocean change as well,” said Alexey Fedorov, an ocean and geophysics expert at Yale University and one of the new study’s authors.
The “warming hole” is a region over the North Atlantic, just south of Greenland, where long-term temperature maps suggest air temperatures have been cooling slightly over the last century, rather than warming like most of the rest of the world. The warming hole is situated over roughly the same part of the ocean where a so-called cold “blob,” a zone of water persistently cooler compared to the surrounding area, was discovered several years ago.
The causes of both have been the subject of some spirited scientific debate in the last few years, and some scientists believe they are related to the same phenomenon. The theory suggests that not as much heat is getting transported to that region of the North Atlantic anymore — and this could indicate a big problem with one of the ocean’s most important currents.
The Atlantic Meridional Overturning Circulation, or AMOC, is a powerful conveyor-like current system that carries warm water north from the equator and sends cool water back down from the Arctic. It’s responsible for transporting heat all over the ocean and regulating weather patterns in places like Europe and eastern North America. But some recent studies have suggested that the AMOC may be slowing down, which could explain why less heat is reaching the North Atlantic.
Whether the AMOC has actually been weakening in recent decades, and to what extent, is still an open question among oceanographers. But many scientists worry that the future effects of global warming, including large influxes of fresh water from melting sea ice and retreating Greenland glaciers, could further disrupt the AMOC’s flow.
The conveyor system relies on a delicate physical process in which warm water travels north to the Arctic, cools down, sinks to the bottom of the ocean and flows back south. Large influxes of less dense fresh water can disrupt the current’s ability to properly “turn over” and ultimately cause it to slow down. Additionally, as sea ice melts and leaves a greater area of liquid ocean exposed to the sun’s radiation, the water may start to absorb more heat — and this can also disrupt the current’s overturning process.
The new study provides further support for this idea, pointing to melting sea ice as a potential major driver of both a weakening AMOC and the North Atlantic warming hole.
Using an ocean circulation model, the researchers explored the way changes to the temperature and fresh water fluxes in different parts of the ocean — the Arctic, the Atlantic, and so on — would affect the AMOC. There are a variety of real-life factors that can cause these changes, according to Fedorov. In the Arctic, melting sea ice can produce an influx of fresh water, while the same is true of the melting Greenland ice sheet in the North Atlantic. And in either case, the more ocean that’s exposed to the sun’s rays, the more heat the water can absorb.
“When we started the study, we didn’t really focus on the Arctic,” he added. But the study’s results suggested that its influence might be more powerful than previously thought.
The researchers, who included Fedorov and colleagues Wei Liu of Yale and lead author Florian Sevellec of the University of Southampton, found that over short time spans of a decade or less, changes in the North Atlantic had the greatest potential to disrupt the AMOC. But over longer periods of 20 years or more — a more relevant time scale for scientists considering the long-term effects of climate change — cold, fresh water influxes in the Arctic had the biggest influence.
Fedorov cautioned that over much longer time scales — say, a century or more — the North Atlantic may again gain the greater influence as the Greenland ice sheet further destabilizes and begins to dump greater quantities of fresh water into the ocean. But throughout the rest of the century, the study suggests that the melting of sea ice in the Arctic may have the more dramatic effect.
The researchers further backed up their findings using a climate model which specifically simulated the melting of Arctic sea ice. In the simulation, the AMOC weakens and a cool spot closely resembling the warming hole appears over the North Atlantic.
Pointing to these results, the researchers suggest that melting Arctic sea ice has likely already had a significant influence on the weakening AMOC, and may continue to play a major role in the coming decades.
That said, some scientists are approaching the results with caution.
“It should be pointed out that these are results from one model only,” said Laura Jackson, an ocean circulation expert at the Met Office, Britain’s national weather service, in an emailed comment. (Jackson was not involved with the new research.) “We know that in different models the AMOC has different sensitivities, so these results may be model dependent. Nevertheless this role of the sea ice is an interesting possibility.”
And, she added, there’s still uncertainty in the scientific community about whether the AMOC has even yet begun to slow down as a result of climate change.
“We only have direct observations for the last decade and, although these have shown a weakening over that time, other evidence has pointed to this being variability rather than an ongoing decline,” she pointed out.
Despite this debate, many scientists are still confident that climate change will still have a significant influence on the AMOC in the future. And the new study reaffirms the idea that multiple seemingly unrelated consequences of global warming may be closely tied to one another after all.
