The warm Gulf Stream is maintained by coldness. The Barents Sea is a cooling machine. To predict how ocean currents in the Atlantic Ocean may develop, one needs to know what drives them. The hunt for driving forces has led researchers to follow the warm water from the Gulf Stream as far north as it gets.
In a new study published in Ocean Science, Jakob Dörr and colleagues from the Bjerknes Center, the University of Bergen, the Nansen Environmental and Remote Sensing Center, Stockholm University and the British National Oceanography Center have looked into what happens to water that flows from the Atlantic Ocean into the Arctic Ocean.
A decreasing Arctic sea ice cover may perhaps limit reductions in ocean currents farther south.
The Gulf Stream is one component in a larger system of currents termed the Atlantic Meridional Overturning Circulation (AMOC). The word overturning refers to water being transported vertically as well as horizontally, as in a loop or conveyor belt between the surface and the abyss.
Whether climate change may reduce the Gulf Stream has been a recurring topic in international news media. The real question concerns possible changes in the overturning circulation in the Atlantic—not one single surface current, but the entire loop.
The overturning involves warm water driven northward by the wind before the water is cooled, gets denser and sinks. Then the water returns southward at depth. In some regions, the water gets denser as salt is released from freezing ice. But the key to sinking is cooling of the surface waters.
With climate change, the water in the northern regions will cool less, while more meltwater and precipitation make the sea fresher. Both factors make the water less dense, reducing the sinking that contributes to keeping the loop going. That less water returns southward at depth will in turn affect the flow of warm water from the Gulf of Mexico across the Atlantic Ocean toward Northern Europe and Norway.
Questions concerning the future of the Gulf Stream are decided by climate changes in the north. But where is north?
In reality, the currents in the Atlantic Ocean do not follow a single loop, but spread like a bloodstream with many branches. North can designate many locations along these branches, but some regions matter more than others.
Originally, it was thought that most water would sink in the Labrador Sea. Later, the Northern Seas were found to be as important. In recent years, researchers like Dörr have glanced even farther north: at the Arctic Ocean.
Theory and climate models suggest that the Atlantic Meridional Overturning Circulation will be reduced in the future, and the Norwegian Sea is one of those regions that will contribute less to maintaining it. Whether Norway will receive less heat from the south is less clear.
Possibly, new regions may take over some of the role that the Norwegian Sea and other cooling regions have played.
One hypothesis is that decreasing sea ice in the Arctic Ocean may open up new regions where water can sink, and thus counteract some of the weakening of the overturning circulation.
“I think it potentially can make a difference for what we will see in Norway, essentially, anything north of Iceland,” says Dörr, though emphasizing that the final result is uncertain.
“We know so little about the Arctic Ocean,” he continues. “There are very few observations of the deep Arctic.”
To be able to follow the movement of water in and out of the Arctic Ocean, Dörr and his colleagues have used a computer model to simulate the ocean currents. They have explored the conditions as they are now, knowledge required when considering possible future changes.
