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 Jakob Dörr and colleagues from the Bjerknes Centre, the University of Bergen, the Nansen Environmental and Remote Sensing Centre, Stockholm University and the British National Oceanography Centre have looked into what happens to water that flows from the Atlantic Ocean and into the Arctic Ocean.

A decreasing Arctic sea ice cover may perhaps limit the reductions in ocean currents farther south.

A ribbon in the sea

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 varm water driven northward by the wind, before the water is cooled, gets denser and sinks. Then the water returns back southward at depths. 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 melt water and precipitation make the sea fresher. Both these factors make the water less dense, reducing the sinking that contributes to keeping the loop going. That less water returns southward at depths will in turn affect the flow of warm water from the Gulf of Mexico across the Atlantic Ocean toward Northern Europe and Norway.

North, north and farther north

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 blood stream with many branches. North can designate many locations along these branches, but some regions mean 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 Jakob Dörr have glanced even farther north: at the Arctic Ocean.

Shrinking ice may counteract a weakening of currents

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 Jakob Dörr, though emphasizing that the final result is uncertain.

"We now 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, Jakob 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.

Water sinks in the Arctic Ocean

From the Atlantic Ocean water from the Gulf Stream enters the Nordic seas between iceland, Scotland and the Faroe Islands. The warm water follows the Norwegian coast northward before the current splits into two branches.

One branch follows the western side of Spitsbergen and enters the Arctic Ocean through Fram Strait. The other turns eastward through the Barents Sea, reaching the main Arctic basin east of Franz Josef Land.

The Arctic Ocean can be delimited in several ways, and in the new study includes the Barents Sea.

In the main basin, near Franz Josef Land, water from the two branches meet again, completing a round before flowing out through Fram Strait and along the east coast of Greenland. But before meeting, each branch has undergone significant changes.

While some water remains near the surface, the rest has cooled, densified and sunk as deep water. Because deep water from the Arctic Ocean contributes to maintaining the Atlantic Meridional Overturning Circulation, it has been particularly important for the researchers to get an overview of how much deep water is produced.

Jakob Dörr and his colleagues' research showed that about one third of the Atlantic water that enters the Arctic Ocean, is transformed into deep water. Most of this water comes from the branch through the Barents Sea.

Ice-free waters open for cooling

"I like to talk about the cooling machine in the Barents Sea," says Jakob Dörr.

In the Barents Sea surface waters cool efficiently. The main reason is that this is ocean is shallow and to a large extent ice-free, even in winter.

While crossing the shelf, the water loses enormous amounts of heat to the cold Arctic air above. The water gets colder and denser. When reaching the deep Arctic basin, it is dense enough to slide far down the continental slope.

The other branch, entering through Fram Strait, meets the ice edge right north of Svalbard. A little bit of deep water is formed before reaching the edge, but that is it. The water continues its journey below the ice, isolated from the cold air above, and noe more cooling occurs.

Inside the Arctic Ocean both branches follow the bathymetry around, anti-clockwise beneath the ice. But the water from the Barents Sea has become much denser and lies below the water from Fram Strait. As the water reaches Fram Strait on its way out, both surface water and deep water flows into the Greenland Sea.

About three fourths of the deep water is formed by the cooling and sinking and surface waters. The rest has been formed because some water from the Fram Strait branch has mixed with denser water from the Barents Sea.

Quiet under the ice

"The oldest water in the depth of the Arctic Ocean has been there for several hundre years," says Jakob Dörr. "The Arctic Ocean is a quiet ocean."

No haste seems to apply farther up in the water column, either. Most of the water spends several decades on its journey through the polar basin.

The oceanographer says he still is surprised by how little the water changes once it has begun circling the basin.

Rain and melt water make the upper layers fresher, while water in regions where ice freezes – the shelf seas along the coast – gets more saline. But except for the mixing of water from the two branches, no processes significantly influence the production of deep water.

The Barents Sea is where the action is.

Uncertain outcome of more open water

"Now that we know the importance of the different regions, we can start to think about what happens when the Atlantic gets warmer and the sea ice edge retreats further," says Jakob Dörr.

As the ice withdraws, new regions of open waters could open for sinking and the formation of deep water. But that requires surface waters to cool. Can we expect an expansion if the southern parts warm correspondingly?

"It might be that the region of deep water formation in the Barents Sea deep will just move northward," says Jakob Dörr.

What could happen when the ice edge withdraws even farther north, out of the Barents Sea and into the polar basin, nobody knows.