New ocean currents do not appear every day. In an article published in Nature Communications yesterday, scientists from Norway, USA and the Faroe Islands present a newly identified deep ocean current
The current carries dense, deep water out of the Nordic Seas, as part of the overturning circulation in the North Atlantic. The overturning circulation’s best-known component is the Gulfstream.
“Some of the measurements we used were old, but no one had noticed this current”, says Stefanie Semper.
Semper is a PhD candidate at the Bjerknes Centre for Climate Research and the Geophysical Institute at the University of Bergen, and has led the work to identify the new current. ar gamle, men ingen hadde lagt merke til denne strømmen, sier Stefanie Semper.
An important current in a major system
The first map of the Gulf Stream was published by Benjamin Franklin in 1786. The map shows a river in the ocean off the coast of North America, heading towards Europe.
It has long been known that the Gulf Stream is part of a loop where surface water that flows northwards in the Atlantic, is cooled and sinks before flowing back south at depth. Gradually, it has become clear that the regions of sinking are the Labrador Sea, the Irminger Sea and the Nordic Seas.
Water enters the Nordic Seas on both sides of Iceland and continues to the Arctic Ocean and the Barents Sea. Some of this water is cooled until it is dense enough to sink and return to the Atlantic through Denmark Strait and the Faroe Bank Channel, between the Faroe Islands and Scotland. From there it rushes down the slope into the abyss of the Atlantic Ocean.
Still, there is much to discover. There have been shifting theories about where in the Nordic Seas the water sinks and about the paths taken back out to the Atlantic. The Iceland-Faroe Slope Jet is the latest addition.
Noticed that water flowed in the opposite direction
In 2011 researchers from Bergen took part in a scientific cruise off the coast of Iceland. They were looking for the source of the current that brings dense water out to the Atlantic on the western side of Iceland. Kjetil Våge, research scientist at the Bjerknes Centre and the Geophysical Institute at the University of Bergen, took part in the cruise, which was led by Bob Pickart from Woods Hole Oceanographic Institution in Massachusetts, USA.
The scientists followed the North Icelandic Jet upstream to a region northeast of Iceland, where they assumed that it emerged. Correctly, their measurements showed that the current gradually got weaker. But when they continued measuring along the continental shelf, they again registered movements in the water.
“Very surprisingly, we saw a distinct current going eastwards”, says Kjetil Våge.
They started to develop a picture of the region north of Iceland as something like a continental divide in the ocean, with water coming from the north splitting into two currents: the well-known North Icelandic Jet that flows to Denmark Strait, and an unknown current flowing in the opposite direction, towards the Faroe Islands.
Assembled old and new time series
To find out whether they were right, they continued measuring southeastwards along the continental shelf. When they returned home, that data set was left untouched until Stefanie Semper started analyzing it last year.
Researchers from the Faroe Islands had noticed that water reached their islands from the west, but they had not looked more into the phenomenon.
«None of these data sets showed the whole length of the current”, says Stefanie Semper, “but together they made it possible to tell a consistent story».
Combining the data from their Faroese colleagues with the data from the 2011 cruise, she could identify a continuous current 800 to 1000 meters below the surface. The water flows from the northern side of Iceland, along the continental self and around the northern side of the Faroe Islands, before continuing into the Atlantic through the Faroe Bank Channel.
Revealed by the fingerprint
To identify the current, the researchers used not only measurements of the water’s velocity, but also of the water itself.
«Water masses have their own fingerprints», says Stefanie Semper.
The combination of the temperature and the salinity of sea water separates water from different sources. Together with measurements of the velocity, this could be used to trace the water backwards in the Nordic Seas.
Alongside the investigation of the new current, researchers from the same institutions, together with Chinese researchers, have looked for the source of the water before it reaches Iceland. These results were also published yesterday.
The scientists show that the water that turns east in the Iceland-Faroe Slope Jet, has the same characteristics as the water that turns to the west. The two currents have the same fingerprint, a characteristic that can be traced to the same region in the Greenland Sea.
Together, these new articles show that dense water from the Greenland Sea follows submarine ridges southwards and splits into two branches when it approaches the continental shelf on the northern side of Iceland. One branch reaches the Atlantic Ocean through Denmark Strait, the other through the Faroe Bank Channel.
A jet stream in the ocean
The term jet stream is normally associated with concentrated bands of strong winds in the atmosphere. The reason that the scientists have used ‘jet’ in the name of an ocean current, is because the current resembles such bands. But water is heavy compared to air, so in the ocean everything goes more slowly.
Compared to the air in the polar jet stream ten kilometers above the Atlantic, which may regularly reach speeds of 50 meters per second, the water in the Iceland-Faroe Slope Jet hardly moves.
“Ten centimeters per second”, says Stefanie Semper. “Fifteen, max. Still it transports about as much water as all the rivers on Earth put together”.
The Nordic Seas are more important than previously assumed
The Nordic Seas have always been important to Norwegian, Icelandic and Faroese oceanographers, but lately the region has also received more attention outside the region. To understand how the currents in the Atlantic are affected by climate change, you have to know the role of the water sinking in the north.
There has been a lot of focus on the Gulf Stream in the southern part of the overturning circulation and on the sinking in the Labrador Sea, which has been considered at least as important. Recent results have drawn attention to the Nordic Seas.
Stefanie Semper and her colleagues hope to look more into how the water moves from the Greenland Sea to the region north of Iceland, and why the flow splits right there.
“You think you’re answering a question”, she says. “Now, we have even more».
Semper, S., Pickart, R.S., Våge, K. et al. The Iceland-Faroe Slope Jet: a conduit for dense water toward the Faroe Bank Channel overflow. Nat Commun 11, 5390 (2020).
Huang, J., Pickart, R.S., Huang, R.X. et al. Sources and upstream pathways of the densest overflow water in the Nordic Seas. Nat Commun 11, 5389 (2020).