Crucial to this warm-to-cold oceanographic choreography is the Denmark Strait Overflow Water (DSOW), the largest of the deep, overflow plumes that feed the lower limb of the AMOC and return the dense water south through gaps in the Greenland-Scotland Ridge.

The North Icelandic Jet in cross-section, adjacent to the continental slope of Iceland. Credit: WHOI. Click to enlarge.

For years it has been thought that the primary source of the Denmark Overflow was a current adjacent to Greenland known as the East Greenland Current. However, this view was recently called into question by two oceanographers from Iceland who discovered a deep current flowing southward along the continental slope of Iceland.

They named the current the North Icelandic Jet and hypothesized that it formed a significant part of the overflow water.

Now, in a paper published in journal Nature Geoscience, the team of researchers—including the two Icelanders who discovered the current—has confirmed that the Icelandic Jet is not only a major contributor to the DSOW but “is the primary source of the densest overflow water.”

We present the first comprehensive measurements of the NIJ. Our data demonstrate that the NIJ indeed carries overflow water into Denmark Strait and is distinct from the East Greenland Current. The NIJ constitutes approximately half of the total overflow transport and nearly all of the densest component.

—Robert Pickart of the Woods Hole Oceanographic Institution (WHOI) in Massachusetts, co-author

The researchers used a numerical model to hypothesize where and how the NIJ is formed.

These results implicate water mass transformation and exchange near Iceland as central contributors to the deep limb of the Atlantic Meridional Overturning Circulation, and raise new questions about how global ocean circulation will respond to future climate change.

—Eric Itsweire, program director in the NSF Division of Ocean Sciences, which funded the research

“We’ve identified a new paradigm,” Pickart said, likely a new, overturning loop of warm to cold water. The results, Pickart says, have “important ramifications” for ocean circulation’s impact on climate.

Scientists have been concerned that this overturning loop—also called a conveyor belt—is slowing down due to a rise in global temperatures. They suggest that increasing amounts of fresh water from melting ice and other warming-related phenomena are making their way into the northern North Atlantic, where it could freeze and decrease the need for the loop to deliver as much warm water as it does now.

Eventually, this could lead to a colder climate in the northern hemisphere.

While this scenario is far from certain, researchers need to understand the overturning process, Pickart said, to make accurate predictions about the future of climate and circulation interaction.

If a large fraction of the overflow water comes from the NIJ, then we need to re-think how quickly the warm-to-cold conversion of the AMOC occurs, as well as how this process might be altered under a warming climate.

—Robert Pickart

Pickart and a team of scientists from the US, Iceland, Norway, and the Netherlands are scheduled to embark on 22 August on a cruise aboard the research vessel Knorr. They will collect new information on the overturning in the Iceland Sea.

The team will deploy an array of year-long moorings across the entire Denmark Strait to quantify the NIJ and distinguish it from the East Greenland Current. They then will collect shipboard measurements in the Iceland Sea to the north of the mooring line to determine more precisely where and how the NIJ originates.

The cruise will be chronicled at the North Icelandic Jet Cruise website.

In addition to Pickart, authors of the Nature Geoscience paper include Michael Spall and Daniel Torres of WHOI; lead author Kjetil Våge, and co-authors Svein Østerhus and Tor Eldevik, all of the University of Bergen, Norway; and Héðinn Valdimarsson and Steingrímur Jónsson—the co-discoverers of the NIJ—of the Marine Research Institute in Reykjavik, Iceland.

The Research Council of Norway also funded the work.

Resources

• Kjetil Våge, Robert S. Pickart, Michael A. Spall, Héðinn Valdimarsson, Steingrímur Jónsson, Daniel J. Torres, Svein Østerhus & Tor Eldevik (2011) Significant role of the North Icelandic Jet in the formation of Denmark Strait overflow water. Nature Geoscience. doi: 10.1038/ngeo1234