A new article in Nature Geoscience revises our current understanding of how the dense overflow waters from the Nordic seas, which represent the headwaters of the Meridional Overturning Circulation's lower limb, are formed. This implies that the timescale for the renewal of the deepest water in the overturning cell, and its sensitivity to changes in climate, could be different than presently envisioned.
Warm Gulf Stream-origin waters flow northward across the Greenland-Scotland Ridge into the Nordic seas and release heat to the atmosphere. The resulting cold and dense waters return southward by flowing through gaps in the ridge as overflow plumes. It is commonly thought that the primary source of the Denmark Strait Overflow Water, the largest of these overflow plumes, is a current flowing southward along the continental slope of Greenland called the East Greenland Current.
Icelandic scientists Steingrímur Jónsson and Héðinn Valdimarsson presented in 2004 the first evidence of a previously unknown current -- the North Icelandic Jet -- that flows along the continental slope of Iceland toward the Denmark Strait. A team of Norwegian, American, and Icelandic scientists -- including dr. Kjetil Våge from the University of Bergen and the Bjerknes Centre for Climate Research, Norway -- conducted dedicated field surveys in October 2008 and August 2009 that establish the existence of the North Icelandic Jet.
The new article by Våge and co-authors shows that the jet supplied approximately half of the total overflow transport and was the primary source of the densest component during the two surveys. The article also includes simulations with an ocean general circulation model that suggests that the import of warm, salty water from the North Icelandic Irminger Current and water-mass transformation in the interior Iceland Sea are critical to the formation of the jet. This represents a new scenario for the formation of Denmark Strait Overflow Water and raises novel questions about a crucial component of Earth's climate system.
The team is currently about to embark on a new cruise to deploy a string of instruments across the Denmark Strait and to search for the origin of the overflow waters carried by the North Icelandic Jet. Their progress can be followed online at: www.whoi.edu/denmarkstrait
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