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Friday, November 25, 2011

Agulhas Ocean Current 'Leakage', Fueled by Global Warming, Could Stabilize Atlantic Overturning Circulation [or maybe not]

Agulhas Ocean Current 'Leakage', Fueled by Global Warming, Could Stabilize Atlantic Overturning Circulation

ScienceDaily (Apr. 27, 2011) — The Agulhas Current which runs along the east coast of Africa may not be as well known as its counterpart in the Atlantic, the Gulf Stream, but researchers are now taking a much closer look at this current and its "leakage" from the Indian Ocean into the Atlantic Ocean. In a study published in the journal Nature, April 27, 2011, a global team of scientists led by University of Miami (UM) Rosenstiel School of Marine & Atmospheric Science Associate Professor Lisa Beal, suggests that Agulhas leakage could be a significant player in global climate variability.

This figure shows the Agulhas system and its leakage into the South Atlantic. The southward shift of the winds in a warming climate (right panel) cause a southward shift of the subtropical front (red arrows), enlarging the "gateway" for Agulhas leakage around the tip of Africa. Increased Agulhas leakage acts to enhance Atlantic overturning, which causes a feedback on climate. (Background colors are dynamic height integrated between 2000 m and the surface.) (Credit: Erik van Sebille, UM/RSMAS)
The Agulhas Current transports warm and salty waters from the tropical Indian Ocean to the southern tip of Africa, where most of the water loops around to remain in the Indian Ocean (the Agulhas Retroflection), while some waters leak into the fresher Atlantic Ocean via giant Agulhas rings. Once in the Atlantic, the salty Agulhas leakage waters eventually flow into the Northern Hemisphere and act to strengthen the Atlantic overturning circulation by enhancing deep water formation.

Recent research points to an increase in Agulhas leakage over the last few decades caused primarily by human-induced climate change. This finding is profound, because it suggests that increased Agulhas leakage could trigger a strengthening in the Atlantic overturning circulation, at a time when warming and accelerated meltwater input in the North Atlantic has been predicted to weaken it.

"This could mean that current IPCC model predictions for the next century are wrong and there will be no cooling in the North Atlantic to partially offset the effects of global climate change over North America and Europe," said Beal, "Instead, increasing Agulhas leakage could stabilize the oceanic heat transport carried by the Atlantic overturning circulation."

There is also paleoceanographic data to suggest that dramatic peaks in Agulhas leakage over the past 500,000 years may have triggered the end of glacial cycles. This serves as further evidence that the Agulhas system and its leakage play an important role in the planet's climate.

"This study shows that local changes in atmospheric and oceanic conditions in the Southern Hemisphere can affect the strength of the ocean circulation in unexpected ways. Under a warming climate, the Agulhas Current system near the tip of South Africa could bring more warm salty water from the Indian to the Atlantic Ocean and counteract opposing effects from the Arctic Ocean," said Eric Itsweire, director of the National Science Foundation (NSF)'s physical oceanography program, which funded the research.

The study establishes the need for additional research in the region that focuses on Agulhas rings, as well as the leakage. Climate modeling experiments are critical, and need to be supported by paleoceanographic data and sustained observations to firmly establish the role of this system in a warming climate.

"Our goal now is to get more of the scientific community involved in research of the Agulhas system and its global effects. The emphasis has been too long in the North Atlantic," said Beal.

The scientific review team included UM's Lisa Beal, Wilhelmus P.M. de Ruijter of Utrecht University in the Netherlands, Arne Biastoch of Leibniz- Institut für Meereswissenschaften (IFM-GEOMAR) in Germany, and Rainer Zahn of the Universitat Autònoma de Barcelona in Spain, as well as members of SCOR Working Group 136 on the Climatic Importance of the Agulhas System, sponsored by the Scientific Committee for Oceanic Research, the International Association for the Physical Sciences of the Oceans, and the World Climate Research Program. The Scientific Committee on Oceanic Research is supported by the National Science Foundation, award no. OCE-0938349. Beal is funded by the National Science Foundation through the ACT (Agulhas Current Time-series) project, award no. OCE-0850891.

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