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Threading the Climate Needle: The Agulhas System

Agulhas Leakage fueled by global warming could stabilize Atlantic Overturning Circulation

Agulhas Current Figure shows 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). Figure produced by Erik van Sebille, UM/RSMAS. Click for larger image.

MIAMI — April 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, 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.

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.

Interview Click to watch Lisa Beal interview

“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," saiid 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.

Lisa Beal is interviewed by radio during a rendezvous between RV Knorr and Clipper Stad Amsterdam off the African Cape. Photo credit Jerry Beard.

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 Autónoma 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.

The ACT ocean observing program was launched in April 2010 to measure the variability of the Agulhas Current using a combination of current meter moorings and satellite data. Beal, who serves as chief scientist, spent one month aboard Research Vessel Knorr in the southwest Indian Ocean deploying oceanographic instruments. The data gathered in situ, when combined with along-track satellite information, will help increase our understanding of how the Agulhas system is changing in a warming climate. For information on the program, please visit http://act.rsmas.miami.edu

About the UM Rosenstiel School
The University of Miami’s mission is to educate and nurture students, to create knowledge, and to provide service to our community and beyond. Committed to excellence and proud of the diversity of our University family, we strive to develop future leaders of our nation and the world. Founded in the 1940’s, the Rosenstiel School of Marine & Atmospheric Science has grown into one of the world’s premier marine and atmospheric research institutions. Offering dynamic interdisciplinary academics, the Rosenstiel School is dedicated to helping communities to better understand the planet, participating in the establishment of environmental policies, and aiding in the improvement of society and quality of life. For more information, please visit www.rsmas.miami.edu.

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agulhas current •  indian ocean •  climate •  research •  dr. lisa beal •  modeling •