Monday, May 9, 2011

The Agulhas system may play a key role in ocean circulation and climate change.

Off the coast of East Africa, the Agulhas current sweeps downward towards the continent's tip and, for the most part, swings back east into the Indian Ocean.  Some of it, however, "leaks" out around the cape and into the Atlantic ocean.  New research, published recently in Nature, suggests that this "Agulhas leakage" may have a huge impact on climate variability.
The Agulhas system.   Credit: Erik van Sebille, RSMAS

While is it not known exactly how much water is leaked into the Atlantic, the salty waters of the Agulhas form giant rings and eddies that ultimately join the Atlantic's main current system, the Atlantic Meridonal Overturning Circulation (AMOC), which includes the more familiar Gulf Stream current.  The AMOC carries warm water northwards, keeping parts of North America and Europe several degrees warmer than they otherwise would be.

Under the current climate change prediction models, ice melt from the Arctic and a weakening of the AMOC due to the buildup of greenhouse gases will result in cooling of the North Atlantic.  But authors of the Nature article say that the Agulhas leakage could compensate for these changes, helping to keep the North Atlantic warm, in contrast to these current prediction models .

In addition, geologic and modern data provide evidence that the Agulhas leakage increases under a warming climate, thus further suggesting that it could play a leading role in climate variability, as our climate continues to warm.

Researchers looked at the abundance of tropical plankton (Agulhas fauna) preserved in marine sediments corresponding to the late Pleistocene epoch, a period of repeated glaciations during the past 500,000 years.  They found less Agulhas fauna in sediments corresponding to glacial periods and more Agulhas fauna in sediments corresponding to periods of a warming climate, thus indicating that the leakage had increased during periods of warming.

Modern data, gathered from satellites, oceanographic instruments, and computer simulations show shifts in wind patterns, a southward expansion of Indian Ocean currents, and a warming of the waters in the Agulhas system that all favor an increase in the Agulhas leakage.

The authors of this study point out that more research into the role of the Agulhas leakage in climate change is sorely needed.  Questions such as whether the Agulhas leakage is a potential climate trigger or whether there is a feedback mechanism involved in its variability still remain to be answered.  Hopefully, research into the Agulhas will shed a new light on an often overlooked piece of the climate puzzle.

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