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Europe's climate fate decided by tussle between oceanic currents

By Martin Leggett - 27 Apr 2011 17:0:0 GMT
Europe's climate fate decided by tussle between oceanic currents

Like the proverbial onion (or ogre..) the unfolding story of climate change for the North Atlantic region has layers. In addition to the counter-play between rising global temperatures, a slackening Gulf Stream-North Atlantic Drift, and melting Arctic waters, a new paper has uncovered another layer to an already complex picture. The so called 'Agulhas leakage' is a warm water current spilling around the tip of southern Africa. And significant changes in its behavior - also driven by global warming - have been reported in a study published in today's Nature.

Those changes could act as a counter to the well-established mechanism for slowing down the Gulf Stream, from the freshwater flooding from a melting Arctic basin. The ultimate effect could be to hasten a heating up of Europe, which scientists has previously predicted would be shielded due to a cooling from a slower 'Atlantic turn-over', and sputtering North Atlantic Drift. A grand battle between northern and southern influences on the future climate of the Northern Atlantic may well be being played out right now.

Oceanic currents have long been known to be important in deciding how regional climes are distributed. And western Europe is a classic case - it is an area benefiting from a warmer-than-expected climate due to the excess heat transported in the Gulf Stream-North Atlantic Drift, which runs from Florida towards northern Europe. This major arm of oceanic circulation is driven by the sinking of cooling salty sea-water near Greenland and Iceland.

But with the Arctic experiencing unprecedented melting, as temperatures rise, more freshwater from rivers and a reducing sea-ice cover are producing pools there of less salty seawater. It is thought that these could threaten the cold-sink that drives the North Atlantic Drift - which would cause Europe to become much cooler, even as the planet warmed; the 'Day After Tomorrow' scenario as dramatically exaggerated by Hollywood. Or would it?

This research, from a team led by Lisa Beal - an Associate Professor at University of Miami - introduces a new player, in the shape of increased 'Agulhas leakage' from the Indian Ocean. The waters from the Agulhas current, that curls round southern Africa, are warm and salty - and they run partly up into the north Atlantic. This study shows that the Agulhas leakage northwards is increasing, as global warming drives the subtropical front, stretching across the southern hemisphere, southwards

That 'opens the gate' for more salty waters to flow up into the Atlantic, which could help speed up the salty-water engine that is driving the North Atlantic Drift. This could have major implications for the evolution of regional climate. ''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.'' It seems that Europe may have lost its global warming 'opt out' card.

Image Credit: Erik van Sebille, UM/RSMAS. Image Caption: 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).