Atlantic currents not slowing after all

Posted by Emma Woollacott

New NASA measurements show that the Gulf Stream has not slowed down over the past 15 years. In fact, it may even have sped up slightly.

The Atlantic overturning circulation is a system of currents, including the Gulf Stream, that bring warm surface waters from the tropics northward into the North Atlantic. There, the water cools, sinks to great depths and changes direction.

Without the heat carried by this circulation system, the climate around the North Atlantic - in Europe, North America and North Africa - would probably be a great deal colder.

Until recently, the only direct measurements of the circulation's strength have been from ship-based surveys and a set of moorings in the mid-latitudes. The new technique is based on data from NASA satellite altimeters, which measure changes in the height of the sea surface, as well as data from Argo robotic floats.

Oceanographer Josh Willis of NASA's Jet Propulsion Laboratory in Pasadena was able to calculate changes in the northward-flowing part of the circulation at about 41 degrees latitude, roughly between New York and northern Portugal.

Combining satellite and float measurements, he found no change in the strength of the circulation from 2002 to 2009. Looking further back with satellite altimeter data alone, Willis found evidence that the circulation had sped up about 20 percent from 1993 to 2009.

The latest climate models predict the overturning circulation will slow down as greenhouse gases warm the planet and melting ice adds fresh water to the ocean. "Warm, fresh water is lighter and sinks less readily than cold, salty water," Willis explained.

For now, however, there are no signs of a slowdown in the circulation. "The changes we're seeing in overturning strength are probably part of a natural cycle," said Willis. "The slight increase in overturning since 1993 coincides with a decades-long natural pattern of Atlantic heating and cooling."

If or when the overturning circulation slows, the results are unlikely to be dramatic. "No one is predicting another ice age as a result of changes in the Atlantic overturning," said Willis. "Even if the overturning was the Godzilla of climate 12,000 years ago, the climate was much colder then. Models of today's warmer conditions suggest that a slowdown would have a much smaller impact now."