Is Gulf Stream slowing down ?

The Gulf Stream does not appear to be slowing down, say US scientists who have used satellites to monitor tell-tale changes in the height of the see. Confirming work by other scientists using different methodologies, they found dramatic short-term variability but no longer-term trend.A slow-down - dramatised in the film The Day After Tomorrow - is projected by some models of climate change. The research is published in the journal Geophysical Research Letters.

It forms part of a bigger movement of water, the Atlantic Meridional Overturning Circulation, which is itself one part of the global thermohaline process of currents.

The stream is a key process in the climate of western Europe, bringing heat northwards from the tropics and keeping countries such as the United Kingdom 4-6C warmer than they would otherwise be.

"The changes they are seeing in overturning strength are probably part of a natural cycle," said Josh Willis from Nasa's Jet Propulsion Laboratory (JPL) in Texas.

Between 2002 and 2009, the team says, there was no trend discernible - lots of variability on short timescales. The satellite record going back to 1993 did recommend a little increase in flow, although the researchers cannot be positive it is significant.

Short measures

"The slight increase in overturning since 1993 coincides with a decades-long natural pattern of Atlantic heating and cooling."

The first observations proposing the circulation was slowing down emerged in 2005, in research from the UK's National Oceanography Centre (NOC).

Using an array of detectors across the Atlantic and comparing its readings against historical records, scientists suggested the volume of chilled water returning southwards could have fallen by as much as 30% in half a century - a significant decline.

However, later observations by the same team showed that the strength of the flow varied hugely on short timescales - from one season to the next, or even shorter.

The surface water sinks in the Arctic and flows back southwards at the bottom of the ocean, driving the circulation.

But they have not found any clear trend since 2004.Rapid relief

The NOC team now has a chain of instruments in place across the Atlantic, making measurements continuously.

The quantities of water involved are immense, varying between two million and 35 million tonnes of water per second.

"In four-and-a-half years of measurement, they have found there is a lot of variability, and they are working to document it," said NOC's Harry Bryden.

Professor Bryden's team calculates that their process is nice to detect a long-term change in flow of about 20% - but it is not happened yet.

The array is part of the UK-funded Speedy project, which aims to refine understanding of potentially large climate change impacts that could happen in short periods.

But, they points out: "The process concentrates only on the upper [northward] flow - it doesn't give you much information on the returning flow southward."

They believes the JPL approach - using satellite altimeters, instruments that can measure sea height exactly, and the Argo array of autonomous floating probes - could potentially add useful information to that coming from long-term onsite monitoring arrays.

Driven by Hollywood, a popular picture of a Gulf Stream slowdown shows a sudden catastrophic event driving snowstorms across the temperate lands of western Europe and eastern North The united states.

Fantasy and reality

"But the Atlantic overturning circulation is still an important player in today's climate," they added.

That has always been fantasy - as, said Josh Willis, is the idea that a slow-down would trigger another ice age.

"Some have suggested cyclic changes in the overturning may be warming and cooling the whole North Atlantic over the coursework of several decades and affecting rainfall patterns across the US and Africa, and even the number of hurricanes in the Atlantic."