A slow-down of the Gulf Stream, as dramatized in the movie The Day After Tomorrow, is projected by some models of climate change.
The stream is a key process in the climate of western Europe, bringing heat northwards from the tropics and keeping countries such as the UK 4-6C warmer than they would otherwise be.
It forms part of a larger movement of water, the Atlantic Meridional Overturning Circulation, which is itself one component of the global thermohaline system of currents.
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 The Atlantic overturning circulation is still an important player
in today's climate  Josh Willis, Nasa |
Short measures
The first observations suggesting 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 cold water returning southwards could have fallen by as much as 30% in half a century - a significant decline.
The surface water sinks in the Arctic and flows back southwards at the bottom of the ocean, driving the circulation.
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.
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.
"In four-and-a-half years of measurement, we have found there is a lot of variability, and we're working to explain it," said NOC's Harry Bryden.
The quantities of water involved are huge, varying between four million and 35 million tonnes of water per second.
The array is part of the UK-funded Rapid project, which aims to refine understanding of potentially large climate change impacts that could happen in short periods.
Professor Bryden's team calculates that their system is good enough to detect a long-term change in flow of about 20% - but it has not happened yet.
He believes the JPL approach - using satellite altimeters, instruments that can measure sea height precisely, and the Argo array of autonomous floating probes - could potentially add useful data to that coming from long-term on-site monitoring arrays.
But, he points out: "The method concentrates only on the upper [northward] flow - it doesn't give you much information on the returning flow southward."
Fantasy and reality
Driven by Hollywood, a popular image of a Gulf Stream slowdown shows a sudden catastrophic event driving snowstorms across the temperate lands of western Europe and eastern North America.
That has always been fantasy - as, said Josh Willis, is the idea that a slow-down would trigger another ice age.
"But the Atlantic overturning circulation is still an important player in today's climate," he added.
"Some have suggested cyclic changes in the overturning may be warming and cooling the whole North Atlantic over the course of several decades and affecting rainfall patterns across the US and Africa, and even the number of hurricanes in the Atlantic."
