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 sea.
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 movie The Day After Tomorrow – is projected by some models of climate change.
The research is published in the journal Geophysical Research Letters.
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.
Between 2002 and 2009, the team says, there was no trend discernible – just a lot of variability on short timescales.
The satellite record going back to 1993 did suggest a small increase in flow, although the researchers cannot be sure it is significant.
“The changes we’re seeing in overturning strength are probably part of a natural cycle,” said Josh Willis from Nasa’s Jet Propulsion Laboratory (JPL) in California.
“The slight increase in overturning since 1993 coincides with a decades-long natural pattern of Atlantic heating and cooling.”
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.