The Gulf Stream does not appear to be slowing down, according to scientists who have used satellites to monitor changes in the height of the sea.
The stream brings heat northwards from the tropics and is a key factor in the climate of western Europe. Some models of climate change predict a slow down. Although the scientists, from Nasa's Jet Propulsion Laboratory in California, noticed dramatic short-term variability, there was no longer-term trend, they said. In fact since 1993 the overall levels of flow looks to have increased.
"The changes we're seeing in overturning strength are probably part of a natural cycle," said Josh Willis from JPL. "The slight increase in overturning since 1993 coincides with a decades-long natural pattern of Atlantic heating and cooling."
Between 2002 and 2009, the team says, there was no trend discernible, just a lot of variability on short timescales.
The research is published in the journal Geophysical Research Letters.
The stream forms part of a larger movement of water, known as the Atlantic Meridional Overturning Circulation, which is itself one component of the global large scale ocean circulation.
The first observations suggesting that the AMOC was slowing down emerged in 2005, in research from the UK's National Oceanography Centre. At this time scientists suggested the volume of cold water returning southwards could have fallen by as much as 30 per cent 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. They have since not found any clear trend since 2004.