By Richard Black
Environment correspondent, BBC News website
Scientists have painted the first detailed picture of Atlantic ocean currents crucial to Europe's climate.
Instruments were deployed during an ocean voyage in 2004
Using instruments strung out across the Atlantic, a UK-led team shows that its circulation varies significantly over the course of a year.
Writing in the journal Science, they say it may now be possible to detect changes related to global warming.
The Atlantic circulation brings warm water to Europe, keeping the continent 4-6C warmer than it would be otherwise.
As the water reaches the cold Arctic, it sinks, returning southwards deeper in the ocean.
Some computer models of climate change predict this Atlantic Meridional Overturning Circulation, of which the Gulf Stream is the best-known component, could weaken severely or even stop completely as global temperatures rise, a scenario taken to extremes in the Hollywood movie The Day After Tomorrow.
Last year the same UK-led team published evidence that the circulation may have weakened by about 30% over half a century.
But that was based on historical records from just five sampling expeditions, raising concerns that the data was not robust enough to provide a clear-cut conclusion.
The key for scientists, then, has been to measure and understand how the circulation varies naturally, making it much easier to pick out any changes related to man-made global warming.
This has been the goal of the Rapid/Mocha (Rapid Climate Change/Meridional Overturning Circulation and Heatflux Array) project; and its first results show that the circulation varies substantially, by a factor of eight, even during a single year.
"I think this is a major step forward for our understanding of ocean circulation," said Stuart Cunningham from the National Oceanography Centre (NOC) in Southampton, one of the project's senior scientists.
"The Atlantic Ocean carries a quarter of the global northwards heat flux, so having the information to plug into climate models will be a major addition," he told the BBC News website.
But measuring long-term variation is, if anything, even more important. Man-made warming could drive the flow downwards, but so could natural climate cycles such as the Atlantic Multidecadal Oscillation.
All five of the historical flow values documented in last year's paper, for example, fit within the range of variability measured here, making it very hard to argue that these observations found a long-term trend.
"We will measure very quickly any sudden shifts," commented NOC's Professor Harry Bryden.
Northwards-moving warm water sinks on encountering Arctic cold
"We already think we can define changes bigger than two Sverdrups (about 10% of the average flow; one Sverdrup (Sv) is defined as a flow of one million cubic metres of water per second).
"But the reality is that anything we measure over 10 years even is going to be labelled interannual variability at the moment."
Making the measurements has not been a trivial matter.
Early in 2004, NOC researchers deployed 19 sets of instruments during a voyage across the Atlantic at 26.5 degrees North, from the north-western coast of Africa to the Bahamas.
US investigators subsequently installed further moorings on the western side of the ocean.
Each set of instruments is strung out along a cable which is tethered to the sea floor at the bottom end, and to a float at the top.
The exact instruments used vary between moorings, but typically they measure flow, salinity, temperature and water pressure.
The instruments were left in place for just over a year, then the team made a second cruise to recover data.
This has given researchers a real-time picture of water flows inside the ocean, from top to bottom and side to side.
But this is just part of the mechanism transporting heat northwards from the tropics to the western shores of Europe.
At 26.5N, the Gulf Stream itself shoots along a narrow channel between the Bahamas and the coast of Florida. The strength of this has been measured for decades using a disused submarine telephone cable - as sea water, an electrical conductor, flows over the cable, it induces a voltage which is continuously measured by scientists in Miami.
A third component of the circulation is movement at the ocean's surface driven by winds, which can be measured nowadays by satellite.
The scientists had to combine these three datasets to calculate the average flow northwards, and by how much it varies.