By Mark Kinver
Science and nature reporter, BBC News
Measurements from a network of monitors stretching across the Atlantic Ocean could offer an early warning of "sudden climate change", scientists have said.
The monitoring devices provide a detailed picture of ocean changes
Underwater instruments measuring the temperature and salinity of seawater will detect any change to currents that regulate Europe's climate, they said.
A UK-led team of researchers said the data offered the most detailed picture of the ocean's circulation patterns.
The first set of results were presented at a climate conference in Birmingham.
The array of 19 "moorings" is positioned at points 26.5 degrees north in the Atlantic Ocean, providing an insight to the Meridional Overturning Circulation (MOC) that acts as the Earth's "heat pump", distributing heat via the ocean from the equator to northern regions.
By the time it reaches the northern latitudes around Greenland and Iceland, the water has cooled so much that it sinks towards the ocean's floor and heads south again, a process known as "overturning".
The process is density driven, so is sensitive to changes in the temperature and salinity of the water.
Computer climate models predict that the overturning may weaken or switch off altogether as freshwater from melting ice sheets flow into the sea.
An increased flow of freshwater makes the ocean less saline and lowers the density, meaning the waters cannot sink and the overturning weakens.
"We have deployed an array to monitor and observe the Atlantic Overturning Circulation," the project's principal investigator, Harry Bryden, said.
The array of moorings reaches across the ocean from Africa to the US. Each mooring consists of a wire up to 5,000m long that stretches from the sea floor to about 15m below the surface.
The Rapid team monitored at roughly 26 degrees north
An assortment of instruments are attached to the wire at fixed depths, taking a reading every 15 minutes.
The data is stored until it is uploaded by researchers when the moorings are recovered after 12 months at sea.
The moorings are positioned in three locations: off the coast of west Africa, just south of the Canary Islands; the mid-Atlantic ridge; and to the east of the Bahamas.
"The first year's measurements really established the size of the short-term variability," Professor Bryden told BBC News.
"This is really important because we never knew this before."
Professor Bryden, from the UK's National Oceanography Centre, was the lead author on a paper published in the journal Nature last year that suggested the overturning circulation had declined by 30% over the past 50 years. That was based on a snapshot of data from the array.
The moorings were first put into position in 2004
The findings were based on five historical "snapshot" measurements of overturning in 1957, 1981, 1992, 1998 and 2004.
"The issue was how big was the variability in the five snapshot measurements, and that was something we needed to know," he said.
Using the first year's data from the array, the researchers were able to adjust their calculations.
"We concluded that there was some evidence of a small decrease but not as big as we reported in the Nature paper last year," Professor Bryden observed.
"But we have had a decrease... in the order of 10% of the overturning circulation in the past 25 years."
Rapid climate change
He said the findings demonstrated that they were able to monitor changes, and act as an alert to any sudden shifts in the ocean circulation that could have a profound impact on the UK's and north-west Europe's climate.
"The ice core records suggest sudden warming or cooling happen on a scale of about a decade. The timescale for temperature change, which are in the order of 5-10C (9-18F), also happen in about a 10-year period," he explained.
The project forms the centrepiece of the Rapid research programme, led by the UK's Natural Environment Research Council (Nerc).
The programme has 36 research projects that will run until at least 2008, and aims to develop a better understanding of the causes of rapid climate change and what might happen in the future.
Professor Bryden presented the UK-US team's findings at a four-day conference, organised by Nerc, where scientists working on the Rapid project discussed the ongoing research.