Could the Atlantic Current Switch Off?

The Atlantic Ocean overturning that maintains Europe’s moderate climate has slowed by 30 per cent according to scientists from the National Oceanography Centre, Southampton in research published today in Nature

Professor Harry Bryden, Dr Stuart Cunningham and University of Southampton research student Hannah Longworth have been researching the flow of the Atlantic Ocean across latitude 25 degrees north – comparing measurements across the Atlantic taken in 2004 with records from 1957, 1981, 1992 and 1998. Ocean flow is measured in Sverdrups, equivalent to one million tonnes of water a second. The team estimate a decrease in the overturning from 20 Sv in earlier surveys to 14 Sv in 2004.

Professor Bryden said, ‘The Atlantic meridional overturning circulation, sometimes called the Conveyor Belt, carries warm upper waters into northern latitudes and returns cold deep waters southward across the equator. It is a massive system that includes the Gulf Stream and it carries heat northward out of the tropics into the northern Atlantic warming the atmosphere and helping to provide northern Europe with a moderate climate.

‘In previous studies over the last 50 years the overturning circulation and heat transport across 25°N were reasonably constant. We were surprised that the circulation in 2004 was so different from previous estimates.

‘Estimates of the size of the overturning circulation have been based primarily on hydrographic sections – where a ship takes measurements at regular intervals across the Atlantic. Because of the importance of the overturning circulation for European climate, we have set up a continuous observing system to monitor the overturning using moored instruments. Last year's expedition was meant to provide a traditional observation of the overturning to initialise the new observing system.’

Climate models suggest that increased levels of carbon dioxide in the atmosphere will result in a slowdown of the Atlantic overturning circulation. Scientists fear that disruption to this circulation could result in a several degree drop in temperatures in as little as 20 years. The Natural Environment Research Council, NERC, has funded a £20 million climate change research programme called RAPID, which is co-ordinated by scientists at the National Oceanography Centre, Southampton. A primary goal of RAPID is to continuously monitor the Atlantic meridional overturning circulation.

Since 2004 scientists at the National Oceanography Centre, Southampton led by Dr Stuart Cunningham have deployed an instrument array across the Atlantic at 25°N – running just south of the Canaries, from the Saharan coast of Africa to Florida. The monitoring array involves close collaboration with American scientists at University of Miami and NOAA's Atlantic Oceanographic and Meteorological Laboratory. To work out the circulation and heat transport, it is essential to measure the temperature and salinity of the water and the speed it is moving.

An array of 22 moorings continuously monitors the Atlantic’s circulation, nine across the Deep Western Boundary Current east of the Bahama Islands, four across the Mid-Atlantic Ridge and nine across the continental slope off the coast of Africa. The moorings are anchored to the seabed on wires up to 5000 metres long. Instruments that measure salinity, temperature, pressure and currents take continuous readings that are collected on annual visits by ship. The project is due to end in 2008 giving four years of continuous observation.

Dr Stuart Cunningham, who has just returned from an expedition to retrieve data from the moored instruments near the Canaries, said, ‘Continuous monitoring could alert us to potential rapid climate change. This is an early warning system in operation for the Atlantic overturning. Before this array was set up, the only means of gathering data was to make transatlantic research cruises taking measurements that took a snapshot of ocean conditions at a specific time giving little information on seasonal variability’.