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Monitoring the Atlantic Conveyor

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The Atlantic Conveyor
(About the Gulf Stream,
the THC and the MOC)

Monitoring the Atlantic Conveyor

 
Is the Atlantic conveyor slowing down?     The 25°N monitoring array
Links to articles, papers and media coverage.


Finding a reliable and cost-effective way to monitor the Atlantic heat conveyor is a primary aim of the RAPID programme. The team of Prof. Harry Bryden and Dr. Stuart Cunningham of the National Oceanography Centre, Southampton are tackling this problem in their RAPID project 'Monitoring the Atlantic Meridional Overturning Circulation at 25°N'.

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Is the Atlantic conveyor slowing down?

On December 1st, 2005, Prof. Bryden and his team reported in Nature that the Atlantic meridional overturning circulation (MOC) seems to have weakened by about 30% in the last decade or so.

The team analysed measurements taken along a line at about 25°N from the Bahamas to Morocco in 2004, and compared these to measurements taken along approximately this line in 1957, 1981, 1992 and 1998. The results suggest that the Gulf Stream flow across 25°N has remained nearly constant in this period, but the MOC has slowed from about 20 Sv in 1957 to 14 Sv in 2004.

The evidence for the slow-down can be found both in the increase in southward recirculation of warm water in the surface layer, and in a decrease in southward transport of lower North Atlantic Deep Water between 3000 and 5000m depth.

Some climate models have suggested that we could expect such a slowdown as a result of global warming, and that this could cool northwest Europe by a few °C, with the change happening relatively fast, perhaps over just a decade. These measurements are evidence that the models could be right.

The five sections are just 'snapshots' and cannot tell us for sure whether the changes are a result of natural variability. Very little is known about how the MOC varies seasonally or from year to year. To be sure that the observed change is the result of a long-term trend rather than natural fluctuations we will need to monitor flow across 25°N continuously for about a decade.

The MOC monitoring array at 25°N

In April 2004 Prof. Bryden's team deployed an array of instruments across the Atlantic at 25° North. The array measures temperature, salinity, currents and pressure. The first full year of measurements were collected in April 2005, and analysis of this data is now underway. The mooring array will continue to give data until the end of the project in 2008. However, the team are hopeful that they will be able to continue the work beyond this to collect a full decade's worth of data.

To build up a picture of the natural variability in the Atlantic overturning circulation the team intends to combine data from the MOC array with measurements from oceanographic cruises, satellite images and data from floats. Armed with this knowledge they should be able to say with greater certainty if changes observed in the 'snapshot' data from 1957 to 2004 represents actual long-term change in the MOC.

Based on their analysis and their experiences with the prototype array the team are also hoping to develop recommendations for a longer term 'early warning system', which could continue to monitor the health of the Atlantic conveyor in a cost-effective way.

Links: Articles, scientific papers and media coverage

Slowing of the Atlantic meridional overturning circulation at 25°N.
Scientific paper in Nature (letters) 1 December 2005.
Bryden et al., Nature 438, 655-657.

Oceanography: The Atlantic heat conveyor slows.
Nature News and Views article by Detlef Quadfasel, 1 December 2005.

Climate change: A sea change.
News Feature by Quirin Schiermeier, 19 January 2006. Nature 439, 256-260.

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Maintained by Val Byfield     Last modified: January 18 2007 16:46