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Unique monitoring system for the Atlantic circulation proves its worth

Published: 
17 August 2007

A team of international scientists, led by researchers from the National Oceanography Centre, Southampton, have established a unique system of instruments that measure the Atlantic Ocean’s circulation – which is largely responsible for Europe’s warm climate – and its likelihood of ever ‘switching off’.

For the first time they are able to continuously monitor the daily variations in the Atlantic’s strength and provide early detection of change in the Atlantic Meridional Overturning Circulation (MOC). The system will help scientists to verify simulations of the MOC from climate models, and to produce more realistic climate change predictions, which are essential for the development of adaptation plans.

In March 2004, as part of the UK Natural Environment Research Council’s £20 million RAPID Climate Change Programme and the US National Science Foundation MOCHA project, the scientists deployed an instrument array across the Atlantic at 26°N from the Saharan coast of Africa to the Bahamas.

Since then, the instruments have provided a continuous record of the salinity, temperature and density of the ocean. In combination with current measurements of the Gulf Stream in the Straits of Florida (provided by scientists from the US National Oceanic and Atmospheric Administration) and satellite measurements of the wind-driven flow across 26°N, the MOC has been calculated on a daily basis.

Analyses of the first year’s data are published this week in the journal Science, with papers from Dr Stuart Cunningham and Dr Torsten Kanzow of the National Oceanography Centre, Southampton (NOCS).

The research was carried out in close collaboration with other colleagues from the NOCS, from Germany (the Max-Planck Institute for Meteorology in Hamburg), and the United States (the Rosenstiel School of Marine and Atmospheric Science and the NOAA Atlantic Oceanographic and Meteorological Laboratory in Miami.)

Their work shows that the remarkable instrument array is working effectively and is bringing in some surprising results, demonstrating that it provides a major contribution to the global climate observing system. The continuous monitoring of the ocean over a full year has pinpointed some major variations in the vertical structure of the Atlantic MOC.

Dr Cunningham explains, "The most astonishing event occurred in early November 2004 when the southward, colder flow of deep water essentially stopped completely. However, the flow at upper levels increased to compensate this reduction in deep flow, so there was no overall change at that time. The array is providing unprecedented observations of the range of ocean variability. The impact of these variations for global and regional climate can now be investigated."

The full range of variability measured in the Atlantic MOC is large. It can range from as little as 4 Sverdrups to as much as 35 Sverdrups. (A Sverdrup is a measure of ocean flow, with one Sverdrup equivalent to one million tonnes of water a second.) The year–long average of the MOC calculates at around 19 Sverdrups, in agreement with earlier estimates.

With the RAPID array it is, however, feasible to monitor the annual average MOC to a resolution of about 1.5 Sverdrups, or about 8 percent of the mean value. This would be sufficient to detect any large, abrupt changes in the circulation, critical to planning for future climate change.

Dr Steven Wilson, Director of Science & Innovation for NERC, said, "Funding this ambitious project was a high risk as nothing like it had ever been done before but, through very strong international collaboration, it has paid off and is providing some spectacular results. Understanding the climate is a very high priority for NERC."

Notes for editors

1. Two papers are published in the journal Science on 17 August 2007:

‘Temporal Variability of the Atlantic Overturning Circulation at 26.5°N’ – Stuart A Cunningham, et al.

‘Observed flow compensation associated with the Meridional Overturning at 26.5°N in the Atlantic’ – Torsten Kanzow et al

Full list of authors: Stuart A Cunningham, Torsten Kanzow, Darren Rayner, Hannah R Longworth, Elizabeth M Grant, Joël J-M Hirschi and Harry L Bryden, National Oceanography Centre, Southampton; Jochem Marotzke, Max-Planck Institute for Meteorology, Hamburg (formerly at NOCS); Molly O Baringer and Christopher S Meinen, Rosenstiel, School of Marine and Atmospheric Sciences, Miami; William E Johns and Lisa M Beal, NOAA Atlantic Oceanographic and Meteorological Laboratory/ PHOD, Miami.

2. These papers build on an earlier research paper from Harry L Bryden, Stuart A Cunningham and Hannah R Longworth, published in the journal Nature on 1 December 2005.

A perspective on the two papers, from John A Church, CSIRO Marine and Atmospheric Research and Antarctic Climate and Ecosystems Cooperative Research Centre, appears in the journal Nature on 17 August 2007.

3. Broadcasters: Footage of data being retrieved from the RAPID instrumentation array is available and can be obtained from the Science Media Centre, 21 Albermarle Street, London W15 4BS, tel. 020 7670 2980.

4. The main objective of the RAPID programme is to establish a pre-operational prototype system to continuously observe the strength and structure of the Atlantic MOC. Strong collaboration between the UK and the USA has been a major factor in enabling the success of the programme. The joint effort is known as RAPID-MOC / MOCHA (Meridional Circulation and Heat Flux Array).

5. The National Oceanography Centre, Southampton, is the UK’s focus for oceanography and represents an unparalleled investment in marine and earth sciences and technology in the UK. It is a collaboration between the Natural Environment Research Council and the University of Southampton. www.noc.soton.ac.uk

6. The Natural Environment Research Council funds world-class science in universities and in its own research centres, that increases knowledge and understanding of the natural world. It is tackling the 21st century's major environmental issues such as climate change, biodiversity and natural hazards. It uses an allocation of around £370 million a year from the Government’s science budget. www.nerc.ac.uk

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