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In deep water: the Greenland Sea

Deep water formation in the Greenland Sea is an important driving force behind many global ocean currents. Of particular interest to Europe is the way the process influences the Gulf Stream, a current that warms our climate significantly. ESOP-2 is learning more about the Greenland Sea using a unique combination of novel experimental techniques, computer modelling, and experience developed under ESOP-1. Two years into the project, scientists in ESOP-2 have already discovered at least two new mechanisms that carry water from the surface layers of the sea to depths of 2-3 kilometres and are making progress in determining the role of this sub-Arctic sea as a carbon dioxide sink.


Deep water formation in the Greenland Sea is important because it keeps the climate of Europe relatively mild by encouraging the northward flow of the warm waters of the Gulf Stream. It also has implications for the global environment, since water that descends into the deep ocean takes carbon dioxide from the atmosphere with it. When this important greenhouse gas is trapped in the ocean depths for long periods, it cannot contribute to global warming.

Despite its importance, the physical processes involved in deep water formation are not well understood. And what we do not know, we cannot put into the computer models designed to predict weather patterns and long-term climate change. In order to add this important missing piece to the global climate jigsaw puzzle, scientists from ESOP-2 decided to put to sea.

All hands on deck
In the first cruise of ESOP-2, 24 scientists from Norway, Sweden, Britain, France, and the USA set out in the British ship, RRS James Clark Ross. In the summer of 1996, they injected 320 kg of an inert tracer, sulphur hexafluoride, into the centre of the Greenland Sea at a depth of about 300 metres. The tracer can be detected in tiny amounts. By following its progress, ESOP-2 researchers hoped that they would be able to monitor the formation of deep water over a period of three years. They also hoped to track the fate of deep water leaving the region. Eystein Jansen, co-ordinator of ESOP-2 commented, "The tracer release experiment of ESOP-2 is the largest experiment of this type so far undertaken by oceanographers and the first to be attempted in a dynamically active region. 1997 was a key year for ESOP-2 and I was very pleased that the project did so well through this critical phase. With most of the field work now complete, we are now concentrating our efforts on data interpretation and management and, most importantly, dissemination of our results."

Three full-scale surveys of the tracer distribution have now been completed and analysis of the data generated is providing estimates of how water mixes in the upper waters of the sea. Rates of summer-time mixing are much more rapid than those detected by earlier tracer release experiments in quieter regions of the ocean, and winter-time rates are five times faster still. Although the tracer spread horizontally within the Greenland Sea itself within a few months of its release, subsequent movement out of the sea has been slow, suggesting that it is isolated from the surrounding water bodies.

Main features of ocean circulation in the Nordic Seas. Mixing of cold and warm water in the Iceland and Greeland Seas with subsequent convection to form deep and intermediate waters create deep overflows injecting cold, ventilated water into the global ocean across the sills between Greenland and Scotland.

New water circulation mechanisms
Some of the results were quite surprising: strong currents transported the tracer-labelled water from the 300 m level down to the deep sea much more rapidly than expected. After the first winter, significant amounts of SF6 tracer were found in a 100-200 metre-thick layer of water that covered a large area at a depth of about 3500 metres. March's Håkon Mosby cruise produced evidence of deep-water convection currents, catching the mechanism almost 'in the act'. Currents were detected which carried the tracer down to a depth of more than three kilometres, causing chaotic storms on the seabed.

In addition to this large-scale movement of water, the team also discovered a localised chimney effect - thin currents which take water from the surface straight down to the bottom. The second ESOP-2 cruise found one chimney stretching down 1600 metres and the Valdivia cruise three months later detected another chimney taking surface water down to a depth of two kilometres. The April-May Johan Hjort cruise also located a deep chimney stretching to about 2000m, similar in both form and location to the one reported in earlier cruises.

The Greenland Sea as a carbon sink
In addition to the tracer experiment, mid-water floats were released and an intensive survey of the chemistry and physics of the ocean in the region was made. The carbon cycle studies undertaken as part of ESOP-2 have shown that the Greenland Sea is a year-round sink for carbon dioxide. Dissolved carbon is transported from the surface of the sea to its deeper layers, where it can be trapped for several hundred years. Data obtained from field experiments have been used in newly developed computer models, and it is now possible for the team to calculate how the rates of air-sea carbon dioxide exchange change with the seasons, and from year to year, and to estimate more accurately how much of the carbon dioxide currently being produced by human activity will be taken up by the oceans. ESOP-2 computer models should also be able to predict the impact of this process on the greenhouse effect and on global warming.



Project Title:  
European Subpolar Ocean Programme (ESOP) - Phase 2: The Thermohaline Circulation in the Greenland Sea


CORDIS databaseFor more information on this project,
go to the Cordis database Record