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.