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TRACTOR - TRAcers and Circulation in the NORdic Seas region

Summary information

Funding:FP5 - Research project
Total cost:2342632
Ec contribution:1887530
Start date:2001-02-01
End date:2004-01-31
Duration:36 months
Coordinator:Truls Johannessen (
Organisation:University of Bergen and Bjerkens Center for Climate Research – Norway
Themes:Ocean circulation changes; deep circulation changes
Regio:Arctic; North Atlantic
Keywords:Resources of the Sea; Fisheries; Environmental Protection; Meteorology; Forecasting
Project name:TRACTOR - TRAcers and Circulation in the NORdic Seas region
Project summary:Primary Objectives
- Describe and quantify the present strength and variability of the circulation and oceanic processes of the Nordic Seas regions using primarily observations of the long term spread of a tracer purposefully released into the Greenland Sea Gyre in 1996.
- Improve our understanding of ocean processes critical to the thermaholine circulation in the Nordic Seas regions so as to be able to predict how this region may respond to climate change.
- Assess the role of mixing and ageing of water masses on the carbon transport and the role of the thermohaline circulation in carbon storage using water transports and mixing coefficients derived from the tracer distribution.

Specific Objectives
Perform annual hydrographic, chemical and SF6 tracer surveys into the Nordic regions in order to:
- Measure lateral and diapycnal mixing rates in the Greenland Sea Gyre and in the surrounding regions.
- Document the depth and rates of convective mixing in the Greenland Sea using the SF6 and the water masses characteristics.
- Measure the transit time and transport of water from the Greenland Sea to surrounding seas and outflows. Document processes of water mass transformation and entrainment occurring to water emanating from the central Greenland Sea.
- Measure diapycnal mixing rates in the bottom and margins of the Greenland Sea basin using the SF6 signal observed there. Quantify the potential role of bottom boundary-layer mixing in the ventilation of the Greenland Sea Deep Water in absence of deep convection.

Monitor the variability of the entrainment of water from the Greenland Sea using time series auto-sampler moorings at strategic positions i.e., sill of the Denmark Strait, Labrador Sea, Jan Mayen fracture zone and Fram Strait. Relate the observed variability of the tracer signal in the outflows to convection events in the Greenland Sea and local wind stress events. Obtain a better description of deepwater overflow and entrainment processes in the Denmark Strait and Faeroe Bank Channel overflows and use these to improve modelling of deepwater overflows.

Monitor the tracer invasion into the North Atlantic using opportunistic SF6 measurements from other cruises: we anticipate that a number of oceanographic cruises will take place in the north-east Atlantic and the Labrador Sea. It should be possible to get samples from some cruises for SF6 measurements.

Use process models to describe the spread of the tracer to achieve better parameterisation for three-dimensional models. One reason that these are so resistant to prediction is that our best ocean models are as yet some distance from being good enough, to predict climate and climate change.
Project outputs:Scientific achievements and conclusions
As by spring of 2004 , the main results of TRACTOR are:
- The vertical mixing rates in the Greenland Sea gyre system have been quantified and found to be much higher than in other regions.
- The main pathways from the Greenland Sea into the ambient oceans have been identified.
- Information on the exchange and exchange rates between basins of the water marked by the tracer have been received from budgeting the tracer patch.
- Two modes of ventilation are identified in the Greenland Sea. The first is the formation of eddies with subsequent ventilation at mid depth during the collapse of the eddies and the second is overturning. The eddy mode seems to play an important role during the tracer experiment with a volume transport estimated to 0.01 to 0.02 Sv while the limited overturning present during the main part of the experiment seemed to contribute to 0.1-0.2 Sv.
- Different pathways from the Greenland Sea dominate during different modes of the North Atlantic Oscillation, the natural variability in atmospheric forcing. In the NAO + mode a bifurcation of the pathways occur with one route from the Greenland Sea through the Jan Mayen channel, along the Jan Mayen Ridge and then into the Faeroe-Shetland channel and the other route along the shelf rise to the Greenland Shelf on route to the Denmark Strait. In the NAOmode waters from the Greenland Sea will mostly select the western path.
- The first appearance of the tracer at the Greenland-Scotland Ridge was in the Faeroe Bank channel after 2.5 years (± 2 months). This is consistent with the different pathways for the water from the Greenland Sea connected to the NAO modes as mentioned above, since NAO + forcing greatly dominated during the experiment.
- The circulation and mixing characteristics of the 20 km resolution Nansen Centre version of the Miami Isopycnic Coordinate Ocean General Circulation Model (MICOM), lead to greatly improved and a representative time-space distribution of the released SF6. It is concluded that simulated distributions of tracers provide detailed insight into isopycnal and diapycnal diffusion parameterizations, and that a data-model comparison can be used to constrain the strength of the mixing schemes.
- The 40 km, and in particular the 20 km, resolution versions of the model system are able to describe the main features of the observed CFC-11, CFC-12, 137Cs and SF6 concentrations in a realistic way. In fact, the performed simulations show that OGCMs used in climate research should be evaluated based on some of the key tracers, including SF6, to assess, and by that to improve, the model's ability to reproduce the main features of the key ventilation processes of the ocean climate system.
- The results from the tracer release experiment will be used for a modelling inter-comparison exercise. Preliminary results from a GCM indicate that it seems to accelerate the general circulation relative to what is observed from the tracer field. A clear need for more detailed intercomparison between the tracer field and the most frequently used GCMs are needed to improve these models from which results are frequently used in guidance of policy making (see IPCC-report).
- New information of the transformation of dissolved inorganic carbon along the East Greenland Current, and of the anthropogenic carbon inventory within the Nordic Seas, and a new hypothesis on how ice formation enhances air-sea flux of CO2.
- During the TRACTOR period an auto-sampler collecting deepwater has been developed. One successful deployment and retrieval of the auto-sampler was done in the Denmark Strait. The water is now being measured for CFC's. The titanium bags used need to be of a larger volume to be able to sample for SF6 and a new design of water collecting bags are under development.

Socio-economic relevance and policy implications
European dimension of the problem
TRACTOR has investigated the circulation of the Nordic Seas. The area has been considered crucial to the general circulation of the oceans, and this interlinked with climate change processes. A better understanding of the processes involved in climate change that affects Europe specifically and the globe in general, and a better predictability of climate change, are important for many aspects of the economy, politics, welfare and cultural and social affairs. Better detection and prediction of anthropogenically forced climate changes, as well as understanding the natural backdrop of changes, will form a better basis for policy making.
Contribution to developing S&T co-operation at international level
The consortium of TRACTOR has brought together skills and analytical methods which are unique in the world. It was built on existing capacities brought together in a scientific task that has not been attempted before. With the contributions from the different partners, a critical mass of personal resources and infrastructure (such as research vessels and computers) was achieved. The project has fostered closer ties between the observational and climate modelling communities in Europe, and laid the ground for further development of European modelling institutions to improve the model skills to describe global ocean circulation and its impact on climate.
Contribution to policy design or implementation
An improved capability to model oceans and oceanic processes will reduce uncertainties in the projected forecasts of the climatic response to greenhouse gas forcing.
- It will form a better basis for choosing the most relevant policies to deal with the problems of climate change and the implementation of the Kyoto protocol.
- The model improvements will specifically address uncertainties related to processes of high relevance for the regional climate; the project has contributed to more reliable regional climate forecasts.
- The improved ocean modelling capabilities is of relevance for the use and management of oceans and ocean resources, in particular living resources.
- The improved ocean modelling capabilities will also be of high relevance for the potential forecasting of water management in Europe.
Quality of life
There is considerable variability in the climate of Europe. Most areas of human activity are affected by climate change in the region. It is necessary to improve our understanding of this variability and the range of the variability in future scenarios. If ocean circulation should change or the decadal variability should tend to shift into a state where one of the NAO modes is dominant, this would impact strongly on life in our part of the globe, in a way which is economically quantifiable. Although parts of this variability may be chaotic and nonpredictive, the project has contributed to a better understanding of the patterns, which may lead to some type of predictability. A better understanding of oceanic processes lies at the heart of this problem.