CAVASSOO
Objectives
The summary objective can be divided into 6 objectives:
- Establish a basic North Atlantic surface pCO2 observing system
- Produce and make available a North Atlantic pCO2 database
- Assess errors and uncertainties in existing pCO2 climatologies
- Estimate seasonal air-sea CO2 fluxes for the North Atlantic
- Evaluate Ocean carbon models with regard to air-sea flux variability
- Improve atmospheric inversion estimates of carbon sources and sinks.
Establish a basic North Atlantic surface pCO2 observing system
4 routes covering the major physical and biogeochemical provinces of the North Atlantic have been identified (see map in overview). Seawater and atmospheric pCO2, temperature and salinity measurement will be measured along these transects. This will initiate the European component of a North Atlantic Carbon Observing System. Joint planning with American and Canadian colleagues is underway to expand this network in the future. This network will resolve, for the first time across the whole region, the annual seasonal cycle of CO2 and its interannual variability. Some of the ships already record hydrographic properties along the route so that complementary measurements (e.g. upper ocean thermal structure) will also be obtained. The implementation of the CO2 network in the North Atlantic lays the foundation for long-term monitoring of CO2 uptake in the region to enable CO2 fluxes to be directly assessed year-by year.
Produce and make available a North Atlantic pCO2 database
Extensive data for pCO2 and associated variables already exists for the North Atlantic, but it has been collected sporadically over the past 30 years. Some of the data are publicly available, for example at CDIAC (Carbon dioxide Information and Analysis Center, Oak Ridge National Laboratory, USA). A CO2 database for the North Atlantic will be constructed with the public data and as much non-public data as these investigators will allow, and will be used to construct new historical climatologies under various assumptions. The database will be made available initially to project members and those others who have contributed to it. The data will be used to test with independent data the assumptions made in combining many years of data into climatologies for a single year. These assumptions very significantly affect the results obtained for the climatologies.
Assess errors and uncertainties in existing pCO2 climatologies
Climatologies derived from the existing database will be compared directly with our new measurements, and the assumptions made in deriving those climatologies tested. Information on pCO2 variability will form the basis for long-term optimization of a carbon observing system in the North Atlantic. The new data will be used to assess the CO2 variability at different time and space scales, and to investigate the causes of that variability.
Estimate seasonal air-sea CO2 fluxes for the North Atlantic
Updated quarterly gridded (1olatitude by 1olongitude) DpCO2 fields will be obtained by independent methods. The uncertainty in the fields will be estimated statistically within each method, and by comparison of different methods. Best estimates of the DpCO2 fields will be made, and seasonal estimates of the CO2 flux derived using a modern parameterization of the gas exchange rate5.
Evaluate Ocean carbon models with regard to air-sea flux variability
Observations will be compared with the output of an ocean carbon model. The international project OCMIP (Ocean Carbon Model Intercomparison Project) already provides results of 13 models which can be used for models-data comparisons. One ocean carbon model will be used in the project to provide information on inter-annual variability of the fluxes, as controlled by individual processes (ocean circulation, marine biology, solubility).
Improve atmospheric inversion estimates of carbon sources and sinks
The better-constrained North Atlantic carbon fluxes calculated in objective 4 will be used in atmospheric inversion models to improve the estimates of the European and North American fluxes. We will also use the ships of opportunity to take selected atmospheric measurements for CO2 and isotopes. The lack of samples for atmospheric concentrations over the North Atlantic is an important source of error in the inversions. These new atmospheric measurements, as well as the oceanographic data collected, will be integrated within a global atmospheric transport model to calculate a "Grand Inversion" of North Atlantic fluxes. Uncertainties linked to the inverse technique, to sparse and uneven atmospheric sampling and to the a priori information introduced in the model will be quantified.
