Three global sets of CO2 exchange coefficients corresponding to the above k-U relationships have been computed since 1991 using ERS and QSCAT wind speeds. Global fields of K are derived from the above parametrisations as follows:
- Sc was deduced from SST using polynomial coefficients of Table A1 of Wanninkhof (1992).
- For each relationship, K was deduced as:
K(mol/(m2 yr microatm))= k(cm/hr)*solubility(mole/(kg atm)/11.1371
- Solubility was deduced from SST and assuming SSS=35 using polynomial coefficients of Table A2 of Wanninkhof (1992). 11.1371 is a conversion factor computed assuming seawater density of 1025 kg/m3.
In order to correctly take into account the non-linearity of the k-U relationships in the K averages, K is computed for each U10 retrieved at 25km resolution from instantaneous 25km scatterometer measurement; gas solubility and Schmidt number are computed for a constant salinity of 35 and for a sea surface temperature, SST, averaged over 1° and 1 month. The SST fields are the ones generated at the National Meteorological Center (NMC) by an optimal interpolation of AVHRR and in situ data [Reynolds and Smith .
Then, the 25km K estimates are interpolated on weekly and monthly 1° resolution grids using a kriging method described in [Bentamy et al., 1996]. Comparison between fields interpolated using this kriging method and the objective analysis described in [Boutin and Etcheto, 1995] shows differences inside the variability of wind speed within two satellite passages.