Seasonal variation of the CO2 exchange coefficient over the global ocean using satellite wind speed measurements

We used one year (July 1987 to June 1988) of wind speed measurements made by the microwave radiometer SSM/I to determine the CO₂ exchange coefficient between air and sea on a global scale through the Liss and Merlivat relationship. We determined the seasonal variation in every region of the world ocean. It can be as high as a factor of 4 in some areas. An estimate of the accuracy of the retrieved wind speeds showed that this variation is likely to be underestimated. The global averaged CO₂ exchange coefficient obtained is 3.34 · 10^-2 mol m^-2 yr^-1 μatm^-1, close to previous estimates. A study of the errors on the retrieved wind speed showed that the exchange coefficient is likely to be underestimated and could be as high as 4 10^-2 mol m^-2 yr^-1 μatm^-1. We combined these results with various estimates of the annual CO₂ partial pressure gradient at the air sea surface and get a net flux absorbed by the ocean. The flux so determined is not meaningful, however, since the covariant variability of the exchange coefficient and of the CO₂ partial pressure gradient is not taken into account.