The air-sea CO2 flux can be determined by the product of the air-sea CO2 exchange coefficient by the air-sea CO2 partial pressure gradient. The exchange coefficient can be related to the wind speed at the ocean surface; since several relationships have been proposed, we compare global CO2 exchange coefficient retrieved using several relationships and ERS wind speeds. Then, using the Liss and Merlivat relationship, we monitor the spatial and temporal variability of the exchange coefficient in the equatorial Pacific ocean. Concerning the CO2 partial pressure gradient, we derive a time series from 1992 to 1996 in the eastern equatorial Pacific around 100W, from in-situ measurements of oceanic CO2 partial pressure and satellite sea surface temperature. In this region, the oceanic CO2 partial pressure is usually well above the atmospheric CO2 partial pressure because the upwelling brings large amounts of CO2 in surface water; this leads to large CO2 flux evading to the atmosphere. However, since the exchange coefficient exhibits a strong seasonal variability, being almost zero at the beginnig of the year, the largest outgassing fluxes are usually observed in the second part of the year. During El Niño events, like at the beginning of 1992, because the upwelling activity is reduced, the oceanic partial pressure decreases and is almost in equilibrium with the atmosphere. In that case, the variability of the flux is mainly controlled by the partial pressure variability. In summary, most of the temporal variability of the flux is driven by the variability of the exchange coefficient except during El Niño year when the decrease of oceanic partial pressure leads to a small air-sea flux.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry