Estimation of air–sea CO2 fluxes in the Bay of Biscay based on empirical relationships and remotely sensed observations

An empirical algorithm has been developed to compute the sea surface CO2 fugacity (fCO2sw) in the Bay of Biscay from remotely sensed sea surface temperature (SSTRS) and chlorophyll a (chl aRS) retrieved from AVHRR and SeaWiFS sensors, respectively. Underway fCO2sw measurements recorded during 2003 were correlated with SSTRS and chl aRS data yielding a regression error of 0.1 ± 7.5 µatm (mean ± standard deviation). The spatial and temporal variability of air–sea fCO2 gradient (ΔfCO2) and air–sea CO2 flux (FCO2) was analyzed using remotely sensed images from September 1997 to December 2004. An average FCO2 of − 1.9 ± 0.1 mol m− 2 yr− 1 characterized the Bay of Biscay as a CO2 sink that is suffering a significant long-term decrease of 0.08 ± 0.05 mol m− 2 yr− 2 in its capacity to store atmospheric CO2. The main parameter controlling the long-term variability of the CO2 uptake from the atmosphere was the air–sea CO2 transfer velocity (57%), followed by the SSTRS (10%) and the chl aRS (2%).