Distribution of surface carbon dioxide and air‐sea exchange in the upwelling system off the Galician coast

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Data on the distribution of the partial pressure of CO2 (pCO2) were obtained during six cruises off the Galician coast, a region characterized by a seasonal upwelling. The values of pCO2 over the continental shelf are highly variable and range between 265 and 415 μatm during the upwelling season and between 315 and 345 μatm during the downwelling season. Both the continental shelf and off‐shelf waters behave as significant net sinks of atmospheric CO2. The computation of the air‐sea fluxes of CO2 over the continental shelf yields a net influx in the range of −2.3 (±0.6) to −4.7 (±1.0) mmol C m−2 d−1 during the upwelling season and −3.5 (±0.8) to −7.0 (±1.5) mmol C m−2 d−1 on an annual basis. During the upwelling season and on an annual basis, although the observed air‐sea gradients of CO2 over the continental shelf are significantly stronger than those in off‐shelf waters, the computed air‐sea CO2 fluxes are not significantly different because of the important incertitude introduced in the calculations by the estimated error on wind speed measurements. The presence of upwelling filaments increases the influx of atmospheric CO2 in the off‐shelf waters. During summer, important short‐term variations of pCO2 are observed that are related to both upwelling and temperature variations. During winter the cooling of water causes important undersaturation of CO2 related to the effect of temperature on the dissolved inorganic carbon equilibrium constants.