Air-Sea CO₂ fluxes in the Atlantic as measured during boreal spring and autumn

Abstract. A total of fourteen hydrographic cruises from 2000 to 2008 were conducted during the spring and autumn seasons between Spain and the Southern Ocean under the framework of the Spanish research project FICARAM. The underway measurements were processed and analysed to describe the meridional air-sea CO₂ fluxes (FCO₂) in the covered sector of the Atlantic Ocean. The data has been grouped into different biogeochemical oceanographic provinces based on thermohaline characteristics. The spatial and temporal distributions of FCO₂ followed expected distributions and annual trends reproducing the recent climatological ΔfCO₂ estimations with a mean difference of −3 ± 18 μatm (Takahashi et al., 2009). The reduction in the CO₂ saturation along the meridional FICARAM cruises represented an increase of 0.02 ± 0.14 mol m⁻² yr⁻¹ in the ocean uptake of atmospheric CO₂. The subtropical waters in both Hemispheres acted as a sink of atmospheric CO₂ during the successive spring seasons and as a source in autumn. The coarse reduction of the ocean uptake of atmospheric CO₂ observed in the North Atlantic Ocean was linked to conditions of negative phase of the North Atlantic Oscillation that prevailed during the FICARAM period. Surface waters in the North Equatorial Counter Current revealed a significant long-term decrease of sea surface salinity of −0.16 ± 0.01 yr⁻¹ coinciding with a declination of −3.5 ± 0.9 μatm yr⁻¹ in the air–sea disequilibrium of CO₂ fugacity and a rise of oceanic CO₂ uptake of −0.09 ± 0.03 mol m⁻² yr⁻¹. The largest CO₂ source was located in the equatorial upwelling system. These tropical waters that reached emissions of 0.7 ± 0.5 and 1.0 ± 0.7 mol m⁻² y⁻¹ in spring and autumn, respectively, showed an interannual warming of 0.11 ± 0.03 °C yr⁻¹ and a wind speed decrease of −0.58 ± 0.14 m s⁻¹ yr⁻¹ in spring cruises which suggest the weakening of upwelling events associated with warm El Niño – Southern Oscillation episodes. Contrary the surface waters of the Patagonian Sea behaved as an intense sink of CO₂ in March and November. The oceanic waters of the convergence of Falkland and Brazil Currents showed the strongest CO₂ absorption with a rate of −5.4 ± 3.6 mol m⁻² yr⁻¹ in November. The Southern Oceans sampled in the Drake Passage behave as an average uptake rate of −1.1 ± 0.9 mol m⁻² yr⁻¹ while the distal shelf of the Livingston Island acted as a slight source of CO₂ to the atmosphere.