Spatio-temporal variability of the air-sea CO2 fluxes in the strait of Gibraltar basedon high-frequency data collected by a VOS

Abstract

The spatio-temporal variability of the surface ocean CO2 system and the air-sea CO2 fluxes were studied in the Strait of Gibraltar based on high-resolution underway field data collected between February 2019 and March 2021 by a surface ocean observation platform (SOOP) aboard a volunteer observing ship (VOS). The changes in the depth of the Atlantic- Mediterranean Interface layer and the upwelling of deep-water drove by tidal and trade winds strongly influenced the surface CO2 distribution. The variability of the CO2 fugacity (fCO2,sw) and fluxes were mainly driven by temperature despite the significant influence of non-thermal processes in the southernmost part. The fCO2,sw increased with temperature by 9.02 ± 1.99 μatm ºC-1 (r2=0.86) and 4.51 ± 1.66 μatm ºC-1 (r2=0.48) in the northern and southern sections, respectively. The thermal to non-thermal effect ratio (T/B) was higher in the northern section (>1.8) compared with the southern section (<1.30) due to the enhancement of biological activity and vertical mixing related to the seasonal wind-induced upwelling along the African coast. The annual cycle (referenced to 2019) of total inorganic carbon normalized to a constant salinity of 36.7 (NCT) was attended. The net community production processes described 93.5-95.6% of the total NCT change, while the contribution of air-sea exchange and horizontal and vertical advection was found to be minimal (<4.6%). According to the seasonality of air-sea CO2 fluxes, the region behaved as a strong CO2 sink during the cold months and as a weak CO2 source during the warm months. The Strait of Gibraltar acted as annual net CO2 sink, with higher net ingassing along the southern section (-1.01 mol C m-2) compared to the northern section (-0.82 mol C m-2). The calculated average CO2 flux for the entire area was -7.12 Gg CO2 yr-1 (-1.94 Gg C yr-1).