Variability of the air-sea CO2 exchange in the Strait of Gibraltar based on measurements from a VOS line.

The spatio-temporal variability of the surface CO₂ system and its air-sea 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 surface CO₂ distribution was strongly influenced by the seasonal and spatial variability in the depth of the Atlantic-Mediterranean Interface layer and by upwelling of deep-water drove by the tidal and easterly winds. The variability of the CO₂ fugacity (fCO_2,sw) and fluxes were mainly driven by temperature despite the significant influence of non-thermal processes in the southernmost part. The thermal to non-thermal effect ratio (T/B) reached higher values values 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 fCO_2,sw increased with temperature by 9.02 ± 1.99 µatm ºC (r²=0.86) and 4.51 ± 1.66 µatm ºC (r²=0.48) in the northern and southern sections, respectively. The annual cycle (referenced to 2019) of total inorganic carbon normalized to a constant salinity of 36.7 (NC_T) was attended. The net community production processes described 93.5-95.6% of the total NC_T 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).

Keywords: Air-sea CO₂ fluxes, CO₂ system, VOS line, Surface Ocean Observation Platform, Strait of Gibraltar.