Volunteer Observing Ships in transitional oceanic regions: the Northeast African upwelling and the Strait of Gibraltar

The spatio-temporal variability of the surface ocean CO₂ system and the air-sea CO₂ fluxes were studied in the Northeast Atlantic and the Strait of Gibraltar, including the northwest African coastal transitional region and the easternmost archipelagic waters of the Canary Islands. The surface underway high-frequency data was collected by a surface ocean observation platform (SOOP) aboard a volunteer observing ship (VOS). The variability of the CO₂ fugacity in seawater (fCO_2,sw) was strongly driven by the seasonal pattern of the sea surface temperature (SST), which increased with latitude and was lower throughout the year in the high-intense African coastal upwelling. In the Strait of Gibraltar, the changes in the depth of the Atlantic-Mediterranean Interface layer and the tidal and wind-induced upwelling influenced the surface CO₂ distribution. The variability of the CO₂ fugacity (fCO_2,sw) in this high-variable semi-enclosed area was mainly driven by temperature despite the significant influence of non-thermal processes in the southernmost part. The fCO_2,sw increased from winter to summer by 11.84 ± 0.28 µatm ºC^-1 in the Canary archipelago and by 11.71 ± 0.25 µatm ºC^-1 along the northwest African continental shelf. In the Strait of Gibraltar, the gradient was lower and showed differences between the northern and southern sections (9.02 ± 1.99 and 4.51 ± 1.66 µatm ºC-1, respectively). The annual cycle (referenced to 2019) of total inorganic carbon normalized to a constant salinity of 36.7 (NC_T) indicated that the net community production in the Northeast Atlantic represented >90% of the reduction of inorganic carbon while air-sea CO₂ exchange described <6%. The net community production processes in the Strait of Gibraltar 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 entire region behaved as a strong CO₂ sink during the cold months and as a weak CO₂ source during the warm months. A net annual CO₂ sink behaviour was observed in the Canary basin (-0.26 mol C m^-2 yr^-1), in the northwest African coastal transitional area (-0.48 mol C m^-2 yr^-1) and in both the northern and southern section of the Strait of Gibraltar (-0.82 and -1.01 mol C m^-2 yr^-1). The calculated average air-sea CO₂ flux for the area of study in Northeast Atlantic and in the Strait of Gibraltar was, -2.65 ± 0.44 TgCO₂ yr^-1 (-0.72 ± 0.12 TgC yr^-1) and -7.12 Gg CO₂ yr^-1 (-1.94 Gg C yr^-1), respectively.