Spatio-temporal variations in surface Marine Carbonate System properties across the Western Mediterranean Sea using Volunteer Observing Ship data

Abstract. The surface physical and Marine Carbonate System (MCS) properties were assessed along the western boundary of the Mediterranean Sea. An unprecedent high-resolution observation-based dataset spanning 5 years (2019–2024) was built through automatically underway monitoring by a Volunteer Observing Ship (VOS). The MCS dynamics were strongly modulated by physical-biological coupling dependent on the upper-layer circulation and mesoscale features. On a seasonal scale, the variations in CO₂ fugacity (fCO_2,sw) were mainly driven by sea surface temperature (SST) fluctuations (45–83 %) and partially offset by the processes controlling total inorganic carbon (C_T) distribution (25–38 %). On an interannual scale, the SST trends (0.26–0.43 ºC yr^-1) have accelerated by 78–88 % in comparison with previous decades. The ongoing surface warming was the main factor (with a contribution of ~76–92 %) increasing fCO_2,sw (4.18 to 5.53 µatm yr^-1) and, consequently, decreasing pH (-0.005 to -0.007 units yr^-1) in the surface waters. The seasonal SST, becoming larger due to progressively warmer summers, was the primary driver of the observed slope up of interannual trends. The evaluation of the air-sea CO₂ exchange shows the area across the Alboran Sea (14,000 Km²) and the eastern Iberian margin (40,000 Km²) acting as an atmospheric CO₂ sink of -1.57 ± 0.49 mol m^-2 yr^-1 (0.97 ± 0.30 Tg CO₂ yr^-1) and -0.70 ± 0.54 mol m^-2 yr^-1 (-1.22 ± 0.95 Tg CO₂ yr^-1), respectively. The net annual CO₂ sink has reduced by 40–80 % since 2019 due to the ongoing strength of the source status during summer and the weakening in the sink status during spring and autumn.