Air-sea CO2 fluxes: seasonal and interannual variability along the Canary Region, the Strait of Gibraltar and the Western Mediterranean for a six-year time series

Abstract

Understanding spatial and temporal variability in air–sea CO₂ exchange remains a critical priority in global carbon budgeting, particularly across dynamic transition zones such as the Canary–Mediterranean corridor. This study examines six years (2019–2024) of high-frequency underway CO₂ data collected from a Volunteer Observing Ship (VOS) along four subregions: the Canary Islands corridor (CAN), the Northwest African Upwelling System (NWA EBUS), the Strait of Gibraltar (SG), and the Western Mediterranean (WM). After rigorous calibration and correction, seasonal harmonic fitting and linear trend analysis were applied to fCO₂ and FCO₂ across selected sites. Results reveal persistent seasonal cycles across all subregions, with summer outgassing and winter uptake driven primarily by SST. However, clear interannual increases in fCO₂sw (+2–6 μatm yr⁻¹) and warming trends (+0.1–0.4 °C yr⁻¹) were observed across most sites, especially in the WM and CAN sections. FCO₂ trends revealed a general weakening of the oceanic CO₂ sink across all regions. In the CAN and WM sections, annual FCO₂ declined by up to –0.55 mmol m⁻² d⁻¹ yr⁻¹ at several sites, while some WM locations transitioned toward seasonal outgassing, reaching fluxes of +12.4 mmol m⁻² d⁻¹ in summer. SST trends ranged from +0.10 to +0.43 °C yr⁻¹, particularly steep in WM and the northern NWA EBUS, and were strongly correlated with rising fCO₂sw (+4 to +6.7 μatm yr⁻¹). These thermal increases have reduced CO₂ solubility and likely intensified stratification, limiting vertical mixing and biological drawdown. Although all sites remained net sinks on an annual scale, results point to accelerating thermal control over FCO₂ dynamics and early signs of sink weakening in subtropical and marginal seas. Confirming the growing importance of high-resolution monitorization in understanding climate-driven shifts in ocean carbon uptake capacity.