Drivers of CO2 flux variability in the Cape Basin, South Africa.

Despite recognition of the South Atlantic Ocean as a significant CO₂ sink, the variability of carbon fluxes (FCO₂) at small temporal and spatial scales remains poorly understood. This gap is especially evident in transitional regions like the Cape Basin, where mesoscale oceanic features and localized atmospheric processes strongly influence ocean-atmosphere CO₂ exchange. Current observational and modeling approaches lack the resolution to capture these fine-scale fluctuations, potentially biasing global CO₂ flux estimates. To address this, our study examines short-term (1–10 days) and small-scale (0.1–10 km) drivers of CO₂ flux variability in the Cape Basin using 2-month high-resolution time-series data from a Wave Glider during late summer. By decomposing the carbon flux equation and applying Reynolds decomposition, we show that wind-driven gas transfer velocity (Kw) dominates the periods of enhanced FCO2, accounting for approximately 78% of flux variability on average in the Cape Basin. A secondary contribution of 11% comes from changes in the air-sea pCO₂ gradient (ΔpCO₂). . However, during localized periods - over the course of hours to days - mesoscale eddies and fronts enhance the ΔpCO₂ to the order of 60 µatm. During moderate winds (5 m s-1 < U10 < 15 m s-1), this sets ΔpCO2 as the dominant driver in FCO2 variability (>50%). However, when U10 < 15 m s-1, Kw dominates variability of FCO2 irrespective of ΔpCO₂. These findings underscore the importance of small-scale ocean processes in CO₂ exchange, their nuanced relationship with wind speed - dominated by large scale extratropical cyclones in the Cape Basin - and the need for high-resolution observations to improve global flux estimates.