Spatiotemporal variability and drivers of pCO₂ and air–sea CO₂ fluxes in the California Current System: an eddy-resolving modeling study

Abstract. We quantify the CO₂ source/sink nature of the California Current System (CalCS) and determine the drivers and processes behind the mean and spatiotemporal variability of the partial pressure of CO₂ (pCO₂) in the surface ocean. To this end, we analyze eddy-resolving, climatological simulations of a coupled physical–biogeochemical oceanic model on the basis of the Regional Oceanic Modeling System (ROMS).

In the annual mean, the entire CalCS within 800 km of the coast and from ∼33° N to 46° N is essentially neutral with regard to atmospheric CO₂: the model simulates an integrated uptake flux of −0.9 ± 3.6 Tg C yr⁻¹, corresponding to an average flux density of −0.05 ± 0.20 mol C m⁻² yr⁻¹. This near zero flux is a consequence of an almost complete regional compensation between (i) strong outgassing in the nearshore region (first 100 km) that brings waters with high concentrations of dissolved inorganic carbon (DIC) to the surface and (ii) and a weaker, but more widespread uptake flux in the offshore region due to an intense biological reduction of this DIC, driven by the nutrients that are upwelled together with the DIC.

The air–sea CO₂ fluxes vary substantially in time, both on seasonal and sub-seasonal timescales, largely driven by variations in surface ocean pCO₂. Most of the variability in pCO₂ is associated with the seasonal cycle, with the exception of the nearshore region, where sub-seasonal variations driven by mesoscale processes dominate. In the regions offshore of 100 km, changes in surface temperature are the main driver, while in the nearshore region, changes in surface temperature, as well as anomalies in DIC and alkalinity (Alk) owing to changes in circulation, biological productivity and air–sea CO₂ fluxes dominate. The prevalence of eddy-driven variability in the nearshore 100 km leads to a complex spatiotemporal mosaic of surface ocean pCO₂ and air–sea CO₂ fluxes that require a substantial observational effort to determine the source/sink nature of this region reliably.