Open Ocean Versus Upwelling Regimes: Air-Sea CO2 Fluxes and pCO2 Inter-Annual Variability in the Southern California Current System

Two moorings equipped with autonomous air-sea CO₂ instrumentation located in the Southern California Current System were used to examine the seasonal and interannual variability of the surface partial pressure of carbon dioxide in seawater (pCO_2,sw) and the air-sea CO₂ flux between 2008 and 2022. These two moorings are in two distinct oceanographic regimes: offshore, centered in the California Current (CCE1), and nearshore within the coastal upwelling regime (CCE2). The offshore seasonal cycles of the surface pCO_2,sw and CO₂ flux are driven by sea surface temperature (SST) seasonality and at the nearshore site by dissolved inorganic carbon (DIC) concentration changes linked with seasonal upwelling. The resulting net annual CO₂ flux at CCE1 is −0.52 molC m⁻² year⁻¹ (sink), while at CCE2, the best estimate for the long-term CO₂ flux mean is 0.23 molC m⁻² year⁻¹ (source). The interannual variability at the offshore site is mainly controlled by SST, where warm anomalies (El Niño and Marine Heatwaves) cause anomalous CO₂ outgassing, and cold anomalies (La Niña) increase CO₂ ingassing. Conversely, at the nearshore site, the strength (or absence of) upwelling of DIC-rich water associated with cold (or warm anomalies) results in increased outgassing (or ingassing) of CO₂. Long-term trends in pCO_2,sw approximately follow the atmospheric CO₂ increase. At the offshore site, the DIC trend is consistent with air-sea fluxes, keeping the CO₂ equilibrium between air and water. At the nearshore site, the DIC trend has a similar magnitude but could also result from changing water-mass composition or concentration due to freshwater loss.