Open Ocean Versus Upwelling Regimes: Air‐Sea CO 2 Fluxes and p CO 2 Inter‐Annual Variability in the Southern California Current System

Abstract Two moorings equipped with autonomous air‐sea CO 2 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 ( p CO 2,sw ) and the air‐sea CO 2 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 p CO 2,sw and CO 2 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 CO 2 flux at CCE1 is −0.52 molC m −2  year −1 (sink), while at CCE2, the best estimate for the long‐term CO 2 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 CO 2 outgassing, and cold anomalies (La Niña) increase CO 2 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 CO 2 . Long‐term trends in p CO 2,sw approximately follow the atmospheric CO 2 increase. At the offshore site, the DIC trend is consistent with air‐sea fluxes, keeping the CO 2 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.