Open Ocean Versus Upwelling Regimes: Air‐Sea CO2 Fluxes and pCO2 Inter‐Annual Variability in the Southern California Current System Journal of Geophysical Research: Oceans

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 (pCO2,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 pCO2,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 pCO2,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. Grant no. NA20OAR4320278 Grant no. NA15OAR4320071 Grant no. NA10OAR4320156 Grant no. NA17RJ1231