FY 2025

Open ocean versus upwelling regimes: Air-sea CO2 fluxes and pCO2 inter-annual variability in the Southern California Current System

Frazão, H.C., U. Send, A.J. Sutton, M.D. Ohman, M. Lankhorst, T.R. Martz, and J. Sevadjian

J. Geophys. Res., 130(7), e2024JC022126, doi: 10.1029/2024JC022126, View open access article at AGU/Wiley (external link) (2025)

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 pCO₂ and CO_2,sw 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.

Plain Language Summary. The ocean is a good reservoir for the atmosphere's excess carbon dioxide (CO₂). It absorbs CO₂ when the concentration in water is lower than the atmosphere and outgasses CO₂ when the concentration in water is higher than the atmosphere. When the seawater temperature increases, the ocean CO₂ increases; therefore, temperature changes affect CO₂ exchange between ocean and atmosphere. However, other factors influence the amount of CO₂ the ocean absorbs. To identify the relevant processes, we placed two stations in the Southern California Current measuring CO2 in the air and seawater since 2008—CCE1 is in the open ocean while CCE2 is closer to the coast. These measurements show how seawater CO₂ changes on multiple timescales. In the open-ocean area, the seawater CO₂ is driven by surface temperature, whereas closer to the coast, it is affected by vertical movement of water during spring, which injects CO₂-rich water into the surface. Annually, the open-ocean station absorbs CO2 while the nearshore station releases CO₂ on average. Furthermore, we observe that warm anomalies, such as marine heatwaves or El Niño, change the average behavior of these two stations: the ocean releases CO₂ into the atmosphere offshore but absorbs CO₂ closer to the coast.