Long High-Resolution Records of Mixed-Layer Carbon Budget Variability in the Southern California Current System

Two moorings in the California Current Ecosystem (CCE) obtained long records of biogeochemical (O₂, pCO₂, pH, nutrients, Chl-a) and physical parameters (currents, temperature, salinity). Here, the data are used to investigate the seasonal variability of the mixed-layer carbon budget from 2008 to 2022. The moorings are located in the California Current offshore region (CCE1) and the upwelling region on the continental shelf (CCE2), recording high-resolution data at both the surface and the base of the mixed layer. On average, air-sea CO₂ fluxes at the surface showed that the open ocean site acts as a sink for atmospheric CO₂, with a net annual mean flux of -0.52 molC m^-2 year^-1, while the upwelling site is a carbon source, with a net outgassing of 0.56 molC m^-2 year^-1. At CCE1, sea surface temperature is the primary driver of seawater pCO2 and CO₂ fluxes, whereas, at the upwelling site, dissolved inorganic carbon (DIC) associated with non-thermal processes acting on seawater pCO₂ is the main driver of seasonal variability. To study which non-thermal processes, such as lateral advection, entrainment/detrainment, biological effects, and CO₂ flux, affect the mixed layer DIC, we first quantify a climatological annual carbon budget via a mass balance at each site. Using this budget, we compute the anomalies that events such as La Niña, El Niño, and Marine Heatwaves create in relation to the observed mean conditions. Specifically, events such as Marine Heatwaves can reverse the mean surface CO₂ flux at both sites, with the CCE1 site switching from a net CO₂ sink to a net CO₂ source and CCE2 from a CO₂ source to a sink. The relevance of each driver during these events is explored with respect to the climatological annual carbon budget at each site. This study highlights the importance of long-term monitoring for accurately capturing the variability of marine carbon fluxes.