Response of O-2 and pH to ENSO in the California Current System in a high-resolution global climate model

Coastal upwelling systems, such as the California Current System (CalCS), naturally experience a wide range of O-2 concentrations and pH values due to the seasonality of upwelling. Nonetheless, changes in the El Nino-Southern Oscillation (ENSO) have been shown to measurably affect the biogeochemical and physical properties of coastal upwelling regions. In this study, we use a novel, high-resolution global climate model (GFDL-ESM2.6) to investigate the influence of warm and cold ENSO events on variations in the O-2 concentration and the pH of the CalCS coastal waters. An assessment of the CalCS response to six El Nino and seven La Nina events in ESM2.6 reveals significant variations in the response between events. However, these variations overlay a consistent physical and biogeochemical (O-2 and pH) response in the composite mean. Focusing on the mean response, our results demonstrate that O-2 and pH are affected rather differently in the euphotic zone above similar to 100 m. The strongest O-2 response reaches up to several hundreds of kilometers offshore, whereas the pH signal occurs only within a similar to 100 km wide band along the coast. By splitting the changes in O-2 and pH into individual physical and biogeochemical components that are affected by ENSO variability, we found that O-2 variability in the surface ocean is primarily driven by changes in surface temperature that affect the O-2 solubility. In contrast, surface pH changes are predominantly driven by changes in dissolved inorganic carbon (DIC), which in turn is affected by upwelling, explaining the confined nature of the pH signal close to the coast. Below similar to 100 m, we find conditions with anomalously low O-2 and pH, and by extension also anomalously low aragonite saturation, during La Nina. This result is consistent with findings from previous studies and highlights the stress that the CalCS ecosystem could periodically undergo in addition to impacts due to climate change.