Abstract. Coastal upwelling systems, such as the California Current System (CalCS),
naturally experience a wide range of O2 concentrations and pH values due
to the seasonality of upwelling. Nonetheless, changes in the El
Niño–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
O2 concentration and the pH of the CalCS coastal waters. An assessment of
the CalCS response to six El Niño and seven La Niña events in ESM2.6
reveals significant variations in the response between events. However, these
variations overlay a consistent physical and biogeochemical (O2 and pH)
response in the composite mean. Focusing on the mean response, our results
demonstrate that O2 and pH are affected rather differently in the euphotic
zone above ∼ 100 m. The strongest O2 response reaches up to
several hundreds of kilometers offshore, whereas the pH signal occurs only
within a ∼ 100 km wide band along the coast. By splitting the
changes in O2 and pH into individual physical and biogeochemical
components that are affected by ENSO variability, we found that O2
variability in the surface ocean is primarily driven by changes in surface
temperature that affect the O2 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 ∼ 100 m, we find conditions
with anomalously low O2 and pH, and by extension also anomalously low
aragonite saturation, during La Niña. 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.
New Insights into the Controls and Mechanisms of Plankton Productivity in Coastal Upwelling Waters of the Northern California Current System