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

Turi, Giuliana; Alexander, Michael; Lovenduski, Nicole S.; Capotondi, Antonietta; Scott, James; Stock, Charles; Dunne, John; John, Jasmin; Jacox, Michael

Coastal upwelling systems, such as the California Current System (CalCS), naturally experience a wide range of Oinline-formula2 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 Oinline-formula2 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 (Oinline-formula2 and pH) response in the composite mean. Focusing on the mean response, our results demonstrate that Oinline-formula2 and pH are affected rather differently in the euphotic zone above inline-formula∼ 100 inline-formulam. The strongest Oinline-formula2 response reaches up to several hundreds of kilometers offshore, whereas the pH signal occurs only within a inline-formula∼ 100 inline-formulakm wide band along the coast. By splitting the changes in Oinline-formula2 and pH into individual physical and biogeochemical components that are affected by ENSO variability, we found that Oinline-formula2 variability in the surface ocean is primarily driven by changes in surface temperature that affect the Oinline-formula2 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 inline-formula∼ 100 inline-formulam, we find conditions with anomalously low Oinline-formula2 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.

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Turi, Giuliana / Alexander, Michael / Lovenduski, Nicole S. / et al: Response of O2 and pH to ENSO in the California Current System in a high-resolution global climate model. 2018. Copernicus Publications.

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