Abstract
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.
Highlights
Ocean deoxygenation and ocean acidification are considered to be major oceanic ecosystem stressors that can severely reduce habitat suitability in benthic and pelagic ecosystems (e.g., Doney et al, 2009a; Doney, 2010; Gruber, 2011; Bopp et al, 2013; Breitburg et al, 2015)
While some of the sea surface temperatures (SSTs)/sea level pressure (SLP) responses are similar to the signals typical during an Eastern Pacific-type El Niño in the observational record, the responses to event 2 and event 6 are drastically different from each other and from observed signals
We delved into the processes through which El Niño–Southern Oscillation (ENSO) can influence O2 and pH in the California Current System (CalCS), and explained how physical variability can influence the carbon and oxygen systems of the CalCS
Summary
Ocean deoxygenation (decreasing O2 concentration) and ocean acidification (decreasing pH) are considered to be major oceanic ecosystem stressors that can severely reduce habitat suitability in benthic and pelagic ecosystems (e.g., Doney et al, 2009a; Doney, 2010; Gruber, 2011; Bopp et al, 2013; Breitburg et al, 2015) Coastal upwelling ecosystems, such as the California Current System (CalCS), support some of the world’s most productive fisheries due to the seasonal, wind-driven upwelling of nutrient-rich waters (e.g., Chavez and Messié, 2009; Messié et al, 2009). Turi et al (2016) used a regional ocean model to demonstrate that a recent intensification of upwellingfavorable winds is linked to a drop in coastal pH and arag It currently remains unclear whether these observed and modeled changes in the CalCS ocean biogeochemistry are ongoing signals of anthropogenic climate change, and could continue into the future, or whether they are driven by natural fluctuations in the climate system (e.g., Bakun, 1990; Narayan et al, 2010; Bakun et al, 2015; Rykaczewski et al, 2015; Wang et al, 2015)
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