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

Abstract

Abstract. 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 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 O 2 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 (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 ∼ 100 m. The strongest O 2 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 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 ∼ 100 m, we find conditions with anomalously low O 2 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.

On the role of climate modes in modulating the air-sea CO<sub>2</sub> fluxes in Eastern Boundary Upwelling Systems

“…Along with standard atmosphere, ocean, land, and sea ice components, the CESM-LENS simulations include land and ocean biogeochemistry. The ocean biogeochemical component of CESM1(CAM5) is the Biogeochemical Elemental Cycling (BEC) model, which has three phytoplankton functional groups and tracks the cycling of C, N, P, Fe, Si, 10 and O in the ocean. Further information on the implementation of BEC in CESM1 can be found in Moore et al (2013) and Lindsay et al (2014).…”

“…where k represents the gas transfer velocity (dependent on the wind speed squared), K 0 the solubility of CO 2 in seawater, and 10 pCO o 2 and pCO a 2 the partial pressures of CO 2 in the surface ocean and atmosphere, respectively. We use a linear Taylor expansion to quantify the relative contribution of each variable to the overall CO 2 flux anomaly in response to internally generated variability following Lovenduski et al (2007) and Turi et al (2014),…”

“…The mean state of Pacific EBUS CO 2 fluxes is particularly well-modeled in CESM-LENS ( Figure 1). The CESM-LENS 10 captures the meridional gradient of poleward uptake and equatorward outgassing of CO 2 in the CalCS (Figure 1e). In this and the other EBUS, the coarse resolution model is not capable of resolving the narrow band of nearshore CO 2 outgassing found in high resolution model solutions (e.g., Turi et al, 2014;Fiechter et al, 2014).…”

Observational Needs Supporting Marine Ecosystems Modeling and Forecasting: From the Global Ocean to Regional and Coastal Systems

“…Thus, prescription of biased lateral boundary conditions to a regional CCS model may distort these important influences. In this application, a global high-resolution physical biogeochemical model with a realistic representation of the smallscale features of the CCS was used to assess the response of ocean pH and oxygen, important ecosystem stressors, to ENSO events (Turi et al, 2018).…”

“…Due to the small spatial scale of key regionally specific coastal processes (e.g., upwelling and topographically stirred along-shore transport), understanding ecosystem dynamics in coastal regions requires observations and ocean models with a sufficiently high spatial and temporal resolution to resolve those scales (Fiechter et al, 2014;Rudnick, 2016;Turi et al, 2018). For example, upwelled water is typically high not only in nutrients but also in inorganic carbon; therefore, recently upwelled waters both outgas CO 2 to the atmosphere and subsequently stimulate primary production that draws down CO 2 .…”