Abstract. Due to seasonal upwelling, the upper ocean waters of the California Current System (CCS) have a naturally low pH and aragonite saturation state ( arag ), making this region particularly prone to the effects of ocean acidification. Here, we use the Regional Oceanic Modeling System (ROMS) to conduct preindustrial and transient simulations of ocean biogeochemistry in the CCS. The transient simulations were forced with increasing atmospheric pCO 2 and increasing oceanic dissolved inorganic carbon concentrations at the lateral boundaries, as projected by the NCAR CSM 1.4 model for the IPCC SRES A2 scenario. Our results show a large seasonal variability in pH (range of ∼ 0.14) and arag (∼ 0.2) for the nearshore areas (50 km from shore). This variability is created by the interplay of physical and biogeochemical processes. Despite this large variability, we find that present-day pH and arag have already moved outside of their simulated preindustrial variability envelopes (defined by ±1 temporal standard deviation) due to the rapidly increasing concentrations of atmospheric CO 2 . The nearshore surface pH of the northern and central CCS are simulated to move outside of their present-day variability envelopes by the mid-2040s and late 2030s, respectively. This transition may occur even earlier for nearshore surface arag , which is projected to depart from its present-day variability envelope by the early-to mid-2030s. The aragonite saturation horizon of the central CCS is projected to shoal into the upper 75 m within the next 25 yr, causing near-permanent undersaturation in subsurface waters. Due to the model's overestimation of arag , this transition may occur even earlier than simulated by the model. Overall, our study shows that the CCS joins the Arctic and Southern oceans as one of only a few known ocean regions presently approaching the dual threshold of widespread and near-permanent undersaturation with respect to aragonite and a departure from its variability envelope. In these regions, organisms may be forced to rapidly adjust to conditions that are both inherently chemically challenging and also substantially different from past conditions.

No abstract

[1] We investigate the diurnal carbon cycle in the near surface ocean and atmosphere of Santa Monica Bay, California on the basis of hourly measurements of the oceanic and atmospheric partial pressures of CO 2 (pCO 2 oc and pCO 2 atm ) and related parameters from a moored platform. The power spectrum of the data from three deployments during late spring, summer, and fall reveal a strong peak at 1 cycle/day for both oceanic and atmospheric pCO 2 . While the average diurnal peak-to-peak amplitude is about 15 to 20 matm for pCO 2 oc and about 10 matm for pCO 2 atm , the 10% largest amplitudes exceed 55 matm and 42 matm, respectively. The diurnal cycle of oceanic pCO 2 is primarily controlled by temperature, but biological processes substantially modify it. The contribution of lateral processes, such as tides, is likely small. For the fall deployment, our data suggest an average net primary production of about 30 mmol C m À2 day À1 . The diurnal cycle of atmospheric pCO 2 is primarily controlled by the air-sea breeze. Neglect of the diurnal variations in the flux calculations may result in biases of more than 0.2 mol C m À2 a À1