Abstract. Due to seasonal upwelling, the upper ocean waters of the California Current System (CCS) have a naturally low pH and aragonite saturation state (Ωarag), 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 (1995–2050) simulations of ocean biogeochemistry in the CCS. The transient simulations were forced with increasing atmospheric pCO2 as projected by the NCAR CSM 1.4 model run under either the IPCC SRES A2 or B1 scenarios. Using ROMS, we investigate the timing of transition decades during which pH and Ωarag depart from their modeled preindustrial (1750) and present-day (2011) variability envelopes. We report these transition decades by noting the midpoint of the ten-year transition periods. In addition, we also analyze the timing of near permanent aragonite undersaturation in the upper 100 m of the water column. Our results show that an interplay of physical and biogeochemical processes create large seasonal variability in pH (∼ 0.14) and Ωarag (∼ 0.2). Despite this large variability, we find that present-day pH and Ωarag have already moved out of their preindustrial variability envelopes due to the rapidly increasing concentrations of atmospheric CO2. The simulations following the A2 emissions scenario suggest that nearshore surface pH of the northern and central CCS will move out of their present-day variability envelopes by 2045 and 2037, respectively. However, surface Ωarag of the northern and central CCS subregions are projected to depart from their present-day variability envelopes sooner, by 2030 and 2035, respectively. By 2025, the aragonite saturation horizon of the central CCS is projected to shoal into the upper 75 m for the duration of the annual cycle, causing near permanent undersaturation in subsurface waters. Overall, our study shows that the CCS joins the Arctic and Southern Oceans as one of only a few known ocean regions presently approaching this dual threshold of 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 prior conditions.
Persistent spatial structuring of coastal ocean acidification in the California Current System