BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-10-193-2013 Spatiotemporal variability and long-term trends of ocean acidification in the California Current System Hauri C. 6 1 Gruber N. https://orcid.org/0000-0002-2085-2310 1 Vogt M. 1 Doney S. C. 2 Feely R. A. 3 Lachkar Z. 1 Leinweber A. 4 McDonnell A. M. P. 6 1 Munnich M. 1 Plattner G.-K. 5 1 Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland Dept. of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA Pacific Marine Environmental Laboratory/National Oceanic and Atmospheric Administration, Seattle, WA, USA Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA, USA Climate and Environmental Physics Group, Physics Institute, University of Bern, Bern, Switzerland now at: School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Fairbanks, AK, USA 14 01 2013 10 1 193 216 Copyright: © 2013 C. Hauri et al. 2013 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/10/193/2013/bg-10-193-2013.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/10/193/2013/bg-10-193-2013.pdf

Due to seasonal upwelling, the upper ocean waters of the California Current System (CCS) have a naturally low pH and aragonite saturation state (&Omega;<sub>arag</sub>), 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 <i>p</i>CO<sub>2</sub> 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 &Omega;<sub>arag</sub> (~ 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 &Omega;<sub>arag</sub> 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<sub>2</sub>. 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 &Omega;<sub>arag</sub>, 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 &Omega;<sub>arag</sub>, 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.

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