Upwelling intensity and source water properties drive high interannual variability of corrosive events in the California Current

Ocean acidification is progressing rapidly in the California Current System (CCS), a region already susceptible to reduced aragonite saturation state due to seasonal coastal upwelling. Results from a high-resolution (~ 3 km), coupled physical-biogeochemical model highlight that the intensity, duration, and severity of undersaturation events exhibit high interannual variability along the central CCS shelfbreak. Variability in dissolved inorganic carbon (DIC) along the bottom of the 100-m isobath explains 70–90% of event severity variance over the range of latitudes where most severe conditions occur. An empirical orthogonal function (EOF) analysis further reveals that interannual event variability is explained by a combination of coastal upwelling intensity and DIC content in upwelled source waters. Simulated regional DIC exhibits low-frequency temporal variability resembling that of the Pacific Decadal Oscillation, and is explained by changes to water mass composition in the CCS. While regional DIC concentrations and upwelling intensity individually explain 9 and 43% of year-to-year variability in undersaturation event severity, their combined influence accounts for 66% of the variance. The mechanistic description of exposure to undersaturated conditions presented here provides important context for monitoring the progression of ocean acidification in the CCS and identifies conditions leading to increased vulnerability for ecologically and commercially important species.

Methods
This dataset is a subset of a coupled physical-biogeochemcial model for the central California Current upwelling system. The physical model is an implementation of the Regional Ocean Modelling System (ROMS; www.myroms.org) coupled to NEMUCSC, a customized version of the North Pacific Ecosystem Model for Understanding Regional Oceanography (NEMURO). The NEMUCSC biogeochemical model includes three limiting macro-nutrients, two phytoplankton functional groups, three zooplankton size-classes, three detritus pools, as well as oxygen cycling and carbonate chemistry. To better represent the combined effects of regional circulation patterns and local upwelling intensity on physical and biogeochemical properties, the model downscales a data-assimilative physical reanalysis at 1/10° (~10 km) resolution for the broader California Current system to a higher 1/30° (~3 km) resolution domain for the central California Current region. Ferret V7.1 (http://ferret.pmel.noaa.gov/Ferret) was used to extract a subset of variables along the 100-m isobath and on the 26.0 kg/m3 isopycnal for 1988–2010. The dataset is in NetCDF4 format (CF-1.4 compliant) with metadata describing each variable name and associated units on the native 1/30° ROMS grid.