Natural variability in the surface ocean carbonate ion concentration

The simulation is run with a prescribed, pre-industrial atmospheric CO₂ concentration for 1000 years, permitting investigation of natural [CO₃²⁻] variability on interannual to multi-decadal timescales. We find high interannual variability in surface [CO₃²⁻] in the tropical Pacific and at the boundaries between the subtropical and subpolar gyres in the Northern Hemisphere, and relatively low interannual variability in the centers of the subtropical gyres and in the Southern Ocean.

Statistical analysis of modeled [CO₃²⁻] variance and autocorrelation suggests that significant anthropogenic trends in the saturation state of aragonite (Ωaragonite) are already or nearly detectable at the sustained, open-ocean time series sites, whereas several decades of observations are required to detect anthropogenic trends in Ωaragonite in the tropical Pacific, North Pacific, and North Atlantic. The detection timescale for anthropogenic trends in pH is shorter than that for Ωaragonite, due to smaller noise-to-signal ratios and lower autocorrelation in pH.