Attributing seasonal pH variability in surface ocean waters to governing factors

On-going ocean acidification and increasing availability of high-frequency pH data have stimulated interest to understand seasonal pH dynamics in surface waters. Here we show that it is possible to accurately reproduce observed pH values by combining seasonal changes in temperature (T), dissolved inorganic carbon (DIC), and total alkalinity (TA) from three time series stations with novel pH sensitivity factors. Moreover, we quantify the separate contributions of T, DIC, and TA changes to winter-to-summertime differences in pH, which are in the ranges of -0.0334 to -0.1237, 0.0178 to 0.1169, and -0.0063 to 0.0234, respectively. The effects of DIC and temperature are therefore largely compensatory, and are slightly tempered by changes in TA. Whereas temperature principally drives pH seasonality in low-latitude to midlatitude systems, winter-to-summer DIC changes are most important at high latitudes. This work highlights the potential of pH sensitivity factors as a tool for quantifying the driving mechanisms behind pH changes.