The oceanic uptake of atmospheric carbon dioxide (CO2) since pre-industrial times has increased acidity levels, resulting in a decrease in the pH and aragonite saturation state (Ωar) of surface waters. Aragonite is the predominant biogenic form of calcium carbonate precipitated by calcifying organisms in tropical reef ecosystems. Values of Ωar are often used as a proxy for estimating calcification rates in corals and other calcifying species. We quantify the regional and seasonal variability of Ωar and its main drivers for the Pacific island region (120°E:140°W, 35°S:30°N). The calculation of Ωar uses a seasonal climatology of the surface-water partial pressure of CO2, combined with values of total alkalinity (TA) estimated from a relationship between surface water salinity and total alkalinity that is derived from measurements in the region. This relationship is valid for all phases of El Niño/Southern Oscillation and for mixed layer waters with less than 15 μmol kg− 1 dissolved nitrate.
The influence of seasonal changes in sea surface temperature (SST) on Ωar is small except in the subtropical waters on the northern and southern boundaries of the study region. Here, SST seasonal variability is large (> 5 °C), causing a change in Ωar of greater than 0.1. Seasonal changes in mixed layer depth and net evaporation (precipitation) do influence the seasonality in TCO2 and TA. However, these processes tend to increase (decrease) the TCO2 (TA) in unison, resulting in a small net effect on seasonal change in Ωar for most of the region. Net biological production and sea–air gas exchange were also found to have only a small impact on the seasonal change in Ωar through the region. Changes in Ωar of between 0.1 and 0.2 occurred where variations in wind-driven upwelling in the Central Equatorial Pacific and the transport of Eastern Pacific waters into the South Equatorial Current region caused a change in the TCO2/TA ratio of the surface waters. In contrast to these two regions, the combined effect of biological and physical processes in the West Pacific Warm Pool and North Equatorial Counter Current subregions resulted in Ωar variability of less than 0.1.