Climatological mean monthly distribution of pH and the degree of aragonite saturation have been determined for the surface water of the global oceans using a data set for pCO2, TCO2, alkalinity and nutrient concentrations in surface waters (depths < 50m), which is built upon the GLODAP, CARINA and LDEO database. The mutual consistency among these parameters is demonstrated using the inorganic carbon chemistry model. The global ocean is divided into 33 regions, and the potential alkalinity-salinity relationships (linear regressions) in 32 of these regions (excluding the equatorial Pacific El Nino zone) are established. Using the mean monthly pCO2 data adjusted to a reference year 2005 and the alkalinity estimated using the potential alkalinity-salinity relationships, the mean monthly distributions of pH and aragonite saturation in surface ocean waters are obtained for the year 2005.

The pH in the global ocean surface waters ranges from 7.9 to 8.2 in the year 2005. Lower values are located in the upwelling regions in the equatorial Pacific and in the Arabian and Bering Seas; and higher values are found in the subpolar and polar waters during the spring-summer months of intense photosynthetic production. The vast areas of subtropical oceans have seasonally varying pH values ranging from 8.05 during warmer months to 8.15 during colder months. The warm tropical and subtropical waters are supersaturated by a factor of as much as 4.2 with respect to aragonite and 6.3 for calcite, whereas the cold subpolar and polar waters are less supersaturated only by a factor of 1.2 for aragonite and 2 for calcite because of the lower pH values resulting from greater TCO2 concentration. In the western Arctic Ocean, aragonite undersaturation is observed. Decadal time-series data at the Bermuda (BATS), Hawaii (HOT) and Drake Passage show that pH has been declining at a mean rate of about 0.002 per year.

Takahashi T. & Sutherland S. C., 2013. Final report submitted to the National Science Foundation, Washington, D. C. for Grant: OCE 10-38891 Climatological mean distribution of pH and carbonate ion concentration in global ocean surface waters in the unified pH scale and mean rate of their changes in selected areas. Report.