value of CO2 concentrations in dry air measured concurrently with the seawater pCO2, barometric pressure, and water vapor pressure at seawater temperature. The sea–air pCO2 difference, ∆pCO2, was computed by subtracting it from the oceanic pCO2 value. Although the non-ideal behavior of CO2 gas due to CO2–CO2 as well as CO2–N2–O2–H2O molecular interactions has been estimated by Weiss and Price (41), its effect is about 1 µatm in the concentration range of CO2. Furthermore, since the corrections are similar for the air and seawater pCO2, the non-ideal effect cancels due to the differencing for ∆pCO2. Therefore, CO2 has been treated as an ideal gas.
The pCO2 database assembled for this study consists of about 250,000 individual measurements made during about 250 expeditions. Many of the observations used have been published in scientific journals and in technical reports (12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37 and 38; 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56 and 57). In addition, many unpublished data in the archives of the authors at the Lamont–Doherty Earth Observatory, the Scripps Institution of Oceanography, the Pacific Marine Environmental Laboratory (National Oceanic and Atmospheric Administration), and the Atlantic Oceanographic and Meteorological Laboratory (National Oceanic and Atmospheric Administration) have been used.
In assembling the global data set for ∆pCO2, the original measurements have been processed through the four steps which are discussed below.
Averaging of the Underway pCO2Data. There are two sources of pCO2 data in surface waters: those obtained for discrete water samples at each hydrographic station and those obtained semicontinuously (several times per hour) using underway systems. Because the number of the latter measurements far exceeds the number for discrete water sample measurements, the latter would statistically overwhelm the former. To prevent this, we have computed a 6-h mean value (e.g., a mean over 100 km if ship’s speed is 8 knots) for underway measurements and counted this mean with an equal statistical weight as a discrete measurement. This averaging scheme has been shown to represent a spatial variation of pCO2 in seawater even in areas of strong gradients such as the equatorial Pacific.
Exclusion of the Equatorial Pacific Data for El Nino Periods. The objective of this study is to obtain a representative distribution of global ocean ∆pCO2 during non-El Nino periods. Although El Niño Southern Oscillations (ENSO) events could affect a wide range of global meteorological and oceanographic conditions including those in the Southern Ocean (58, 59), the extent of its effects on the carbon chemistry beyond the equatorial Pacific belt has not been documented. Therefore, we have assumed that the effects are limited to the equatorial Pacific between 10°N and 10°S and removed the equatorial Pacific data from the data set for the following El Nino periods, which have been identified on the basis of the Southern Oscillation Index (less than −1.5) and sea surface temperature (SST) changes (NINO3 and NINO4) (59): March 1972–March 1973; May 1976–March 1977; June 1982–June 1983; August 1986–July 1987; October 1991–May 1992; October 1992–October 1993; and April 1994–February 1995.
Through these two processes described above, the original 250,000 individual measurements were reduced to about 16,500 data points; their spatial distribution is shown in Fig. 1. Although the global ocean appears to be well covered over the 12-month period with the exception of the southern Indian Ocean, monthly data distributions (February and August are shown as examples) show large oceanic areas without measurements.
Normalization of ∆pCO2to the Year 1990 With the exceptions of well-studied areas such as the western North Pacific, the available observations are not sufficient to resolve the interannual variability of ∆pCO2 over the global oceans. Therefore, our approach is to combine all the observations onto a single virtual calendar year (chosen arbitrarily to be 1990). However, since the mean atmospheric CO2 concentration has increased by about 30 ppm from about 326 ppm in 1972 to 356 ppm in 1994, the secular increase in atmospheric CO2 must be taken into consideration when ∆pCO2 data are assembled to represent a single year. In subtropical gyres, vertical mixing of surface layer waters with subsurface waters is limited due to the strong stratification. Hence, as the surface water takes up more atmospheric CO2, the mean pCO2 in surface waters tends to increase with a rate similar to the atmospheric CO2 increase. This has been demonstrated over the Sargasso Sea (60) and over the western North Pacific between 3°N and 35°N along the 137°E meridian by Inoue et al. (21). R.A.F. (unpublished data) analyzed the surface water pCO2 data obtained in the equatorial Pacific along 140°W during the years 1984, 1988, 1990, and 1995, and observed that the surface water pCO2 values have been increasing at a mean rate of 1.3 ± 0.5 µatm·yr−1, which is consistent with the atmospheric rate of about 1.8 ppm·yr−1. This implies that the strong CO2 source in the Pacific equatorial belt is caused by the warming of recently ventilated, colder subsurface waters derived primarily from depths about 100–300 meters. Hence ∆pCO2 values for the temperate gyre and equatorial areas are nearly independent of the year of measurements and thus, those measured in different years may be treated as though those were measured in the same year.