Surface-water pCO has been determined with a high precision (±2 µatm) using underway equilibrator-CO analyzer systems over the global ocean since the International Geophysical Year of 195659. As a result of recent major oceanographic programs, including the global CO survey and other international field studies, the database for surface-water pCO observations has been improved to about 1 million measurements with several million accompanying measurements of SST, salinity and other necessary parameters such as barometric pressure and atmospheric CO concentrations. Based upon these observations, a global, monthly climatological distribution of surface-water pCO in the ocean was created for a reference year 1995, chosen because it was the median year of pCO observations in the database. The database and the computational method used for interpolation of the data in space and time will be briefly described below.
For the construction of climatological distribution maps, observations made in different years need to be corrected to a single reference year (1995), based on several assumptions explained below (see also Takahashi et al., 2002). Surface waters in the subtropical gyres mix vertically at slow rates with subsurface waters because of strong stratification at the base of the mixed layer. As a result, they are in contact with the atmosphere and can exchange CO for a long time. Consequently, the pCO in these warm waters follows the increasing trend of atmospheric CO concentrations, as observed by Inoue et al. (1995) in the western North Pacific, by Feely et al. (1999) in the equatorial Pacific and by Bates (2001) near Bermuda in the western North Atlantic. Accordingly, the pCO measured in a given month and year is corrected to the same month in the reference year 1995 using the following atmospheric CO concentration data for the planetary boundary layer: the GLOBALVIEW-CO2 database (2000) for observations made after 1979 and the Mauna Loa data of Keeling and Whorf (2000) for observations before 1979 (reported in CDIAC NDP-001, revision 7).
In contrast to the waters of the subtropical gyres, surface waters in high-latitude regions are mixed convectively with deep waters during fall and winter, and their CO properties tend to remain unchanged from year to year. They reflect those of the deep waters, in which the effect of increased atmospheric CO over the time span of the observations is diluted to undetectable levels (Takahashi et al., 2002). Thus no correction is necessary for the year of measurements.
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