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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