Improved estimates of the variability in air-sea CO2 fluxes on seasonal and
interannual time scales are necessary to help constrain the net partitioni
ng of CO2 between the atmosphere, oceans and terrestrial biosphere. Few dir
ect measurements of the carbon system have been made in the main Indian Oce
an basin. In the mid 1990s, several global carbon measurement programs focu
sed on the Indian Ocean, greatly increasing the existing carbon database fo
r this basin. This study examines the combined surface CO2 measurements fro
m three major US programs in the Indian Ocean: the global carbon survey cru
ises, conducted in conjunction with the World Ocean Circulation Experiment
(WOCE), the NOAA Ocean-Atmosphere Carbon Exchange Study (OACES) Indian Ocea
n survey and the Joint Global Ocean Flux Study (JGOFS) Arabian Sea Process
Study. These data are fit with multiparameter linear regressions as a funct
ion of commonly measured hydrographic parameters. These fits are then used
with NCEP/NCAR reanalysis and Levitus 94 gridded values to evaluate the sea
sonal variability of surface seawater CO2 in the tropical and subtropical I
ndian Ocean and to estimate the magnitude of the Indian Ocean as a net sink
for atmospheric CO2.
The net annual flux for the Indian Ocean (north of 36 degrees S) was - 12.4
+/- 0.5 X 10(12) mol of carbon (equivalent to -0.15 Pg C) in 1995. The rel
atively small net flux results from the very different surface water pCO(2)
distributions and seasonal variations in the northern and southern Indian
Ocean. The equatorial and northern hemisphere regions have values that are
generally above atmospheric values. During the S-W monsoon, pCO(2) values i
n the Arabian Sea coastal upwelling region are among the highest observed i
n the oceans. The upwelling is seasonal in nature, however, and only affect
s a relatively small area. The Indian Ocean equatorial region generally has
values slightly above atmospheric. Unlike the Pacific and Atlantic Oceans,
however, no clear equatorial upwelling signature was observed in 1995. The
Southern Hemisphere Indian Ocean, which represents the largest region by a
rea, generally has values below atmospheric. The strongest undersaturations
are observed in the austral winter, with summer values reaching near or sl
ightly above atmospheric. (C) 2000 Elsevier Science B.V. All rights reserve
d.