Assessment of the air‐sea exchange of CO₂ in the south Pacific during austral autumn

Abstract

Measurements of CO 2 concentrations in the atmosphere and in the surface waters of the South Pacific Ocean were made by NOAA scientists between 1984 and 1989. These basin-wide measurements were all taken during austral autumn and provide data for evaluation of the seasonal flux of CO 2 from this region. The sensitivity of this flux to the uncertainty in the CO 2 gas exchange coefficient was evaluated using four different wind data sets and two formulations for the wind dependence of gas transfer velocity. The integrated net flux of CO 2 to the atmosphere during austral autumn (February to May) ranges from -0.03 (ocean influx) to +0.09 (ocean effiux) GT of carbon depending on the combination of wind field and wind-dependent exchange coefficient used. INTRODUCTION Gas flux across the air-sea interface is dependent on twoMajor spatial and temporal gaps in the measured distribution factors: the difference in air and sea concentrations (ApCO2), of CO 2 in the surface waters of the South Pacific Ocean have and the rate of gas transfer across the boundary. The former hindered an assessment of this ocean as a source or sink for determines the potential for gas flux and the sign of the flux, CO 2. The first annually averaged, global ApCO 2 map was made by Keeling [1968] using relatively sparse data. The large-scale features for the South Pacific are equatorial oversaturation, undersaturation in the subtropical gyre regions, and oversaturation in the polar region. Takahashi et al. [1986] i.e., flux into the ocean (negative) or flux out of the ocean (positive). The other component of flux, gas transfer rate, determines to what extent gas transfer will actually occur. It is not measured directly but is calculated from wind speed using an empirical, independently determined relationship subsequently filled in some of the data gaps and extrapolated between wind speed and gas transfer velocity. As more where necessary to create an annually averaged, zonal map of seasonal measurements of CO 2 become available, the limiting ApCO 2. This map confirms the equatorial oversaturation found factor in accurately assessing fluxes becomes that of assessing by Keeling [1968] but shows undersaturation in both the the gas transfer rate. Current formulations of gas transfer subtropical and polar regions. Additional measurements of velocity depend on wind speed, which is poorly known in the southern hemisphere. Wind speeds from direct observations ApCO 2 have permitted separation into seasonal maps [Tans et al., 1990], but large data gaps in the South Pacific still exist. are available sporadically, and very infrequently, in the South Since the role of this ocean is poorly understood, it is one of Pacific. Wind speed can also vary enormously over short time the major uncertainties in estimating global fluxes and scales, and in consequence, a large number of measurements assessing the role of the global ocean in absorbing atmospheric CO 2 [Tans et al., 1990]. Recently, general circulation models have been used to infer oceanic sources and sinks...

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