SEASONAL-VARIATIONS IN THE ATMOSPHERIC O-2 N-2 RATIO IN RELATION TO THE KINETICS OF AIR-SEA GAS-EXCHANGE/

Observations of seasonal variations in the atmospheric O-2/N-2 ratio a re reported at nine baseline sites in the northern and southern hemisp heres. Concurrent CO2 measurements are used to correct for the effects of land biotic exchanges of O-2 on the O-2/N-2 cycles thus allowing t he residual component of the cycles due to oceanic exchanges of O-2 an d N-2 to be calculated. The residual oceanic cycles in the northern he misphere are nearly diametrically out of phase with the cycles in the southern hemisphere. The maxima in both hemispheres occur in summer. I n both hemispheres, the middle-latitude sea level stations show the cy cles with largest amplitudes and earliest phasing. Somewhat smaller am plitudes are observed at the high-latitude stations, and much smaller amplitudes are observed at the tropical stations. A model for simulati ng the oceanic component of the atmospheric O-2/N-2 cycles is presente d consisting of the TM2 atmospheric tracer transport model [Heimann, 1 995] driven at the lower boundary by O-2 fluxes derived from observed O-2 saturation anomalies in surface waters and by N-2 fluxes derived f rom the net air-sea heat flux. The model is optimized to fit the obser ved atmospheric O-2/N-2 cycles by adjusting the air-sea gas-exchange v elocity, which relates O-2 anomaly to O-2 flux. The optimum fit corres ponds to spatially and temporally averaged exchange velocities of 24+/ -6 cm/hr for the oceans north of 31 degrees N and 29+/-12 cm/hr for th e oceans south of 31 degrees S. These velocities agree to within the u ncertainties with the gas-exchange velocities expected from the Wannin khof [1992] formulation of the air-sea gas-exchange velocity combined with European Centre for Medium-Range Weather Forecasts winds [Gibson et al., 1997] but are larger than the exchange velocities expected fro m the Liss and Merlivat [1986] relation using the same winds. The resu lts imply that the gas-exchange velocity for O-2, like that of CO2, ma y be enhanced in the open ocean by processes that were not systematica lly accounted for in the experiments used to derive the Liss and Merli vat relation.