Air-sea CO2 flux by eddy covariance technique in the equatorial Indian Ocean

Direct measurements of the air-sea CO₂ flux by the eddy covariance technique were carried out in the equatorial Indian Ocean. The turbulent flux observation system was installed at the top of the foremast of the R/V MIRAI, thus minimizing dynamical and thermal effects of the ship body. During the turbulent flux runs around the two stations, the vessel was steered into the wind at constant speed. The power spectra of the temperature or water vapor density fluctuations followed the Kolmogorov −5/3 power law, although that of the CO₂ density fluctuation showed white noise in the high frequency range. However, the cospectrum of the vertical wind velocity and CO₂ density was well matched with those of the vertical velocity and temperature or water vapor density in this frequency range, and the CO₂ white noise did not influence the CO₂ flux. The raw CO₂ fluxes due to the turbulent transport showed a sink from the air to the ocean, and had almost the same value as the source CO₂ fluxes due to the mean vertical flow, corrected by the sensible and latent heat fluxes (called the Webb correction). The total CO₂ fluxes including the Webb correction terms showed a source from the ocean to the air, and were larger than the bulk CO₂ fluxes estimated using the gas transfer velocity by mass balance techniques.