GasEX-II Experiment: Processes of Gas Exchange in Low Wind Conditions
Collaborators: James B. Edson (Woods Hole Oceanographic Institution, presently at University of Connecticut)
Christopher W. Fairall (NOAA)
Wade B. McGillis (Woods Hole Oceanographic Institution, presently at Lamont)
An important question to be answered is which physical processes control the kinetics of air-sea CO2 exchange underlow wind conditions [McGillis et al., 2004]. GasEx-II was a 15-day air-sea carbon dioxide (CO2) exchange study conducted in the equatorial Pacific in January and February of 2001 board the NOAA ship Ronald H. Brown, and used a combination of ships, buoys, and drifters equipped with ocean and atmospheric sensors to assess variability and surface mechanisms controlling air-sea CO2 fluxes. As part of the work during GasEx-II, C. Zappa led the team responsible for the surface processes instrument platform (SPIP), a 15-foot, remotely operated catamaran used to measure the atmospheric profiles of CO2, temperature, water vapor, and momentum very close to the air-water interface. SPIP has the advantage of measuring the atmospheric boundary layer near the surface with less flow distortion than the Brown. Results from both the R/V Ronald H. Brown and the catamaran show that the measured fluxes reasonably agree with existing models for air-sea gas exchange. The GasEx-II results show a weak wind dependence but a strong variability in processes governed by the diurnal heating cycle. This implies that any changes in the incident radiation, including atmospheric cloud dynamics, phytoplankton biomass, and surface ocean stratification may have significant feedbacks on the amount and variability of air-sea gas exchange. This is in sharp contrast with previous field studies of air-sea gas exchange, which showed that wind was the dominating forcing function. The results suggest that gas transfer parameterizations that rely solely on wind will be insufficient for regions with low to intermediate winds and strong insolation.