Air-sea CO sub(2) exchange in the equatorial Pacific

• Published in: Journal of Geophysical Research. C. Oceans 2004-08, Vol.109 (C8)
• Main Authors: McGillis, Wade R, Edson, James B, Zappa, Christopher J, Ware, Jonathan D, McKenna, Sean P, Terray, Eugene A, Hare, Jeffrey E, Fairall, Christopher W, Drennan, William, Donelan, Mark, DeGrandpre, Michael D, Wanninkhof, Rik, Feely, Richard A
• Format: Article
• Language: English
• Subjects: Marine
• ISSN: 0148-0227; 2169-9275; 2169-9291
• DOI: 10.1029/2003JC002256

GasEx-2001, a 15-day air-sea carbon dioxide (CO sub(2)) exchange study conducted in the equatorial Pacific, used a combination of ships, buoys, and drifters equipped with ocean and atmospheric sensors to assess variability and surface mechanisms controlling air-sea CO sub(2) fluxes. Direct covariance and profile method air-sea CO sub(2) fluxes were measured together with the surface ocean and marine boundary layer processes. The study took place in February 2001 near 125 degree W, 3 degree S in a region of high CO sub(2). The diurnal variation in the air-sea CO sub(2) difference was 2.5%, driven predominantly by temperature effects on surface solubility. The wind speed was 6.0 +/- 1.3 m s super(-1), and the atmospheric boundary layer was unstable with conditions over the range -1 < z/L < 0. Diurnal heat fluxes generated daytime surface ocean stratification and subsequent large nighttime buoyancy fluxes. The average CO sub(2) flux from the ocean to the atmosphere was determined to be 3.9 mol m super(-2) yr super(-1), with nighttime CO sub(2) fluxes increasing by 40% over daytime values because of a strong nighttime increase in (vertical) convective velocities. The 15 days of air-sea flux measurements taken during GasEx-2001 demonstrate some of the systematic environmental trends of the eastern equatorial Pacific Ocean. The fact that other physical processes, in addition to wind, were observed to control the rate of CO sub(2) transfer from the ocean to the atmosphere indicates that these processes need to be taken into account in local and global biogeochemical models. These local processes can vary on regional and global scales. The GasEx-2001 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.