The effect of wind speed products and wind speed–gas exchange relationships on interannual variability of the air–sea CO₂ gas transfer velocity

Abstract

A B S T R A C TThe lack of a firm relationship between wind speed (U 10 ) and gas transfer velocity (k) is considered to be one of the factors that hinders accurate quantification of interannual variations of ocean-atmosphere CO 2 fluxes. In this paper the interannual variations of k of using four different k-U 10 parametrizations are examined using wind speed data from the NCEP/NCAR reanalysis project. The extent to which interannual variations are faithfully reproduced in the NCEP/NCAR data is also investigated. This is carried out through comparison with QuikSCAT data. Compared with 4 yr of QuikSCAT data, NCEP/NCAR data reproduce interannual k variations, although the absolute magnitude of k is underestimated. Interannual k variation shows great sensitivity to selection of k-U 10 parametrization, and in the Westerlies it changes by a factor of three depending on k-U 10 parametrization. Use of monthly mean winds speeds leads to overestimation of interannual k variations compared with k variations computed using 6-hourly wind speeds and the appropriate k-U 10 parametrization. Even though the effect of changing k-U 10 parametrization is large enough to be an issue that needs to be considered when computing interannual air-sea CO 2 flux variations through combining estimates of k with data for the air-sea CO 2 gradient, it is not sufficient to bridge the gap between such estimates and estimates based on analyses of atmospheric oxygen, CO 2 and δ 13 C data. Finally it is shown that the ambiguity in the relationship between wind speed and k introduces an uncertainty in interannual flux variations comparable to a bias of interannual pCO 2 variations of at most ±5 µatm.