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

摘要

The lack of a firm relationship between wind speed (U10) and gas transfer velocity (k) is considered to be one of thefactors that hinders accurate quantification of interannual variations of ocean–atmosphere CO2 fluxes. In this paperthe interannual variations of k of using four different k–U10 parametrizations are examined using wind speed datafrom the NCEP/NCAR reanalysis project. The extent to which interannual variations are faithfully reproduced in theNCEP/NCAR data is also investigated. This is carried out through comparison with QuikSCAT data. Compared with4 yr of QuikSCAT data, NCEP/NCAR data reproduce interannual k variations, although the absolute magnitude ofk is underestimated. Interannual k variation shows great sensitivity to selection of k–U10 parametrization, and in theWesterlies it changes by a factor of three depending on k–U10 parametrization. Use of monthly mean winds speedsleads to overestimation of interannual k variations compared with k variations computed using 6-hourly wind speedsand the appropriate k–U10 parametrization. Even though the effect of changing k–U10 parametrization is large enoughto be an issue that needs to be considered when computing interannual air–sea CO2 flux variations through combiningestimates of k with data for the air–sea CO2 gradient, it is not sufficient to bridge the gap between such estimates andestimates based on analyses of atmospheric oxygen, CO2 and δ13C data. Finally it is shown that the ambiguity in therelationship between wind speed and k introduces an uncertainty in interannual flux variations comparable to a bias ofinterannual ΔpCO2 variations of at most ±5 µatm.