Inferring Air‐Sea Carbon Dioxide Transfer Velocities from Sea Surface Scatterometer Measurements
Journal of Geophysical Research - Oceans
C-band microwave sea surface radar backscatter observations from the FINO-2 tower in the western Baltic are analyzed with respect to their relevance for air-sea CO 2 transfer velocity parameterizations. The scatterometer measurements observed from a height of 25 m above the sea surface using a multifrequency scatterometer instrument of the University of Hamburg were obtained quasi-simultaneously with eddy covariance CO 2 flux measurements. Both data sets are merged here to derive a gas transfer
... velocity parameterization based on radar backscatter measurements. At the location of the FINO-2 tower, the resulting time-averaged gas transfer velocity amounts to 26.95 cm/hr. In combination with ΔP CO 2 measurements available from the vicinity of the FINO-2 platform, a time-mean CO 2 flux of 0.23 μmol•m −2 •s −1 into the Baltic was estimated. Applied to monthly mean satellite-based C-band ASCAT scatterometer data, the newly derived gas transfer velocity parameterization provides estimates of seasonal and annual mean global maps of air-sea transfer velocities. The new results agree in their general pattern with previous estimates using wind speed parameterization. However, the backscatter-based transfer velocities appear smaller at higher latitudes. Globally averaged air-sea CO 2 fluxes would thereby be reduced by 20%. To what extent this is a robust result, or if it depends on the fact that the training data set did not represent conditions, has to be investigated in the future.