Measurement and parameterisation of the air-sea CO2 flux in high winds

Abstract

During a three year occupation of Station Mike (66°N 2°E), the Norwegian Ocean Weather Ship Polarfront was equipped with a range of meteorological and seastate measuring instruments, including the autonomous air-sea flux system “AutoFlux” (Yelland et al., 2009) and an underway ?pCO2 system. An extensive set of direct, eddy covariance measurements of momentum, latent heat, sensible heat and CO2 flux was obtained over a wide range of open ocean conditions. The maximum recorded 20-minute mean wind speed was 25 m.s-1. The maximum significant wave height was 11 m.

The initial CO2 flux results were subject to a large, commonly observed humidity cross-sensitivity error. A novel iterative correction procedure was developed, tested against an independent data set and proved to be robust (Prytherch et al., 2010a). Open-path sensors may now be used for air-sea CO2 flux measurement, greatly increasing the number of measurements available for analysis.

There are large differences between existing gas transfer to wind speed relationships, particularly at high wind speeds, and there is significant uncertainty over the form (quadratic or cubic) of the relationship. From the 3938 direct CO2 flux measurements made onboard Polarfront, a new
relationship between gas transfer velocity, k660 , and wind speed, U10n has been obtained:

k660 = ?0.51+ 0.095U10n
2.7 0 ?U10n ? 20 m.s-1

The motion corrected fluxes were found to have a large signal at frequencies associated with platform motion. This signal is also apparent in results from previous air-sea experiments from both fixed and moving platforms. The cause of this signal, whether error or real wind-wave nteraction, remains unknown. The gas transfer relationship obtained after removal of this signal is:

k660 = ?0.09 + 0.02U10n
3.1 2 ?U10n ? 20 m.s-1

demonstrating that the observed near cubic dependence on wind speed, also reported in some previous experiments over a more limited wind speed range (McGillis et al., 2001a), is a robust result. This suggests a significant role for wave breaking and bubble-mediated exchange in air-sea gas
transfer.

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