Comparison of NSCAT, ERS 2 active microwave instrument, special sensor microwave imager, and Carbon Interface Ocean Atmosphere buoy wind speed: Consequences for the air‐sea CO 2 exchange coefficient

Wind speeds U obtained using three different sensor types are compared to assess which might be most useful for calculating gas fluxes across the air‐sea interface. U remotely sensed using scatterometer (ERS 2 and NASA scatterometer (NSCAT)) and microwave radiometer (special sensor microwave imager (SSM/I)) are compared with each other and with U measured in situ (Carbon Interface Ocean Atmosphere (CARIOCA) drifting buoy). The effect of different gridding schemes is also evaluated for the satellite data. Global comparisons for January 1997 indicate that 1° and monthly ERS 2 U are systematically 0.3 to 2 m s −1 lower than NSCAT U at mid and low latitudes. The NSCAT‐SSM/I comparisons are similar for the three SSM/Is. SSM/I U appear to be higher by 1 to 2 m s −1 than NSCAT U in the tropics and to be lower at high latitudes. When individual satellite measurements are collocated, ERS 2 U appear to be more coherent with NSCAT (rms of the difference equal to 0.8 m s −1 ) than SSM/I U (rms of the difference equal to 1.3 m s −1 ). However, when comparing 1° one‐week interpolated fields, NSCAT‐ERS 2 differences are more scattered than NSCAT‐SSM/I differences, because of the high ERS 2 undersampling. This flaw is attenuated when monthly interpolated 1° fields are compared. The CO 2 exchange coefficients K deduced from these remotely sensed U are compared. Longitudinal profiles of monthly K are consistent within 0 to about 25% depending on the regions and on the instruments. Comparisons with in situ measurements in a region of very low U in the eastern equatorial Pacific show excellent agreement between NSCAT and buoy U (bias of 0.3 m s −1 ) whereas ERS 2 U are underestimated by 1.3 m s −1 and SSM/I U are overestimated by 0.7 m s −1 on average.