Impact of ocean currents on scatterometer winds in the tropical Pacific Ocean

The SeaWinds scatterometer on QuikSCAT has been measuring wind vectors globally since mid-July, 1999. The scatterometer signal is generated by backscatter from centimeter-scale wind-generated waves. When the wind blows against (with) the surface currents, the scatterometer will measure higher (lower) winds than the absolute wind measured by an anemometer at a buoy. We estimate time-varying surface currents by differencing the scatterometer and buoy winds from the Tropical Atmosphere-Ocean array. We compare the wind differences with climatological and actual currents derived from drifters at 15-m depth, acoustic Doppler current profiles (ADCP) at 25-m depth and the TOPEX/Poseidon altimeter. We find the zonal wind differences in good agreement with the zonal currents, particularly with the geostrophic zonal currents from the altimeter. Climatological current estimators that include an annual and semiannual component as well as a linear dependence on the Southern Oscillation Index (SOI) show good skill in predicting the zonal wind differences. The meridional wind differences have magnitudes as large as the zonal differences; however, the meridional wind differences are not well predicted by the estimators. Previous studies of the meridional currents also show less predictability in the meridional component than in the zonal component. The relative motion winds from the scatterometer are precisely what are needed in bulk algorithms for air-sea fluxes. Therefore, ignoring the effect of ocean currents will contribute to errors in the flux fields. Comparisons of flux fields derived using QuikSCAT winds with those derived using NCEP Reanalysis winds show that, while the current effect can be quite large, other NCEP model errors exceed those from currents