Examining the impact of surface currents on satellite scatterometer and altimeter ocean winds

A five-year dataset collected over two surface current and meteorological moorings allows rigorous evaluation of questions surrounding wave/current interaction and the scatterometer. Results demonstrate that scatterometer winds represent winds relative to the moving sea surface, affirming previous observational efforts that inferred the phenomenon using climatological approaches over larger time and space scales in equatorial and Western boundary currents. Comparisons of wind residuals between Ku-band QuikSCAT and buoy measurements show near one-to-one correlation with ocean surface velocity for 5, 12.5, and 25 km resolution wind speed products, especially under conditions of moderate wind speed and near-neutral atmospheric stability. No measurable correlation is observed between wind direction residuals and current vectors, indicating a weak effect of surface currents on derived wind direction at the length scales observed by this scatterometer. Similar analyses are applied to C-band ASCAT satellite wind measurements at the same sites as well as to satellite altimeter winds, and overall confirm the results seen with QuikSCAT; differences are likely the combined result of sampling, satellite wind algorithms, and geophysical windwave coupling in the presence of currents. On the whole, this study affirms that at length scales of 10 km and longer the scatterometer wind can be considered to be current-relative. Observed differences between earth-relative and current-relative wind of order 10-20% of the wind velocity are not uncommon in this and other ocean regions and this study more fully validates that microwave remote sensing winds lie closer to wind stress in the presence of larger scale currents.