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. This paper analyzes the differences between ERA-Interimand ERA5 surface winds fields relative to Advanced Scatterometer (ASCAT) ocean vector windobservations, after adjustment for the effects of atmospheric stability anddensity, using stress-equivalent winds (U10S) and air–sea relative motionusing ocean current velocities. In terms of instantaneous root mean square (rms) wind speedagreement, ERA5 winds show a 20 % improvement relative to ERA-Interimand a performance similar to that of currently operational ECMWF forecasts.ERA5 also performs better than ERA-Interim in terms of mean and transient winderrors, wind divergence and wind stress curl biases. Yet, both ERA productsshow systematic errors in the partition of the wind kinetic energy intozonal and meridional, mean and transient components. ERA winds arecharacterized by excessive mean zonal winds (westerlies) with too-weak meanpoleward flows in the midlatitudes and too-weak mean meridional winds (trades)in the tropics. ERA stress curl is too cyclonic in midlatitudes and high latitudes,with implications for Ekman upwelling estimates, and lacks detail in therepresentation of sea surface temperature (SST) gradient effects (along the equatorial cold tonguesand Western Boundary Current (WBC) jets) and mesoscale convective airflows (along the Intertropical Convergence Zone and the warmflanks for the WBC jets). It is conjectured that large-scale mean windbiases in ERA are related to their lack of high-frequency (transient wind)variability, which should be promoting residual meridional circulations inthe Ferrel and Hadley cells.

Characterizing ERA-Interim and ERA5 surface wind biases using ASCAT