Udrifter = Udeep + UEkman + UStokes + Ulee + Ulog + Uerror
Udeep is the background flow and UEkman is the velocity near the top of the Ekman layer. The remaining suB.Sc.ripts refer to the surface corrections for Stokes drift, the leeway effect, and the logarithmic sublayer (the effect of the surface sublayer was assumed negligible). The calculation of UStokes, Ulee and Ulog is straightforward and based on physical principles. The background flow (below 20m) was modelled using the adjoint method to assimilate current data from moored instruments and a shipborne acoustic doppler current profiler. UEkman was modelled statistically, using complex regression. The background flow correction gave the greatest reduction in the variance between model and observation of the drifter velocities. The variance of the error was 53% that of the observed (i.e. uncorrected) velocity variance. Subtracting the surface correction factors further reduced the residual variance to 45%. Complex regression of the corrected drifter velocity on the wind stress reduced the residual variance to 37%.