  A bypass transition scenario in a wind-stress driven aqueous flow is analysed
using a temporally developing boundary layer model with accelerating surface
drift velocity. The parameters of the model are selected to mimic a wave-tank
experiment with a reference wind speed of 5 m/s. To study the boundary layer
processes in isolation, a flat free surface is adapted, which inhibits the
initiation of waves. First, preferred initial perturbations to which the
boundary layer is the most sensitive are identified using linear non-normal
growth theory. These perturbations are arranged as streamwise-constant vortex
pairs located adjacent to the free surface. Subsequently, direct numerical
simulations are initialized with these optimal perturbations and streamwise
streaks are generated. High-speed streaks penetrate into deeper water layers
and undergo sinuous instabilities reminiscent of the instabilities developing
on low-speed streaks in wall-bounded flows. Streak instabilities induce lateral
undulations at the free surface, which closely resemble the dye patterns before
the onset of waves in wind-wave-tank experiments. The present analysis provides
a theoretical background for these experimental observations.
