  Surface ships operating in ocean waves are shown to generate transverse
surface currents which persist long after the initial ship-induced currents
have decayed. These surface currents are due to the formation of large-scale
Langmuir-type circulations and are suggested to be the mechanism for the
concentration of surface-active substance into streaks or bands, which are
regularly seen in synthetic aperture radar imagery of the ocean surface. These
circulations are generated by the Craik-Leibovich vortex force, which results
from interaction of ship-induced current with ambient surface waves. Once
generated, the circulations persist due to their large-scale nearly-inviscid
nature. Numerical simulations of the wake behind a surface ship in calm, head,
and following seas are presented and show good agreement with radar imagery
from at-sea experiments. It is demonstrated that surface currents do not
persist in the absence of surface waves, but can persist for tens of kilometers
through the formation of Langmuir-type circulations. Since the circulations are
driven by the cross product of the Stokes drift and the wake vorticity, the
structure of the persistent wake is a function of relative heading of the ship
with respect to the direction of surface-wave propagation.
