Bulk, Spectral and Deep-Water Approximations for Stokes Drift:
Implications for Coupled Ocean Circulation and Surface Wave Models
Abstract
Surface waves modify upper ocean dynamics through Stokes drift related
processes. Representation of these processes at either resolved or
parameterized scales in an ocean model depends on accurate estimation of
Stokes drift profiles. Stokes drift estimated from a discrete wave
spectrum is compared to Stokes drift approximations as a monochromatic
profile based on bulk surface wave parameters, and to two additional
super-exponential functional forms. The impact of these different
methods on resolved-scale ocean processes is examined in the context of
two test-bed cases of a wave-current coupled system: (1) a shallow water
inlet test case and (2) an idealized deep water hurricane case. In case
(1), tidal currents can modify the waves and significantly affect Stokes
drift computed from the wave spectrum. In both cases, large deviations
in ocean current response are produced when the Stokes drift is
approximated monochromatically from bulk wave parameters, rather than
from integration over the wave spectra. Deep water simulations using the
two super-exponential approximations are in better agreement with those
estimated from wave spectra than are those using the monochromatic,
exponential profile based on bulk wave parameters. In order to represent
the impact of Stokes drift at resolved scales, we recommend that for
studies of nearshore processes and brief deep water events, like
wave-current interactions under storms, the Stokes drift should be
calculated from full wave spectra. For long simulations of open ocean
dynamics, methods using super-exponential profiles to represent
equilibrium wind seas might be sufficient, but appear to be marginally
more computationally efficient.