Abstract:
Third order spectra computed from the rotary components of
winds and currents at the TOTEM buoy site off the Oregon coast provide
an insight to the nonlinear, quadratic interactions within and between
these vector fields. The topography of bispectra and cross-bispectra
as displayed on four tri-frequency planes shows that significant nonlinear
interaction takes place between triples of oscillatory components
over wide ranges of frequencies.
A quantitative evaluation of the energy transferred linearly and
non-linearly from wind to current is obtained through the development
of a set of general, complex-valued transfer functions for two-dimensional
vector random processes.
Using a postulated "cause-effect" relation between measured
surface wind stress and current at 14 m (within the mixed layer), the
inertial frequency energy in the current field is attributed solely to
the non-linear transfer of momentum from the wind field. The
transfer occurs from multiple pairs of frequencies (in the intermediate
frequency range) in the wind field, with the dominant transform
attributed to the wind stress oscillations at half-day and quarter-day
periods. In the intermediate frequency range, 63% of the total
kinetic energy in the 14 m current is derived from the wind stress
through nonlinear processes, but only about 7% of the total kinetic
energy in the current at 34 m, below the mixed layer, is related
indirectly to this source.
