Wednesday, 6 June 2001
The motion of an initially quiescent, incompressible,
stratified and/or rotating fluid of
semi-infinite extent due to surface forcing
is considered.
The stratification parameter N and the Coriolis parameter f
are constant but arbitrary and all possible combinations are
considered, including
N=0 (rotating homogeneous fluid) and f=0 (non-rotating
stratified fluid).
The forcing is suction or pumping at an upper rigid surface
and the response consists
of geostrophic flows and inertial-internal waves.
The response to finite-sized
circularly symmetric impulsive forcings is considered.
Initial conditions are unbalanced geostrophic vortices which
adjust through
radiation of internal waves.
At early times transient
internal waves change the vortices that are created by
pumping/suction at the
surface. The asymptotically remaining vortices are
determined, a simple expression
for what fraction of the initial energy is converted into
internal waves
is derived, as well as wave energy fluxes and the dependence
of the flux direction
on the value of N/f. The internal wave field is to leading
order in time a
distinct pulse, and rules for the arrival time of the pulse,
its amplitude,
its motion along a ray of constant frequency and decay with
time, are given for
the far-field. A simple formula for the
total wave energy distribution as a function of frequency is
derived for when
all waves have propagated away from the forcing. Remarkably,
the amplitude
of the vortices amplifies by a factor N/f after the
adjustment is completed,
which can put them well into the inertially (centrifugally)
unstable range if anticyclonic and N >> f.
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