Inertial Gravity Waves in Eddying Flows

Recent estimates based on high-resolution OGCMs suggest that about
three fourth of the kinetic energy of oceanic motions reside in
time-varying flows, predominantly related to mesoscale eddies. Since
mesoscale eddies tend to transfer energy upscale within the paradigm
of geostrophic turbulence, the constant generation through barotropic
and baroclinic instabilities of large-scale circulations must be
balanced by processes that transfer energy from mesoscales to smaller
scales, at which small-scale turbulence can complete the down-scale
cascade of energy to dissipation. Here we investigate a route to
dissipation via IGWs that are emitted by eddying flows and propagate
subsequently within the eddying flows. The investigation is based on a
0.1-degree simulation, in which the general circulations including the
related eddy field are realistically simulated, but leaving out tides
and, to a considerable degree, also wind-induced near-inertial waves. We
show that this route to dissipation is capable of transferring energy
from mesoscale eddies to the smaller scales, despite of the fact
that the wave emission can be small for flows having small Rossby
numbers. We argue that the wave intensity is not only determined by
the wave generation itself. After being generated by eddying flows,
the waves, presumably low-mode internal waves, will propagate within
the flows, whereby being refracted and captured by the flows. During
the wave capture process, the intrinsic group velocity vanishes and
the wavenumber and wave amplitude grow exponentially. It is this wave
capture process that makes the emitted waves 'visible' and allows the
energy transfer to smaller scales, whereby supplying power needed for
interior mixing.