| Improvements in observational and numerical resolution of the ocean have led to a greater
interest in the role of large-scale coherent internal waves, particularly those generated by tidal
forcing over topography. Analysis of the global energy budget of the ocean has also identified
tidal forcing as a major energy input and has suggested internal wave breaking as a primary
mechanism for abyssal mixing. As a result, there has been a renewed interest in
these long scale waves and their transfer of energy to smaller scales, ultimately
leading to overturning and mixing. Recent observational and numerical studies
have shown that the majority of internal wave energy from generation sites have
wavelengths on the order of the deformation radius and can propagate across hundreds
and possibly even thousands of kilometers. Additionally, there is strong numerical
evidence that such waves are unstable in latitudes near 29-, where this coherence
contributes to an accelerated nonlinear cascade to smaller scales and increased abyssal
mixing.
One aspect of this problem which has not yet been addressed is the role of the geostrophic
mesoscale. Coherent internal waves propagating across the open ocean are likely to encounter
regions of vigorous geostrophic turbulence. We present numerical evidence showing that the
potential vorticity (PV) anomalies contained in large scale geostrophic currents are
responsible for the transfer of energy between different waves. In this way, a single coherent
signal propagating through geostrophic turbulence will tend to lose most of its energy to a
broad spectrum of gravity waves propagating in different directions, producing a
more isotropic and incoherent wavefield. Furthermore, this forcing is dominated
by resonant triad interactions within the nonlinear advection terms, where the PV
modes are essentially unchanged but act as catalysts for the redistribution of energy
within the wave spectrum. These results place constraints on the lifetime of these
tidally forced internal waves and the relative importance of their role in the ocean. |