Oceanic Internal Wave Field: Theory of Scale-invariant Spectra

TL;DR

This paper investigates scale-invariant solutions of a kinetic equation for oceanic internal waves, revealing divergence issues, identifying convergent solutions near the Garrett--Munk spectrum, and exploring effects of Earth's rotation.

Contribution

It introduces a detailed analysis of divergence in scale-invariant wave turbulence spectra and finds convergent solutions close to observed oceanic internal wave spectra, including effects of rotation.

Findings

Divergence in collision integral for most spectral exponents.

Existence of a convergent power-law solution near Garrett--Munk spectrum.

Rotation introduces an induced diffusion regime.

Abstract

Steady scale-invariant solutions of a kinetic equation describing the statistics of oceanic internal gravity waves based on wave turbulence theory are investigated. It is shown in the non-rotating scale-invariant limit that the collision integral in the kinetic equation diverges for almost all spectral power-law exponents. These divergences come from resonant interactions with the smallest horizontal wavenumbers and/or the largest horizontal wavenumbers with extreme scale-separations. We identify a small domain in which the scale-invariant collision integral converges and numerically find a convergent power-law solution. This numerical solution is close to the Garrett--Munk spectrum. Power-law exponents which potentially permit a balance between the infra-red and ultra-violet divergences are investigated. The balanced exponents are generalizations of an exact solution of the scale-invariant kinetic equation, the Pelinovsky--Raevsky spectrum. A balance between oppositely signed divergences states that infinity minus infinity may be approximately equal to zero. A small but finite Coriolis parameter representing the effects of rotation is introduced into the kinetic equation to determine solutions over the divergent part of the domain using rigorous asymptotic arguments. This gives rise to the induced diffusion regime. The derivation of the kinetic equation is based on an assumption of weak nonlinearity. Dominance of the nonlocal interactions puts the self-consistency of the kinetic equation at risk. Yet these weakly nonlinear stationary states are consistent with much of the observational evidence.