Plane internal gravity wave ($e^{i({\mathbf{k}}_h\cdot {\mathbf{x}}_h+mz-\omega t)}$)

A monochromatic internal gravity wave in uniform stratification has a spatially linear phase. In the nonrotating inviscid Boussinesq approximation, its dispersion relation is ${\omega }^2=N^2|{\mathbf{k}}_h{|}^2/(|{\mathbf{k}}_h{|}^2+m^2)$. The wave vector is normal to its phase planes, while the group velocity lies along them.

The displayed displacement convention is the negative of the usual positive-frequency wave pseudomomentum $k\overline{E}/\omega$ for a rightgoing internal gravity wave. Here energy per unit reference mass density is $\overline{E}=|\hat{\sigma }{|}^2/(2N^2)$. From the buoyancy polarization of a plane internal gravity wave, horizontal averaging gives $\overline{{\zeta }_{x}p}=km|\hat{\sigma }{|}^2/(2N^2{K}^2)=c_{gz}P$, where $p$ is kinematic pressure. The sign convention changes both density and flux and leaves their propagation at the group velocity unchanged.

With weak kinematic viscosity $\nu$, zero buoyancy diffusion and locally unchanged plane internal gravity wave polarization, the displacement pseudomomentum of an internal gravity wave obeys $P_t+{\partial }_z(c_{gz}P)=-\nu K^{2}P$. For constant wavenumbers and a stationary wave envelope, $P\propto |\hat{\sigma }{|}^2$ proves the displayed amplitude decay. Decay is along the group velocity, so $c_{gz}<0$ describes a source above the receiving level. In a slowly varying shear flow, use wave action and the changing intrinsic frequency rather than treating $P/|\hat{\sigma }{|}^2$ as constant.

At a jump in buoyancy frequency with continuous background density, incident, reflected and transmitted internal gravity waves share frequency and horizontal wavenumber. Continuity of pressure and normal velocity determines their amplitudes. If amplitudes multiply unit particle-displacement vectors directed along each ray with positive horizontal component, the reflection coefficient is $[\sin {\theta }_2/\sin {\theta }_1-\cos {\theta }_2/\cos {\theta }_1]/[\sin {\theta }_2/\sin {\theta }_1+\cos {\theta }_2/\cos {\theta }_1]$. Using vertical-displacement amplitudes reverses the reflected coefficient's sign because the reflected polarization reverses its vertical component.

A reflecting bottom beneath a jump in buoyancy frequency creates a standing field from repeated transmitted and reflected internal gravity waves. For ray angles ${\theta }_1=\pi /3$ and ${\theta }_2=\pi /6$, the downward particle-displacement amplitude has magnitude $\sqrt{3}|{\eta }_i|/[1+8{\sin }^2(m_{2}H){]}^{1/2}$. It reaches $\sqrt{3}|{\eta }_i|$ when $m_{2}H$ is an integer multiple of $\pi$. The upper reflected wave has the same amplitude magnitude as the incident wave because the lossless closed cavity admits no net downward energy flux.

For a plane internal gravity wave with vertical displacement $\zeta$, linear density advection gives ${\rho }^{\prime }=-{\bar{\rho }}_z\zeta$. In a constant stable gradient, ${\rho }_z={\bar{\rho }}_z(1-{\zeta }_z)$, so static overturning begins when the maximum of ${\zeta }_z$ exceeds one. A monochromatic displacement has maximum $|m{A}_{\zeta }|$. The criterion predicts the failure of the small-amplitude field, rather than a valid continuation of linear theory beyond overturning.