Evolution of tsunami-induced internal acoustic-gravity waves

The authors present an idealized theoretical and numerical study of tsunami-induced internal waves in the atmosphere. These are gravity waves modified by acoustic effects that can propagate rapidly from the ocean surface up to the ionosphere, where they are well known to leave a detectable fingerprint in airglow patterns and other remote sensing observables. Accurate modeling of the wave propagation is a prerequisite for being able to detect and decode this transient observational fingerprint by remote sensing methods. The authors study this problem by formulating the initial-value problem for linear waves forced by an idealized tsunami at the lower boundary and then employing a semianalytic Fourier-Laplace method to solve it. This approach allows them to compute the detailed time evolution of the waves while ensuring that the correct radiation condition in the vertical is satisfied at all times, a nontrivial matter for these transient waves. The authors also compare the predictions of an anelastic model with that of a fully compressible model in order to discern the importance of acoustic effects. The findings demonstrate that back-reflection at the tropopause is a significant factor for the structure of these waves and that the earliest observable signal in the ionosphere is, in fact, a fast acoustic precursor wave generated by the nearly impulsive formation of the tsunami itself.