High-Altitude Responses to Tsunami Forcing: 1. Modeling the Thermospheric Response in the Presence of Tidal Winds

Abstract:

Tsunamis in the deep ocean are small-amplitude, long-wave, fast-propagating disturbances. Their small amplitudes (10’s of cm) make them difficult to detect and track in the ocean. Despite their small amplitudes, they generate vertically propagating atmospheric gravity waves. Decreasing atmospheric density with altitude and conservation of energy result in these initially small disturbances attaining appreciable perturbations in the thermosphere and ionosphere. Because these initial amplitudes are small, the subsequent wave propagation is linear, predisposing tsunami generated waves to survive into the thermosphere and ionosphere rather than breaking at lower altitudes. The large length-scales associate with tsunami-generated waves also contributes to their attaining high altitudes.

Using an anelastic finite-volume numerical model, we have performed simulations of tsunami-generated gravity waves and their propagation through a variety of background conditions in order to assess the viability of detecting and tracking tsunamis in the deep ocean. We have considered simple mean wind shears as well as representative oscillatory tidal structures. Results indicate that large-amplitude waves readily propagate to the ionosphere for a variety of wind profiles. The presence of turning levels in the wind profile partially block the waves, reducing but not eliminating the perturbations in the thermosphere and ionosphere. The wind's average shear both deforms the wave and, dependent on the wind direction, can enhance or reduce perturbation amplitudes.