Atmospheric Gravity Waves and Effects in the Upper Atmosphere Associated with Tsunamis

Abstract

Tsunamis propagate at the surface of the deep ocean horizontal phase speeds of approximately 200 m/s, which is about two-thirds of the lower atmospheric sound speed. They have large horizontal wavelengths that are typically of a few hundred kilometers, and they remain coherent over large propagation distances. They also have large horizontal extents (sometimes a few thousand kilometers) parallel to their wave fronts. They can traverse great distances over a span of several hours, so that large areas of the oceanatmosphere interface are impacted. Typical dominant wave periods associated with tsunamis are a few tens of minutes. In the deep ocean their amplitudes are usually quite small with surface displacements being only a few centimeters, but occasional large events can have amplitudes of a few tens of cm. The speeds, wavelengths and periods of tsunamis lie within the range of those of atmospheric gravity waves. These are vertically transverse waves with motions of air parcels mainly influenced by gravity and buoyancy. The vertical displacement of the water acts like a moving corrugation at the base of the atmosphere and so very effectively generates atmospheric gravity waves. In general a spectrum of waves will be produced by a tsunami. Most of the power in the spectrum resides in internal gravity waves, with acoustic waves and evanescent waves being less efficiently generated. Internal waves can transport energy and momentum vertically through the atmosphere. Due to the decrease of mean atmospheric density with increasing altitude, the amplitude of these waves increases as they propagate upward in order to conserve wave energy. At sufficiently high altitudes molecular viscosity and thermal conductivity damp the waves, and their amplitudes then decrease with increasing altitude. Because the waves have high phase speeds (commensurate with the tsunami speed), they are deep waves with vertical wavelengths of ~ 100 km. This allows them to reach the middle thermosphere (~ 250 km altitude) before the molecular dissipation becomes severe. Atmospheric winds also influence the upward propagation of atmospheric gravity waves. Because the winds vary with height the waves may be propagating with the wind at some heights and against the wind at other heights. In the former case the vertical wavelengths are shortened, which increases the velocity shears and thereby increases the viscous damping rate. In the latter case the vertical wavelengths are increased, which decreases the velocity shears and decreases the viscous damping rate. At these heights the tsunami-driven atmospheric gravity waves have large amplitudes so that their interaction with the ionosphere is likely to produce detectable traveling