Infrasound Observations of Variability During Stratospheric Warmings

Abstract

Observations of natural infrasound produce a continual record of the sound velocity, a function of wind and temperature, at the reflection level in the upper atmosphere. Under normal conditions in winter the reflection level, for sound generated by ocean waves to the east of Palisades, N. Y., is in the lower thermosphere. During the circulation changes associated with stratospheric warmings, winds near the stratopause may become east or north, allowing infrasound to be reflected from this level. We are then provided with a continuous record of sound velocity near the stratopause. The methods which are used to distinguish between stratosphere and thermospheric sound reflection are discussed, and circulation changes for each year are cataloged. During the warming event sound velocities in the stratosphere are shown to vary radically, with fluctuations of up to 60 m s?1 in a few hours time period. These short time period variations, observable only because of the continuous nature of infrasound recording, are greater than expected and indeed constitute a significant fraction of the total wind and temperature variation associated with the event at our latitude. As such they imply significant energy variations on shorter time scales than those usually considered important in stratospheric dynamics. Some possible explanations for these observations are given.