Observational Evidence of Internal Inertia-Gravity Waves in the Tropical Stratosphere

Abstract

Twenty-nine highly accurate wind profiles up to 30 km obtained during four days of the GARP Atlantic Tropical Experiment (GATE) by tracking vertically ascending balloons with radar are presented and analyzed. The existence of short, vertical wavelength easterly waves propagating upward in the tropical stratosphere, as suggested by a quick look at the profiles, is demonstrated and the wave characteristics are determined. The period of the waves is determined from the power spectra of the u (eastward) and v (north-ward) components at different heights, computed by the lag correlation method and from the rotary spectra computed by the maximum entropy method (35 h). Clockwise rotation of wind vector with time at various levels indicates westward propagation of the phase relative to the medium. Cross spectra between different levels are computed and phase differences demonstrate the downward propagation of phase. From the chronological sequence of u and v profiles, the vertical profile of vertical phase speed is determined and the dependence of this wave parameter on the mean u profile is shown. As the vertical wavelength of a wave propagating upward in a changing wind profile is modified, a spectrum analysis taking this property into account has been performed. Then the vertical wavelength in a zero-wind profile is determined (5 km±1 km). The waves may correspond to an internal inertia-gravity mode (n=1) suggested by theoretical studies on tropical waves but never resolved in radiosonde data. The wave characteristics as determined by the theory of tropical stratospheric waves agree well with those observed in this study. Some results strongly support the idea of wave generation by jet stream instabilities. As the profile of vertical phase speed shows absorption of the waves at a given level above the jet stream, the relationship with the quasi-biennial oscillation of the tropical stratosphere is investigated. The acceleration of the mean flow due to the waves is computed based on several assumptions; the maximum acceleration is about ?5 m s?1 (32 days)?1.