Stochastic Forcing of Small-Amplitude Oscillations in the Stratosphere

Abstract

A quasi-geostrophic ?-plane channel model is used to study the response of the stratosphere to planetary waves forced at the ground. The forcing consists of a standing field of eddy vertical velocity whose amplitude fluctuates randomly about a time-average value. Cross-spectrum analysis of model results reveals the presence of westward traveling waves and, among these, of normal modes in the model solution. The superposition of the traveling waves and a stationary wave maintained by the time-average forcing gives rise to oscillating eddy heat fluxes. The relationship of these fluxes to changes in the zonal-mean temperature gradient is investigated by means of the squared coherence statistic. We examine how the occurrence of high levels of coherence depends on the existence of the external normal mode and on the presence of noise introduced into the model output.