Dynamics and Predictability of Downward-Propagating Stratospheric Planetary Waves Observed in March 2007

Abstract

The predictability of a downward-propagating event of stratospheric planetary waves observed in early March 2007 is examined by conducting ensemble forecasts using an AGCM. It is determined that the predictable period of this event is about 7 days. Regression analysis using all members of an ensemble forecast also reveals that the downward propagation is significantly related to an amplifying quasi-stationary planetary-scale anomaly with barotropic structure in polar regions of the upper stratosphere. Moreover, the anomaly is 90° out of phase with the ensemble-mean field. Hence, the upper-stratospheric anomaly determines the subsequent vertical-propagating direction of incoming planetary waves from the troposphere by changing their vertical phase tilt, which depends on its polarity. Furthermore, the regressed anomaly is found to have similar horizontal structure to the pattern of greatest spread among members of the predicted upper-stratospheric height field, and the spread growth rate reaches a maximum prior to the occurrence of the downward propagation. Hence, the authors propose a working hypothesis that the regressed anomaly emerges as a result of the barotropic instability inherent to the upper-stratospheric circulation. In fact, the stability analysis for basic states constituting the ensemble-mean forecasted upper-stratospheric streamfunction field using a nondivergent barotropic vorticity equation on a sphere supports this hypothesis. Thus, the barotropic instability inherent to the distorted polar vortex in the upper stratosphere forced by incoming planetary waves from the troposphere determines whether the planetary waves are eventually absorbed or emitted downward in the stratosphere.