Interference of the Diurnal Tide and Normal-Mode Rossby Waves in the Summer Stratosphere

Abstract

An investigation was conducted into the interference among the tidal wave, the 5-day, and 10-day normal-mode Rossby waves in the summer upper stratosphere, making use of a general circulation model. It was found that the EP flux due to the interference has a period of slightly longer than one day, and that two kinds of periodic structures of the EP flux also exist, having about 10-day periods. One structure is an amplitude modulation of the quasi-diurnal variation while the other is a long-term variation in the low-pass filtered data. It was also found that the acceleration of the zonal-mean zonal wind is mainly composed of the zonally symmetric diurnal tide. The EP flux due to the diurnal tide for zonal wavenumber 1 and the normal-mode Rossby waves of zonal wavenumber 1 plays a minor role in the acceleration of the zonal-mean zonal wind. When the Rossby waves simultaneously amplify, however, the EP flux brings about a noticeable long-term forcing of the zonal-mean zonal wind.
The interference of the tide and normal-mode Rossby waves appearing in the NMC analysis data were also examined. The diurnal tide was analyzed as a quasi-stationary wave due to the once-daily sampling. It was found that the observed 5-day and 10-day variations in the EP flux are superficial and result from aliasing phenomena due to the once-daily sampling.