An Amplification Mechanism of Medium-Scale Tropopausal Waves

Abstract

The mechanism for the amplification of medium-scale tropopausal waves (with horizontal wavelengths of 2000?3000 km and wave periods of 1?2 days) that are basically neutral is investigated using a three-dimensional wave-activity flux. Distinctive upward wave-activity fluxes are found in the midtroposphere (400 hPa) upstream of the region where the medium-scale waves are dominant, that is, where the horizontal components of the wave-activity flux are large. These upward fluxes converge in the upper troposphere (300 hPa). This convergence is a major source of medium-scale waves, producing large values of the horizontal components of the wave-activity flux in the active regions. Examination of the spatial structure of the heat flux shows that the upward fluxes are due to occasional baroclinic interactions between upper-level medium-scale waves and lower-tropospheric disturbances with the same temporal scales. The seasonal and geophysical variations of the waves depend mostly on the amount of baroclinic coupling with the lower disturbances.