Spectrum of wave forcing associated with the annual cycle of upwelling at the tropical tropopause

The zonal-wavenumber spectrum of atmospheric wave forcing is examined in order to understand the annual cycle of the upwelling near the tropical tropopause. The tropopause-level upwelling is derived using angular momentum balance and mass conservation, and wave forcing is computed from ERA-Interim reanalysis for 34 years (1979-2012). The momentum-based calculation agrees well with thermodynamic-based estimates, and accurately captures the annual cycle in upwelling. The spectrum of wave forcing reveals that the annual cycle of upwelling is largely driven by zonal wavenumber-3 forcing in the Northern Hemisphere and wavenumbers 2-4 in the Southern Hemisphere. The Eliassen-Palm flux shows that the associated waves primarily originate from the Northern Hemisphere extratropics and partially from the Southern Hemisphere tropics during December-February. On the other hand, these wave activities are nearly absent during June-August. Wavenumber-1 and synoptic-scale waves have a significant contribution to the tropopause upwelling, but have little influence on the annual cycle. Calculations of the Rossby wave refractive index suggest that the wavenumber-3 wave can effectively force the shallow branch of the Brewer-Dobson circulation, as it is the lowest wave number that can not propagate deeply into the stratosphere, but instead is trapped and tends to break in the lowermost stratosphere near the subtropical jet.