A Numerical Simulation of the Two-Day Wave near the Stratopause

The middle atmosphere version of the National Center for Atmospheric Research Community Climate Model 2 (MACCM2) mechanistic model is used to simulate several aspects of the observed two-day wave near the stratopause and its possible connection with inertial instability. Model experiments show that, for a prescribed initial wind condition with strong horizontal wind variation in the Tropics, inertial instability can trigger the two-day wave. An increase in horizontal wind curvature fostered by inertial instability circulation barotropically destabilizes the easterly jet in low summer latitudes and allows first the growth of the wavenumber-4 and then the wavenumber-3 component of the two-day wave. Near the stratopause, the two-day wave Eliassen–Palm flux is directed equatorward away from the wave’s critical line source and westerly momentum is transported into the easterly jet core by the wave. While much of the wavenumber-4 energy is confined near the stratopause, the wavenumber-3 energy can propagate upward well into the mesosphere where strong Rayleigh damping is imposed. The model results suggest that the observed disparity in the wavenumber-3 amplitude between the austral and boreal summers is a consequence of the difference in summer easterly jet strength.