The relation between wave forcing characteristics and prolonged sudden stratospheric warmings

Midwinter polar stratospheric variability is dominated by wave driving events wherein tropospherically generated planetary waves with large zonal scales break in the stratosphere. Approximately once every second winter in the Northern Hemisphere, such wave driving forces a sudden stratospheric warming (SSW), an event characterized by significant temperature increases and by zonal wind reversal to easterlies. Dramatic cases of SSWs also display a prolonged period during which vertical propagation of planetary waves is inhibited due to the concomitant easterlies. Given that the existence of easterlies is a necessary condition for all SSWs, it is not immediately clear what differentiates SSWs with and without such long periods of wave inhibition.

Previous work has demonstrated a propensity for SSWs to be forced by wave pulses of moderately large amplitude and long (order 1 week) time scales, but this need not hold on an individual SSW basis. We thus utilize both reanalyses and idealized modeling to show how characteristics of the wave forcing – such as the wave forcing time scales – that leads to a SSW relate to the persistence of critical layers to wave propagation. The model here is the GFDL dry dynamical core primitive equation model with initialization and forcing similar to previous work by Kushner, Polvani, and Gerber, including simplified bottom boundary topography. Initial results suggest a relation between the persistence of SSWs and extended periods of wavenumber 2 forcing.