Is stratospheric variability uniquely controlled by tropospheric wave forcing?

Recently, the authors showed that coherent, internal modes of stratospheric variability, observed previously in highly truncated models, are also found in a fully three dimensional (idealized) model of the stratosphere. These internal modes of stratospheric variability consist of downward propagating patterns of zonal wind anomalies that closely resemble those found recently in the observations, and are associated with large variability in the upward EP fluxes into the stratosphere. Since the lower boundary condition in this model is time independent, one is lead to conclude that the stratosphere itself is able to control the amount of wave flux that can enter into it.

This result naturally leads one to ask to what extent the observed stratospheric variability is controlled by internal stratospheric dynamics, as opposed to being driven by the tropospheric variability. This question is here addressed using a simple variation of the model configuration used to identify the original, internal modes of stratospheric variability. Instead of being time independent, the tropospheric wave forcing is now allowed to vary in a prescribed manner as a simple, controlled representation of natural tropospheric variability.

Comparing the characteristics of the stratospheric variability obtained with and without tropospheric variability provides an estimate of the importance of the latter. We consider both periodic and random modulations of the tropospheric wave forcing, with the modulation period and amplitude as external control parameters, and use a frequency decomposition to illustrate the character of the stratospheric response. For periodic modulation, although the response typically becomes frequency locked, the character of the variability strongly resembles the internal variability found with in the absence of tropospheric modulation. For random tropospheric modulation, significant power persists at the internal frequency, even for large modulation amplitudes. The results suggest that, even in the presence of large time-dependent external forcing, internal dynamical modes play a fundamental role in determining the total stratospheric variability.