The circulation response to time-dependent tropospheric wave forcing in a simple general circulation model of the stratosphere

Using a simple, stratosphere-only GCM, we have recently shown that robust coherent, internal modes of stratospheric variability, (observed earlier in highly truncated models) exist when all forcings are time-independent. These purely internal modes of stratospheric variability consist of downward propagating patterns of zonal wind anomalies that closely resemble observations and are associated with large variability in the upward EP fluxes into the stratosphere.

Here we extend that study and explore the case when the forcing at the lower boundary of the model is time dependent. The idea is to probe whether the internal variability of the stratosphere survives once the wave forcing from the troposphere is time dependent. Contrasting the characteristics of the stratospheric variability for steady and time-dependent tropospheric forcing 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. For periodic modulation, although the response can become frequency locked, the character of the variability strongly resembles the internal variability found in the absence of tropospheric modulation. Similarly, with random tropospheric modulation, significant power again persists at the internal frequency, even for large modulation amplitudes. In general, the stratospheric response appears to depend on an effective steady forcing equal to the rms of the time varying forcing, with the response period and amplitude similar to the corresponding internal mode. Our 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.