Sudden Stratospheric Warming Formation in an Idealized GCM Using Three Types of Tropospheric Forcing

Sudden stratospheric warmings (SSWs) are the dominant sources of variability in the Northern Hemisphere winter stratosphere, but exactly what causes SSWs is not fully understood. In this presentation we use tropospheric heating perturbations and topography to create Northern Hemisphere winter-like stratospheric variability in an idealized atmospheric GCM, and investigate the extent to which SSWs are caused by anomalous tropospheric wave fluxes. Wave-1 and wave-2 heating perturbations as well as wave-2 topography are used. With appropriate choices of amplitudes, the three forcings produce reasonable SSW frequencies. It is found that large numbers of both split and displacement sudden warmings occur when the model is forced by heating perturbations, regardless of the wavenumber of the forcing. This is different from the wave-2 topographic forcing, which produces almost only splits. We find that SSWs in this model can form both as a direct result of anomalous tropospheric wave activity and due to internal stratospheric processes which alter the propagation of tropospheric wave flux into the stratosphere, and that the fraction of the two mechanisms is similar to that of the observed atmosphere for all three forcings. The processes by which splits and displacements are formed are found to be dependent on the wavenumber of the forcing. We further investigate the circulation differences associated with splits and displacements, and find that splits and displacements have different zonal mean surface signatures when the model is forced by wave-1 heating.