The Role of Eddy–Eddy Interactions in Sudden Stratospheric Warming Formation

The effects of eddy-eddy interactions (EEI) on the zonal mean flow can be removed from model simulations by calculating the advection of eddy fluctuations by eddy winds and replacing them with their zonal mean values. We use this method to investigate the role of EEI in sudden stratospheric warming (SSW) formation in an idealized GCM. Wave-1 and wave-2 heating perturbations are used to produce Northern Hemisphere winter-like stratospheric variability in the model. In addition to the control runs, model runs where EEI are removed throughout the depth of the atmosphere and in sections of the atmosphere are performed. The results show that SSW frequencies can be highly sensitive to EEI throughout the atmosphere, in non-predictable ways. Furthermore, it is shown that large changes in SSW frequencies can be obtained by removing EEI in the upper stratosphere only, even though the climatological wave forcing below remains the same as that of the control runs. Finally, we investigate the structure of the polar vortex during SSWs in the model runs. We find that the vortex does not shed filaments of high vorticity or produce splits and displacements in the absence of EEI. Instead, the vortex tends to cascade meridionally, despite the fact that the climatological wave forcing in the region is similar to that of the control runs. However; splits, displacements and shedding of filaments do occur when EEI are allowed only in the upper stratosphere, indicating that EEI in the troposphere and lower stratosphere are not necessary in order to obtain realistic SSWs. It is argued that displacements and splits can be considered local reactions to wave-1 and wave-2 forcing, respectively.