The preconditioning of major sudden stratospheric warmings

Abstract

The preconditioning of major sudden stratospheric warmings (SSWs) is investigated with two long time series using reanalysis (ERA‐40) and model (MAECHAM5/MPI‐OM) data. Applying planetary wave analysis, we distinguish between wavenumber‐1 and wavenumber‐2 major SSWs based on the wave activity of zonal wavenumbers 1 and 2 during the prewarming phase. For this analysis an objective criterion to identify and classify the preconditioning of major SSWs is developed. Major SSWs are found to occur with a frequency of six and seven events per decade in the reanalysis and in the model, respectively, thus highlighting the ability of MAECHAM5/MPI‐OM to simulate the frequency of major SSWs realistically. However, from these events only one quarter are wavenumber‐2 major warmings, representing a low (∼0.25) wavenumber‐2 to wavenumber‐1 major SSW ratio. Composite analyses for both data sets reveal that the two warming types have different dynamics; while wavenumber‐1 major warmings are preceded only by an enhanced activity of the zonal wavenumber‐1, wavenumber‐2 events are either characterized by only the amplification of zonal wavenumber‐2 or by both zonal wavenumber‐1 and zonal wavenumber‐2, albeit at different time intervals. The role of tropospheric blocking events influencing these two categories of major SSWs is evaluated in the next step. Here, the composite analyses of both reanalysis and model data reveal that blocking events in the Euro‐Atlantic sector mostly lead to the development of wavenumber‐1 major warmings. The blocking–wavenumber‐2 major warming connection can only be statistical reliable analyzed with the model time series, demonstrating that blocking events in the Pacific region mostly precede wavenumber‐2 major SSWs.