Life cycle of stratospheric sudden warmings

Stratospheric Sudden Warmings (SSWs) are of great interest to the climate community for many reasons. They represent unusually fast dynamical events in the stratosphere, where typical time scales are months for the seasonal cycle and years for the overturning circulation. They also serve as excellent examples to study stratosphere-troposphere coupling, which is important in seasonal forecasting as well as climate change predictions. To complement reanalysis and comprehensive GCM studies, this work makes uses of a simple dry GCM with a realistic stratosphere to generate a database of over 1,500 SSWs. With this database, it is shown that in terms of average zonal mean time evolution, there is only little difference between SSWs as defined by either the WMO standard for major SSWs, the crossing of a threshold of the annular mode index, splitting or displacement events. Furthermore, eddy heat fluxes, although significantly extreme in the upper stratosphere during the event, do not show any strong increase in the troposphere prior to the onset date. Similarly, upward Eliassen-Palm fluxes show some “bursting” behavior in the troposphere, but this signal is smaller than its standard deviation over the full simulations. Finally, it is shown that there are no significant common features of SSWs that couple strongly to the troposphere before the onset date, or indeed anytime during their life cycle other than the trivial fact that the annular mode index in the troposphere becomes anomalously negative after the onset.