Sudden stratospheric warmings (SSWs) are dynamically driven events in the polar winter stratosphere that are associated with abrupt regime shifts from strong westerly flow, around the so-called polar vortex, to weak westerlies or even easterlies, as well as a sudden increase in polar cap temperature. SSWs lead to as much as 50 K warming within a few days with important consequences for ozone chemistry amongst other things. Furthermore, there has been evidence in recent years of downward propagating wind and temperature anomalies associated with SSWs that can reach the troposphere with a potential to alter surface weather and climate on seasonal time scales.

After an introduction to relevant aspects of stratospheric dynamics and climate this talk will discuss simple models of stratospheric wave-mean flow interaction suitable to represent SSWs. Conventionally, SSWs are thought to be caused by amplifying planetary-scale waves propagating upward from the troposphere into the stratosphere. Here it will be shown in the context of a low-order model of stratospheric wave-mean flow interaction that small-scale variability, such as related to gravity waves, can trigger SSWs given an amount of planetary wave activity that is by itself not sufficient to cause a SSW.