Sudden stratospheric warmings (SSWs) are midwinter events in which the

primary stratospheric circulation, which is characterised by a strong

cyclonic vortex over the polar cap, abruptly breaks down leading to an

explosive warming of the polar stratosphere. SSWs modify the circulation

throughout the stratospheric column. Furthermore, it is now well established

that they tend to modify the large-scale circulation near the surface for up

to 2 months following the stratospheric event. Fundamentally, SSWs are a

manifestation of anomalously strong two-way interactions between upward

propagating planetary waves and the mean flow. However, the conditions that

trigger anomalously strong wave-mean flow coupling leading to an SSW are

still not well understood. While tropospheric precursors to SSWs have often

been noted (e.g., blocking), SSWs have also been shown to spontaneously

arise due to fortuitous coupling of a fixed wave field provided by the

troposphere and the concurrently evolving state of the stratosphere. Here we

present evidence based on reanalysis data and climate model simulations that

the explosive dynamics associated with SSWs primarily take place within the

stratosphere. Anomalous upward wave fluxes from the lower troposphere may

play a role for some events, but seem less important for the majority of

them. The crucial dynamics for forcing SSWs appear to take place across the

''communication layer'' just above the tropopause. Consequences for

stratosphere-troposphere coupling will be discussed.