Simulations of stratospheric sudden warmings in the Berlin troposphere-stratosphere-mesosphere GCM

Stratospheric sudden warming events in theNorthern Hemisphere of the Berlin TSM GCM are investigated. In about 50% of thesimulated years (13 out of 28), major midwinter warmings occur. This agrees wellwith observations but, whereas real events tend to occur approximately everysecond season, those in the model are clustered, most of them occur in theperiod between years 15/16 and years 24/25. In most other years, minor warmingevents take place. The warming events are found earlier in the winter than inreality. Many of the observed characteristics of warming events are wellcaptured by the model: pulses of wave activity propagate out of the troposphere;these transient events force the zonal-mean zonal wind in the stratosphere andcoincide with increases of the temperature at the North Pole and cooling at lowlevels in the tropics; temperature changes of opposite sign are modelled athigher levels. Synoptically, the modelled stratosphere evolves quiterealistically before the warmings: the cyclonic vortex is displaced from thePole by an amplifying anticyclone. After minor warmings, the stratosphereremains too disturbed as the cyclonic centre does not return to the North Poleas quickly as in reality. In the aftermath of major warmings the cyclonic vortexis not fully eroded and the anticyclonic circulation does not develop properlyover the Pole; furthermore, the wintertime circulation is not properly restoredafter the event