Extreme upper level cyclonic vorticity events in relation to the Southern Hemisphere jet stream

The mean seasonal variation in the frequency of occurrence of extreme upper level cyclonic vorticity events, and its relation to the jet stream, is examined in the Southern Hemisphere. During the austral summer to fall, extreme cyclonic vorticity occurs most frequently at the upper level jet stream core, while during the austral winter to spring, there is a main peak on the poleward flank of the subtropical jet and a secondary peak on the poleward flank of the eddy-driven jet. Composite analysis shows that the extremes in both seasons are associated with wave breaking and the formation of elongated vorticity tongues. In summer, extreme events occur when waves propagating on the eddy-driven jet break nonlinearly, while in winter, extreme events occur when waves on the eddy-driven jet interact with waves on the subtropical jet. In both seasons, these extreme upper level vorticity events are associated with significant positive precipitation anomalies and a pattern of alternating positive and negative surface temperature anomalies.