Synoptic climatology of extratropical transition of tropical cyclones over the Southern Hemisphere

Abstract. Tropical cyclones that move into the midlatitudes undergo changes in their structure and transition into extratropical cyclones. The process is known as extratropical transition (ET). ET can result in severe weather locally and also affect the weather downstream. Although the importance of ET has been recognised primarily in the Northern Hemisphere, there are only a handful of studies focusing on the Southern Hemisphere. The current study conducts a comprehensive synoptic-climatological analysis of ET over the Southern Hemisphere. We use a state-of-the-art low-pressure system detection and classification scheme to objectively track tropical cyclones and detect those that undergo ET based on ERA5 data. Our results show that ET preferentially occurs in the southwest Indian Ocean, off the northwest coast of Australia, and in the southwest Pacific. The ET fraction is higher in March–May and lower in January and February, and the latitude of ET also changes strongly with season. The observed seasonality is associated with meridional shifts in the large-scale circulation and sea surface temperature pattern. The changes in structural characteristics and background environment during ET are investigated via cyclone-centred composites. In general, the transitioning cyclone lies on the equatorward side of the jet entrance, with an upper-level trough approaching from the west and a ridge developing downstream. Highly asymmetric fields of vertical velocity and equivalent potential temperature advection are indicative of warm, moist, ascending (cold, dry, descending) air to the east (west), responsible for an increasingly asymmetric precipitation pattern. Case-to-case variability of the ET structure is further examined by clustering ET events into four clusters. In particular, Clusters 2 and 3 feature the transitioning cyclone with a relatively strong intensity and high precipitation, accompanied by enhanced latent heat release in its southeastern sector. In the upper troposphere, the cyclone-associated divergent outflow impinges on the waveguide and enhances the potential vorticity gradient, leading to downstream jet streak formation and contributing to ridge development.