Downstream development of extratropical transition of tropical cyclones over the Southern Hemisphere

Abstract. Tropical cyclones (TCs) occasionally move into the midlatitudes and transition into extratropical cyclones. The process is known as extratropical transition (ET), which can affect the weather further downstream. The present study conducts a climatological analysis of the dynamics of the downstream development associated with ET over the Southern Hemisphere. These ET events are objectively identified and further grouped into four distinct ET clusters using K-means clustering. ET events in clusters 2 and 3 cause pronounced downstream development. The dynamics of the TC-midlatitude flow interaction are diagnosed from both potential vorticity (PV) and eddy kinetic energy (K_e) perspectives. From the PV viewpoint, the anticyclonic PV tendency due to the PV advection by diabatically-driven divergent outflow contributes substantially to downstream ridge amplification, supplemented by the nonlinearities arising from PV anomalies advected by flow induced by PV anomalies. From the K_e viewpoint, the transitioning TC injects additional K_e through baroclinic conversion into the midlatitude flow. The energy is then distributed by ageostrophic geopotential fluxes downstream, thereby amplifying the downstream wave packet. We identified two different pathways of downstream development. One pathway (cluster 2) exhibits a classic picture of downstream development, with Rossby wave packets amplifying and propagating farther downstream along a strong and straight jet, whereas the other pathway (cluster 3) is more localised and characterised by a pronounced ridge immediately downstream. These distinct pathways appear to be predominantly determined by the characteristics of the jet, in which the more pronounced downstream ridge is associated with a weak, short, and poleward-orientated jet. Furthermore, our results show that the strong ET events in the southwest Indian Ocean downstream can cause significantly enhanced high-impact weather frequency over Australia.