The impact of Typhoon Jangmi (2008) on the midlatitude flow. Part II: Downstream evolution

Extratropical transition (ET) of a tropical cyclone (TC) can modify the midlatitude flow and thus have implications for the weather and predictability in downstream regions such as North America or Europe. Albeit recurving and undergoing ET, the downstream impact of Typhoon Jangmi in September 2008 during the THORPEX Pacific Asian Regional Campaign (T-PARC) was weak. In this study, we explore the physical processes inhibiting the reintensification of Jangmi and governing the forecast uncertainty during ET. The focus is on the relative contribution of the TC and the midlatitude flow. Quasi-geostrophic diagnostics reveal that Jangmi is located south of and in phase with a midlatitude trough. Jangmi does not couple with the region favourable for reintensification ahead of the trough and subsequently decays. Instead, a weak frontal wave-like cyclone develops. The role of the phasing of Typhoon Jangmi and the midlatitude Rossby wave pattern is tested in sensitivity experiments with the COSMO model in which Jangmi has been relocated. The simulations reveal two contrasting groups of scenarios. The first group reflects the weak actual ET and subsequent decay accompanied by a broad ridge over the Pacific. The second group is characterized by a strong ET with extratropical reintensification, the triggering of a pronounced Rossby wave train and downstream cyclogenesis. The displacement of Jangmi's initial position in the two contrasting groups is less than 1.25 degrees (130 km). In the frame moving with the environmental background flow a critical bifurcation point for the track of Jangmi exists. The relative position to this bifurcation point determines whether Jangmi decays or reintensifies and triggers a Rossby wave train. A simple discrimination parameter is derived that allows for classifying the scenarios. The observed behaviour gives a simple explanation for the reduced predictability of the track and of the midlatitude flow in downstream regions during ET.