The role of the "phasing" of the tropical cyclone and the midlatitude flow during extratropical transition: T-PARC Typhoon Jangmi (2008) and other case studies

Tropical cyclones (TCs) that move polewards and recurve may transform into an extratropical cyclone and interact with the midlatitude flow. This interaction can result in a modification of the upper-level midlatitude Rossby wave guide and thus has implications for the weather and predictability in downstream regions such as North America or Europe. Albeit recurving and undergoing extratropical transition (ET), the downstream impact of Typhoon Jangmi that occurred in September 2008 during the THORPEX Pacific Asian Regional Campaign (T-PARC) was weak. In this study, the sensitivity of the downstream flow evolution on the relative position of Typhoon Jangmi and the midlatitude Rossby wave pattern is tested. Therefore, a TC relocation method based on potential vorticity inversion has been developed and applied upon the initial conditions of simulations with the COSMO model. The relocation experiments reveal two completely different scenarios. The first scenario reflects the actual decay of Jangmi accompanied by a broad ridge over the Pacific. The second scenario exhibits a strong (<970 hPa) extratropical reintensification of Jangmi, the triggering of a pronounced Rossby wave train and downstream cyclogenesis. The distance of Jangmi's initial position in the two contrasting scenarios is less than 1.25° (130 km). It turned out that in the frame moving with the environmental midlatitude background flow a critical bifurcation point for the track of Jangmi exists. The relative position to this bifurcation point determines if Jangmi decays or reintensifies. This behaviour gives a simple explanation for the reduced predictability of the midlatitude flow in downstream regions during ET. The findings are expanded on various other cases.