The impact of Typhoon Jangmi (2008) on the midlatitude flow. Part I: Upper‐level ridgebuilding and modification of the jet

Abstract

AbstractThis study identifies the key physical process that governs the direct impact of the extratropical transition (ET) of Typhoon Jangmi on the midlatitude flow in September 2008. Jangmi was the strongest tropical cyclone (TC) in the western North Pacific in 2008 and occurred during the THORPEX Pacific Asian Regional Campaign (T‐PARC). A distinct outflow–jet interaction resulted in ridgebuilding directly downstream of the ET system and a strongly accelerated upper‐level jet streak over Japan. A combination of model and observational data is used to investigate the interaction of the transforming TC with the midlatitude flow and a potential vorticity (PV) surgery technique based on PV inversion is developed to further quantify the contribution of the TC to the modification of the midlatitude flow during ET.The joint interaction of the TC circulation with the midlatitude baroclinic zone and of the TC outflow with the upper‐level jet is found to be responsible for a substantial diabatically enhanced vertical transport of lower tropospheric air that arrives at jet level with nearly zero PV. This process explains the ridgebuilding and evolution of the jet streak to the northeast of the ET system. The quantification of Jangmi's role in the outflow–jet interaction revealed the diabatic reduction and isentropic transport of upper‐tropospheric PV into the jet region as the key physical process responsible for the direct impact of ET. This is manifested in a ridgebuilding with upper‐level PV being 6 PVU lower and a midlatitude jet streak being at least 25 m s−1 faster in the presence of Jangmi. The study corroborates the crucial role that ET has on the modification of the midlatitude flow directly downstream of a transforming TC. The remote, downstream impact of the ET of Jangmi is investigated in a companion paper.