A Climatological Perspective on the Role of the “Phasing” of Tropical Cyclones and Midlatitude Flow Features during Extratropical Transition

Extratropical Transition (ET) describes the transition of a Tropical Cyclone (TC) into an extratropical system, together with the physical and dynamical processes governing the interaction of the former TC with midlatitude flow features such as upper level (UL) troughs. ET may significantly alter the hemispheric-scale flow configuration and trigger high impact weather locally and in downstream regions. Previous studies based on individual cases showed that ET is very sensitive to the phasing between the TC and midlatitude flow features (e.g. Ritchie and Elsberry, 2007; Grams et al., 2013). However, so far there is no detailed climatological study investigating this aspect from a geometrical perspective.

This project aims to give a comprehensive picture on the role of the phasing between the ET system and midlatitude flow features. A novel diagnostic is developed, allowing to classify ET cases with respect to features observed in the geometry of the large-scale flow, such as hyperbolic stagnation points associated with midlatitude upper-level troughs. The relative position of a transitioning TC to such hyperbolic stagnation points is thought to govern track bifurcation in the frame of reference moving with the trough and to determine the impact of ET (e.g. Riemer and Jones, 2014). The diagnostic is applied to all recurving Western North Pacific TCs in the month of September using IBTrACS cyclone track and ERA Interim reanalysis data, allowing to characterize each ET case in terms of the phasing, TC characteristics (e.g. intensity, size), and midlatitude flow properties (e.g. size, shape, trough phase speed, Rossby wave activity).

This approach allows a systematic investigation of the role of the phasing and of TC and midlatitude flow characteristics in the triggering or amplification of downstream Rossby waves, associated high impact weather, and increased forecast uncertainty during ET.