A Composite Perspective of the Extratropical Flow Response to Recurving Western North Pacific Tropical Cyclones

Abstract

his study investigates the composite extratropical flow response to recurving western North Pacific tropical cyclones (WNP TCs), and the dependence of this response on the strength of the TC?extratropical flow interaction as defined by the negative potential vorticity advection (PV) by the irrotational wind associated with the TC. The 2.5° NCEP?NCAR reanalysis is used to construct composite analyses of all 1979?2009 recurving WNP TCs and of subsets that undergo strong and weak TC?extratropical flow interactions.Findings indicate that recurving WNP TCs are associated with the amplification of a preexisting Rossby wave train (RWT) that disperses downstream and modifies the large-scale flow pattern over North America. This RWT affects approximately 240° of longitude and persists for approximately 10 days. Recurving TCs associated with strong TC?extratropical flow interactions are associated with a stronger extratropical flow response than those associated with weak TC?extratropical flow interactions. Compared with weak interactions, strong interactions feature a more distinct upstream trough, stronger and broader divergent outflow associated with stronger midlevel frontogenesis and forcing for ascent over and northeast of the TC, and stronger upper-level PV frontogenesis that promotes more pronounced jet streak intensification. During strong interactions, divergent outflow helps anchor and amplify a downstream ridge, thereby amplifying a preexisting RWT from Asia that disperses downstream to North America. In contrast, during weak interactions, divergent outflow weakly amplifies a downstream ridge, such that a RWT briefly amplifies in situ before dissipating over the western-central North Pacific.