An observational study of the downstream Rossby wave response to recurving western North Pacific tropical cyclones

Tropical cyclones (TCs) that recurve into the extratropical western North Pacific (WNP) can excite or amplify Rossby wave trains that may persist on time scales of a week or longer. Such Rossby wave trains can contribute to reductions in regional and hemispheric model forecast skill and can serve as precursors to high-impact weather events over North America. The downstream Rossby wave response to a recurving western North Pacific TC depends on various factors, including characteristics of the North Pacific jet stream and the presence of a preexisting wave packet interacting with the TC. Since the latitude at which a TC recurves is strongly modulated by the strength and latitudinal position of the jet stream, TCs recurving at different latitudes might be expected to produce different downstream responses. In this study, the relationship between the latitude of WNP TC recurvature and the Rossby wave response is explored from a composite perspective, and the dependence of model forecast skill on the Rossby wave response to recurving WNP TCs is considered from a case study perspective.

Recurving WNP TCs occurring between 1979 and 2009 are grouped by recurvature latitude, and their downstream extratropical flow response is compared using composite analyses constructed from 2.5° NCEP–NCAR reanalysis data. Preliminary results indicate that WNP TCs that recurve at relatively low latitudes (i.e., south of 25°N), when the North Pacific jet stream is strong and displaced equatorward, may be more likely to excite or amplify Rossby wave trains than WNP TCs that recurve at relatively high latitudes (i.e., north of 25°N), when the North Pacific jet stream is weak and displaced poleward. WNP TCs that recurve at relatively low latitudes typically occur in spring and late fall, whereas WNP TCs that recurve at relatively high latitudes typically occur in summer and early fall. Furthermore, WNP TCs that recurve at relatively low latitudes are more likely to be accompanied by a preexisting wave packet over eastern Asia and the western North Pacific than are WNP TCs that recurve at relatively high latitudes.