The observed extratropical flow response to recurving western North Pacific tropical cyclones

Tropical cyclones (TCs) that recurve into the extratropical western North Pacific (WNP) and undergo extratropical transition can initiate or amplify Rossby wave trains that subsequently propagate downstream along the North Pacific jet stream. Such Rossby wave trains may serve as precursors to high-impact weather events and large-scale flow anomalies over North America that can influence the sign and magnitude of regional precipitation and temperature anomalies on seasonal time scales. Additionally, these wave trains may contribute to significant increases in model forecast error and uncertainty over the Pacific–North American sector. The objective of this study is thus to understand the factors modulating whether recurving WNP TCs will be associated with Rossby wave trains and/or episodes of reduced atmospheric predictability.

To study the characteristic downstream extratropical response to recurving WNP TCs, a compositing analysis is performed using the 2.5° NCEP–NCAR reanalysis for 100 selected recurving WNP TC episodes between 1979 and 2009. The composite analyses are used to assess the dependence of the extratropical flow response to recurving WNP TCs on a variety of factors, including time of year, latitude of TC recurvature, and the strength of the interaction between the TC and the North Pacific jet stream. Further, both composite analyses and case studies are employed to investigate how the aforementioned factors influence the evolution of model forecast error downstream of recurving TCs. Results reveal that recurving WNP TCs are favored to initiate or amplify Rossby wave trains in fall and spring when the Pacific jet stream is relatively strong and shifted equatorward relative to its overall climatological position. Preliminary findings also suggest that the strength of the initial interaction between the recurving TC and the North Pacific jet stream (as measured, e.g., by the magnitude of negative potential vorticity advection associated with the divergent TC outflow) is positively correlated with the amplitude of the resulting Rossby wave response over North America.