Recurving Western North Pacific Tropical Cyclones as Precursors to High-Latitude Blocking

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Wednesday, 5 February 2014: 9:15 AM
Room C201 (The Georgia World Congress Center )
Heather M. Archambault, NPS, Monterey, CA; and P. A. Harr

High-latitude blocking is a subseasonal atmospheric phenomenon that has been linked to a variety of high-impact weather, including flooding rains, heat waves, and cold-air outbreaks. Increasing understanding of the precursors to high-latitude blocking thus is an avenue through which subseasonal forecasting of high-impact weather may be improved. In this study, the hypothesis that recurving tropical cyclones over the western North Pacific favor high-latitude blocking is explored from statistical and dynamical perspectives. This hypothesis is based on the observed tendency for recurving tropical cyclones to induce cyclonic Rossby wave breaking, a mechanism for high-latitude block formation. In this study, a standard blocking index is used in conjunction with a database of recurving tropical cyclones over the western North Pacific to assess the frequency of blocking episodes following recurving tropical cyclones, and, conversely, the frequency of recurving tropical cyclones preceding blocking episodes. In addition, composite analyses are constructed from the 0.5 NCEP/NCAR Climate Forecast System Reanalysis data to establish the dynamical mechanisms and physical processes linking recurving tropical cyclones to the onset of blocking. It is found that blocking tends to occur a wavelength or more downstream from the recurving tropical cyclone within the poleward exit region of an elongated, intensified jet steam, where cyclonic wave breaking is favored. Given the importance of the jet stream in the onset of blocking, this study specifically addresses how characteristics of the phasing between the tropical cyclone outflow and extratropical features influence the structure and intensity of the downstream jet stream.