Rossby Wave Initiation by Recurving Tropical Cyclones in the Western North Pacific

Abstract

Abstract The interaction of recurving tropical cyclones (TCs) with midlatitude Rossby waves during extratropical transition (ET) can significantly alter the midlatitude flow configuration. This study provides a climatological investigation of Rossby wave initiation (RWI) by transitioning TCs in the specific configuration of an initially zonal midlatitude waveguide and elucidates physical processes governing ab initio flow amplification during ET. Recurving TCs interacting with a zonally oriented waveguide in the western North Pacific (WNP) basin from 1979 to 2013 are categorized into cases initiating Rossby waves (TC-RWI) or not (TC-noRWI). Interactions with a zonally oriented waveguide occurred for 22.7% of the recurving TCs, and one-third of these resulted in TC-RWI. In the presence of a TC, the probability of RWI on a zonally oriented waveguide is 3 times larger than in situations without a TC. The occurrence of TC-RWI exhibits a seasonality and is relatively more common during boreal summer than in autumn. We further reveal that a strong preexisting upper-level jet stream, embedded in a deformative large-scale flow pattern, hinders TC-RWI as air from the diabatic outflow of the TC is rapidly advected downstream and does not lead to strong ridge building. In contrast, an enhanced monsoon trough favors TC-RWI as the poleward moisture transport strengthens diabatic outflow and leads to strong ridge building during ET. Thus, we conclude that TC-related ab initio flow amplification over the WNP is governed by characteristics of the large-scale flow more so than by characteristics of the recurving TC.