Impact of interannually varying background circulation on summertime wave patterns and tropical cyclone tracks in the western North Pacific

Abstract

This study analyzed summertime wave patterns in the western North Pacific by separating them into those during El Nino (EN) and La Nina (LN) years for 1979–2015. The background flow revealed that in the EN years, the subtropical high was weaker and the East Asian summer monsoon trough was deeper and extended farther southeastward. The composite results indicated that in the EN years, tropical cyclone (TC) tracks were more solid and stably associated with wave pattern propagation routes because the circulation anomalies were significantly stronger. However, in the LN years, when the wave patterns were weak under the shorter and less organized monsoon trough due to the enhanced subtropical high, the TC tracks and cyclonic anomalies were irregular and sporadic. Further background flow analyses indicated that the EN circulation favored the development of the monsoon trough and east–west expansion and contraction of the subtropical ridge, whereas the stronger subtropical ridge dominated most western North Pacific areas in the LN years. Therefore, the monsoon trough was weaker in the LN years and induced recurving TCs to take more scattered routes near Taiwan and China’s east coast. Therefore, Taiwan experienced more rainfall during recurving TC periods in the LN years than in the EN years. For the straight-moving TCs passing through Taiwan, rainfall was concentrated over southern Taiwan in the EN years but more evenly distributed in the LN years. Thus, background flow changes strongly affect wave patterns and TCs.