Impacts of early/late South China Sea summer monsoon withdrawal on tropical cyclone genesis over the western North Pacif
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Impacts of early/late South China Sea summer monsoon withdrawal on tropical cyclone genesis over the western North Pacific Peng Hu1,2 · Jingliang Huangfu1 · Wen Chen1,2 · Ronghui Huang1 Received: 18 October 2019 / Accepted: 13 June 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract This study reveals a significant in-phase relationship between the South China Sea summer monsoon (SCSSM) withdrawal date and tropical cyclone (TC) genesis over the western North Pacific (WNP). The number of TCs generated over the WNP from mid-September to mid-October is positively correlated with the SCSSM withdrawal date during the period of 1979–2016. The decreased (increased) number of TCs generated during early (late) SCSSM withdrawal years is attributed to both internal atmospheric dynamics and external sea surface temperature (SST) forcing. Through dynamic (Rossby wave response) and thermodynamic (increased moisture) processes, the warm SST anomalies during late withdrawal years over the tropical WNP contribute to maintaining the monsoon trough (MT) in the boreal autumn and moisturizing the mid-level atmosphere, providing a favorable environment for TC genesis. The remaining MT can facilitate the conversion of mean kinetic energy into eddy kinetic energy (EKE) and enhance synoptic-scale waves. In addition, upper-level baroclinic energy conversion also contributes to EKE development. Both barotropic and baroclinic processes favor TC genesis over the WNP. In contrast, colder SSTs during early withdrawal years induce the early withdrawal of the MT, leading to depressed enhancement of the EKE and weakening the northwestward propagation of synoptic-scale waves. Hence, fewer (more) TCs tend to be generated during early (late) withdrawal years. Keywords South China Sea · Summer monsoon withdrawal · Tropical cyclone genesis · Monsoon trough
1 Introduction The interactions between the summer monsoon (SM) and tropical cyclones (TCs) have long been a key scientific issue in monsoon meteorology, and thus, they have been studied extensively. For example, both a case study and statistical analysis indicated that TCs can prominently trigger the onset of the South China Sea (SCS) summer monsoon (SCSSM) (Huangfu et al. 2017a). An example of such an event occurred in 2006, when the monsoon was directly triggered by TC Chanchu (Mao and Wu 2008; Chen 2015). Statistical * Jingliang Huangfu [email protected] * Wen Chen [email protected] 1
Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100190, China
University of Chinese Academy of Sciences, Beijing 100049, China
2
studies have also shown that enhanced (reduced) TC activities appear on the interannual timescale during early summer over the western North Pacific (WNP) for most of the early (late) SCSSM onset (Huang and Li 2010; Ren et al. 2016; Huangfu et al. 2017a). Moreover, the increasing number of TCs visiting the SCS-Philippine Sea has been suggested to represent an important contributor to inter
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