Impact of the interannual variability of the South China Sea monsoon trough on tropical cyclone activity over the western North Pacific

Abstract

AbstractThe present study reveals the impact of the interannual variability of the South China Sea monsoon trough (SCSMT) on tropical cyclone (TC) activity over the South China Sea (SCS) and the western North Pacific (WNP) from June to August during the period of 1958–2019. The SCSMT can influence TC genesis and occurrence through affecting the large‐scale environmental conditions and barotropic energy conversion. The intensification of the SCSMT can lead to enhanced low‐level vorticity and upper‐level divergence, decreased vertical wind shear, and increased midlevel humidity over the northern SCS and subtropical WNP. The strengthening of the SCSMT is accompanied by an intensification and eastward extension of the WNP monsoon trough and eastward retreat of the WNP subtropical high. All these changes associated with the SCSMT are favourable for TC genesis and occurrence over the north SCS and WNP. More TCs form over the WNP around 20° N, while higher TC occurrence frequency appears over the north SCS and WNP north of 20° N. Diagnostic results of eddy kinetic energy distribution and barotropic energy conversion provide a possible explanation for the impact of the SCSMT on TC activity. The synoptic‐scale disturbances are more active in the WNP region north of 20° N which might lead to an increase in TC occurrence frequency. While the barotropic energy conversion is intensified in the northern SCS and east of Philippines, more eddy energy converted from the mean flow is favourable for TC genesis. The meridional shear of mean zonal wind and the meridional convergence of mean meridional wind, which are related with the interannual variability of the SCSMT, primarily contribute to the barotropic energy conversion.