资源环境科技发展态势分析平台

This article analysed two typhoon seasons (1998 and 2016) over the western North Pacific (WNP) with similar preceding background conditions of the tropical Pacific sea surface temperature (SST). Following the strong El Nino event in the preceding winter, the typhoon season was inactive in 1998 while active in 2016. Compared with 1998, the monsoon trough (MT) in 2016 shifted more eastwards, leading to an increase of the mid-level water vapour, lower-tropospheric relative vorticity and upper-tropospheric divergence, and a decrease of the vertical wind shear. Additionally, the eastwards extension of the MT in 2016 induced more active tropical depression (TD)-type waves than those in 1998. The analysis of the energy budget suggested that the eastwards shift of the MT during the 2016 typhoon season provided increased eddy kinetic energy for typhoon genesis, due to the joint contributions of the meridional shear of the mean zonal winds and the zonal wind convergence over the WNP. Moreover, the WNP entered a La Nina-like SST pattern after July 2016. The relatively slower developing process of this pattern sustained the warmer SST anomalies and the anomalous cyclonic circulation, which the eastwards extension of the MT can be attributed. Hence, compared to 1998, the active typhoon season in 2016 is attributed to the eastwards extension of the MT, more active TD-type wave activities, and slow developing process of a La Nina-like SST pattern in 2016 typhoon season.