Tropical cyclones and multiscale climate variability: The active western North Pacific Typhoon season of 2018

The 2018 typhoon season in the western North Pacific (WNP) was highly active, with 26 named tropical cyclones (TCs) from June to November, which exceeded the climatological mean (22) and was the second busiest season over the past twenty years. More TCs formed in the eastern region of the WNP and the northern region of the South China Sea (SCS). More TCs took the northeast quadrant in the WNP, recurving from northwestward to northward and causing heavy damages in mainland China (69.73 billion yuan) in 2018. Multiscale climate variability is conducive to an active season via an enhanced monsoon trough and a weakened subtropical high in the WNP. The large-scale backgrounds in 2018 showed a favorable environment for TCs established by a developing central Pacific (CP) El Nino and positive Pacific meridional mode (PMM) episode on interannual timescales. The tropical central Pacific (TCP) SST forcing exhibits primary control on TCs in the WNP and large-scale circulations, which are insensitive to the PMM. During CP El Nino years, anomalous convection associated with the TCP warming leads to significantly increased anomalous cyclonic circulation in the WNP because of a Gill-type Rossby wave response. As a result, the weakened subtropical high and enhanced monsoon trough shift eastward and northward, which favor TC genesis and development. Although such increased TC activity in 2018 might be slightly suppressed by interdecadal climate variability, it was mostly attributed to the favorable interannual background. In addition, high-frequency climate signals, such as intraseasonal oscillations (ISOs) and synoptic-scale disturbances (SSDs), interacted with the enhanced monsoon trough and strongly modulated regional TC genesis and development in 2018.