The Mechanism for Extremely Active Tropical Cyclone Activities in Summer 2018 over the WNP and SCS: Joint Effects of the Decaying La Niña Events and the Intraseasonal Oscillation

In summer 2018, a total of 18 tropical cyclones (TCs) formed in the western North Pacific (WNP) and South China Sea (SCS), among which 8 TCs landed in China, ranking the second and the first highest respectively since 1951. Most of these TCs travelled northwest to northward, bringing in heavy rainfall and strong winds in eastern China and Japan. The present study investigates the impacts of the associated decaying La Niña event and the intraseasonal oscillation (ISO) on extremely active TCs over the WNP and SCS in summer 2018 by use of correlation and composite analyses. It is found that the La Niña episode from October 2017 to March 2018 led to above-normal sea surface temperature over central–western Pacific, lower sea level pressure and 500-hPa geopotential height in the WNP, and abnormally strong convective activities over the western Pacific. These preceding oceanic thermal conditions are favorable to TC genesis in summer. Detailed examination reveals that the monsoon trough was located further north and east, inducing more TCs in northern and eastern WNP; and the more eastward WNP subtropical high as well as the significant wave train with a“－+－+” height anomaly pattern over the midlatitude Eurasia–North Pacific region facilitated the northwest to northward TC tracks. Further analyses reveal that two successive active periods of Madden–Julian oscillation (MJO) occurred in summer 2018 and the boreal summer intraseasonal oscillation (BSISO) was also active over WNP, propagating northward significantly, corresponding to the more northward TC tracks in summer 2018. The MJO was stagnant over the maritime continent to the western Pacific, leading to notably enhanced convection in the lower troposphere and divergence in the upper troposphere, conducive to TC occurrences. In a word, the extremely active TC activities over the WNP and SCS in summer 2018 are closely linked with the decaying La Niña, the MJO and BSISO; their joint effects result in more TC occurrences and the TC tracks being shifted more northwest to In summer 2018, a total of 18 tropical cyclones (TCs) formed in the western North Pacific (WNP) and South China Sea (SCS), among which 8 TCs landed in China, ranking the second and the first highest respectively since 1951. Most of these TCs travelled northwest to northward, bringing in heavy rainfall and strong winds in eastern China and Japan. The present study investigates the impacts of the associated decaying La Niña event and the intraseasonal oscillation (ISO) on extremely active TCs over the WNP and SCS in summer 2018 by use of correlation and composite analyses. It is found that the La Niña episode from October 2017 to March 2018 led to above-normal sea surface temperature over central–western Pacific, lower sea level pressure and 500-hPa geopotential height in the WNP, and abnormally strong convective activities over the western Pacific. These preceding oceanic thermal conditions are favorable to TC genesis in summer. Detailed examination reveals that the monsoon trough was located further north and east, inducing more TCs in northern and eastern WNP; and the more eastward WNP subtropical high as well as the significant wave train with a“－+－+” height anomaly pattern over the midlatitude Eurasia–North Pacific region facilitated the northwest to northward TC tracks. Further analyses reveal that two successive active periods of Madden–Julian oscillation (MJO) occurred in summer 2018 and the boreal summer intraseasonal oscillation (BSISO) was also active over WNP, propagating northward significantly, corresponding to the more northward TC tracks in summer 2018. The MJO was stagnant over the maritime continent to the western Pacific, leading to notably enhanced convection in the lower troposphere and divergence in the upper troposphere, conducive to TC occurrences. In a word, the extremely active TC activities over the WNP and SCS in summer 2018 are closely linked with the decaying La Niña, the MJO and BSISO; their joint effects result in more TC occurrences and the TC tracks being shifted more northwest to northward than normal.