Extremely Late Onset of the 2018 South China Sea Summer Monsoon Following a La Niña Event: Effects of Triple SST Anomaly Mode in the North Atlantic and a Weaker Mongolian Cyclone

Abstract The onset of South China Sea (SCS) summer monsoon (SCSSM) in June 2018 was extremely late following a La Niña event, indicating a breakdown in the traditional positive relationship between ENSO and SCSSM onset. Although the weaker western Pacific subtropical high in early May facilitated the SCSSM establishment, the persistent weaker Mongolian cyclone featured by an anomalous barotropic anticyclone over Northeast Asia during mid–late May dramatically postponed the SCSSM onset in 2018. This not only reduced upper‐tropospheric divergence over the SCS, which suppressed monsoon convection by attenuating the South Asian High, but cooled the mid–upper troposphere to decrease the in‐situ vertical easterly shear. The convection over the Maritime Continent was therefore trapped in the tropics, which further delayed the SCSSM onset via an enhanced local Hadley cell. It suggests a potential effect of triple SSTA mode in North Atlantic on the SCSSM onset by changing the mid‐latitudinal circulation.