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The onset of South China Sea (SCS) summer monsoon (SCSSM) in June 2018 was extremely late following a La Nina 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.

Plain Language Summary The onset of South China Sea (SCS) summer monsoon (SCSSM) with large interannual variation, as the indicator of the beginning of the East Asian summer rainy season, has been widely concerned in the seasonal forecast. The ENSO events have been treated as the most important precursor of the SCSSM onset. An El Nino (La Nina) event was usually followed by the late (early) onset of SCSSM. However, this relationship was failed in 2018, when the SCSSM built up extremely late in June after a previous La Nina in the winter of 2017/18. The present work shows an additional potential precursor, the strong positive triple SSTA mode in North Atlantic, is able to delay the onset of SCSSM in 2018 by weakening the Mongolian cyclone in the mid-high latitude. On the one hand, the weaker Mongolian cyclone could decrease the upper-level divergence to suppress the convection over the SCS by attenuating the South Asian High. On the other hand, the colder troposphere comes up over the northern SCS to inhibit the in-situ vertical easterly shear. Thus, the convection over the Maritime Continent is trapped in the tropics to further postpone the SCSSM onset by enhancing a local Hadley cell.