资源环境科技发展态势分析平台

The late onset of the South China Sea (SCS) summer monsoon (SCSSM) has traditionally been attributed to El Nino events. However, the SCSSM built up around Pentad 26 in 2019, which was 10 days earlier than expected following the 2018/2019 El Nino event. The present study showed that the early onset of the SCSSM in 2019 could be ascribed to tropical cyclone (TC) Fani over the Bay of Bengal (BOB) from 27 April to 3 May 2019. Before the landfall in Pentad 24, the anomalous condensation heating released by the TC not only shifted the South Asian High (SAH) northward but also reinforced the upper-level barotropic trough to the west of the Tibetan Plateau (TP) at midlatitudes. This phenomenon facilitated the early establishment of monsoon convection by intensifying the upper-level pumping over the SCS. In addition, the TC carried abundant moisture to the eastern TP and the SCS when it landed in eastern India in Pentad 25 and strengthened the local rainfall in front of the deeper midlatitudinal trough in the upper troposphere over the TP, which in turn released more condensation heating to warm the tropospheric atmosphere. Afterwards, this warmer air advected downstream to the north of the SCS by the basic flow, resulting in the further northward shift of the SAH and the stronger monsoon onset convection over the SCS. Such an upscaling effect of the TC prevented the onset time of the 2019 SCSSM from being accurately forecasted by the ECMWF S2S model.