Record-breaking summer rainfall in South Korea in 2020: Synoptic characteristics and the role of large-scale circulations

In the summer of 2020, South Korea experienced record-breaking rainfall due to 15 consecutive heavy rainfall events (HREs) from mid-June to early September. Among them, 11 HREs occurred in late June to mid-August with distinct synoptic characteristics depending on the occurrence period. All HREs from 29 June to 27 July (P1) were triggered by extratropical cyclones, while those from 28 July to 15 August (P2) mainly occurred along monsoon rainband. We argue that their transition is associated with atmospheric teleconnections. During P1, the western North Pacific subtropical high (WNPSH) anomalously extended westward, but its northward expansion was hindered by the meridional wave train from the suppressed convection over the South China Sea. This condition prevented a northward migration of the monsoon rainband but allowed more extratropical cyclones to pass over the Korean Peninsula, resulting in four HREs. During P2, the South China Sea convection was enhanced, and its circulation response prompted an abrupt northward expansion of the WNPSH with a large pressure gradient along its northern boundary. With intensified southwesterly moisture transport, a monsoon rainband was activated over the Korean Peninsula, producing six HREs. The opposite phases of the summer North Atlantic Oscillation, i.e., negative in P1 but positive in P2, further contributed to the anomalous monsoon circulation by modulating the mid-latitude circulation response to the South China Sea convection. This study demonstrates that the nature of summertime HREs in East Asia can be strongly modulated by remote forcings.