Exceptionally prolonged extreme heat waves over South China in early summer 2020: The role of warming in the tropical Indian Ocean

Abstract

South China experienced a series of unprecedented extreme heat wave (EHW) events in early summer (June–July) 2020, which broke the historical record for the frequency of EHW events since 1979. Observational analyses showed that the prolonged EHW events were primarily induced by an anomalous tropospheric circulation over South China with a barotropic structure. This was attributed to the exceptional westward displacement of the western Pacific subtropical high (WPSH) and the eastward-extended upper-level South Asia high (SAH). In addition, the value of the SAH–WPSH index, which describes the relative displacement of the SAH and WPSH, also broke the historical record. Further analysis revealed that the EHW events in South China were closely associated with warming of the tropical Indian Ocean (TIO) sea surface temperature (SST), which affected both the SAH and WPSH via atmospheric teleconnections. Sensitivity and pacemaker experiments using an atmosphere-only and a coupled global climate model, respectively, suggested that the anomalous warming of the SST in the TIO directly caused the significant southeastward displacement of the intensified SAH as a result of diabatic heating in the troposphere. This heating also led to a westward-displaced WPSH via an induced mid- to lower tropospheric Kelvin wave and the associated Ekman divergence. In summary, warming in the TIO sustained exceptional shifts in both the SAH and WPSH, which favored the formation of a stationary anomalous high over South China that ultimately contributed to the record-breaking EHW events in early summer 2020.