Exceptionally Prolonged Extreme Heat Waves Over South China in 2020 Early Summer : The Role of Tropical Indian Ocean Warming

Southern China (SC) suffered a series of unprecedented extreme heat waves (EHWs) in early summer (June-July) 2020, which broke the historical record of EHW frequency since 1979. Observational analysis shows that prolonged EHWs were primarily induced by anomalous tropospheric circulation over SC with a barotropic structure, which was attributed to the exceptionally westward-displaced Western Pacific subtropical high (WPSH) and eastward-extended upper-level South Asia high (SAH). In addition, the value of SAH-WPSH index, which describes the relative displacement of the SAH and WPSH, also broke its historical record in 2020. Further analysis reveals SC EHWs were closely associated with Tropical Indian Ocean (TIO) sea surface temperature (SST) warming which affected both the SAH and WPSH via atmospheric teleconnection. Sensitivity and pacemaker experiments using an atmosphere-only and coupled global climate model, respectively, suggest that anomalous TIO SST warming directly causes significantly southeastward displacement of an intensified SAH due to diabatic heating in the troposphere. At the same time, it also results in a westward-displaced WPSH via an induced in mid-to-low tropospheric Kelvin wave and associated Ekman divergence. In summary, TIO warming can sustain the exceptional shifts of both SAH and WPSH which favors the stationary anomalous high over SC, ultimately contributing to the record-breaking SC EHWs in early summer 2020