Causes of the Extreme Hot Midsummer in Central and South China during 2017: Role of the Western Tropical Pacific Warming

This study investigates why an extreme hot midsummer occurred in Central and South China (CSC) during 2017. It is shown that the western North Pacific subtropical high (WNPSH) was abnormally intensified and westward-extending, resulting in anomalous high pressure and consequent extreme heat over CSC. The abnormal WNPSH was favored by the warming of the western tropical Pacific (WTP), which was unrelated to ENSO and manifested its own individual effect. The WTP warming enhanced the convection in-situ and led to anomalous high pressure over CSC via a local meridional circulation. The influence of the WTP was confirmed by CAM4 model experiments. A comparison between the 2017 midsummer and 2010 midsummer (with a stronger WNPSH but weaker extreme heat) indicated that the influence of the WNPSH on extreme heat can be modulated by the associated precipitation in the northwestern flank. The role of the WTP was verified by regression analyses on the interannual variation of the WTP sea surface temperature anomaly (SSTA). On the other hand, the WTP has undergone prominent warming during the past few decades, resulting from decadal to long-term changes and favoring extreme warm conditions. Through a mechanism similar to the interannual variation, the decadal to long-term changes have reinforced the influence of WTP warming on the temperature over CSC, contributing to the more frequent hot midsummers recently. It is estimated that more than 50% of the temperature anomaly over CSC in the 2017 midsummer was due to the WTP warming, and 40% was related to the decadal to long-term changes of the WTP SSTA.