Causes of the Unprecedented Month-To-Month Persistent Extreme Heat Event Over South China in Early Summer 2020: Role of Sea Surface Temperature Anomalies in the Tropical Indo-Pacific Region

An unprecedented month-to-month persistent extreme heat event attacked South China in early summer (May-July) 2020, with an extremely and persistently stronger-than-normal intensity and higher-than-normal number of extreme heat days. The underlying mechanisms were investigated from the month-to-month perspective using observations, reanalysis data, and numerical experiments. The results showed that a local anomalous anticyclone in the mid-lower troposphere over South China and the South China Sea directly contributed to this persistent extreme heat event via anomalous warm advection heating and diabatic warming at near-surface level. Further investigation revealed that the rapid transition from El Nino to La Nina with subsequent development in the central and eastern tropical Pacific, along with persistent warming in the tropical Indian Ocean in the early summer of 2020, were crucial to the maintenance and reinforcement of the local anomalous anticyclone, with greater importance of the latter. On the one hand, rapid cooling in the central and eastern tropical Pacific enhanced the Walker circulation and further triggered an abnormal local Hadley cell, leading to anomalous subsidence over South China and the South China Sea. On the other hand, the persistent warming in the tropical Indian Ocean resulted in an enhanced and westward-extended western North Pacific subtropical high via eastward-propagating Kelvin waves and an intensified and eastward-extended South Asian high by heating the troposphere. These two aspects of atmospheric circulation configuration favored the maintenance and reinforcement of the local anomalous anticyclone in the mid-lower troposphere over South China and the South China Sea.