Spatial-Temporal Characteristics and Causes of Summer Heat Waves in Hunan Province

A comprehensive understanding of heatwaves is of great importance for making scientifically-based emergency plans to reduce their consequent disastrous impacts. Based on daily maximum temperature observations in summer over Hunan Province during 1960-2014, the spatial and temporal distribution characteristics of heatwaves and the possible synoptic causes are discussed. The major conclusions are as follows. High occurrence frequency and intensity of heatwaves are commonly observed in eastern Hunan (near Hengyang Basin), while heat waves are less frequent and with weak intensity in western Hunan. The areas affected by heatwaves are expanding and the intensity of heat waves is also enhancing, whereas the spatial difference in heatwaves has narrowed. The heatwave frequency (intensity) displays obvious stage changes, which decreased (weakened) before 1980s but turned to increase (strengthen) since the 1990s. After entering the 21st century, more frequent and stronger heatwaves take place. The analysis of a heat wave event with extremely high temperature showed that a stable and strong western subtropical high with its position shifting westward and northward, eastward extending South Asian high, and strong and persistent downdrafts are all favorable for the occurrence of high temperature in Hunan.