Contributions of Local Land-Atmosphere Coupling and Mesoscale Atmospheric Circulation to the 2013 Extreme Flash Drought and Heatwave Compound Event Over Southwest China

The flash drought and heat wave compound event of the 2013 summer in Southwest China was exceptional, with profound socio-economic impacts. Existing studies have captured the signal of this compound event; however, its causes are still poorly understood. In this study, we utilized a high-resolution index to examine the life cycle of this compound event from 1980 to 2020 and quantified the relative contributions of land-atmosphere coupling and mesoscale atmospheric circulation using the air parcel backward tracking method. It was found that the dry soil caused by the precipitation deficit further inhibited precipitation and caused the soil to become even drier, which then led to the development of the flash drought. The dry soil induced a substantial increase in the land-atmosphere sensible heat flux and a rise in the near-surface air temperature. During this process, the strength of land-atmosphere coupling was 2.5 standard deviations higher than the mean value of the climate, playing an integral role in the near-surface air temperature increase, accounting for 55.9% of the maximum temperature changes. The mesoscale atmospheric circulation contributed 44.1%, of which horizontal advection accounted for 41.1%, while vertical advection only accounted for 3.0%. The western Pacific subtropical high expanded westward, accompanied by the northward-moving westerly jet stream, which prevented cold air from accessing the Southwest China, and abnormal subsidence interfered with precipitation formation. Furthermore, the control of an abnormally high-pressure system over the region led to a decrease in cloud cover, allowing more surface incident solar radiation and contributing to the increase in the near-surface air temperature.