Acceleration of the Onset Speeds of Heat Waves Over East China by Upwind Flash Droughts

Abstract The summer temperatures in 2022 have exceeded historical records over the Northern Hemisphere, leading to compound extreme events of droughts and heat waves (HWs) with rapid intensification. This type of compound extreme event has raised unprecedented social concerns. Previous studies have found that upwind droughts can enhance the intensity of HWs through advected sensible heat, but how the droughts affect the onset speed of HWs remains unclear, which is key to HW early warning. By using a Lagrangian model to track the air mass trajectory during the HWs, here we show that upwind droughts enhance 21% of the HWs over East China, and the upwind droughts with faster onset speeds (i.e., flash droughts) have a greater impact on HWs than the slowly evolving droughts. The strong negative soil moisture anomaly caused by a persistent anticyclone increases sensible heating, leading to the increases of total sensible heat and atmospheric boundary layer height over East China. Meanwhile, the rapid onset of flash drought is accompanied by the intensified anticyclone and rapid accumulation of advected sensible heat, and these increase the HW onset speed. Compared with the antecedent upwind slow droughts, the antecedent upwind flash droughts and the associated anticyclone accelerate the HW onset speed by 43% significantly. Our research shows that flash drought is a critical precursor for an extreme HW in the downwind region.