Dry Coupling Aggravates Soil Drought and Atmospheric Aridity Over North China

Abstract The north China region frequently experiences drought during the warm season, significantly impacting the local water cycle and socioeconomic life. To better understand the role that local land‐atmosphere (L‐A) coupling plays in the modulation of drought, we utilize 40 years of the fifth generation of European Re‐Analysis (ERA5) data set. With the aid of two metrics, that is, convective triggering potential (CTP) and low‐layer humidity index (HI) that classify the local L‐A coupling into four regimes, we identify days characterized by dry and wet coupling patterns in the region and define a standardized coupling drought index (SCDI) based on the relative number of dry and wet coupling days over a period of time. The results show that on the dry coupling day, the atmosphere exhibits a higher demand for moisture with an increased net radiation and surface sensible heat flux, and a reduced latent heat flux, so the dry coupling promotes drying. Similarly, the wet coupling promotes wetting. During the dry coupling events (i.e., multiple days with similar coupling states), the positive feedback between soil moisture and atmospheric humidity, along with atmospheric conditions unfavorable for precipitation, result in a continuous decrease in soil moisture and an increase in vapor pressure deficit, which sustain and intensify both soil drought and atmospheric aridity. In contrast, wet coupling events are conducive to drought recovery. The SCDI effectively captures the spatiotemporal evolution of drought across varying intensities, showing that dry and wet coupling play an important role in the persistence and recovery of drought in north China.