A Daily Soil Moisture–Temperature Compound Index for Characterising Dry–Hot Extremes

ABSTRACT Compound dry–hot extremes are increasing globally, yet existing indices often overlook the critical role of soil moisture in regulating moisture surplus or deficit. This study proposes the Standardised Soil Moisture Temperature Compound Index (SSTCI), a globally applicable, daily‐scale index derived from the joint probability of the Standardised Antecedent Soil Moisture Index (SASMI) and Standardised Temperature Index (STI). To improve event classification, SSTCI incorporates an objective removal and merging procedure that filters minor spells and consolidates temporally dependent events. As a case study, we apply SSTCI to the Poyang Lake Basin (1961–2020), where it performs better than existing indices in detecting CDHEs, validated by recorded events. Spatiotemporal analyses show a significant rise in event frequency and intensity over recent decades. By integrating soil moisture dynamics and operating at a sub‐monthly scale, SSTCI enhances the accuracy of CDHE monitoring, offering a robust tool for early warning and risk assessment.