Impact of Spring Soil Moisture Over the Eastern Iranian Plateau on Interannual Variability of Persistent Spring‐Summer Dryness/Wetness in Yunnan, China

This study investigates the characteristics and associated mechanisms of the first leading mode of spring‐summer dryness/wetness variation in Yunnan (China), obtained from an extended empirical orthogonal function analysis of the seasonal Standardized Precipitation Evapotranspiration Index during spring and summer. The first leading mode reveals an in‐phase variation during spring‐summer, where a positive (negative) phase of the corresponding time series indicates consistent wetness (dryness) across both seasons. Further analysis demonstrates that soil moisture in spring over the Eastern Iranian Plateau (EIP) plays a crucial role in shaping this consistent mode. During spring, enhanced soil moisture over the EIP boosts local convective precipitation through positive soil moisture‐precipitation feedback. The anomalous diabatic heating, driven by increased precipitation, intensifies with altitude, triggering a mid‐upper tropospheric cyclone anomaly over the EIP. Therefore, the East Asian subtropical westerly jet strengthens and shifts southward during spring. Concurrently, an anomalous anticyclone dominates the lower troposphere over the Bay of Bengal, weakening the Southern Branch Trough and suppressing water vapor transport and precipitation over Yunnan and the Indochina Peninsula (ICP). These circulation anomalies contribute to spring dry conditions and reduced soil moisture over these regions. Persistently low soil moisture from spring to summer over Yunnan and the ICP favors an equivalent barotropic high‐pressure anomaly over the region, reinforcing westward extension of the western Pacific subtropical high and eastward displacement of the South Asian high. This atmospheric configuration maintains dry conditions during summer in Yunnan. Analysis using the Linear Baroclinic Model further supports this physical mechanism.