Impact of Summer Soil Moisture over the Qinghai–Xizang Plateau on Autumn Precipitation in the Upper Hanjiang River Basin, Central China

The Upper Hanjiang River basin (UHRB) serves as the primary water source area for the middle route of the South-to-North Water Diversion Project in China. Understanding the precursors of its distinct autumn precipitation regime is critical for regional water security. This study investigates the lagged impact of summer soil moisture (SM) over the Qinghai–Xizang Plateau (QXP) on autumn precipitation in the upper Hanjiang River basin (UHRB), using observational data from 1961 to 2022 and numerical simulations conducted with the Community Earth System Model version 2.2 (CESM2.2). Statistical analyses identify a key region in the central–western QXP where summer SM exhibits a significant positive correlation with downstream autumn rainfall. Mechanistic diagnostics reveal that the persistence of SM anomalies regulates the surface energy partitioning: antecedent wet soil conditions enhance latent cooling while suppressing sensible heating, leading to a sustained surface cooling anomaly into autumn. This negative diabatic heating acts as a source for a downstream stationary Rossby wave train, which induces an anomalous anticyclonic circulation and enhances southwesterly moisture transport toward the UHRB. Sensitivity experiments with CESM2.2 successfully reproduce the observed precipitation and circulation responses, confirming the causality of this land–atmosphere teleconnection. These findings highlight that the QXP soil moisture serves as a robust and independent potential source of sub-seasonal predictability for autumn hydroclimate in the UHRB.