Relative-Humidity Decomposition of July Rainfall Anomalies over the Middle–Lower Yangtze River Basin Associated with Eastern Mediterranean–West Asian March Precipitation

Seasonal prediction of summer rainfall over the middle and lower reaches of the Yangtze River Basin (MLYRB) remains challenging because the linkage between preceding climate signals and regional precipitation anomalies involves complex dynamic and thermodynamic processes. This study examines March precipitation over the eastern Mediterranean–West Asia region, hereafter referred to as PE, as an upstream spring signal associated with July rainfall anomalies over the MLYRB. The central objective is to determine, through a relative humidity decomposition framework, whether the humidification accompanying PE-related July rainfall anomalies is dominated by moisture changes or by temperature-related saturation effects. The results indicate that high-PE years are associated with a significant increase in July rainfall over the MLYRB. This rainfall enhancement is accompanied by anomalous circulation patterns favourable for moisture transport and convergence over eastern China. Meanwhile, positive relative humidity anomalies extend from the lower to the upper troposphere, with the 400 and 300 hPa levels showing a particularly close spatial correspondence with the significant rainfall anomalies over the rainfall region. Although absolute water vapour content decreases with height, the coherent upper-tropospheric relative humidity response indicates the presence of a deep moist layer, which is favourable for sustained condensation, reduced dry-air entrainment, and persistent monsoon rainfall. A further moisture–temperature decomposition shows that the PE-related relative humidity response is jointly controlled by changes in atmospheric moisture content and saturation vapour pressure. Over the MLYRB, the increase in relative humidity is primarily associated with enhanced moisture content, whereas temperature-induced changes in saturation conditions are more evident in regions with more coherent temperature anomalies. These findings suggest that the PE-related July rainfall anomaly is supported by a combination of dynamic moisture supply and thermodynamic humidification of the atmospheric column. The study provides a physically consistent explanation for the potential precursor relevance of the PE signal and emphasizes the importance of vertical humidity structure in understanding and predicting summer rainfall anomalies over eastern China.