<p>The western Pacific subtropical high (WPSH) constitutes a pivotal component of the East Asian summer monsoon, and a swing of its position directly influences the weather over eastern China. Various indices have been developed to monitor WPSH, but few are designed to depict its northwest boundary. We defined a bivariant index representing the northwest boundary of WPSH (NBI), based on 700-hPa horizontal winds, contour lines of 500-hPa geopotential height (H500) and H500 gradient. This index separately portrays meridional and zonal activities of the northwest boundary of WPSH, which are orthogonal to each other. Compared with the similar indices from National Climate Center of China, NBI is better connected to the processes over the land, especially to soil moisture (SM). Thereby, a significantly negative relationship is identified between leading SM in southern China and the meridional component of NBI (NBI_lat) in the next two pentads in the observations. When soil is anomalously wet in southern China, the northwest boundary tends to stick to south in the next two pentads, and vice versa. To explore the mechanism behind this phenomenon, numerical experiments were carried out. The model results suggest that anomalously positive SM at the initial time in southern China cools the air, causing shallow responses at lower and middle levels of the troposphere. The cooling induces the downward motion and a baroclinic structure around 30°N, leading to the H500 decrease in the next two pentads, which is not in favor of the northward development of the northwest boundary. When SM is anomalously negative, the opposite results are achieved. The numerical results support the negative relationship between SM and NBI_lat in the observations. Additionally, this study also implies that wet/dry soil conditions in southern China can be potentially an important precursor signal for the medium-range prediction of wet/dry conditions over the Yangtze River valley.</p>