Abstract:The study of intraseasonal oscillation (ISO) characteristics is crucial for both understanding internal variability and improving prediction skill at the sub-seasonal time scale. Based on daily precipitation from 108 national meteorological stations across Shanxi and NCEP/NCAR daily atmospheric reanalysis datasets spanning 1980 to 2022, this study investigates the climatological spatiotemporal evolution of summer precipitation ISO in Shanxi and its association with atmospheric circulation by using statistical methods of Lanczos filtering, power spectrum analysis, and composite analysis. The main findings are as follows: (1) Summer precipitation in Shanxi shows a significant 10-50-day ISO signal. The intensity of the precipitation ISO displays a significant and positive correlation with precipitation, with a high temporal correlation value of 0.88 and a consistent increasing trend after 2000. Specifically, precipitation increases at a rate of 35.4 mm/decade, while ISO intensity increases by 0.27 (mm·d-1)/decade. Spatially, areas with higher precipitation generally experience stronger ISO, and the most active ISO centres are located in southern Shanxi and the mountainous areas. (2) Two typical summer precipitation ISO cycles in Shanxi exhibit three-stage evolution according to their composite analysis. The initial stage is characterised by a weakening of drought conditions and the onset of positive precipitation anomalies in parts of Shanxi; the active stage features the expansion and intensification of positive precipitation anomalies until peak precipitation progresses southward to southern Shanxi; and the weakening stage begins when ISO-induced positive precipitation anomalies retreat and terminates when the negative precipitation anomalies re-emerge over most parts of Shanxi. (3) The evolution of precipitation ISO is closely linked to vertically integrated water vapour transport. The enhancement (weakening) and westward-northward (eastward-southward) shift of low-frequency anticyclonic water vapour transport over the subtropical western Pacific plays a vital role in regulating the ISO precipitation cycle. During the dry phase, the water vapour transport is too weak to reach Shanxi. As the anomalous anticyclone shifts westward and northward, the ISO enters its active stage, accompanied by strengthened and persistent southeasterly water vapour transport, placing Shanxi in a water vapour convergence zone. As this transport weakens and retreats eastward, the water vapour convergence zone gradually shifts out of Shanxi. After the weakening stage, a new ISO cycle restarts. This study elucidates the climatological spatiotemporal evolution of summer precipitation ISO in Shanxi and its synergistic interaction with atmospheric circulation, providing a scientific foundation for regional sub-seasonal precipitation forecasting and offering new insights into the ISO characteristics and mechanisms over mid-latitude regions with complex terrain.
