Dipolar Mode of Summer Precipitation over the Upper Yellow River Basin in China and Possible Causes

Summer precipitation over the Upper Yellow River Basin (UYRB), China plays a significant role in the water supply in the upstream region of Yellow River. In this study, the main modes and variability of summer precipitation over the UYRB were diagnosed. Summer precipitation over the UYRB showed an overall increase during 1980–2020, with increasing rate weakening from southwest to northeast. The empirical orthogonal analysis revealed that the first main mode (EOF1) of UYRB precipitation was spatially uniform, while EOF2 had a “northeast-southwest” dipolar structure, both of which showed significant interannual variations. In addition, the potential causes of the dipolar mode in EOF2 were investigated. The results show that the weakened East Asian subtropical jet, South Asian high and West Pacific Subtropical High (WPSH) are favorable for the decreased/increased precipitation over the western/eastern UYRB regions, while the strengthened Ural blocking, East Asian subtropical jet and the southwestward extended WPSH are favorable for the opposite distribution of precipitation during negative EOF2 years.