Understanding the extremely wet autumn over West China in 2017 from intraseasonal oscillation perspective

Abstract West China experienced an extremely wet autumn in 2017, and the amount of September–October precipitation is the greatest during 1961–2018. This study explored the physical processes responsible for two primary intraseasonal precipitation events which contribute greatly to such an extremely wet season. The first event peaking in early September is induced by the southward movement of a frontal system from the northern China, which is attributed to the southward displacement of an intraseasonal anticyclonic anomaly. The northerly anomaly associated with the anticyclone advects cold air into West China and thus triggers frontal precipitation. The second event peaking in early October is related to the northward shift and intensification of an intraseasonal anticyclonic anomaly centred over the northwestern Pacific Ocean. The southerly anomaly along the western flank of the anticyclone transports moisture to West China and thus triggers precipitation. The different initiation processes for the two precipitation events may be due to the distinct dynamical and thermal‐dynamical conditions in September and October. West China has sufficient moisture but less dynamical disturbances in September, while it has adequate dynamical disturbances but less moisture in October.