Analysis on the Interdecadal Transition and Its Causes of the Autumn Precipitation Trend in West China

The West China (25°N-35°N, 100°E-110°E) is one of the main autumn precipitation regions in China. In this study, the monthly precipitation data of 72 stations in West China, NCEP/NCAR reanalysis data, and sea surface temperature and sea ice data of Hadley Centre were analyzed. Accordingly, the interdecadal variability of autumn rain in West China from 1961 to 2014 and its relationship with atmospheric circulation and sea surface temperature were found through correlation and regression methods. The decadal variability of autumn precipitation in West China was decomposed into two periods P1 (1964-1998) with a significant downward trend and P2 (1998-2014) with an upward trend. Results show that the anomalous field of the atmospheric geopotential height, corresponding to the downtrend of the precipitation in P1, has a structure of the positive to negative in west to east inside the study region. The large-scale circulation field shows the quasi-zonal wave train originating from the eastern Atlantic Ocean to East Asia via the Barents-Kara Seas and Kara Sea areas, which reflects the modulation effect of the upstream negative-phase NAO. In P2, the geopotential height field configuration related to the downward precipitation trend is opposite to that in P1.The large-scale wave train structure shows a westward-shifting northwest-southeast pathway over Eurasia, which reflects the modulation effect of positive-phase NAO at the upstream. This circulation structure leads to the formation of a negative anomaly center over the northwest of West China, which is favorable for the southwest warm and humid airflow to enter the study area. The key SST zones that influence the autumn rain in West China are located in central and eastern tropical Pacific and Indian Ocean. In P1, the precipitation trend of autumn rain transition in West China was positively correlated with the SST over tropical Middle East Pacific and Indian Ocean during the same period. In P2, there exists a significant negative correlation between the autumn rain and SST in the tropical Middle East Pacific and Indian Ocean during the last winter. In addition, the positive SST anomaly in the northwest Pacific Ocean during the previous winter also affects the increasing trend of the autumn rain in West China.