Causes Analyses of Precipitation Anomaly in Chongqing in Autumn 2017
This paper analyzes the precipitation in Chongqing that exhibits positive anomaly during the early autumn and negative anomaly during the late autumn in 2017. To diagnose the cause of this extreme anomaly in the precipitation season, this paper uses daily precipitation data observed by 34 meteorological stations in Chongqing since 1961 (from early September to late November), NCEP/NECA and NOAA daily height field, wind field, water vapor field, sea temperature field and other reanalysis data. Using correlation, composite and other statistical diagnostic methods, we analyze the abnormal changes in the autumn precipitation in Chongqing in 2017. The diagnosis of related circulations shows that in 2017, precipitation in Chongqing witnessed a sharp transition from droughts to floods. The cause of this situation is that the subtropical high maintained along the section of Yangtze River in Chongqing and the middle and lower reaches of the Yangtze River for a long time. At the same time the development of the double-resistance type (the high-ridge of the Ural Mountains), together with the establishment and maintenance of the Okhotsk Sea high, slowed the eastward movement of the low trough in the west, resulting in frequent heavy precipitation and abnormal precipitation amount from September to early and mid-October in Chongqing. In late October, the West Pacific subtropical high anomaly weakened southward. In November, when the closed monomer completely exited from the mainland, the high dam in the low latitude cut off the northward transport of water vapor. The mid-high latitude subtropical high showed a west-positive east-negative type anomalous distribution. The area from the east of Lake Baikal to the west of the Okhotsk Sea exhibited a negative anomaly. The south edge of negative anomaly was located in the northern part of Hetao Region. The cold air path was eastward and northward, not conducive to the continued precipitation. Thus, a transition from above normal to below normal precipitation occurred. The analysis of the SST anomaly field indicates that the double-restricted circulation anomaly in the early-autumn and high latitudes may be related to the leading and current warming of the SST and northwestern Pacific Ocean. The SST of tropical Indian Ocean Basin-Wide Mode of the whole region and SST in the equatorial eastern Pacific may be the main causes and external forcing factor of the anomalous atmospheric circulation (West Pacific subtropical high) in the subtropical region in the late autumn.