Circulation Patterns Of Regional Mean Daily Precipitation Extremes And Their Linear Trends Over The Middle And Lower Reaches Of Yangtze River During Boreal Summer

Using the daily data from NCEP/NCAR reanalysis and daily precipitation records from 743 stations of China, the composite analysis has been performed to investigate the anomalous circulation patterns for the regional mean daily precipitation extreme events over the middle and lower reaches of Yangtze River (MLRYR) in June-July-August of boreal summer, and has compared to its non-extreme precipitation events. In addition, each station of MLRYR has been analyzed its linear trend. Results are as follows:(1) There are two types of anomalous circulation patterns related to the daily precipitation extremes (DPEs). Type-I is featured in the main by:i) an anomalous cyclone and anticyclone extend in the zonal dimension; ii) convergence (divergence) occurs in the mid-lower (higher) troposphere, displaying a baroclinic structure; iii) warm, wet air from the Bay of Bengal, South-China Sea (SCS) and western Pacific basin is carried into the MLRBR via the tropical anticyclonic circulation to provide plentiful vapor for the DPEs there; iv) the MLRYR and western Pacific waters act as a high-valued zones of apparent heat source and vapor sink, respectively, and the anomalously negative diabatic heating in the southeastern MLRYR, Yellow Sea and western Pacific plays a role in making the subtropical high extend westward and amplified; v) Pacific SST exhibits a pattern with negative (positive) SSTA in the north (south) and the anomalous pattern resembles that in the El Nino strengthening and damping stages. All these conditions facilitate the DPEs occurrence.Type-II is featured as follows:i) an anomalous cyclonic and anticyclonic circulation extend longitudinally; ii) an abnormal anticyclone is over the SCS at mid-lower levels as opposed to an anomalous cyclonic circulation over the MLRYR that is deep and tilts westward; iii) a vapor convergent belt runs in the SW-NE direction; iv) maximal positive centers of atmospheric column-integrated apparent heat source and vapor sink emerge in the eastern MLRYR, the domains being near-circular and greatly smaller than that of the heating field of composite 93 DPEs; v) the western Pacific east of the Philippines shows distinct anomalously negative diabatic heating to favor air divergence to be in the lower troposphere; vi) weakly negative SSTA appears to the south of the equator and north of the equator, especially in the Kuroshio extension band, there occurs increased warm SSTA, a pattern that is similar to that of SST in the damping La Nina episode. These conditions are responsible for producing DPEs over the MLRYR.(2) The circulation anomalies in DPEs and the non-extreme events (NEDPs) show some similar distributions such as an anomalous anticyclone over the Southern China Sea and northwestern Pacific, an anomalous cyclone over regions northwest of the precipitation area, and the divergent flow emanating from the rainfall area outward at the upper troposphere. However, different features are clearly seen between these two types of events by the big differences in both intensities and locations of the anomalous circulation. The anomalous circulations of the DPEs are intensified as compared to those of the NEDPs. The anomalous circulations in upper troposphere in the DPEs are more effectively matched with the anomalous circulations in lower troposphere than those in the NEDPs. The different sources of the water vapor are found to be related to different types of precipitation events; the vapor is from the Bay of Bengal in DPEs whereas it is from the South China Sea in NEDPs. The apparent diabatic heating and vapor sources in these two types of events are examined, showing the stronger net heating over northern flank of the rainfall area and the stronger cooling in northwestern Pacific in the DPEs. The stronger cooing over oceans facilitates maintenance of the anomalous anticyclone over the southeastern coastal areas. Interestingly, very significant sea surface temperature anomalies (SSTA) are observed in Pacific in DPEs other than the NEDPs, which look similar to the SSTA pattern of El Nino episodes. Both the anomalous circulation and SSTA patterns in DPEs are more significant statistically in large areas than those in NEDPs. Almost no significant signals are found in SSTA in NEDPs. The weaker significance of both circulation anomalies and SSTA in NEDPs suggest that the NEDPs are more probably induced by the local circulation anomalies other induced by both the local and remote forcings from middle latitudes and tropics.(3)The distributions of DPEs frequency are obviously different from one to another in summer over the MLRYR. Trends in DPEs are positive sign in southern and negative sign in norther parts of MLRYR in June. However, their signs are reversed in July. The frequencies of DPEs are higher in Zhejiang coastal and Northern Hubei in August. This scenario is probablely related to the Meiyu rain belt migration.The linear trends in the total rainfall of DPEs over MLRYR are consistent with DPEs frequencies in each month of the summer, though some differences are observed from one month to another. In June, no big geographical differences are focused in linear trends of DPEs.Significant increasing trends in the DPEs appear in the west MLRYR and the decreasing trends in the east MLRYR in July. However, these trends are reversed in signs.