The Characteristic Of Precipitation Anomaly Transition From Spring To Summer Over The Yangtze River Basin And Its Mechanism

The Yangtze River Basin covers national economic and culture centres which are especially vulnerable to the rainfall anomaly. Precipitation changes in spring (MAM) and summer (JJA) over the Yangtze River Basin during1880to2011were analyzed based on observation datasets from21gauge stations. The mechanisms were discussed by using the NCEP reanalysis datasets. Finally, the projected changes in precipitation were evaluated by using nine CMIP5datasets. The major conclusions are as follows:(1) Interannual and interdecadal changes in spring and summer precipitation occurred during1880to2011. The Dry-Dry patterns of seasonal evolution generally occurred and Dry-Wet patterns were rare occurrence over the past132years. Begining in1880s, the first species and third species has decreased and increased respectively, however the changes of other patterns were not obvious.(2) S-EOF analysis was employed to identify the major modes of seasonal precipitation evolution from spring to summer in the Yangtze River Basin, the first three modes were decomposition by S-EOF. The leading mode describes the spatial coincidence of seasonal variability from spring to summer. Almost the entire Yangtze River Basin has been dominated by a positive anomaly, especially in the middle and lower basins from spring to summer. The opposite precipitation anomaly between North and South appeared in the third mode whatever in spring or summer.(3) The first two S-EOF modes were affected by water vapor transport, sea surface temperature,500hPa geopotential height, extratropical westerly jet and monsoon. Six physical factors were selected to design statistical experimental. The result showed that the most significant factors were spring and summer monsoon index and input moisture.(4) A sudden turn from drought to flood resulted in enormous losses occurred in2011over the middle-lower part of the Yangtze River Basin. Persistent negative anomalies were identified during MAM. However, a significant transition was detected by the increasing Meiyu rainfall during the June. The water vapor fluxes, SSTA, extratropical westerly jet, monsoon, the corresponding location of the western Pacific subtropical high (WPSH) and vertical motion anomalies in spring and June were described and examined to investigate the mechanism of the sudden precipitation transition in2011. (5) The ensemble mean precipitation was relatively close to the observation in baseline.The time series of spring precipitation shows a significant positive linear trend in RCP2.6and RCP8.5. But no significant trend was found in RCP4.5. Compared with the baseline, spring precipitation will increase in the mid-low Northern Yangtze River Basin under each RCP. However a similar pattern shows that rainfall will decrease over the middle basin under each RCP in summer. A relatively higher anomaly over a wider area will occur in RCP8.5. In the2040s, the similar patterns are presented in spring and summer under each RCP. Summer rainfall in the middle basins and Northern basin will exhibit a negative anomaly in2020s and2040s under all RCPs respectively.(6) The Wet-Wet pattern will be common under RCP2.6and RCP4.5. The Wet-Wet and Wet-Dry patterns both will become more frequent under RCP8.5.