Inter-decadal And Interannual Change Of Precipitation In The Qiangtang Plateau And Possible Mechanism

The Qiangtang Plateau basin is a closed area in the centre of the Tibetan Plateau(TP)where there are many lakes.Because human activities in the basin are rare,the lake area variability acts as a unique indicator of climate change.Previous studies have found that the lakes have been expanding dramatically since the middle of 1990s and increase in precipitation is the main cause.To understand why precipitation increased during the last two decades,the Global Precipitation Climatology Centre(GPCC)precipitation data and ERA-interim reanalysis were used to analysis the change in summer precipitation and water vapor flux(WVF).(1)An abrupt change in summer precipitation in Qiangtang Plateau occurred at 1994.The WVF on the eastedge of Qiangtang Plateau decreasd since 1984 and the WVF increased on the southedge of Qiangtang Plateau,caused the increasing tendency of the net vapor flux in Qiangtang Plateau since 1994.Inter-decadal difference in atmospheric circulation indicates that after 1990s,a cyclonic circulation anomaly in the Arabian Sea enhanced southerly water vapor transport.Meanwhile,an anticyclonic circulation anomaly over Siberia(called "Siberian high anomaly")weakened the westerly in the eastern TP.The southerly monsoon flow from the Bay of Bengal increased simultaneously.Those three factors lead to the enhancement of WVF convergence in Qiangtang TP eventually.(2)The results of the Ensemble Empirical Mode Decomposition(EEMD)of the Qiangtang Plateau's summer precipitation series show that the interannual variability in precipitation is closely related to the ENSO and the "Siberian high anomaly" index,and its decadal change has a good correlation with Atlantic Multi-decadal Oscillation(AMO).Meanwhile,AMO index shows a good relation with the time serie of the first EOF mode of summer precipitation in Qiangtang Plateau at a significance level of 99%,suggesting that the change in AMO may be attributed to the increase in precipitation in the Qiangtang Plateau.The shift of cold phase to warm phase of AMO excited teleconnection wave in upper troposphere.Under the impact of an easterly anomaly,convergence and WVF increased in Qiangtang Plateau.(3)Detrended summer precipitation in Qiangtang Plateau,signals of interannual changes are extracted.Composite analyses of the abnormal atmospheric circulation in wet and dry years shows that there is a strong cyclonic circulation anomaly in the high latitude of east Asia,in the troposphere of 30°N exists two abnormal high pressure center,enhanced the westerly around 40°N.