Interdecadal autumn drought in Chongqing and its possible cause since 1990

Based on the precipitation and temperature data of 30 observation stations in Chongqing,NCEP / NCAR reanalysis data and NOAA ERSST v3 b during 1961—2012,this study uses the standardized precipitation evapotranspiration index( SPEI) computed by temperature and precipitation data to discussthe interdecadal variability feature of autumn drought in Chongqing and its possible causes. Observational results show that main distribution of Chongqing autumn SPEI is in the same phase and its interdecadal shift happens in 1989. After( before) this shift,Chongqing gets drier( wetter) than normal. It is also found that the tropical eastern Indian Ocean and western Pacific( EIWP) and the North Atlantic( NA) are both key areas,where the interdecadal increases of sea surface temperature anomaly( SSTA) play important roles in interdecadal autumn drought in Chongqing,and EIWP is more important than NA. Their possible mechanisms of effect of SSTA in the two key areas on the interdecadal autumn drought in Chongqing are investigated. When autumn SST in EIWP is warmer than normal on interdecadal time-scale,the northwestern Pacific subtropical high( NWPSH) will get larger and stronger than usual,and extend westward and northward,resulting in higher geopotential heights over Chongqing and weaker south branch trough( SBT)over the Bay of Bengal. These circulation anomalies will be not favorable to water vapor transport from the Bay of Bengal to Chongqing. There exists an anomalous upward motion around the key area,inducing an enhanced regional Hadley cell across Chongqing,and then an anomalous downward motion controls Chongqing,resulting in the interdecadal drought. When autumn SSTA in NA is warmer than normal on interdecadal time-scale,the NWPSH will extend northward,resulting in higher geopotential heights and an anomalous downward motion over Chongqing,which causes the interdecadal drought. The above observational results are verified by the sensitive experiments on effect of SSTA in the two key areas on the interdecadal drought,which are conducted by NCAR CAM5. 1( Community Atmosphere Model 5. 1).