Impact of Madden-Julian Oscillation Activities on Precipitation in Summer over the East of Southwest China and Its Possible Mechanism

Based on the daily precipitation data of 20 stations in eastern Southwest China, real-time multivariate Madden-Julian oscillation(MJO) index, Outgoing Longwave Radiation(OLR) grid data, and NCEP/NCAR reanalysis daily data in summer from 1979 to 2013, the relationships between the different MJO phases and the summertime precipitation over the east of Southwest China and its possible mechanism were investigated using composite and linear regression analysis.The results showed that phase 4 of the MJO is propitious to the appearance of circulation that is conducive to more summer rainfall over eastern Southwest China;the vapor transport and convergent upward velocity are strengthened, and the ridge position of the western Pacific subtropical high leans to the south.The opposite was found to be the case for phase 6 of the MJO.The tropical low-frequency convection center moves eastward from the eastern equatorial Indian Ocean to the western Pacific Ocean area while MJO phase 4 moves to phase 6.When the MJO is located over the North Indian Ocean, the energy of waves forced by the MJO-related convective heating can propagate northeastward and affect the circulation pattern, resulting in increased precipitation over eastern Southwest China.But when the MJO enters the western Pacific, the energy of convective heating can stimulate Pacific-Japan-type waves and cause decreased precipitation.