热带大气季节内振荡对西南地区东部夏季降水的影响及其可能机制

利用1979~2013年6~8月的西南地区东部20个台站日降水量资料、逐日MJO(Madden-Julian Oscillation)指数、全球OLR(Outgoing Longwave Radiation)逐日格点资料以及NCEP/NCAR再分析日资料,采用合成分析和线性回归等方法,对夏季MJO不同位相活动影响西南地区东部夏季降水的原因及其可能机制进行了初步分析。研究表明,MJO与西南地区东部夏季降水之间存在着显著的关系,当MJO处于第4(第6)位相时,由于西太平洋副高位置偏南(偏北)、向西南地区东部的水汽输送偏多(偏少),在异常上升(下沉)气流影响下,西南地区东部夏季降水偏多(偏少)。MJO影响西南地区东部夏季降水的可能原因是:当MJO处于第4位相时,赤道东印度洋地区上空大气释放凝结潜热,其激发东北向传播的异常波动,进而影响东亚环流,使得西南地区东部出现夏季降水偏多的环流形势,西南地区东部夏季降水增多;但在第6位相时,西太平洋地区上空对流释放的凝结潜热,其激发PJ(太平洋-日本)型Rossby波列,出现不利于西南地区东部夏季降水的环流形势,西南地区东部夏季降水偏少。

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.