利用JRA55大气再分析资料和TRMM卫星降水资料,分析了2002年夏季(5～8月)华南地区降水的低频振荡特征,重点揭示了对其影响显著的中高纬大气季节内振荡的环流结构及演变.小波和功率谱分析表明,2002年夏季华南降水表现为主周期为10～30 d的准双周低频振荡.典型低频降水事件及合成分析指出,准双周降水的强(弱)变化除了受低空西北太平洋副热带高压西伸进入(东移退出)南海的影响以外,还显著地依赖于中高纬地区高空大气环流的季节内振荡.在对流层高层,中高纬度地区存在一支自大西洋经欧亚大陆的气旋一反气旋相间排列的低频波列.该波列在欧亚大陆地区向东南传播,当异常反气旋和气旋分别位于青藏高原和华北上空时,这种偶极型环流之间的高空辐散场有利于华南地区上升运动的发展,因而华南降水偏强;反之,华南降水偏弱.研究还表明,低频波列南移造成了对流层异常温度平流和副热带高层异常绝对涡度的变化,使得华南地区上升与下沉运动交替出现以及相应的经向环流圈反转,从而导致华南准双周振荡干湿位相的转换.局地异常感热加热对干湿位相转换也起一定作用.时滞相关分析发现,当青藏高原地区500 hPa位势高度异常场超前于华南异常降水4d(即位相差为1/4周期)时,二者出现显著正相关,表明青藏高原地区500hPa位势高度异常对预测华南地区季节内降水变化有潜在的应用价值.%The satellite-based TRMM rainfall and the JRA55 atmospheric reanalysis datasets are used to investigate the structure and evolution of extratropical atmospheric intraseasonal oscillation and its impact on anomalous rainfall over South China (SC) in the summer (May to August) of 2002.Wavelet and power spectra analyses demonstrate that the intraseasonal variability of the SC rainfall during the summer of 2002 was characterized by a significant quasi-biweekly oscillation (QBWO) with a period of 10-30 days.Composite analyses show that such a QBWO of the SC rainfall was not only related to alternate occurrences of the anticyclone and cyclone anomalies in the lower troposphere over the coastal region of SC that modulate the western North Pacific subtropical high entering or leaving the South China Sea,but also largely dependent on the extratropical atmospheric oscillation in the middle and upper troposphere.The QBWO in the upper troposphere was characterized by a southeastward-propagating wave train consisting of a series of anomalous cyclones and anticyclones,which were aligned in a northwest-southeast direction over Eurasian continent.When the anomalous anticyclone and cyclone were respectively located over the Tibetan Plateau and northern China as the wave train moved,the upper tropospheric divergence between this dipole in anomalies was favorable for the development of ascending motions over SC,leading to above-normal rainfall;in the opposite situation,below-normal rainfall occurred over SC.For the transition between dry and wet episodes of the QBWO,anomalous tropospheric horizontal temperature advection and absolute vorticity anomalies in the upper troposphere above the subtropics caused by the southward migration of the wave train are two dominant factors leading to the alternation between ascending and descending motions over SC as well as the reversal of the corresponding meridional circulations.Local sensible heating anomalies also make certain contributions to the above process.Significantly positive correlations were found when the 500-hPa geopotential height anomalies over the Tibetan Plateau led the SC rainfall anomalies by 4 days (that is,a phase lag of 1/4 period of the QBWO),which implies that the 500-hPa geopotential height anomalies around the Tibetan Plateau may be a potential predictor for the forecast of the intraseasonal variation of rainfall over SC.
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