长江中下游旱涝急转年多尺度低频振荡特征及其对旱涝急转的影响

EFFECTS OF MULTI-SCALE LOW FREQUENCY OSCILLATIONS ON THE DROUGHT-FLOOD ABRUPT TRANSITION OVER THE MIDDLE AND LOWER REACHES OF THE YANGTZE RIVER

利用国家气候中心提供的753站逐日降水及TRMM 3b42卫星降水资料,NOAA向外长波辐射（OLR）资料及NCEP/NCAR再分析数据集,对长江中下游地区春夏出现的旱涝急转现象进行详细分析,探讨了长江中下游旱涝急转年多尺度低频振荡特征及其对旱涝急转的影响.（1） 2011年作为长江中下游夏季旱涝急转的典型年,转折前后大气环流场存在显著差异,副高短期活动、孟加拉湾低槽及中高纬度槽脊对旱涝急转具有重要影响.（2） 2011年长江中下游夏季降水具有多时间尺度周期特征,10～20 d及30 ～ 60 d低频分量是夏季降水的重要组成部分,不同低频分量具有不同的作用关键区域,相应的低频系统也有差别.前者在南海-西太平洋地区最活跃,具体表现为低频反气旋的发生发展;后者则主要活跃在阿拉伯海-孟加拉湾地区,低频对流加强是该地区最主要的特征.（3）副高的短期活动是造成201 1年长江中下游旱涝急转的关键,它的西伸和加强在低频场上表现为南海-西太平洋附近10～20 d低频反气旋的发展加强,后者与孟加拉湾地区30 ～ 60 d低频对流的影响相互叠加,在旱涝急转期作用达到最强;孟加拉湾地区30～60 d低频对流旺盛发展,OLR极值转变要早于南海-西太平洋地区副高极值的变化约5d的时间,可能是影响副高西伸的一个重要因素.

Using precipitation data at 753 observational stations of China provided by the National Climate Center of China Meteorological Administration,TRMM 3b42 daily rainfall,OLR data from National Oceanic and Atmospheric Administration and global reanalysis data provided by NCEP/NCAR,we analyzed abrupt drought-flood transitions during summer in the middle and lower reaches of the Yangtze River,especially the multi-scale low frequency oscillations and their effects on the transitions.Results are shown as follows.（1） The most obvious transition occurred in 2011 and there are significant differences in the associated atmospheric circulation before and after this transition.The short-term activity of a westem Pacific subtropical high and a low tough over the Bay of Bengal region has an important role in the transition.（2） Summer precipitation in 2011 has several different timescale periods,in which 10 ～ 20 d and 30 ～ 60 d are the most remarkable low-frequency components.The two low-frequency oscillations affect summer rainfall in different ways; 10 ～ 20 d low-frequency systems are active mostly over the South China Sea and the western Pacific,typically with an 10 ～ 20 d low frequency anticyclone developing and strengthening,while 30 ～ 60 d low-frequency convection strongly deepens over the Arabian Sea and the Bay of Bengal.（3） The short-term activity of western Pacific subtropical high is the key factor for the abrupt drought-flood change in 2011.Corresponding to the 10～ 20 d low-frequency anticyclonic circulation in the South China Sea,the western Pacific subtropical high strengthens and moves westward,together with 30 ～ 60 d low frequency convection over the Bay of Bengal.The latter is about 5 days ahead of the change of the westem Pacific subtropical high,indicating that the 30 ～ 60 d low frequency convection over the Bay of Bengal plays an important role in the westward movement of the subtropical high.