【作者】 吴志伟；

【导师】 何金海； 李建平；

【作者基本信息】 南京信息工程大学 ， 气象学， 2006， 硕士

【摘要】 本文利用欧洲中心(ECMWF)的ERA-40再分析资料、中国台站降水资料以及美国国家海洋和大气管理局(NOAA)的ERSST海温资料,通过统计分析,研究了长江中下游地区夏季旱涝并存、旱涝急转的发生演变规律及其若干气候特征。主要结果如下: (1) 正常季风年长江中下游夏季早涝并存的气候特征 从夏季(5—8月)季节平均的尺度出发对长江中下游夏季“旱涝并存”的总体统计特征进行了诊断分析,发现:近50年来正常夏季风年长江中下游夏季旱涝并存异常的发生频率呈现上升趋势;夏季旱涝并存异常强年,夏季西太平洋副高呈现出异常的季节内振荡,从赤道印度洋直至赤道西太平洋存在显著的西风异常,索马里和苏门答腊附近为显著的北风异常;其前一年11月份至当年1月份南半球环状模(SAM)往往偏强,从前一年11月至当年4月份,阿拉伯海、盂加拉湾和中国南海海温显著偏高,另外赤道东太平洋海温呈现上升趋势,对应着El Ni(?)o或El Ni(?)o的发展阶段。 (2) 长江中下游夏季长周期旱涝急转的气候特征 对长江中下游地区夏季长周期(旱和涝时间尺度均在2个月左右)旱涝急转现象(LDFA)的研究表明:旱转涝年旱期西太平洋副高主体偏南,长江流域低层负涡度中心发展伴随着辐散、下沉的加强和水汽输送减弱,同时南亚高压和高空西风急流位置偏南,长江中下游地区干旱少雨;涝期西太平洋副高北抬,该地区低层有正涡度发展并伴随辐合、上升运动和水汽输送增强,南亚高压和高空西风急流北抬,导致该地区降水增多。涝转旱年涝期北方冷空气活跃,冷暖空气交汇于长江中下游地区,低层正涡度发展同时辐合、上升运动和水汽输送较强,南亚高压位于高原南侧上空,长江中下游地区偏涝;旱期低层有负涡度中心发展伴随着辐散、下沉的加强和水汽输送减弱,副高异常显著的北跳,南亚高压向东向北扩展至华北地区,长江中下游转为东风急流所控制,干旱少雨。另外,长江中下游夏季LDFA与其前期2月份的SAM和NAM存在显著的负相关。 (3) 长江中下游夏季短周期旱涝急转的气候特征 对长江中下游夏季短周期(旱和涝时间尺度大致在10天-40天左右)旱涝急转(SDFA)的研究结果显示:强和弱SDFA夏季分别对应着南北型和中间型两种不同的表层土壤湿度分布模态,强SDFA夏季受大尺度大气环流异常的影响较大,而弱SDFA夏季受表层土壤湿度的影响显著;长江中下游夏季SDFA与其前期12月份的南半球环状模(SAM)存在显著的正相关,与1月份的北太平洋涛动(NPO)和3月份的北大西洋涛动(NAO)存在显著的负相关。更多还原

【Abstract】 The daily precipitation data at 720 stations over China for the 1957-2003 period during summer (May-August), ERA-40 reanalysis datasets and extended reconstructed sea surface temperature (ERSST) data taken from NOAA are used to investigate the droughts-floods coexistence (DFC) and droughts-floods abrupt alternation (DFA) phenomena during the normal summer monsoons in the middle and lower reaches of the Yangtze River valley. The variation and climatic features are analyzed and the potential affecting factors and the preceding signals are explored. First, the DFC and DFA climatic features are diagnosed statistically on different time scales. Then, the ocean-atmospheric features for the anomalous DFC and DFA years are demonstrated. Finally, some possible factors related to the DFC and DFA are suggested. The main results are as follows:1. Climatic Features of the Summer DFC during the Normal Summer Monsoons in the MLYRVThe results show that the occurrence of strong summer DFC displays an increasing trend from 1957 to 2000 and the anomalous subseasonal oscillation of the western Pacific subtropical high (WPSH) might be one factor contributing to the strong summer DFC in the MLYRV. The anomalously low-level westerly winds anomalies prevailing over the near-equatorial Indian Ocean and the western Pacific offer the necessary condition for the cross-equatorial propagation of Rossby wave from the Southern Hemisphere (SH) to the Northern Hemisphere (NH). Strong Southern Hemisphere annual mode (SAM) during the preceding November through January (NDJ) is often followed by the strong summer DFC in the MLYRV. In the preceding NDJ of the strong DFC summers in the MLYRV, high sea surface temperature (SST) in the oceanic areas from the Arabian Sea to the South China Sea (SCS) and El Nifio or the developing phase of El Nifio are often observed. All these phenomena offer some predictive signals for the strong summer DFC in the MLYRV. Nevertheless, the physical mechanism of the interannual variability of summer DFC is still not clear. Based on the results thus far, the cross-equatorial propagating of the preceding strong SAM signal from the SH to the NH, associated with the anomalous subseasonal oscillation of the WPSH, might also be a factor contributing to the strong summer DFC in the MLYRV. High SST developing in the tropical eastern Pacific might also play a role in the strong summer DFC in the MLYRV. This paper examines the summer DFC in the MLYRV on seasonal timescale. There is no doubt that if the timescale gets shorter, the possible mechanisms related to the DFC might be different.2. Climatic Features of the Summer Long-cycle Droughts-floods Abrupt Alternation (LDFA) in the MLYRVResults show that the summer droughts-to-floods (DTF) in the MLYRV often go with the more southward WPSH, negative vorticity, strong divergence, descending movements developing and the weak moisture transport in the low level, and the more southward position of the South Asia high (SAH), the westerly jets in the high level during May—June;While during July—August it usually accompanies with the northward shift of the WPSH, positive vorticity, strong convergence, ascending movements and strong moisture transport in the low level, and the northward shift of the SAH and the westerly jets in the high level. The summer floods-to-droughts (FTD) in the MLYRV is often related to the active cold air mass from the high latitude, positive vorticity, strong convergence, ascending movement developing and strong moisture transport in the low level, and the SAH over the Tibetan Plateau in the high level, while during July—August it is often connected with negative vorticity, strong divergence, descending movements developing and weak moisture transport in the low level, the remarkable northward shift of the WPSH, the SAH extending northeastward to North China and the easterly jets prevailing in the high level over the MLYRV. In addition, the summer LDFA in the MLYRV is significantly related with the Southern Hemisphere annual mode and the Northern Hemisphere annual mode in the preceding February.3. Climatic Features of the Summer Short-cycle Droughts-floods Abrupt Alternation (SDFA) in the MLYRVStrong and weak SDFA summers are often related to the south-north pattern and the middle pattern of surface soil humidity, respectively. The former is more likely to be influenced by the singularities of large scale atmospheric circulation, whereas the latter is often linked with the anomalous surface soil humidity. The SDFA phenomena also have significantly positive correlation with SAM in the preceding December, significantly negative correlation with NPO in the preceding January and NAO in the preceding March.更多还原