近52年长江中下游地区夏季年代际尺度干湿变化及其环流演变分析

利用中国气象局国家气象信息中心提供的长江中下游地区353站1961~2012年逐月降水资料,通过计算得到各站点夏季标准化降水指数(SPI)。根据长江中下游地区夏季中旱及以上等级站点数目及其突变检测(Mann-Kendall方法,MK)结果,将时间序列划分为三个时段。在此基础上,利用NCEP/NCAR再分析资料及NOAA海洋表面温度重建资料,分析了各个时段前冬至夏季环流背景场的异常特征及其演变过程,并建立了各时段的概念模型。结果表明:(1)长江中下游夏季在第一时段(1961~1973年)呈明显干旱状态;第二时段(1974~1986年)为干旱向湿润转变的阶段;第三时段(1987~2012年)基本转为湿润状态。(2)第二时段为第一时段与第三时段的过渡期,环流背景场在该时段发生明显变化,使得第一时段与第三时段所对应季节的环流距平场相位相反。(3)第一时段,前冬至夏季全球海温持续偏冷,印度洋海温冷异常在夏季尤为显著,南亚高压与西太平洋副热带高压偏弱;前冬,青藏高原北部脊偏弱,蒙古高压明显偏弱;夏季,印度低压偏强、南支槽加深,夏季风水汽输送偏强,而亚洲中高纬度为平直西风气流,北方冷空气不易南下至我国南方地区,冷暖空气交绥受阻,使得长江中下游夏季出现大范围的干旱。第三时段相对于第一时段,前冬至夏季全球海温暖异常,印度洋海温显著偏暖,西太平洋副热带高压偏强;前冬,青藏高原北部脊偏强,蒙古高压异常偏强;夏季,印度低压减弱、南支槽异常偏弱,夏季风水汽输送较弱,水汽滞留在长江流域,且贝加尔湖高压脊发展,脊前冷空气南下,使得长江中下游夏季降水偏多。

Decadal Variation of Summer Dryness/Wetness over the Middle and Lower Reaches of the Yangtze River and the Evolution of Atmospheric Circulation in the Last 52 Years

Monthly precipitation records at 353 stations in the middle and lower reaches of the Yangtze River during 1961-2012, provided by the National Climate Center of the China Meteorological Administration, were used to calculate the Standardized Precipitation Index in summer. The Mann-Kendall (MK) statistical test was applied to detect the trend in the number of stations at which summer SPI reached a value of -1.0 or less. According to the MK test results, the whole time series could be divided into three stages, separated by abrupt change points. Furthermore, based on National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data and National Oceanic and Atmospheric Administration extended reconstructed sea surface temperature (SST) data, the evolution of the large-scale atmospheric circulation was analyzed from pre-winter to summer, and a conceptual model established for each stage. The results showed that: (1) the first stage (1961-1973) was in an obvious state of drought, the second stage (1974-1986) was a dry-to-wet transition phase, and the third stage (1987-2012) was basically wet; (2) the atmospheric circulation changed notably in the second stage, leading to an opposite phase of the circulation anomaly between the first and third stages; and (3) global SST remained abnormally cold from pre-winter to summer, and Indian Ocean SST was especially colder in summer, which led to a dramatically weaker South Asian high and western Pacific subtropical high. The high pressure ridge over the northern Tibetan Plateau was weaker, resulting in weaker high pressure over Mongolia in pre-winter. Low pressure over India was stronger, the southern branch of the trough deepened, and southerly moisture transported by the summer monsoon prevailed over eastern China. Meanwhile, there was straight westerly wind over the mid-high latitudes of Asia, which was unfavorable for the cold air to extend into southern China. Under the above evolution and configuration of the atmospheric circulation, the middle and lower reaches of the Yangtze River were prone to widespread drought in summer during the first stage. However, the circulation from pre-winter to summer during the third stage was opposite to that during the first stage. Global SST was warmer from pre-winter to summer, especially in the Indian Ocean, resulting in a stronger western Pacific subtropical high. The stronger high pressure ridge over the northern Tibetan Plateau reinforced the high pressure in pre-winter over Mongolia. Moreover, the low pressure over India and the southern branch of the trough weakened, and water vapor stagnated over the Yangtze River basin because of the weaker summer monsoon; plus, the high ridge over Lake Baikal was favorable for the cold air to extend into southern China. Therefore, the circulation from pre-winter to summer during the third stage led to more precipitation in summer over the middle and lower reaches of the Yangtze River.