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Continuous Spring and Meiyu Rainfall in the Mid-lower Reaches of the Yangtze During the Past 50 Years
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摘要: 利用1961—2009年长江中下游地区52个气象站逐日降水资料，研究了该地区春季降水与梅雨期降水的连续变化特征，划分了连续旱、连续涝、先旱后涝和先涝后旱4类连续性事件，并探讨其成因。结果表明：长江中下游地区春季降水量年际和年代际变化较为显著，其中连续旱和连续涝事件发生较多。前冬Niño3区的海温与春季和梅雨期降水量相关性超过0.05显著性水平，前冬青藏高原积雪深度与6月西太平洋季风指数与梅雨期降水量相关性均达到0.05显著性水平。当春季水汽丰富，同时春季与6月副热带高压中心位置持续偏西可能导致春季和梅雨期降水持续偏多；春季水汽丰富，但春季至6月副热带高压中心位置由偏西向偏东转变，可能造成先涝后旱；春季水汽偏少，且春季与6月副热带高压中心位置持续异常偏东，易造成持续干旱。2011年水汽突变可能是导致旱涝急转的直接原因，前冬的La Niña事件不利于春季降水而6月副热带高压位置异常西伸, 则容易引发旱涝急转。Abstract: Using the daily precipitation data from 52 meteorological stations in the mid-lower reaches of the Yangtze and NCEP/NCAR reanalysis data from 1961 to 2009, the characteristics of spring and Meiyu precipitation are analyzed. It shows that spring precipitation takes on significant inter-annual and inter-decadal variations while Meiyu precipitation doesn't. Based on the annual spring and Meiyu precipitation characteristics, it is divided into four classes: Flood-flood, flood-drought, drought-drought and drought-flood. Generally, drought-drought and flood-flood events take place frequently. The variation of circulation features affects the precipitation anomaly too. The mechanism of different type is also discussed, finding the correlation coefficient between previous winter Niño3 index and spring, Meiyu precipitation to be 0.42 and 0.30, which reach 0.01 and 0.05 levels, respectively. The previous winter snow of Tibetan Plateau and western Pacific summer monsoon index are shown both significantly correlated with Meiyu precipitation. The abundant water vapor is carried by the anomalous southerly from the South China Sea and the western Pacific, the western Pacific subtropical anticyclone strengthens, and its position leans westward in June. That would make a successive flood event. When water vapor over the reaches is abundant but insufficient in source region, position of the western Pacific subtropical anticyclone appears from west to east anomaly in spring and Meiyu periods, and that would cause the flood-drought events. When water vapor is insufficient in both areas, drought event occurs. In 2011, a sudden turn of drought and flood takes place in the mid-lower reaches of the Yangtze, leading to big economic losses of agriculture. The factors which may cause the sudden turn of drought and flood events are analyzed. The sea surface temperature anomaly of the equatorial eastern Pacific happens in precious winter and less water vapor cause spring drought, and with Meiyu occuring, the western Pacific subtropical anticyclone moving westward and precipitation increasing greatly. On the basis of preliminary analysis of related impact factors, numerical experiments are needed to evaluate the result further.
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图 1 长江中下游地区气象站点分布
Fig. 1 Location of the selected stations in the mid-lower reaches of the Yangtze
图 2 1961—2009年长江中下游地区梅雨期降水与6月、6—7月降水量的变化趋势
Fig. 2 The variations of Meiyu period, June and June—July precipitation in the mid-lower reaches of the Yangtze during 1961—2009
图 3 1961—2009年长江中下游地区春季 (a) 和梅雨期 (b) 降水量变化趋势
Fig. 3 The trend of spring (a) and Meiyu (b) precipitation in the mid-lower reaches of the Yangtze during 1961—2009
图 4 1961—2009年长江中下游地区春季与梅雨期降水变化散点分布
Fig. 4 The quadrant pattern of spring and Meiyu precipitation anomaly in the mid-lower reaches of the Yangtze during 1961—2009
图 5 前冬海温与春季 (a)、梅雨期 (b) 降水相关系数分布
(阴影区表示达到0.05显著性水平)
Fig. 5 The distribution of correlation coefficients between previous winter SST and spring (a), Meiyu (b) precipitation, respectively
(shaded areas represent passing the test of 0.05 level)
图 6 前冬青藏高原积雪深度、6月西太平洋夏季风指数与梅雨期降水量的变化趋势
Fig. 6 The trends of previous winter snow depth of Tibetan Plateau, western Pacific summer monsoon index of June and Meiyu precipitation
图 7 春季850 hPa风场 (矢量) 及比湿分布 (阴影)
(a) 气候平均场，(b) 先涝后旱年距平，(c) 连续旱年距平，(d) 连续涝年距平
Fig. 7 The spatial distribution of wind (vector) and specific humidity (shaded) at 850 hPa in spring
(a) climate mean, (b) anomaly in flood-drought events, (c) anomaly in drought-drought events, (d) anomaly in flood-flood events
图 8 春季500 hPa副热带高压 (实线：气候平均态；虚线：典型年份；单位：gpm )
(a) 连续涝年, (b) 先涝后旱年, (c) 连续旱年
Fig. 8 500 hPa WPSH in the climate mean (solid line) and average of abnormal years (dotted line) in spring (unit:gpm)
(a) flood-flood events, (b) flood-drought events, (c) drought-drought events
图 9 6月500 hPa副热带高压 (实线：气候平均态；虚线：典型年份；单位：gpm)
(a) 连续涝年, (b) 先涝后旱年, (c) 连续旱年
Fig. 9 500 hPa WPSH in the climate mean (solid line) and average of abnormal years (dotted line) in June (unit:gpm)
(a) flood-flood events, (b) flood-drought events, (c) drought-drought events
图 10 2011年850 hPa风场距平 (矢量) 及比湿距平分布 (阴影) (a) 春季，(b)6月
Fig. 10 The spatial distribution of wind anomaly (vector) and specific humidity anomaly (shaded) at 850 hPa in 2011(a) spring, (b) June
图 11 500 hPa副热带高压气候平均态及2011年位置比较 (实线：气候平均态；虚线：2011年；单位：gpm )
(a) 春季, (b)6月
Fig. 11 The comparison of WPSH between climate state and 2011(solid line: climate state; dotted line: 2011; unit: gpm)(a) spring, (b) June
表 1 连续性降水事件分类表
Table 1 The classification of common abnormal precipitation event
事件 春季 梅雨期 年份 连续涝 ≥+0.5倍标准差 ≥+0.5倍标准差 1975，1980，1983，1991，1995，1998，1999 先旱后涝 ≤-0.5倍标准差 ≥+0.5倍标准差 连续旱 ≤-0.5倍标准差 ≤-0.5倍标准差 1965，1971，1978，1985，2000，2001，2005 先涝后旱 ≥+0.5倍标准差 ≤+0.5倍标准差 1967，1973，1977，1992，2002 
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