湖南省夏季极端降水异常时空特征及其成因分析

利用湖南省88站常规观测资料、NCEP全球再分析资料,采用EOF分解和合成分析方法研究了湖南省1961—2014年极端降水的年际变化异常和时空特征,以及极端降水异常的环流成因。并主要分析了由EOF方法分析得出的第一类极端降水形态(一致偏多或偏少型)与环流异常之间的联系。结果表明,湖南省夏季极端降水分布有较大的空间差异,且湖南省极端降水呈明显增强的趋势。由于极端降水的变化周期与大尺度环流固有的2～4 a周期相同,因此大尺度环流的变化是湖南省极端降水异常的主要成因。大尺度环流的变异引起中国东部经向排列的环流异常分布,使得高度场和风场等气候场产生变异,从而引起湖南省极端降水的异常。从极端降水偏多年与偏少年大气环流的差值场可以看出:湖南省处于显著差异区,湖南省极端降水偏多(偏少)主要是由于北方的偏北风加强(减弱)和经西太平洋—南海进入湖南地区的西南风加强(减弱)并在湖南地区产生辐合(辐散)。当夏季东亚副热带西风急流偏南时,湖南正处在偏南较强西风急流的南侧,较强西风急流所产生的抽吸现象使得上层空气辐散,增加了湖南省极端降水的强度。副热带高压强度偏强、位置偏南、偏西,使得东亚夏季风偏弱,导致气流辐合稳定在湖南省,造成极端降水偏强。前冬中东太平洋海温偏暖(EI Ni1o)/偏冷(La Ni1a),湖南地区的极端降水容易一致偏多/偏少。同时黑潮暖流区也是影响湖南极端降水变化的可能原因。

Characteristics of summer extreme precipitation anomaly and the cause of concurrent anomaly pattern in Hunan province

Based on the convective observational data from 88 stations in Hunan province and NCEP/NCAR dataset, by using EOF decomposition and composite analysis, the characteristics and the cause of extreme precipitation anomaly in Hunan province were analyzed. The relationship between the EOF1 of anomaly extreme precipitation (concurrent rich/seldom rainfall pattern) and the large circulation were studied. The results show that, a large difference of spatial-extreme precipitation and extreme precipitation in Hunan has significantly increased. Due to the cycle of extreme precipitation are similar to the cycle of large circulation (2-4 years), the change of circulation may be the inherent reason for the anomaly of extreme precipitation. The variation of large-circulation make the meridional circulation anomaly distribution in eastern China, then the geopotential height and the wind fields changed in future, which causes the change of extreme precipitation in Hunan province. The change of circulation between more extreme precipitation and less extreme precipitation can be seen that, Hunan province located in the significant difference areas. More (less) extreme precipitation in Hunan province are mainly due to the strengthening (weakening) of northly and southly convergence (divergence) in Hunan province. When the location of East Asia subtropical westerly jet is southward, the extreme precipitation in Hunan province will be strengthened, as Hunan province is located the north of East Asia subtropical westerly jet, the strong East Asia subtropical westerly jet cause the upper air divergence. When the intensity of subtropical anticyclone is strong, the location is south-westward, The EASM will be weakened and the convergence is located in Hunan province, the extreme precipitation are enhanced. The pre-winter SST of mid-east Pacific is warmer (cooler), the extreme precipitation in Hunan province is concurrently rich(seldom). Meanwhile, the Kuroshio current region is another possible reason for the change of extreme precipitation in Hunan province.