夏季风水汽输送对云南夏季旱涝的影响

利用欧洲中期天气预报中心的1961-2002年逐月再分析资料(ERA-40),计算夏季80°E~130°E, 0°~35°N区域内水汽通量及其散度,并使用EOF分解和小波分析等统计方法,对云南夏季水汽通量及其散度的时空特征进行诊断分析,结果表明:云南夏季风降水有两支水汽来源,最强一支来自越赤道气流转向后在孟加拉湾北上形成西南水汽输送,其水汽源地为印度洋、孟加拉湾,反映南亚季风对云南的影响;另一支为副热带高压南侧的东风气流,其源地是西太平洋、南海,反映东亚季风对云南的影响;云南夏季降水的主要水汽通道是南亚夏季风水汽输送。夏季风水汽通量矢量的第一特征向量分布型大致呈反气旋式水汽输送,对应云南夏季降水的一致偏多;水汽通量矢量场第二特征向量的分布大致为气旋式水汽输送,对应云南夏季降水偏少;第三特征向量为气旋式－反气旋式,造成云南夏季降水南北分布差异,反映云南夏季降水的南北差异。第一模态的水汽通量有很强的年际变化,在年代际变化上13~15 a周期较为显著,在年际变化上2~3 a的周期较显著;1960年以来水汽通量呈减少趋势为-0.105 kg(m·s)^-1/a。第二模态的水汽通量呈增加趋势,为0.566 kg(m·s)^-1/a,在年代际变化上11~15 a周期最为显著,从1970年代中期后水汽通量呈2~3 a的周期振荡。夏季异常旱涝年与来自孟加拉湾和南海地区的西南水汽输送的强度及位置有关,并且对流层中层的水汽输送起决定因素。

Impacts of the Vapor Transportation by Summer Monsoon on Drought and Flooding in Summer of Yunnan

By making use of the reanalysis monthly data(ERA-40) by ECMWF from 1961 to 2002,we calculate the vapor flux and its divergence in summer in the area of 80°-130 °E and 0°-35°N.And the temporal and spatial features of vapor flux and its divergence in summer of Yunnan are diagnosed by employing statistical methods of EOF and wavelet analysis.The result shows that,there are two vapor sources of the summer monsoon rainfall in Yunnan,while the strongest one is the southwest vapor transportation resulted from the cross equatorial flow turning and proceeding northward over the Bay of Bengal,sourcing from the Indian Ocean and the Bay of Bengal,reflecting the influence of South Asian monsoon on Yunnan.The other is the easterly jet in the south of Subtropical High,sourcing from West Pacific Ocean and South Sea,reflecting the influence of East Asian monsoon on Yunnan.And the major vapor passage of summer rainfall in Yunnan is the vapor transportation by South Asian summer monsoon.The first EOF distribution pattern of the vapor flux and its divergence by summer monsoon is roughly the anticyclone vapor transportation,which corresponds to the identical more summer rainfall in Yunnan.The second EOF distribution is roughly the cyclone vapor transportation,which corresponds to the less summer rainfall in Yunnan.The third EOF distribution is cyclone-anticyclone pattern,which may result in the summer rainfall difference from south to north in Yunnan,reflecting the summer rainfall difference from south to north in Yunnan.There is an extensive annual variability in the vapor flux of the first EOF,and 13-15 years time is its more significant period in the interdecadal variability,while 2-3 years time is its more significant period in the annual variability.And the vapor flux has been decreasing by-0.105 kg(m·s)-1/a since the 1960s.The vapor flux of the second EOF has been increasing by 0.566 kg(m·s)-1/a,and 11-15 years is its most significant period in the interdecadal variability,while its period turns to be 2-3 years since the mid 1970s.The abnormal drought and flooding years in summer have some connection with the intensity and location of the southeast vapor transportation from the Bay of Bengal and the South China Sea,and the vapor transportation in the mid-troposphere is the decisive factor.