我国低纬高原地区初夏强降水天气研究II·2005与2001年5月云南旱涝成因的对比分析

2001年和2005年的5月云南分别出现了严重的洪涝和干旱天气。通过分析研究表明,这两年在大尺度环流形势、季风爆发的早晚和强弱以及ENSO背景的影响等方面都有着显著的不同。2001年5月对流层中层的东亚槽位置偏西且强度偏强,同时南支槽稳定维持且较强,致使冷暖空气易于在云南地区交汇,这可能是在该地区产生较强降水的主要环流形势,而2005年5月东亚槽位置偏东,南支槽较弱,致使云南地区几乎没有水汽来源和缺乏冷暖空气的交汇,这是产生干旱的主要环流形势;同时南海季风爆发的早晚和强弱与云南5月降水也有着较好的关系,2001年5月南海季风爆发偏早且强度偏强,而2005年5月南海季风爆发偏晚且强度偏弱,这两年5月的降水差异较大;对水汽分布的分析还表明,2001年5月云南地区为水汽辐合区,且较常年平均偏大,水汽通量为西南向,且强度较强。而2005年5月云南为水汽辐散区,孟加拉湾的水汽向东进入南海,云南地区非常干燥;从ENSO大背景对其影响的分析可知,2001年5月的洪涝和2005年5月的干旱与这两年的前期海温变化似乎也存在一定的联系。

Study of Strong Heavy Rainfall in Low Latitude Plateau Area in China during Early Summer.Part II:Comparison Between Circulation and Climatology Causing Flood in May 2001 and Drought in May 2005

The severe flood and drought occurred in Yunnan in May 2001 and May 2005,respectively.In this paper,the comparison between the two events has been conducted. It can be seen,through analysis and study,that there existed very different characteristics in the circulation,early or later about monsoon onset and ENSO events.At middle troposphere in May 2001,East Asian trough lean to west and more stronger,and southern westerlies trough was stationary as well as stronger, made the cold air and warm air meet in Yunnan easily,and caused heavy rainfall.In May 2005,East Asian trough lean to east and southern westerlies trough weaken,which induce to water vapor and cold air and warm air,not meeting in Yunnan.These were the possible reasons for flood and drought in the Yunnan area.Meanwhile,the monsoon onset and its strength were associated with precipitation in Yunnan in May.The monsoon onset was early and it was stronger in May 2001,on the contrary,in May 2005.So the precipitations between two years were quite different.In addition,the analysis of the water vapor transportation in Yunnan showed,the water vapor was convergence in Yunnan in May 2001,and it was more than that in normal years,the direction of moisture flux was southwest and strong.But the water vapor in Yunnan in May 2005 was divergence,the water vapor was transported from the Bay of Bengal to South China Sea,and it was very dry in Yunnan.It can also be found from the analysis for ENSO,the flood in May 2001 and drought in May 2005 were related with the sea surface temperature in the last several months.