东部和中部型El Nino事件对云南冬季降水影响的差异分析

利用1981-2017年云南125个站逐日降水观测资料和NCEP再分析资料,对比分析了两类El Nino事件期间云南冬季的降水差异,发现东部型El Nino有利于冬季云南大部地区降水显著偏多,而中部型El Nino的影响却不明显。进一步从两类事件相应的大气环流等方面分析了造成降水差异的物理原因。研究表明:(1)东部型El Nino年,Walker环流在西太平洋为异常强的下沉气流,西太平洋副热带高压偏强偏西,Hadley环流在20°N为异常上升气流,造成云南水汽输送和辐合加强,中高纬度巴尔喀什湖和贝加尔湖脊发展,冷空气沿高原东侧南下影响云南,冷暖空气在云南频繁交汇,导致云南冬季降水日数明显偏多,相应降水偏多;(2)中部型El Nino年,Walker环流在西太平洋下沉支强度较东部型弱,相应的西太平洋副热带高压较东部型偏弱偏东,Hadley环流在20°N为下沉气流,对云南区域水汽输送影响较弱,同时中高纬度环流不利于冷空气南下影响云南,除对应冬季云南东部降水日数偏少外,对其他地区降水的影响不明显。

Difference Analysis on Influence of EP and CP El Nino of Precipitation in Yunnan in Winter

There are few analyses on the effect of different types of SST changes in the equatorial Pacific Ocean of precipitation in Yunnan in winter. Based on daily precipitation observation data of 125 stations in Yunnan and NECP reanalysis data from 1981 to 2017,the authors analyze the differences influence of eastern Pacific(EP)and central Pacific(CP)El Nino of precipitation in Yunnan in winter by Partial correction,Composite analysis.It was found that the rainfall in most areas of Yunnan in winter will be more than annual year in EP El Nino year,but there was no obvious unusual of rainfall in CP El Nino year. With deep study on the physical causes of two types El Nino events affect winter precipitation differences,results demonstrate that:Firstly,in the EP El Nino year,the Walker circulation has an unusually strong downdraft in the western Pacific,the West Pacific Subtropical High(WPSH)is stronger,and the position is westward than normal. The Hadley circulation has an abnormal updraft at 20°N,which offer an advantage for water vapor transport and convergence enhance in Yunnan. At the same time,the ridge developing between the Balkhash Lake and Baikal in the middle and high latitudes which leader to cold air move south and invade Yunnan along the east side of the plateau. Meeting of cold and warm air in Yunnan causes heavy rain in winter. Secondly,in the CP El Nino year,the intensity of downdraft of the Walker circulation in the western Pacific is weaker than EP El Ni?o year. The WPSH is stronger and the position is westward than regular ones but weaker and eastward than EP El Nino year. The Hadley circulation has a downdraft at 20°N,which against the water vapor transport and convergence enhance in Yunnan. Meanwhile,the circulation in the middle and high latitudes result no obvious cold air move south and influence Yunnan. So,except the rainfall-days is less than normal in eastern of Yunnan,there is no obvious effect on precipitation in winter in Yunnan in CP El Nino year.