Composite Characteristics of the Abnormal Circulation in May between Extreme Drought Years and Rainy Years of Yunnan
Using the NCEP/NCAR reanalysis data from 1961 to 2010,the discrepancies of atmospheric circulation between the extreme-drought years and above-normal rainfall years in May of Yunnan are analyzed.The results show that the characteristics of atmospheric circulation in May of above-normal rainfall years are different from those of extreme-drought years in both low and high latitude.The extreme-drought-year circulation shows two-trough-one-ridge pattern in mid-high latitude on 500 hPa,with the corresponding anomaly field of geopotential height presenting ‘-+-' pattern and the westerlies moving northward are later than normal years; rather straight flows cover mid-low latitude and large-scale negative anomalies of sea-level pressure cover Asia; cold air activities that might influent Yunnan are weak.On the contrary,it exhibits two-ridge-one-trough pattern in above-normal rainfall years,with the ‘ +-+ ' pattern of the anomaly field over the mid-and high-latitude and the westerlies moving northern are earlier.The trough over the Ural maintain and Caspian Sea tends to lead cold air intruding China,positive anomaly center of sea-level pressure locating at the eastern part of the Qinghai-Xizang Plateau,which makes cold air activities strong.At the low latitude,the west Pacific Subtropical High (WPSH) is strong and shifts westward,so that the northward and eastward advancement of the tropical westerlies is inhibited and the onset of summer monsoon over the Bay of Bengal and Indo-China peninsula is late and weak,vice versa.The south Asian High in above-normal years is strong and big and locates westward compared to that in extreme-drought years.There is anomaly large-scale and deep subsidence flow at low-latitude in the northern hemisphere(including Yunnan) in extreme-drought years,which is contrary to that in above-normal years.In above-normal rainfall years,the southwest water vapor transport is dominant over Yunnan province and the water vapor converge is comparatively strong than normal,with moisture content above normal and and the shift between dry season and wet season is early,the WPSH shifting eastwards.In extreme-drought years,the WPSH locates westward and southward,the west water vapor transport and water vapor divergence covers Yunnan,with moisture content below normal,and the shift between dry season and wet season is later.The correlation coefficient of Asian summer monsoon index WYI and the precipitation in May is significant positive,especially it is related to the extreme rainfall events.
Rainfall in MayExtreme droughtAtmospheric circulation characteristicWater vapor transportWYI monsoon index
NETL
NSTL
万方数据
维普
