| The Characteristics and Causation of the Southwest China (including Sichuan, Yunnan, Guizhou and Chongqing) regional meteorological drought events were analysis in this paper. An objective identification technique for regional extreme events (OITREE) and the daily composite drought index(CI) of101stations in Southwest China were used to detect the Southwest China regional meteorological drought events during1960-2010.Values of the parameters of the OITREE method which suitable for Southwest China were determined. The drought Characteristics and temporal characteristics were discussed, the mean circulation anomalies were also studied. Further analysis reveals that:(1) OITREE could be apply to Southwest China.87events were identified during1960-2010, including9extreme events and17sever events. The2009/2010severe drought in Southwest China is the most serious meteorological drought event for the past50years, which lasted for231days and had sever influence in most area of Southwest China.(2) The durations of the Southwest China regional meteorological drought events are generally between10-80days, with the longest period being231days, the impacted area are generally in70×104km2-100×104km2.The drought are mostly start in last autumn to early spring.November to April is the dry season in Southwest China. In regional distribution, Yunnan and southern Sichuan have the highest drought frequency and intensity, and strong (extreme and severe) Southwest China regional meteorological drought events could be divided into five types, with the Southern type occurring more frequently.(3) During the past50years, Southwest China regional meteorological drought events became more frequent and stronger, for which the main reason maybe the significant decreasing of precipitation in this region, and the increasing of temperature also has a contribution.(4)The drought circulation anomalies in four seasons were analyzed base on composite analysis, the results show:mean Surface air temperature in drought month is higher than normal. Geopotential height anomalies show "+-"distribution in mid and high latitudes in500hPa in both autumn and winter, result in a weaker European shallow trough and Lake Baikal ridge, the East Asia westerlies are straighter, cold air which could affect Southwest China is weak. In dry spring, ridge in north of the Mongolian plateau is weaker than normal while the East Asia deep trough is wider. During dry summer, the ridge in Urals is weak as well as the through near Lake Baikal. In short, high latitude circulation anomalies weaken the meridional wind transport, which against the cold air blowing to the south and lead to inactivity of cold air in southwest China. Wind and water vapor flux divergence in850hPa have different activities in different season, but the factors that cause drought were alike----Lacking of confluence of warm and cold air and weak water vapor delivery were the main reason of droughts. The vertical velocity over Southwest China was downward motion in dry season, which could inhibit the precipitation and continue the droughts.(5) While southwest China dry cases happened under a La Nina condition,the SSTA pattern would enhance the Walker circulation, result in anomalous ascending motion over Maritime Continent and tropical west Pacific Ocean, further induces a anomalous Hadley circulation. The downward flow of Hadley circulation over southwest China leads to a persistent descending anomaly, which leads to less precipitation. Meanwhile, a negative phase of the Tropical Indian Ocean Dipole (TIOD) tends to further strengthen the Hadley circulation. While southwest China dry cases happened under a El Nino condition, the downward motion anomaly over southwest China was little effected by the tropical circulation anomalies but chiefly dominated by circulation anomalies in the north. |
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