Study On Characteristics Of The Drought In Nearly 50 Years And Causes Of Continuous Drought During Winter Half Year In Southwest China

Under the background of global warming, more and more drought disasters have occured. Both the duration and coverage of droughts have shown an increased trend, thus regional drying has became one of the major environmental problems that human must confront currently. The drought disasters have occurred many times in Southwest China since 21th century, especially for the sustainedly severe droughts occurred in autumn, winter, spring for four consecutive years from 2009 to 2013. These have caused widespread attentions due to the huge economic losses with characteristics of long duration、large scope、heavy drought degree、serious impact on agricultural production and people’s life in arid regions and need a further study.Firstly, the characteristics of drought variation in Southwest China for nearly 50 years are studied with the compound index (CI) of meteorological drought. Features of the drought trend, drought frequency, drought coverage and variations of different drought intensity levels have been analyzed. Then, causes of the continuous droughts occurred in autumn, winter, spring in Southwest China from 2009 to 2013 have been analyzed in detail by diagnosing the influence of large scale atmospheric circulation, water vapor transportation, tropical sea surface temperature, Arctic Oscillation (AO) and stratospheric polar vortex with the NCEP/NCAR monthly reanalysis data, NOAA monthly global sea surface temperature data and NOAA AO index. Finally, composite analysis has been applied by selecting typical dry years and wet years of Southwest China in order to reveal the key factors which influence the continuous droughts in Southwest China during winter half year. The main conclusions are summarized as follows:(1)It has presented a climate change feature in Southwest China with increases of temperature and drying trend and decreases of precipitation overall in nearly 50 years, especially for the variations during winter half year. However, the higher temperatures and severely less precipitations during summer half year for the recent 10 years make the drying trend obviously sharpened since entering 21th century. Droughts occurred in winter and spring show a higher frequency than those in autumn and summer and present a west high-east low tendency. The summer drought frequency is the lowest. Drought coverages of every season can achieve above 50% and the widespread droughts mainly occur in winter and spring. The yearly mean of drought days which drought intensity level exceeds light are 112.5d, most of the droughts have occurred in winter and droughts appeared in spring take the second place,and the drought days during winter half year accounted for 64% of all year. In terms of the seasonal drought days of different levels, light and middle droughts are mostly like to occur in winter, but heavy and most droughts prefer to occur in spring.(2)The continuous droughts occurred in autumn, winter, spring in Southwest China from 2009 to 2013 are closely related to the atmospheric circulation anomalies of the tropical western Pacific, the south China sea, Indo-China Peninsula, the Bay of Bengal and the eastern Tibet Plateau. In the major periods of the four years droughts, Southwest China is controlled by the abnormal northwest airflow and sinking motion. The common backgrounds of the four years continuous drought are the sustainedly weaker south branch trough, fewer water vapor transportation over the Bay of Bengal and the pathway of the boreal cold air shifts eastwardly due to the weakened Lake Baikal ridge caused by the sustained negative phases of AO which are produced by the abnormal wave activities during winter weak polar vortex. The whole layer of west wind and the southern water vapor transportation are significantly weaker in the 2009/2010 and 2010/2011 seasonal sustained droughts. Although the southern water vapor transportation is strong in the year 2011/2012 and 2012/2013, but water vapor transport flux divergence over the Southwest China in most time of this four years is divergent and this is bad for the precipitation formation. The continuous drought in 2009/2010 occurred with an El Nino event, so that the lower troposphere over the tropical western Pacific generates anticyclonic circulation anomalies which are not conducive to the southwestern water vapor transportation. While continuous droughts in 2010/2011,2011/2012 and 2012/2013 are affected by La Nina events, especially the strength of the La Nina event in 2010/2011 is stronger than the rest of the two years. Thus, the western Pacific subtropical high anticyclone circulation is weaker and locate southeasterly that will weaken the water vapor transportation from the south China sea. And the persistently anomaly high sea surface temperatures of the tropical Indian Ocean in winter and spring for these three years have significant effects on the continuous droughts in Southwest China.(3) The composite analysis of typical dry years and wet years during winter half year in Southwest China show that in terms of the large-scale atmospheric circulation in the typical dry years, strength of the Lake Baikal ridge is weaker that is unfavorable to the cold air moving southward and Southwest China is controlled by the northwest airflow and sinking motion. Long wave trough lying east of the Caspian sea and the south branch trough are all weaker, western Pacific subtropical high expands westward and strengthens so that block transportation of the southwestern warm moist airflow to the Southwest China. On the whole, the cold air activities, water vapor conditions and dynamic factors are all not conducive to precipitation. During winter half year, the water vapor transport is mainly depend on the zonal water vapor transportation. Both the Zonal and meridional water vapor transport flux are relatively less than normal, and the whole layer of the water vapor transport flux divergence is divergent over Southwest China in the typical dry years. Continuous droughts during winter half year in Southwest China are closely related to the sea surface temperature anomalies of two key areas, one is the tropical Indian Ocean and the other is the tropical eastern Pacific. During the typical dry years, there will be positive sea surface temperature anomalies of the tropical Indian Ocean and negative sea surface temperature anomalies of the tropical eastern Pacific. In addition, winter stratospheric polar vortexes are significantly weaker in the typical dry years. The sustained negative phases of AO make the south branch trough intensity weakening and locating westerly so as to reduce the southwestern water vapor transportation and the strength of the Lake Baikal ridge is weaker as well so that the moving path of the boreal cold air shifts eastward and the cold air strength is weakened. All of these are not beneficial to the intersection of warm and cold air over Southwest China, thus leading to drought in Southwest China.In this paper, some meaningful conclusions are drawn through the research on characteristics of the drought variations and causes of continuous droughts during winter half year in Southwest China. These will help us to further deepen the understanding of the variation characteristics of droughts in Southwest China and have a certain reference value to improve the level of the regional drought monitoring and the ability of drought forecasting and warning.