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过去十年，全球多个地区发生了极端干旱事件，西南地区是极端干旱发生的热点地区之一(图1上)。中国西南地区干旱对当地的农业、水资源、生态系统以及经济社会造成了深远的影响(图1下,Wang et al., 2012)。西南干旱已成为政府和学术界高度关注的热点问题。中国科学院大气物理研究所东亚中心的王林与研究员陈文、黄刚以及香港城市大学副教授周文在揭示西南干旱特征、成因和干旱化趋势预估中取得系列研究进展。
虽然每一次西南干旱过程都伴随着降水减少，但西南极端干旱产生的根本原因是不同时间尺度干旱，即气象、农业、水文干旱相互叠加所致。同时，传统的干旱指数由于缺乏对干旱现象的综合考虑，难以刻画西南极端干旱。因此，作者提出了新的干旱指数——多尺度综合指标(CMI)，以检测不同时间尺度的干旱是否同步发生 (Wang et al., 2015b)。
未来百年，西南地区的降水和蒸发都表现为增加趋势，但蒸发增加的速率超过了降水，即降水对干旱的缓解作用完全被强烈的蒸发所抵消。虽然干旱化趋势明显以及极端干旱发生的频率增加，但极端偏湿事件的发生率也显著地增加，即西南地区未来的气候将变得更加极端。就全国而言，西南地区未来的干旱风险是其它区域的2倍(图2, Wang et al., 2014a, b)。
最近，作者还对西南干旱的相关研究进展进行了综述(Wang et al., 2015a)。目前而言，对西南干旱的研究主要可以分为两类：一是对西南地区过去和未来干旱演变特征的分析和预估，二是对西南干旱事件发生的大尺度环流背景和成因分析(图3，Wang et al., 2015c)。虽然该领域的研究进展较快，但仍缺乏综合性和系统性，因此作者对中国西南干旱的科研成果和发展动态进行了系统的归纳和总结，并希望对今后的研究有所启示。 
图1 过去十几年全球不同地区发生的极端干旱事件(上)，西南地区是热点地区之一。下图是西南干旱灾情照片(图片收集自网络)。 
图2 中等温室气体排放情景(RCP4.5，左)和高排放情景(RCP8.5,右)下，西南干旱未来的演变。褐色表示干旱加剧，而蓝色表示干旱缓解。 
图3 影响中国西南降水的关键海洋和大气因子。
Drought in Southwest China: Past and Future The last decade has witnessed a number of extreme droughts around the world, and Southwest China (SWC) is one of these hotspots (Fig. 1, top). The severe and sustained droughts in SWC during the last decade resulted in tremendous losses includingcrop failure, lack of drinking water, ecosystem destruction, health problems,and even deaths (Fig. 1, bottom; Wang et al., 2012). These devastating droughtsin SWC have attracted great concern from both the Chinese government and thescience community.
Dr. Lin WANG, Prof. Wen CHEN, and Prof. Gang HUANG from the Institute of Atmospheric Physics, and Associate Prof. ZHOU Wen from theCity University of Hong Kong, have carried out a series of studies to revealthe key characteristics and causes of droughts in SWC in the past, as well as the possible tendency in the future. They have found that, although the droughtsin this region are associated with deficient precipitation, the overall essentialdeterminant is in fact a combination of multiple stresses on water resources. Meanwhile, traditional drought indices, as well as the precipitation amount,fail to recognize these causes, owing to a lack of integrated treatment ofdrought. Hence, the team has proposed a new drought metric, termed the Comprehensive Multiscalar Indicator, to detect whether severe droughts at multiple timescales occur alone or in concert (Wang et al., 2015b).
Regarding future projection, towards the end of the21st century, precipitation and evaporation are both expected toincrease. In comparative terms, the rate of increase of evaporation outweighsthat of precipitation, producing an overall drying tendency in SWC. Furthermore,Dr. Wang and colleagues estimate that not only will incidences of severe andextreme drought increase dramatically in the future, but extremely wet eventswill also become more probable. Also of note is that the future drought risk inSWC is nearly twice that in other parts of China (Fig. 2; Wang et al., 2014a, b).
Through this avenue ofresearch, the team has summarized and integrated the accumulated but fragmentedfindings to aid understanding of multiple aspects of drought in SWC (Wang etal., 2015a), including both historical and future perspectives of drought andsea surface temperature, together with variation in circulation patternstriggering deficient precipitation in the region (Fig. 3; Wang et al., 2015c). Thislatter publication (which is a review paper) also highlights key issues andchallenges to be addressed in the future.
References Wang Lin, Chen Wen, Zhou Wen, Huang Gang. 2015a. Drought inSouthwest China: A review. Atmos. OceanicSci. Lett., 8, 339–344, DOI:10.3878/AOSL20150043.
Wang Lin, Chen Wen, Zhou Wen, Huang Gang. 2015b.Understanding and detecting super extreme droughts in Southwest China throughan integrated approach and index. Q. J.R. Meteorol. Soc., DOI: 10.1002/qj.2593.
Wang Lin, Chen Wen, Zhou Wen, Huang Gang. 2015c.Teleconnected influence of tropical Northwest Pacific sea surface temperatureon interannual variability of autumn precipitation in Southwest China. Clim. Dyn., 45, 2527–2539, DOI: 10.1007/s00382-015-2490-8.
Wang Lin, Chen Wen, Zhou Wen. 2014a. Assessment of futuredrought in Southwest China based on CMIP5 multimodel projections. Adv. Atmos. Sci., 31, 1035-1050, DOI: 10.1007/s00376-014-3223-3.
Wang Lin, Chen Wen. 2014b. A CMIP5 multimodel projection offuture temperature, precipitation, and climatological drought in China. Int. J. Climatol., 34, 2059–2078, DOI: 10.1002/joc.3822.
Wang Lin, Chen Wen. 2012. Characteristics of multi-timescalevariabilities of the drought over last 100 years in Southwest China. Advances in Meteorological Science andTechnology, 2(4), 21-26. (in Chinese)
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