2022年夏季长江流域重大干旱特征及其成因研究

Study on characteristics of severe drought event over Yangtze River Basin in summer of 2022 and its causes

干旱是影响范围最广的自然灾害之一。2022年夏季发生在长江流域的异常高温干旱事件不仅强度大,而且持续时间长,是一次罕见的重大干旱事件,对我国的社会经济造成了十分严重的影响。鉴于这次事件的极端性,本文在客观分析此次事件演变特征的基础上,揭示大气环流和外强迫异常对此次高温干旱的可能影响。研究发现,气象干旱指数及土壤湿度监测结果一致表明本次旱情从6月开始出现,7月迅速发展,进入8月后范围进一步扩展、强度进一步加剧。与此同时,流域内整体气温偏高,部分地区高温日数超过40 d。此外,夏季整个流域的蒸散量距平是1960年以来的历史第二高值(仅次于2013年高温伏旱),进一步加剧了长江流域的水分亏缺程度。从环流特征来看,夏季西太平洋副热带高压异常偏强西伸、极涡面积偏小及强度偏弱、南亚高压偏强东移,共同导致长江流域的水汽输送条件偏弱、下沉气流盛行,使得整体条件不利于降水发生。而前期拉尼娜事件的持续、印度洋偶极子负位相的出现以及春季青藏高原西北部积雪负异常的持续,可能是导致今年夏季环流异常的主要外强迫因子。

Drought is one of the natural disasters with the widest global impact.The anomalous drought and heatwave event that occurred in the Yangtze River Basin in summer of 2022 is not only of high intensity but also of long duration,it is a rare and significant drought event leading to very serious socioeconomic impacts in China.In view of the extreme nature of this event,this paper reveals the possible influence of atmospheric circulation and external forcing anomalies on this drought event based on an objective analysis of the evolutionary characteristics of this event.It is found that the meteorological drought index and soil moisture monitoring results consistently indicate that this drought event started to appear in June,developed rapidly in July,and further expanded and intensified in both extent and intensity in August.At the same time,the overall temperature in the basin was high,with the number of high temperature days exceeding 40 days in some areas.In addition,anomaly of evapotranspiration over the basin in summer was the second highest on record since 1960,second only to the high temperature drought event in 2013,which further exacerbated the degree of water deficit in the Yangtze River Basin.From the perspective of circulation characteristics,the abnormal intensifying and westward extension of the western Pacific subtropical high pressure,the small area and weak strength of the polar vortex and the intensifying and eastward shift of the South Asian high pressure in summer jointly led to weak water vapor transport conditions and prevailing sinking air currents in the Yangtze River Basin,making the overall conditions unfavorable for the occurrence of precipitation.The persistence of the La Ni?a event,the appearance of negative Indian Ocean Dipole(IOD) and the persistence of the negative snow cover anomaly in the northwestern Tibet Plateau in spring may be the main external forcing factors leading to the circulation anomaly in this summer.