Anthropogenic influence on the occurrence of extreme drought like that in eastern China in 2019

Abstract

Eastern China (EC) experienced extreme drought during August–November 2019, with notable features of a particularly prolonged duration and record-breaking intensity, leading to huge agricultural and economic losses. In the context of anthropogenic climate change, the authors investigated the anthropogenic impacts on the occurrence of extreme drought like that in 2019 (2019Drought-like event) and the related physical mechanism. The authors first selected relatively better-performing models by evaluating their historical simulations, and found that the occurrence probability of a 2019Drought-like event increased by eight times due to anthropogenic forcing. It seems that anthropogenic forcing induces background mean state changes over EC, including a decline in mean moisture content and enhanced mean descending motion. These two mean state changes are favorable for the formation of extreme drought. Diagnosis of the moisture budget showed that the decline in mean moisture mainly stems from the enhanced mean descending motion via the term −δω¯·∂q¯∂p (vertical advection of mean moisture by the change in mean vertical motion). The enhanced descent is linked to the anthropogenic forcing–induced mean sea surface temperature warming pattern, which exhibits a maximum near the equatorial region. This reinforces the mean ascending motion over the Maritime Continent, as in “the rich-get-richer” mechanism, and further causes strengthened subsidence over the EC region through a “regional Hadley circulation”. This study suggests that anthropogenic forcing can sharply increase the occurrence probability of a 2019Drought-like event by modulating large-scale circulations.摘要2019年夏秋季中国东部遭遇了七十年一遇的强持续性极端干旱事件. 本文借助于CMIP6检测归因模式比较计划 (DAMIP) 数据, 量化了人为强迫对类似2019年极端干旱事件发生概率的影响. 通过比较不同外强迫因子作用下此类极端干旱事件的发生概率, 发现人为强迫可使此类极端干旱事件的发生概率提高8倍左右. 进一步研究发现人为强迫引起的海表温度增暖是经向不均匀的, 对应海洋大陆附近存在一个增暖大值中心, 从而增强海洋大陆地区平均上升环流, 通过局地Hadley环流作用导致中国东部地区平均垂直环流出现异常下沉, 最终有利于中国东部极端干旱事件发生概率增加. 该研究表明, 人为强迫通过调制气候平均背景场从而引起极端事件发生频次的变化是人类活动影响极端气候事件的重要途径之一.