基于日干旱指数的青藏高原1979-2020年干湿变化特征分析

A Daily Drought Index-based Dry and Wet Variation Analyses over the Qinghai-Xizang Plateau from 1979 to 2020

青藏高原是对全球气候变化响应最为敏感的地区之一,在区域水循环中扮演着重要角色。尽管目前关于青藏高原气候干湿变化已有大量研究,但大多是基于月干旱指数,对月以下时间尺度干湿变化的研究还相对较少,不利于对更短时间尺度干湿变化的认识。本文使用日干旱指数,即日蒸散发差指数DEDI(Daily Evapotranspiration Deficit Index),基于ERA5再分析数据分析了1979-2020年青藏高原的干湿变化,尤其是不同强度干湿类型(特旱、重旱、中旱、轻旱、特湿、重湿、中湿和轻湿)发生日数的季节差异。结果表明:(1)青藏高原1979-2020年的干湿变化具有较为明显的年代际转换特征,大部分地区呈现出干旱-湿润-干旱-湿润的年代际变化特征。(2)过去40多年间,春季和冬季发生的4种强度干旱类型呈增加趋势,夏季4种强度干旱类型呈减小趋势,秋季特旱、重旱和中旱呈增加趋势,轻旱呈减小趋势。与干旱发生日数的变化趋势基本相反,春、秋和冬季特湿呈增加趋势,重湿、中湿和轻湿呈减小趋势,夏季4种强度湿润类型呈增加趋势。相比其他季节,夏(冬)季4种强度的湿润(干旱)发生日数增加趋势最为显著,即夏季和冬季分别具有最强的湿润和干旱化趋势。此外,对于任何季节,极端情况的干旱和湿润类型发生日数都远少于其他强度的干湿类型,中等和轻度情况的干旱和湿润最为频繁发生且发生日数比较接近。

The Qinghai-Xizang Plateau is one of the most sensitive areas to global climate change and plays an important role in regional water cycle.Many studies have been conducted on the dry and wet variations over the Qinghai-Xizang Plateau in recent decades.However,the previous studies were based on monthly drought indices,the daily day and wet variations over the Qinghai-Xizang Plateau in the last decades are still unclear.In this study,a daily drought index named Daily Evapotranspiration Deficit Index(DEDI)calculated using the ERA5 reanalysis data was utilized to analyze the daily dry and wet variations over the Qinghai-Xizang Plateau from 1979 to 2020,especially the seasonal differences in the number of occurrence days for dry and wet categories with different intensities(i.e.,extreme,severe,moderate,and abnormal conditions).The main results are as follows:(1)The dry-wet variations over the Qinghai-Xizang Plateau experienced remarkable interdecadal transition from 1979 to 2020,and basically characterized as dry-humid-dry-humid in the last 40 decades.(2)In the past 40 years,daily droughts with four intensities(i.e.,extreme,severe,moderate,and abnormal droughts)over the Qinghai-Xizang Plateau showed increasing tendencies in spring and winter,and showed decreasing tendencies in summer.In autumn,extreme,severe,and moderate droughts showed increasing trends,and abnormal drought had a decreased trend.In contrast to the tendencies of drought occurrence days,extreme wetness in spring,autumn,and winter showed increasing trends,while severe,moderate,and abnormal wetness had decreased trends.Wetness in all the four intensities over the Qinghai-Xizang Plateau in summer showed increasing trends.Compared with other seasons,the number of wet/drought days with four intensities had the most significantly increasing trend in summer/winter,i.e.,summer/winter had the strongest wetting/drying tendency.In addition,the occurrence days of extreme drought/wetness were much smaller than drought/wetness in other intensities in each season.Drought/wetness in moderate and abnormal intensities occurred most frequently and had close numbers of occurrence days.