Analysis Of Spatial-temporal Variation And Influencing Factors Of Seasonal Meteorological Droughts In Dongting Lake Basin

Drought is the most common natural disaster which exerts the greatest impacts on human society.It is caused by water deficiency during a long time.Dongting Lake Basin is located on the middle and lower reaches of the Yangtze River,which is a typical monsoon climate region,extremely sensitive to climate change and frequently suffers from meteorological droughts.Researches on meteorological droughts in Dongting Lake Basin would improve the understanding of the spatial-temporal variation of seasonal meteorological droughts and their controls,and provide supports for the prevention and mitigation of synoptic and climatic drought/flood disasters over the basin.Dongting Lake Basin is alternately controlled by winter monsoon,southwest monsoon and southeast monsoon in a year,and meteorological droughts would occur in each season.In addition,the leading controls of these droughts are distinctive.Therefore,based on the monthly precipitation data from 103meteorological stations in the basin(from 1960 to 2017),this paper analyzed the spatial-temporal distribution characteristics of meteorological droughts over the basin and determined the influencing factors of meteorological droughts by monthly reanalysis data.The results showed that:(1)Based on the statistics in recent 58 years,the frequency of winter droughts was higher in the 1960s,and the basin had been getting wetter in winter since then.There was an increase of the autumn drought in the 1970s,and the frequency of it was higher in the 1990s and 2000s.In autumn,there was a trend of getting wet near the lake area,and the western basin became drier.In spring,the basin showed a drying trend,but in summer,it showed the opposite.The upper reaches of the rivers in the basin were prone to drought,and the frequency of spring and summer drought was higher in the 1980s and 2010.(2)The summer and winter precipitation changed abruptly in 1985 and 1978,respectively,then maintained an increasing trend.The Z-index of autumn may gradually increase after the abrupt change in the 2010s.The dry/wet phase changed or remained in two adjacent years during spring and winter respectively,and it reversed in autumn and winter at?=23 and?=28-30,respectively.(3)In typical drought years,at the 850h Pa geopotential height,the downdraft anomalies occurred in all seasons over the basin.At the 500h Pa geopotential height,there were EU-like teleconnections in the middle and high latitudes in each season.Besides,there was an EAP teleconnection in summer and autumn,simultaneously,the western pacific subtropical high northward.The centers of geopotential height anomalies at 200h Pa in each season were almost corresponding to the centers of horizontal wind anomalies in the middle troposphere.There was a Silk Road pattern teleconnection in summer,and the South Asia high was weaker than the average.(4)In terms of water vapor transport,there were obvious water vapor flux divergence over the basin in typical drought years.Easterly water vapor flux anomalies existed when droughts occurred in spring.During the summer drought,the water vapor flux anomalies showed a strong summer monsoon pattern.As for autumn drought,the mean westward water vapor flux received by the basin was less than average.In winter droughts,the mean water vapor flux anomaly was northward.The overall Z index of the basin was significantly positively correlated with the north-south net fluxes.(5)Preceding ENSO events exerted significant effects on drought/flood in spring,autumn and winter.With the ENSO signal removed,SST near Aleutian Islands(S3)and East Australia(S4),IOD and IOBW were still closely related to drought/flood in spring,summer,autumn and winter over the basin respectively.The impact of SST over S3 on spring drought/flood was exerted from westerly circulation.The higher SST over S4 seemed to be the manifestation of weak East Asian summer monsoon.The mature IOD was one of the important controls of autumn drought/flood,and the IOBW in positive phase contributed to the formation of precipitation dynamic conditions in winter.(6)There were significant and sustained statistical linkages between the ENSO-related indexes in preceding summer and the overall Z-index in spring,autumn and winter.QBO took the leading role of the fitting equation of the overall Z-index over the basin in spring.GMLOT of the preceding summer was the most important factor in the fitting equation in summer.The preceding ENSO-related indexes accounted for a large proportion in the fitting equations of autumn and winter overall Z-indexes.(7)In summer and autumn,there was a significant amount effect on stable isotopes over the basin,and there were negative correlations between precipitation?¹⁸O,precipitation and GX.Besides,precipitation?¹⁸O was close to the upper atmospheric circulation pattern over the basin.The sea areas near the Philippines and Australian northeast coast were important water sources,besides,the precipitation and precipitation?¹⁸O over the basin and that over the maritime continent were in phase.