| Under the background of global warming,extreme events of weather and climate have occurred more and more frequently,as well as regional events of drought and flood have shown an increased trend.In recent years,anomalous winter precipitation in China have occurred off and on,which cause serious impact on people's productions and lives,so winter precipitation draw more and more attention from people,studying the pattern of spatial-temporal variation and corresponding cause could provide decision support for regional water resources management and use,as well as for drought and flood disaster prevention.This paper,based on winter precipitation data of 96 meteorological stations from 1961 to 2014 in Dongting Lake Basin,analyze spatial-temporal variation characteristics of winter precipitation in recent 54 years in Dongting Lake Basin by using methods of empirical orthogonal function(EOF),Morlet wavelet analysis,and so on,monthly mean reanalysis dataset from NCEP/NCAR,monthly mean sea surface temperature dataset from Met Office Hadley Centre are used to analyze causes of winter precipitation especially anomalous winter precipitation in Dongting Lake Basin.The results show that:(1)In spatial,mean winter precipitation in Dongting Lake Basin exhibit decreasing trend gradually from central reaches of Xiangjiang River to southeastern,northwestern areas of Dongting Lake Basin,the distribution of standard deviation of winter precipitation is the same as mean winter precipitation's,namely,areas of more precipitation are with greater value of standard deviation,vice versa.In temporal,winter precipitation in Dongting Lake Basin exhibits remarkable interannual and interdecadal variation characteristics in the past 54 years,from interdecadal variation,there were less rainfall in 1960s and 1970s,more rainfall started from 1980s,the most rainfall occurred in 1990s,after entering 21 century,winter precipitation exhibited fluctuant decreasing trend.In recent 54 years,winter precipitation of each station in Dongting Lake Basin was mainly with increasing trend,however,which was not significant,stations with significant trend were Huarong,Yueyang,Linxiang,Zixing Station.(2)There were three leading modes by using EOF methods to decompose winter precipitation in Dongting Lake Basin:the mode of winter precipitation is consistent over the whole area,the mode of out-of-phase relationship between south Dongting Lake Basin and north Dongting Lake Basin,the mode of out-of-phase relationship between central Dongting Lake Basin and south/north Dongting Lake Basin,these three modes reflect various distribution characteristics of winter precipitation in Dongting Lake Basin.Wavelet analysis shows that,there are long-period of 12a,16a,7a and short-period of 3 a,respectively.According to REOF methods,Dongting Lake Basincan be divided into four subareas,area with the most winter precipitation is subarea I(Southerm region),with the least winter precipitationis subarea IV(Westem region).(3)Winter precipitation in Dongting Lake Basin is closely associated with atmospheric circulation situation in mid-high latitude areas of Eurasia,spatial distribution are roughly opposite during typical winter flood years and typical winter drought years,for concrete manifestation,during typical winter flood years,500hPa geopotential heightin Eurasia exhibit“positive-negative-positive'' over zonal and c“north low-south high,over meridional,East Asia trough is weaker than normal and the position is more easterly than normal,high pressure ridge near to the Urals is weaker than normal,meridional circulation is also weaken,finally cause weak winter monsoon which couldn't penetrate into Dongting Lake Basin,this condition is favorable for low latitude ocean's warm and wet air flow transport northward,leading to more winter precipitation in Dongting Lake Basin;during typical winter drought years,500hPa geopotential height in Eurasia exhibit "negative-positive-negative" over zonal and "north high-south low" over meridional,Europen trough and East Asia trough is deeper than normal,their positiorn is more westerly than normal,high pressure ridge near to the Urals moves northward and is stronger than normal,meridional circulation is enhanced,leading to stronger winter monsoon,this condition is favorable for southward invasion of cold air mass,under the control of northerly wind,less winter precipitation occurred in Dongting Lake Basin.(4)From prespective of water vapor transport,during typical winter flood years,water vapor transport from low latitude ocean to Dongting Lake Basin,at the same time,ascending atmospheric motion in Dongting Lake Basin is stronger than normal,which is favorable for water vapor convergence to form precipitation;during typical winter drought years,generally,Dongting Lake Basin is under the control of northerly wind,with water vapor divergence,ascending atmospheric motion suppressed,which is not favorable for precipitation.Analyzing each boundary's water vapor flux can we find that,during typical winter flood years,more anomalous water vapor import exist in southern boundary of Dongting Lake Basin,as oppsite to the condition during typical winter drought years,water vapor import from southern boundary of Dongting Lake Basin is a key factor casuing drought or flood.Compared with precipitable water,anomalous winter precipitation in Dongting Lake Basin is much influenced by water vapor convergence and divergence.(5)Winter precipitation in Dongting Lake Basin is closely related with ENSO and sea surface temperature of South China Sea,correlation coefficient are respectively 0.39,0.43,both are significant above the 99%significance level;the correlation signal between ENSO and winter precipitation in Dongting Lake Basin has occurred at preceding summer,with ENSO mainly affecting south Dongting Lake Basin,whereas the correlation signal between South China Sea and winter precipitation in Dongting Lake Basin has occurred at synchronous winter,and South China Sea mainly affecting east Dongting Lake Basin;there are not marked correlation 'between time coefficients of the firs EOF mode of winter sea surface temperature over South China Sea but with ENSO signal removed and winter precipitation in Dongting Lake Basin,which is other than time coefficients of the second EOF mode of winter sea surface temperature over South China Sea but with ENSO signal removed,this is related to corresponding water vapor transport.(6)ForNino3.4 sea surface temperature,Southern Oscillation Index(SOI),Indian Ocean Basin Mode(IOBM),all of them have stable and lead correlation with winter precipitation in Dongting Lake Basin,for Nino3.4 sea surface temperature,SOI,significant correlation with winter precipitation in Dongting Lake Basin is started from preceding summer,eontinued to synchronous winter,for IOBM,significant correlation is started from preceding Novmber,further discuss the contributions of EUCI,PW,vertical-integrated water vapor flux divergence,IOBM,ENSO and sea surface temperature of South China Sea to winter precipitation in Dongting Lake Basin,the results show that vertical-integrated water vapor flux divergence contributes most,EUCI contributes secondly. |
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