A Study On The Subseasonal Precursor Information Of Persistent Drought Events In Southwestern China

Based on integrated meteorological drought index(CI) and NCEP/NCAR griddata, by using EOF and CEOF methods, we analyse the characteristics of persistentdrought event in southwestern China; by using WPD, EEOF and correlation coefficientmethods, we analyse the subseasonal precursor information of seasonally persistentdrought events. The results show that:(1) Winter and spring drought events is prone occurred in southwestern China, theaverage drought area is more than30%in these two seasons. There are11yearswithin the total54from1956to2009that winter and spring droughts occurred,and the percentage is more than20%. In these years, warm and cool air from bothsouth and north sides are weak in500hPa geopotential height, and hardly meet.(2) Based on the CI index of Kunming and Guiyang stations, we divide the seriousmeteorological drought lasting from2009to2010into6stages: September21th toNovember5th, moderate drought; November6th to November18th, heavydrought; November19th to December20th, moderate drought; December21th toJanuary10th, light drought; January11th to February22th, moderate drought;February22th to March20th, heavy drought. And as well, we can sum thesestages up into3stages, those are early November, heavy drought; December toFebruary, light-moderate drought; and March, heavy drought.(3) From the view of wave, there are obvious characteristics of drought appear in theserious meteorological drought lasting from2009to2010. The results of EOFshow that the heavy drought in early November may dominant by the firsteigenvector of700hPa wind, which is northwest wind; the moderate drought inlate November and December may dominant by the dry700hPa relative humidityfield; and the moderate-heavy drought after January may dominant by both the500hPa geopotential height and700hPa north wind. The results of CEOF showthat the main factors of this event is high amplitude(geopotential height pleion)over Qinghai-Tibet Plateau eastward propagating and stay in southwestern region. (4) From the view of wave energy, blocking high in the high latitudes and westernpacific subtropical high in the low latitudes, which are the most important factorsof this drought event, the intensity and range of their wave packet are anomalousfor a long time. Blocking high in the high latitudes strong and east results in theeast path of the polar cold air; western pacific subtropical high strong and northand also west leads to the warm and moist air hardly go north. Cool and warm airhardly flow together for a long time, so it is difficult to form precipitation insouthwest region, thus the drought continued. Besides, during the entire event, thewave-packet of the southwest region continuely low, show up obviouscharacterastics of drought.(5) Extracting3most important key region of this event, they are polarregion(60-80°N,100-140°E), blocking high region(40-60°N,60-100°E) andsubtropical high region(10-30°N,110-140°E).(6) The EEOF results(10-30days in advance) show that, at the stage of heavy drought,cool air in200hPa starting to become weak from20-10days before, wave energyof blocking high region and subtropical high region enhance from10days beforethe event; at the stage of drought alleviate, the wave-packet center of blockinghigh region in200hPa is strong, east, and north10days before, at the same timethe wave-packet center of subtropical high region in200hPa is west, thewave-packet of polar region is also strong in30days; at the stage of droughtmaintain, the wave-packet center of blocking high region in200hPa becomingintenseness in west and weakness in east, the wave-packet center of subtropicalhigh region is normal, and the wave-packet of polar region is still strong in30days.(7) Comparing wave-packet and CI index, and also considerating the correlationcoefficient, we conclude some primary index of the drought events. Testing thisindex by drought events between1998and2003, finally we generalize it asfollows: When the wave-packet of polar region and blocking high region exceedstheir own threshold(polar region0.4in200hPa,0.7in500hPa,0.9in700hPa;blocking high region0.4-0.5in these3levels) before70-80days and80-90daysrespectively, drought is likely to develop in southwest China.