Theactivityoflowfrequencyoscillation(LFO)widelyexistsinEastAsiaduringtheperiodof1991excessivelyheavyrainovertheChangjiang-HuaiheRiverBasin(Jianghuai).BoththerainfallamountofJianghuaiandtheatmospherefromsubtropicalareatomid-highlatitudeshavethedominantperiodof10—20d,whiletheatmospheresintropicalareaandhighlatitudeshavethedominantperiodof30—60d.ComparedwithnormalMeiyuseason,theanomalyofthe1991Meiyuprocessmaybereflectedinthefollowingtwolowfrequencysynopticevents:(a)TheMeiyuprocessonsetsextremelyearly(intheseconddekadofMay,whichisnearlyonemonthearlierthaninnormalMeiyu)andisimmediatelyfollowedbythefirstepisodeofheavyrain.(b)InthefirstdekadofJuly,thereoccurstheheaviestepisodeofrainfallofthewholeMeiyuseason,anditiseventheheaviestrainfallfortherecent30yearsinChina.Forthesetwoperiods,correspondingtotheadjustmentoflarge-scalesituationfrom“doubleblockinghigh”to“bipolarblockinghigh”,thepropagationdirectionofLFOinEastAsiahasadistinctseasonalvariation,fromeastward/northwardpropagation(passingthroughJianghuai)duringthefirstepisodetowestward/southwardpropagationduringthethirdepisode.Oscillationsofdifferentfrequencybandsaresuperposedinphase.TheLFOactivityofthecoldandwarm/moistairsoverJianghuaicanbestrengthenedthroughthoseLFOpropagationprocessesinEastAsia,althoughtheymayhavedifferentdirectionsinthreeepisodes.Particularly,theeastward(westward)propagationinlowlatitudesmakesthesouthwest(southeast)airflowtransportintensivelythelowfrequencywarm/moistairtothesouthofJianghuaifromtheIndianOcean(tropicalWestPacificOcean)inthefirst(third)episode.Suchwarm/moistairsmeetandinteractwiththecoldairwhichvigorouslyinvadesJianghuaipersistently,andfinallythreeepisodesofheavyrainoccurinmodeofLFO.
