The northeast monsoon and its effects to the weather of ...

The northeast monsoon in China is basically attributed to the cold air and high pressure area residing over the Asian Continent in the latter part of a year. SkipContent ThenortheastmonsoonanditseffectstotheweatherofsouthernChinainautumn EducationalResources false ThenortheastmonsoonanditseffectstotheweatherofsouthernChinainautumn HOPak-singDecember2010 ThenortheastmonsooninChinaisbasicallyattributedtothecoldairandhighpressurearearesidingovertheAsianContinentinthelatterpartofayear. Theairthatflowsfromhightolowpressureregionsiscalledthenortheastmonsoon. Innormalcircumstances,thenortheastmonsoonstartstoaffectsouthernChinainautumn. InHongKong,autumnusuallybeginsfrommid-SeptembertolateOctober. Sinceautumnthisyear(2010),severalsurgesofthenortheastmonsoonandtheirreplenishmentshadaffectedHongKong. Localtemperaturesdroppedfromover30oCinsummertoaround20oC. Hotweatherwasreplacedbymildconditionswithwindsturningfromsouthwesterlies/southeasterliestonortheasterlies/northerlies. Onewouldfindthatcumulonimbuscloudsandtheassociatedshowersbecamerare,whilestratocumuluscloudswouldoccasionallybringraintotheterritory. Allthesechangesarerelatedtothenortheastmonsoon.   Fromearlytomid-September,thesubtropicalridge(1) shiftssouthwardsfrom29-30to24-25degreeNorth(latitude)inconcertwiththeweakeningofthesummermonsoon,whilethecoldhighpressuresystemestablishesitselfovernorthernChina. Northerlywindsatthe850hecto-pascal(hpa)level(around1500metresaboveground)spreadfromChangjiangregionsoutheastwards,signifyingtheestablishmentofthenortheastmonsoon. BytheendofSeptember(aroundtheAutumnalEquinox),coldairfromthenorthcouldrideovertheNanlingmountainandintrudeintoGuangdong. InearlyOctober,thesubtropicalridgefurtherretreatstothesouth,dryandcoldairfromthenorthcouldthenfrequentlyreachthecoastalareasofsouthernChina. Thedailymeantemperatureduringthisperiodgoesbelow23oCwithdryandwindyweather. Apartfrombringingaboutchangesinwindsandtemperatures,thenortheastmonsooncanalsoaffecttropicalcyclonesovertheSouthChinaSea. Ifthenortheastmonsoonhappenstomeetthewarmandmoistairassociatedwithtropicalcyclones,windyandrainyweathercouldfollowandisoftencalled"WetColdDewWind"(WCDW)weatherinMainlandChina. OnesuchexampleistheheavyrainepisodeatHainaninearlyOctoberthisyear. Theheavyrainseverelyaffectedtheharvestoflatecropandvegetable. TheintervalbetweentwoWCDWoccasionsisusually6to8days,beingshorterinnorthernGuangdongandlongerinthesouth. DuringaWCDWoccasion,theaveragedailyrainfallisingeneralintheorderof30mmbutitcouldbeover100mminthecloseproximityofatropicalcyclone. Thenortheastmonsoonhelpstointensifytherain-bandsassociatedwiththetropicalcycloneandsignificantlyraisestheprobabilityofdailyrainfallover100mmintheHainanprovinceandthecoastalareasofGuangdong. SimilarweatheraffectedHongKongbetween10and11October2010. Atthattime,thecombinedeffectofalowdepressionareaandthenortheastmonsoonbroughtover50mmofrainfalltoShatininHongKong(Fig.1). TheStrongMonsoonWarningSignalwasinforceduringthatperiod. Althoughalowpressureareainsteadofatropicalcycloneisinvolvedinthisoccasion,itisagoodexampleoftheinteractionofthenortheastmonsoonwithwarmandmoistairassociatedwiththelowpressurearea,bringinganintenserainbandtosouthernChina. Fig1:Weatherchartfor1400HKT10Oct2010 Apartfromenhancingtherainbandsoftropicalcyclones,thenortheastmonsooncanalsoaffecttheirmovement. TropicalcyclonesovertheSouthChinaSeaaroundtheautumnequinoxfrequentlymakeslandfallalongthecoastalareasofGuangdongandtheHainanprovince. TyphoonDamrey(25/9/2005),TropicalStormFrancisco(24/9/2007)(Figures2and3),andTyphoonKetsana(28/9/2009)areafewexamples. Fig2:WeatherChart(withobservation)for0800HKT24Sep2007  Fig3:TrackofTropicalStormFRANCISCO TropicalcycloneusuallymovesalongthesouthernedgeofthesubtropicalridgeinOctoberandNovember,crossingtheBashiStraitorBalintangChannelorthenorthernpartofPhilippinesandenteringtheSouthChinaSea. TheywouldthenbesteeredtothesouthwestbythestrengtheningnortheastmonsoontoaffectthecoastalareasofGuangdong. Whenthetropicalcycloneencountersthenortheastmonsoon,itusuallymovesonawestwardtrackandmakeslandfalltothewestofthePearlRiverDeltaregionorcoastalareasofHainan. However,thereareexceptionsandSevereTyphoonMegiin2010wasanexample. Movementoftropicalcyclonesinsummerisusuallydominatedbythesteeringflowatthe500hpalevel(around6000metresaboveground)whiletheroleofthe850hpalevelflowbecomesmoreprominentinautumn. Becauseofthedifferentairflowatdifferentaltitudes,thetrackofatropicalcycloneisnotstraightforward. Between20and22October2010,theeffectofthenortheastmonsoontothemovementofMegiovertheSouthChinaSeapresentedachallengetoweatherforecasters(Figures4and5). IteventuallytookanortherlytrackandlandedonthecoastofFujian,bringingseveredamagestonearbyareas. Fortunately,therewas nomajorimpacttoHongKong.  Fig4:WeatherChart(withobservation)for0800HKT21Oct2010  Fig5:TrackofSeverTyphoonMEGI Reference ThereferencesinChineseonly,areavailable here.