Dominant Synoptic Disturbance in the Extreme Rainfall at Cherrapunji, Northeast India, Based on

15 Pages·2017·3.69 MB·English

15OCTOBER2017 MURATA ET AL. 8237
Dominant Synoptic Disturbance in the Extreme Rainfall at Cherrapunji,
Northeast India, Based on 104Years of Rainfall Data (1902–2005)
FUMIEMURATA,aTORUTERAO,bHATSUKIFUJINAMI,cTAIICHIHAYASHI,d
HARUHISAASADA,eJUNMATSUMOTO,fANDHIAMBOKJ.SYIEMLIEHg
aFacultyofScienceandTechnology,KochiUniversity,Kochi,Japan
bFacultyofEducation,KagawaUniversity,Takamatsu,Japan
cInstituteforSpace-EarthEnvironmentalResearch,NagoyaUniversity,Nagoya,Japan
dCenterforSoutheastAsianStudies,KyotoUniversity,Kyoto,Japan
eNaraWomen’sUniversity,Nara,Japan
fTokyoMetropolitanUniversity,Hachioji,andJapanAgencyforMarine-EarthScienceand
Technology,Yokosuka,Japan
gDepartmentofGeography,North-EasternHillUniversity,Shillong,India
(Manuscriptreceived5June2016,infinalform18July2017)
ABSTRACT
Thecharacteristicsofactiverainfallspells(ARSs)atCherrapunji,northeastIndia,whereextremehigh
rainfallisexperienced,andtheirrelationshipswithlarge-scaledynamicswerestudiedusingdailyrainfalldata
from1902to2005andJapanese55-YearReanalysisfrom1958to2005.Extremehighdailyrainfallsoccurin
associationwithARSs.Theextremelylargeamountsofrainfallinthemonsoonseasonaredeterminedbythe
cumulativerainfallduringARSs.ARSsstartwhenanomalousanticycloniccirculation(AAC)at850hPa
propagateswestwardfromtheSouthChinaSeaandwesternNorthPacific,andcoversthenorthernBayof
Bengal.TheAACpropagatesfartherwestwardandsuppressesconvectionovercentralIndiaduringARSsat
Cherrapunji,andcontinuesfor3to14days.Consequently,anorthwardshiftofthemonsoontroughduring
the‘‘break’’intheIndiancoreregionoccurs.Thewesterlywind,whichprevailsinthenorthernportionofthe
AAC,transportsmoisturetowardnortheastIndiaandenhancesmoistureconvergenceovernortheastIndia
withsoutherlymoisturetransportfromtheBayofBengal,andgreatlyintensifiestheorographicrainfall.In
theuppertroposphere,theTibetanhightendstoextendsouthwardwiththeonsetofARSs.Alinearre-
lationshipcanbeseenbetweenthelengthandtotalrainfallofanARS.LongerARSstendtoresultingreater
totalrainfall.AACswithagreaterzonalscaletendtoproducelongerandmoreintenseARSs.Thisstudy
providesevidencefortheeffectofwesternNorthPacificAACsontheIndiansummermonsoon.
1. Introduction The synoptic condition during break spells has been
described as a northward shift of the trough of low
The Indian summer monsoon has remarkable intra-
pressuretowardthefoothillsoftheHimalayas.
seasonalvariation.Rainfallperiodswithaboveandbe-
Ohsawaetal.(2000)studiedtheintraseasonalrainfall
lownormalrainfallamountshavebeencalled‘‘active’’
variationduringthe1995summermonsoonseason,and
and ‘‘break’’ spells, respectively. A detailed review of
found that rainfall in Bangladesh increased when the
studies of active and break spells was conducted by
monsoontroughwaslocatedatthefootoftheHimalayas.
Rajeevan et al. (2010). Traditionally, the active and
Shrestha et al. (2012) analyzed rainfall over the central
breakspellsoftheIndiansummermonsoonhavebeen
Himalayanregionusing11years’worthofdatafrom
based on the convective activity over northwest and
the Tropical Rainfall Measuring Mission (TRMM)
central India [referred to as the ‘‘Indian core region’’
PrecipitationRadar,andshowedthatrainfallwasheavy
following Rajeevan et al. (2010)]. Negative (positive)
overthesouthernmosthillsoftheHimalayas[calledthe
rainfallanomaliesovernortheastIndiacorrespondedto
Siwalik Range or sub-Himalayas; approximately 500–
active (break) spells over northwest and central India.
700mabovemeansealevel(MSL)]whenthemonsoon
trough was lying in the foothills of the Himalayas.
Correspondingauthor:FumieMurata,[email protected] Goswamietal.(2010)analyzedtheextremerainfallover
DOI:10.1175/JCLI-D-16-0435.1
(cid:1)2017AmericanMeteorologicalSociety.Forinformationregardingreuseofthiscontentandgeneralcopyrightinformation,consulttheAMSCopyright
Policy(www.ametsoc.org/PUBSReuseLicenses).
8238 JOURNAL OF CLIMATE VOLUME30
northeastIndia, and showedthat these extreme events fromtheHimalayanrangesbytheBrahmaputraValley.
occurred in association with monsoon synoptic events As the plateau is the first elevated terrain that warm,
rather than isolated thunderstorms. In general, studies moistairfromtheBayofBengalencounters,maximum
that have focused on intraseasonal variation over precipitation tends to occur (Houze 2012). It is con-
northeastIndiahavebeenlimited. nected to the Indo-Burma Range in the east. The
Hartmann and Michelsen (1989) performed spectral southernmarginoftheMeghalayaPlateauissteepand
analysis of rain gauge data for 70 years from 3700 sta- ruggedduetothetectonicmotionalongtheDaukifault
tionsinIndia,andnotedthatseveralstationsalongthe at the southern boundary of the plateau. There are
southern flank of the Himalayas had spectral peaks of severaldeepvalleysthatopentowardthesouthwithsteep
nearly 15 days. The 40–50-day spectral peak was not cliffsonbothsides(Fig.1b):thetownofCherrapunjiis
significant along the southern flank of the Himalayas, situatednearonesuchvalley.Orographicamplification
whereas it was dominant at most stations elsewhere in isthemajorplausiblecausefortheextremeprecipitation
India. Ohsawa et al. (2000) detected a periodicity of at thislocation (e.g., Romatschke et al. 2010), although
around 20 days in an analysis of the 1995 summer otherfactors,suchasalocal frontbetweenthesouth-
monsoon season. Fujinami et al. (2011) analyzed rain westmonsoonflowandeast–northeastmountainwinds
gaugedatafor20yearsinBangladesh,andshowedthata intheBrahmaputraValley,mayalsohaveaninfluence
7–25-dayspectralpeakwasdominant,whilea30–60-day (Yoshino1975).
spectralpeakwasnotprominent.Fujinamietal.(2014) NortheastIndiareceivestheheaviestrainfallinIndia.
further investigated the spatiotemporal structure of a Astheregionincludeshillyareasandadjoinsthegreat
7–25-daymodeovertheMeghalaya–Bangladesh–western Himalayas, orographic effects are important in rainfall
Myanmarregion. innortheastIndia(Goswamietal.2010).Ourstudyre-
A 10–20-daymode has beendetected in various ele- vealstherelationshipbetweenthelocalorographicrain-
ments of the Indian summer monsoon system (e.g., fall and large-scale circulation patterns at intraseasonal
Krishnamurti and Bhalme 1976). Past studies suggest time scales. Understanding the rainfall patterns over
thatthe10–20-daymodeintheAsiansummermonsoon northeast India might be crucial in understanding the
region originates over the equatorial west Pacific hydrological processes over the Ganges–Brahmaputra–
(Annamalai and Slingo 2001), propagates westward MeghnaRiverbasin(HoferandMesserli2006).Forex-
(Krishnamurti and Ardanuy 1980) with a double-cell ample,heavyrainfallduringARSsatCherrapunjidirectly
structure(ChenandChen1993),andisamanifestation runs off toward Bangladesh and triggers flash floods.
of the moist equatorial Rossby waves modified by the Also, the high monsoon rainfall years at Cherrapunji
climatological mean flow (Kikuchi and Wang 2009). tendedtocorrespondwithseverefloodyearsinBangla-
Fujinami et al. (2014) revealed that a westward- desh(Murataetal.2007).
propagating anticyclonic anomaly related to the equa- Causes of the negative correlation between rainfall
torial Rossby wave from the western North Pacific anomaliesintheIndiancoreregionandnortheastIndia
promotes rainfall over the Meghalaya–Bangladesh– intheactive–breakcyclehavenotbeendiscussed.The
westernMyanmarregion. ARSsatCherrapunjicanberepresentativeinnortheast
Thisstudyanalyzesactiverainfallspells(ARSs)inthe India because the extreme rainfall amount provides
monsoonseasonatCherrapunji,whichisatownlocated noiseless signalfor favorable synoptic-scaleevents. The
in northeast India. Cherrapunji is a unique station be- objectiveofthisstudyistoanswerthefollowingquestions
cause it holds the world record for the highest rainfall by understanding the characteristics of the Cherrapunji
in a calendar month and for longer time scales than a ARSs(CARSs),anddiscusstherelationshipwithactive–
calendarmonth(Jennings1950;KiguchiandOki2010), breakcycleintheIndiancoreregion:
as well as the highest daily rainfall records in India
1) WhatistherelationshipbetweenCARSsandlarge-
(Guhathakurta2007).RainfallatCherrapunjihasclear
scaledynamics?
ARSs, and daily rainfalls with more than several hun-
2) What is the contribution of CARSs to the extreme
dredmillimetersarecommonduringARSs(Fig.3).The
rainfall?
large daily rainfall at Cherrapunji during the ARSs is
suitable to detect the synoptic-scale events with high Section 2 describes the datasets used in this study.
sensitivity becauseitcanminimizeothernoisescaused The definition and characteristics of CARSs are de-
byfewlocalcumulonimbusclouds. scribedinsection3.Section4givesthespatiotemporal
Cherrapunji is located on the southern slope of the structure of CARSs. The synoptic condition during
MeghalayaPlateau(Fig.1a)withamaximumelevation CARSs,therelationshipwiththeIndiancoreregion,and
ofaround2000mabovesealevel.Theplateauisdivided thecontributiontotheextremerainfallarediscussed
15OCTOBER2017 MURATA ET AL. 8239
FIG.1.(a)TopographicalmapoftheregionaroundCherrapunji(shownwiththestar).
(b)LandscapeofthesoutherncliffoftheMeghalayaPlateaunearbyCherrapunjitown.
in section 5. Finally, the results are summarized in from 1902–70, and the Guwahati regional office of the
section6. IMD for daily rainfall data from 1970 to 2003. Daily
rainfalldatafor2003–05wereobtaineddirectlyfromthe
CherrapunjiObservatoryoftheIMD.Fiveyearsofdaily
2. Data
rainfall data at Cherrapunji are missing completely
Data on the daily rainfall observed at Cherrapunji (1965,1982–84and1987),and7years(1946,1960,1961,
observatory of the India Meteorological Department 1963,1964,1972,and1986)havemorethan10%missing
(IMD) were obtained from the following different data for June–September. To study the relationship
sources: the Research Data Archive at the National between rainfall at Cherrapunji and that at other
CenterforAtmosphericResearch(Computationaland regions, the IMD rainfall dataset (Pai et al.2014) ona
Information Systems Laboratory; ds.480.0) for data 0.258 latitude–longitude grid was used for the period
8240 JOURNAL OF CLIMATE VOLUME30
Fujinamietal.(2011,2014)examinedthecharacteristics
ofthe10–20-daymodeovertheMeghalaya–Bangladesh–
westernMyanmarregion.Thisstudyfocusedonthetiming
ofonset,length,andtotalrainfallamountofCARSsbe-
causetheyaretheimportanthydrologicalcharacteris-
ticsofthisregion.
Figure 3 shows examples of the daily rainfall from
21 May to 10 October in the years 1915 and 1974. The
annual rainfall in 1915 was almost mean value and in-
cludedfouractivespells.Theannualrainfallin1974was
the highest between 1902 and 2005. The solid curve il-
lustrates the daily climatological rainfall: it shows the
averagedailyrainfallineachyear,andsmoothingbythe
15-day running mean twice. The daily climatological
rainfall reaches a maximum in late June, which was
more than 1 month earlier than the rainfall over the
FIG.2.The92-summerensemblespectrumoftheCherrapunji
dailyrainfall(mm).Arednoisespectrum(dashedcurve)and95% Indiancoreregion(Rajeevanetal.2010;Paietal.2016).
levelofsignificance(solidcurve)basedonalag-1autocorrelation Heavyrainfalldays(HRDs)aredayswithmorethan
arealsoshown. 1.5timesthedailyclimatologicalrainfallbetweenJune
and September. CARSs are defined as consecutive
1902–2005,andtheAPHRODITEV1101rainfalldata- HRDsonwhichthetotalrainfallexceeds 500mm. For
set (Yatagai et al. 2009, 2012) on a 0.258 latitude– HRDperiodsthatinclude1Juneor30September,the
longitude grid was used for the period 1958–2005. The number of consecutive HRDs extended into May or
streamfunction,horizontalwindcomponentsandverti- October. There were only three (two) cases of 1-day
cally integrated water vapor flux components of the (2-day) intervals between the two adjacent CARSs.
Japanese 55-Year Reanalysis (JRA-55) (Ebita et al. Definitions of active–break spells can vary (Rajeevan
2011;Kobayashietal.2015)ona1.258latitude–longitude et al. 2010). Fujinami et al. (2014) defined active and
grid were used to analyze the features of circulation breakmonsoonspellsaccordingtofiltereddata.Inthis
patterns and moisture transport during CARSs. Daily study unfiltered data are used to define ARSs, with no
interpolated outgoing longwave radiation (OLR) data assumptionregardingfrequency.Theshadedperiodsin
ona2.58latitude–longitudegridwereusedtoshowlarge- Fig. 3 represent ARSs in this study. There were nine
scale convective activity, including ocean areas from CARSs in 1974, which was the maximum number re-
1979 to 2005 (Liebmann and Smith 1996). In the com- cordedduringtheanalysisperiod.TheCARSsrepeated
posite analysis, the statistical significance of the differ- periodically at intervals of 10–20 days, which is consis-
encesfromclimatologicalvaluesateachgridpointwas tentwiththeresultofFig.2.
estimatedusingtheStudent’sttest. Table 1 shows the relationship between the total
rainfallonconsecutiveHRDsandthedurationofcon-
secutiveHRDs.ThenumbersinthecolumninTable1
3. FeaturesofARSsatCherrapunji
indicatetheperiodsofconsecutiveHRDsbetween1902
In this section CARSs are defined and described. and2005.Thenumberofperiodsdecreasesasthetotal
Figure2showsthe92-summerensemblespectrumofthe rainfall increases. Regarding the length of a period,
Cherrapunji daily rainfall. The years for which more there was a greater percentage of consecutive 3-day
than10%oftheJune–Septemberdailydataaremissing HRDs. A linear relationship is observed between the
were excluded; in total, 92 years were used, the first number of consecutive HRDs and the total rainfall.
threeharmonicsoftheannualcyclewereremoved,and Greater rainfall is observed in the periods with more
the fast Fourier transform technique was used to cal- consecutiveHRDs.Therearesomeextremecaseswhen
culate the spectra. A 5-point running mean in the fre- thetotalrainfallwasbetween3500and5000mmduring
quencydomainwasappliedtotherawspectratoreduce 7-or9-dayconsecutiveHRDs.Inthisstudy,CARSsare
estimate errors. Pronounced peaks appear around a definedaseventswithmorethan500mmtotalrainfall,
10–30-day period and a peak of about 11 days exceeds andtheminimumlengthofaCARSis3days.
the95%confidencelevel.Thespectrumissimilartothat Variation between intra- and interannual scales has
of all Bangladesh daily rainfall (Fujinami et al. 2011) been considered. For example, Gadgil and Joseph
and represents the dominance of the 10–20-day mode. (2003)showedthattheinterannualvariationinall-India
15OCTOBER2017 MURATA ET AL. 8241
FIG.3.Dailyrainfall(histogram;mm)for21May–10Octoberin(a)1915and(b)1974,andtheclimatologicaldaily
rainfall(solidcurve;mm).Thegray-shadedareasrepresentCherrapunjiactiverainfallspells(CARSs).
summer monsoon rainfall is related to the number of Figure5showstherelationshipbetweenthecumulative
daysofrainbreaksandactivespells.Fujinamietal.(2011) number of CARSs and the cumulative rainfall amounts
showedthatmorefrequentandstrongsubmonthly-scale duringCARSsoveroneyear.AsintheanalysisofFig.2,
oscillations, and a greater number of rainy days, were data for 92 years were used. The bottom and top of
relatedtothewetmonsoonyearsinBangladesh.Figure4 theboxarethefirstandthirdquartiles,respectively.The
shows that the cumulative rainfall of CARSs is pro- thickbandinsidetheboxisthemedian.Theendsofthe
portionaltototalmonsoonrainfallamountfromJuneto whiskersshowtheminimumandmaximum.Thenumber
September. The cumulative rainfall during the period of CARSs ranges from one to nine per year. The years
withoutCARSsrangesfrom2000to6000mmandhasa with four to nine CARSs over one year compose ap-
negative tendency with monsoon total rainfall because proximatelyhalfofthetotalsampleyears.Intherangeof
the number of days without CARSs decreases when onetofour spellsperyear, there isa tendencyfortotal
CARSsincreases. rainfalltoincreasewiththenumberofCARSs.However,
TABLE1.Relationshipbetweenthetotalrainfallofconsecutiveheavyrainfalldays(HRDs;horizontalline)andthelengthofconsecutive
HRDs(verticalline).Asterisksdenotecellswithmorethan10periods.
14-day 1 1
13-day 0
12-day 3 1 2
11-day 0
10-day 5 1 3 1
9-day 15 5 3 1 5 1
8-day 13 1 3 5 3 1
7-day 28 1 3 *13 4 3 2 1 1
6-day 57 1 *10 *30 *14 2
5-day 90 2 *46 *28 *11 3
4-day 103 *15 *70 *18
3-day 167 *103 *64
2-day 82 *82
1-day 68 *68
Total 632 272 194 98 38 15 11 1 1 1 1
Total ,500 ,1000 ,1500 ,2000 ,2500 ,3000 ,3500 ,4000 ,4500 ,5000
Totalrainfall(mm)
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FIG.5.Thedependenceofthecumulativetotalrainfallamounts
(mm) during CARSs over 1yr on the number of CARSs. The
FIG.4. Scatterplotsof monsoonrainfall(totalrainfallamount bottomandtopoftheboxarethefirstandthirdquartiles.Thethick
fromJunetoSeptember;mm)andthecumulativerainfallwithand bandinsidetheboxisthemedian.Theendsofthewhiskersshow
without CARSs (solid and open circles, respectively; mm). The theminimumandmaximumvalues.Thedatainparenthesesnext
regressionlineisshownforwithinCARSs. tothenumberofCARSsonthehorizontalaxisshowthenumberof
periods.
thereisnocleartrendtowardanincreaseinrainfallwhen
therearemorethanfourCARSsperyear.Itisnotalways andnorthoftherelativelylowrainfallarea.Theareaof
thecasethatyearswithagreaternumberofCARSsare relativelylowrainfallisconsistentwiththelocationofthe
wet monsoon years.Itimplies thatCARSswith greater monsoontroughorcontinentaltropicalconvergencezone
totalrainfallscontributetowetmonsoonyearsinaddition inpaststudies(Gadgil2003).Usually,typhoonsandother
toanumberofCARSs. cyclonicdisturbances,suchasdepressionsorlows,move
Figure6showsacompositeoftheanomalousrainfall westward along the monsoon trough zone and produce
from daily climatological rainfall over India during rainfalloverthemonsoontrougharea.
CARSs.ThismapwasproducedbyusingtheIMDgrid Toexplaintheanomalousrainfalldistributionduring
rainfall data for the period 1902 to 2005. Positive and CARSs(Fig.7),datafromcompositeanomalousstream-
negativevaluesshowregionsofexcessandlessrainfall functionandcompositeanomalouswind(Figs.8a,b)and
during CARSs, respectively. The hatched regions are
significant at the 95% confidence level. While excess
rainfallisobservedovernortheastIndiaduringCARSs,
less rainfall is observed over central India and the
Western Ghats. Usually, the active and break spells of
the Indian monsoon are defined based on the rainfall
over central India. Thus, it is important to note that
northeast India generally has the opposite tendency to
that of central India: the spatial distribution is very
similartothatofthebreakmonsooncompositionofthe
Indiancoreregion(e.g.,Paietal.2016).
4. Relationshipwithlarge-scalecirculationfield
Figure7isthesameasFig.6,buttheAPHRODITE
datasetisusedtoprovideanextendedviewovertheAsian
monsoon region. Although the analysis period (1958–
2005)isshorterthanthatinFig.6,thedistributionofhigh
andlowrainfallareasoverIndiaissimilar.Theregionwith
significantly lower rainfall during CARSs extends longi-
FIG. 6. Composite of anomalous rainfall (mm) from daily cli-
tudinally over Pakistan, central India, the Indochina
matologicalrainfallduringCARSsoverIndia.TheIndiaMeteo-
Peninsula,andthenorthernPhilippines.Theregionwith
rologicalDepartment(IMD)griddatasetwasused(1902–2005).
significantly excessive rainfall during CARSs lies south Hatchedareasarestatisticallysignificantatthe95%level.
15OCTOBER2017 MURATA ET AL. 8243
analysisperiod,andthecompositethatexcludesCARSs
is almost same as the climatological mean. There is a
largedifferenceinthemoisturefluxfieldover208–308N,
whilethereisalmostnodifferencesouthof208N,except
that the moisture flux is somewhat smaller during
CARSs. Climatologically, cyclonic circulation due to a
monsoon trough appears over central India, and mois-
ture transport is limited over this area (Fig. 9a). In
contrast, a strong westerly moisture transport appears
over central India, which intensifies moisture conver-
genceovernortheastIndiaduringCARSs(Fig.9b).The
location of moisture convergence over the foot of the
central Himalayas is shifted slightly northward and in-
tensifiesduringCARSs.
Figure 10 is the same as Fig. 8, but shows the com-
posites of the CARS onset days. In the lower tropo-
sphere(Fig.10b),AACisdistributedoverthenorthern
FIG. 7. Composite of anomalous rainfall (mm) from daily cli-
matological rainfall during CARSs over the Asian monsoon re- Bay of Bengal, the Indochina Peninsula, the South
gion.TheAPHRODITEdatasetwasused(1958–2005). China Sea, and the western North Pacific along 108–
208N.ComparedwithFig.8b,theAACdoesnotcover
the Indian subcontinent. The location of the AAC is
composite vertically integrated anomalous moisture flux almost consistent with the convective suppression rep-
anditsdivergence(Fig.8d)wereproducedbyusingJRA- resentedbypositiveOLRanomalies(Fig.10c)andthe
55reanalysisdata.Theanalysisperiodwas1958–2005.The divergence of the anomalous moisture flux (Fig. 10d).
compositeofanomalousOLRisalsoshown(Fig.8c)for The anomalous negative streamfunction is located to
comparison with the convective activity, including ocean the south of the AAC over the eastern Indian Ocean,
areas.Theanalysisperiodis1979–2005,duetothelimited andshowsthedoublecellstructure.Anotheranomalous
satellitedata.Anomalousanticycloniccirculation(AAC) negative streamfunction is located around Japan. In
appearsovercentralIndiaandthenorthernBayofBengal, the upper troposphere (Fig. 10a), the AAC is located
atboth200-and850-hPalevels.Anomalouscycloniccir- just over northeast India where convection is active
culationsappearoverthewesternequatorialIndianOcean (Fig. 10c). There are two other anomalous negative
andtheTibetanPlateauatthe200-hPalevel.Thepositive streamfunctions situated in the northeast and south of
OLR anomaly (Fig. 8c) and anomalous moisture di- theAAC.ThenegativeOLRanomaly(Fig.10c)andthe
vergence (Fig. 8d) show intense convective suppression convergenceoftheanomalousmoistureflux(Fig.10d)
over the western Indian subcontinent. In the lower tro- are distributed not only in northeast India but also ex-
posphere(Fig.8b),theAACisextendedfarthereastover tend to southern China along the 208–308N zone; this
the South China Sea; this location is consistent with the locationisconsistentwiththenorthernedgeoftheAAC
area of relatively low rainfall in Fig. 7 and the positive atthe850-hPalevel(Fig.10b).
OLRanomalyovertheSouthChinaSeaandthePhilip- Table 1 shows that greater total rainfall tends to be
pines in Fig. 8c. The convergence of the anomalous observed during longer CARSs. The data were then
moisture flux over northeast India (Fig. 8d) corresponds divided by the length of CARSs, and compared with
well with active convection over northeast India during thoseinFig.11.Thecompositenumbersofperiodswere
CARSs(Figs.6and7),althoughnegativeOLR(Fig.8c) 21,40,and9for3-,5-,and7-dayCARSs,respectively.
does not represent well the convective activity over Figure 11 shows the composite of anomalous stream-
northeastIndia.Theanomalousnegativestreamfunctionis function and anomalous wind data at 850hPa for
located over the central equatorial Indian Ocean at the 3- (Figs. 11a,d,g), 5- (Figs. 11b,e,h), and 7-day CARSs
850-hPalevel,andrepresentsadouble-cellstructure. (Figs. 11c,f,i), respectively. The time evolution is also
To further investigate how synoptic conditions pro- shownfromday22today12.Onday22(Figs.11a–c),
duce moisture convergence over northeast India, a an AAC forms over the South China Sea. This AAC
compositeofverticallyintegratedmoisturefluxanddi- propagates westward, and the onset of CARSs
vergence during CARSs (Fig. 9b) is compared with a (Figs.11d–f)occurswhentheAACcoversthenorthern
composite of climatological means throughout June to Bay of Bengal. The AAC moves farther westward on
September(Fig.9a).CARSsoccupyonly8.5%oftotal day12(Figs.11g–i).Thewestwardpropagationspeedis
8244 JOURNAL OF CLIMATE VOLUME30
FIG.8.Compositeofanomalousstreamfunction(shading;106m2s21)andanomalouswind(vector;ms21)at
(a)200-and(b)850-hPalevelsfromdailyclimatologicalvaluesduringCARSs.TheJRA-55datasetwasused
(1958–2005).(c)Asin(a)and(b),butforoutgoinglongwaveradiation(OLR)(shading;Wm22).(d)Asin(a)and
(b),butforverticallyintegratedmoistureflux(vector;kgm21s21)anditsdivergence(shading;1024kgm22s21).
Hatchedareasarestatisticallysignificantatthe95%confidencelevel.Onlyanomalouswindvectorsthatshowed
a95%statisticallysignificantdifferenceareplotted.
intherangeof5–10ms21.Onday22,theAACextends A significant AAC over the Tibetan Plateau tends to
morezonallytowardthewesternNorthPacificin7-day movesouthwardwiththeonsetofCARSs,andliesover
periodsthanin 3-dayperiods.Furthermore,theAACs the convectively active area. The comparison between
aremoreintensein7-dayperiodsthanin3-dayperiods. theonsetdayoftheCARSsandday12showsawest-
Onday12,thewesterlyanomaliesstillremainoverthe ward propagation. The composite of 3- and 7-day
western North Pacific along 208N in the 5- and 7-day CARSs showed similar AAC movement toward the
periods.Inthe7-dayperiods,thecenteroftheAACis south, but they did not reach significance (figures not
stilllocatedinthewesternNorthPacific. shown), possibly due to the relatively weak signal for
Thetimeevolutionoftheverticalstructurewasana- 3-day CARSs and insufficient number of samples for
lyzed.Thecompositesoftheanomalousstreamfunction 7-dayCARSs.
and anomalous wind at 500hPa were almost the same
as in Fig. 11 (figures not shown). Figure 12 shows
the composite of the anomalous streamfunction and 5. Discussion
anomalouswindat200hPain5-dayperiodsonday22
a. SynopticconditionsduringCARSs
(Fig. 12a), the onset of CARSs (Fig. 12c), and day 12
(Fig.12e).TheyarecomparedwithanomalousOLRon The composite of the CARS onset days implies that
day 22 (Fig. 12b), the onset of CARSs (Fig. 12d), and the CARSs begin when the AAC in the lower tropo-
day 12 (Fig. 12f) to examine the effect of diabatic sphereisovertheBayofBengal(Fig.10b).ThisAAC
heatingonthecirculationfieldintheuppertroposphere. signal formed in the South China Sea and the western
15OCTOBER2017 MURATA ET AL. 8245
FIG.9.Compositeofverticallyintegratedmoistureflux(vector;kgm21s21)anditsdivergence(shading;
1024kgm22s21)for(a)June–Septemberclimatologyand(b)duringCARSs.
North Pacific propagates westward along 108–208N propagatesfartherwestwardovercentralIndia(Fig.8b),
(Fig. 11). An anomalous negative streamfunction is and causes convective suppression (Fig. 8c). The AAC
situatedtothesouthoftheAAC,andmakesadouble- over central India has a barotropic structure (Fig. 8a).
cell structure (Fig. 10b). During the CARSs, the AAC ThesefeaturesareconsistentwiththeresultsofFujinami
FIG.10.AsinFig.8,butfortheonsetdaycompositesofeachCARS.
8246 JOURNAL OF CLIMATE VOLUME30
FIG.11.AsinFig.8b,butforthecompositesfor2daysbeforetheonsetof(a)3-,(b)5-,and(c)7-dayCARSs,respectively.(d)–(f)Asin
(a)–(c),butforthecompositesoftheonsetdayofCARSs.(g)–(i)Asin(a)–(c),butforthecompositesofthe2daysaftertheonset
ofCARSs.
et al. (2014) and other studies in the 10–20-day mode the model developed by Wang and Li (1994). It is an
analysis. The AAC signal in the upper troposphere equatorial beta-plane channel model with a model
movedsouthwardfromtheTibetanPlateauandcovered atmosphereconsistingofatwo-layerfreetroposphere
overcentralandnortheastIndia(Fig.12). and a well-mixed boundary layer, and includes sur-
DuringCARSs,suppressedconvectiveanomaliesare faceevaporationinthemoisturebudget.Theinstabil-
distributedoverazonallyelongatedbandfromsouthern ity generated by convection–frictional convergence
Pakistan,viacentralIndia,thenorthernBayofBengal, produceda low-frequency multiscale convectivecom-
the Indochina Peninsula, and the Philippines, to the plexinwhichthecondensationheatingcoupledmoist
westernNorthPacific(Figs.7and10c).Asimilarelon- Kelvin wave and moist Rossby wave with the gravest
gatedband,butwithoppositesign,wasshownbyMoon meridional structure (n 5 1 mode). Wang and Xie
etal.(2013)duringtheactivephaseoftheIndiansum- (1997)introducedtheclimatologicalJulymeanflowat
mer monsoon, and by Takahashi et al. (2015) during 200-and850-hPalevelsandJulymeansurfacespecific
above-normal precipitation over the Indochina Penin- humidity over Asian monsoon region as the model’s
sula. The zonally elongated band is formed due to the basicstate.
westwardpropagationofAAC. Thepath,propagatingspeed,andhorizontalscaleof
WangandXie(1997)simulatedthesimilarelongated theobservedwestwardpropagatingAACsinthisstudy
precipitation band over northwest India toward the (Fig.11)aresimilartothatofthesimulatednortherncell
equatorial central Pacific. They used the extension of ofthen51equatorialRossbywavesinWangandXie