JOURNAL
Evidence
OF GEOPHYSICAL
of meridional
RESEARCH,
motion
VOL. 100, NO. D8, PAGES 16,675-16,688, AUGUST
in the summer
20, 1995
lower
stratosphere adjacent to monsoon regions
Timothy J. Dunkerton
Northwest ResearchAssociates,Bellevue,Washington
Abstract. Twenty-oneyearsof rawinsondedata were used togetherwith 8 years of
uninitializedEuropeanCentre for Medium-RangeWeather Forecasts(ECMWF) analyses
to describethe climatologicalstructureof large-scalecirculationsadjacentto monsoon
regionsin northern and southernhemispheresummers.In the upper troposphereand
lower stratosphere,equatorwardand polewardmotionsoccuron the easternand western
sidesof monsoonregions,respectively.It is shownthat significantmeridionalvelocities
(>1 ms-1) penetrate
thelowerstratosphere
up to a maximum
heightof 50-30 mbar.
Largestmeridionalvelocitiesare observedin connectionwith the Asian monsoonin
northern summer.Although evanescentin height, thesemotionsare relativelyimportant
for horizontaltransportof constituentsin the summerlower stratosphere,when planetary
wavesare otherwisesmall.Asian and Mexican monsoonsin this seasonare displaced
sufficientlyfar from the equator,in closeproximityto the tropopausebreak, to have a
significantrole in stratosphere-troposphere
(S/T) exchange.The companionpaper by
Chen (1995) providesevidenceof irreversibleS/T exchangein the "upper middle world"
during northern summer.
1.
Introduction
Large changesof circulation,cloudiness,and precipitation
accompanythe seasonalmarch of the tropical and subtropical
troposphere.These variations are especiallyprominent over
the easternhemisphere(attributedto African, Asian,andAustralian monsoons)and Americas.Althoughcomplexin detail,
and varyingsomewhatfrom year to year, the seasonalcycle
follows a predictablecourse.Deep convectionand rainfall
occur in the summer hemisphere,attracted to the warmest
land surfacesand sea surfacetemperature.Troposphericcirculationsprovide moisture for convectionand are in turn
drivenby latent heat release.The upper troposphericresponse
is approximatelyoppositethat near the surface,containinga
significantcross-equatorial
divergentflow, aswell asrotational
circulationsin the subtropicsadjacentto the convection.Midlatitude jet streamsare enhancedlocally as part of the rotational response,with meridional transport into jet entry regionsand away from jet exit regions.
Although monsoon circulations have been described in
manytextbooks,relativelylittle attentionhasbeen givento the
structureof climatologicalcirculationin the upper troposphere
and lower stratosphere.For reasonsthat are not entirelyclear,
we find numerous
illustrations
of 200 mbar circulation
in the
literature, as if this level adequatelydescribedthe upper troposphericflow. Among mandatoryrawinsondelevels,the 200
mbar level contains
the maximum
zonal wind of midlatitude
jets (u), and largestzonallyaveragedmeridionalwind at the
equator(•); this level alsoprovidesa qualitativelycorrectpicture of the rotational responseto tropical convection.However, the magnitudeof responsein northern summeris generallylargestin the 200-100 mbarlayer,especiallyoverAsia, and
is best illustratedusing150 or 100 mbar data.
Circulation patterns of the subtropicalupper troposphere
are visiblein the lower stratosphereduringsummer,decaying
rapidly with height. These circulationsare able to transport
constituentshorizontally in the lowest layers of the stratosphere,asillustrated,for example,in the polewarddispersalof
volcanic aerosol following the eruption of Mount Pinatubo
[Trepteet al., 1993]. Although summer monsooncirculations
and quasistationary
wavesare evanescentin height (significant
onlyup to approximately20-25-km altitude),they are primarily responsiblefor lateral mixing of the lower stratospherein
this season,when planetarywavesare otherwisesmall.
In a companionpaper, Chen [1995] emphasizesfor the first
time the role of summermonsooncirculationsin stratospheretroposphere(S/T) exchange.Chen findsthat isentropicmixing
and S/T exchangein the "upper middle world" are significant
in northern summer, in connection with Asian and Mexican
monsoons.Extending the terminologyof Hoskins [1991], the
upper (lower) "middle world" containssurfacesof constant
potentialtemperature0 intersectingthe tropopauseabove(below) the jet maximum.
The purposeof this paper is to describethe climatological
circulationof the upper troposphereand lower stratosphereat
the solstices,emphasizingthe penetration of monsooncirculationsand quasistationary
wavesinto the lower stratosphere
in the northern hemisphereduring summer.Rawinsondedata
for 1973-1993 are compared to analysesobtained from the
European Centre for Medium-Range Weather Forecasts
(ECMWF) in 1985-1992 (the data and analysismethodsare
describedin section2). The upwardpenetrationis more dramatic in northern summer(section3) and displacedfarther
from the equator, in comparisonto southernsummer(section
4). Variabilityof monthlymeansis discussed
brieflyin section5.
Copyright1995 by the American GeophysicalUnion.
2.
Paper number 95JD01263.
Rawinsond½
wind and temperaturedata for January1, 1973,
throughNovember6, 1993,were usedin this study.Mandatory
0148-0227/95/95JD-01263505.00
16,675
Data Analysis
16,676
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leveldataweresubjectto qualitycontrolandlinearlyinterpo- Because of possible trends in the data, as well as different
lated to 4 timesdailyas described
by Dunkerton[1993].The sampling
times,observational
input and analysis
techniques,
mandatorylevels are 1000, 850, 700, 500, 400, 300, 250, 200,
exact agreementbetween rawinsondeand ECMWF climatolo-
150, 100, 70, 50, 30, 20, and 10 mbar. For monthlymeans,a
minimumof eight soundings
per monthwere required.For
eachstation'sclimatology,a minimumof four Januarys,four
giesshouldnot be expected.
For the purposes
of thispaper,
theyagreewell andmaybe usedinterchangeably.
Februarys, etc., were needed in order to be included in the
analysis.
Stations
selected
for studymetoneof twocriteria:(1) 3. Climatology: Northern Summer
havinga minimumof 90,000datarecords(onerecordis equiv- 3.1. Regionsof Strong Meridional Flow
alentto windor temperature
datareportedat a singlepressure Figures1 and2 showthezonalandmeridional
components
level), or (2) havinga minimumof 45,000 data recordsand of velocityat 150 mbar for July,derivedfrom ECMWF analmore recordsthan anyadjacentstationin a 10ø x 2øgridbox. ysesfor 1985-1992.In the summerhemisphere
we noticetwo
At least 1000 wind observations at the 200 mbar level were
concentratedpositive v maxima over western North America
requiredin either case.Altogether,971 stationswere selected andeasternMediterranean.Comparable
valuesare attainedin
in thistime period.Mostwerein the firstcategory;
all stations June and August.In June,anotherpositivemaximumis lo-
in thiscategorywereusedregardless
of stationdensity.
Rawinsondeanalyses
were constructed
by mappingstation
climatologies
onto a 10ø x 2ø grid usingBarnes'algorithm
[Daley,1991]with anisotropic
weightfunctionexp -r, where
catedoffnorthwestern
Africa.Thisfeatureis reducedbymidsummer,asupperleveleasterlies
encroach
fromthe east.Negative valuesof meridionalwind appearover easternNorth
AmericaandChina,complementing
the polewardflow.These
r2 = (X/Lx)
2 q- (y/Ly)2. Theeccentricity
of ellipse
Rf =
circulationsare part of the Mexican and Asian summermon-
L x/Ly wasdeterminedfor a particularvariablef (- u, v, T) soons,
respectively,
if theterm"monsoon"
canbeusedbroadly
by the ratio of rmslatitudinalgradientto rmszonalgradient, to includequasistationary
wavesadjacent
to convectively
active
usinguninitializedECMWF climatological
data as a guide, regions.Closerinspection
of Figure2 indicatesequatorward
suchthat1 _<Rf _<5. The areaof ellipserrLxLywasheld flow eastof the CaspianSea and weakpolewardflow over
constant,equalto that of a circlewith radius6øat the equator. westernIndia. The latter occursat the westernedgeof the
This value gavea reasonablecompromise
betweenanalysis moist Indian monsoon.As shownin section5, this feature is
coverage
(requiring
largeradius)andsmall-scale
structure
(re- quitevariablefromyearto year.Most of the polewarddiverquiringsmallradius).To compensate
for stationdensity,the sionof massoccurswestof the Arabianpeninsula,
arounda
weightfunctionat a particularstationwasreducedby a factor secondary
anticyclone
westof themainTibetananticyclone
(cf.
equal to the numberof adjacentstationswithin a normalized PlateIII of Ramage[1971]).Thisconcentrated
maximumover
distancer - 1. Two iterationswere appliedafter the first the eastern Mediterranean is a robust feature of the northern
guess.The choice of weight function and other nuancesare summercirculation[0ort, 1983,p. 121].
unimportant for analysisnear stations since the iteration is
The zonal componentat 150 mbar for July is shownin
guaranteed
to converge
to the stationvaluesat theirrespective Figure1 fromECMWF data.Eachof thepositivev anomalies
locations. Analysis between stations is more realistic with
in Figure 2 coincideswith a local accelerationof the zonal
anisotropic
weighting,
particularly
for zonalwindandtemperaturewhichhavestronglatitudinalgradientsnearjet streams.
This proceduregave good overall agreementwith ECMWF
climatology,exceptfor slightlysmootherfields.In regions
whereno stations
werenearby,suchthatthesumof allweights
fell below somethreshold,rawinsondeanalysisvalueswere
discarded.Latitude-heightcrosssectionsintegratedover a
rangeof longitudesare thereforeincompleteif analysis
values
are "missing."
Thisproblemis unavoidable
in certainregions,
component
u, including
theanomalynorthof India.Relatively
strongspeeds
areattainedoverthe Caspian
Sea(-25 ms-•),
althoughthesevaluesareweakcompared
to thoseof northern
winteror southernhemisphere.
The overallweakness
of northernsummer
jetsisimportantfor isentropic
mixingin theupper
troposphereas discussedin section6.
The meridionalcomponentof velocityderivedfrom rawinsondesin 1973-1993(not shown)is very similarto that of
Figure2. All of thefeatures
identified
in Figure2 arepresent.
suchasthe easternPacificand southernmidlatitudes,but does Theweakpoleward
flownorthof Indiain Figure2 appears
as
not affectany of the conclusions
in this paper.
a weaklynegativelocalmaximum.Strongpositiveanomalies
UninitializedECMWF analyses
on a 2.5ø x 2.5ø grid were over western North America and eastern Mediterranean are
obtainedas monthlymeansfor 1985-1992and averagedto similarto thosederivedfrom ECMWF data. Examplesof
yield climatologicalfieldsfor each level and month. Identical stationclimatologies
near v extremaof Figure2 are shownin
mandatorylevelswere used except for 20 mbar which was Figures3a-3e.Maximumpolewardflowoverthe easternMedunavailable.Stratosphericanalysesprior to June 1986 were iterranean
exceeds
12ms-• in summer.
Stations
in thevicinity
discarded
for reasons
explained
by Trenberth
[1992].Compar- of thisspatialmaximumhavetheirtemporalmaximumv at 150
isonwith climatologiesderivedfrom initializedECMWF fields mbar in June,July,or August.It is worth notingthat the
for 1980-1989showedgoodagreement,althoughtheuninitial- tropical easterlyjet attainsits maximumvalue over southern
ized fieldshad slightlymore varianceat the smallesthorizontal India at the 100 mbar level.The relativelyhigh altitudeof
scales.
uppertropospheric
circulations
inducedby the Asianmonsoon
When comparingrawinsonde
and ECMWF climatological suggests
thatAsiais the mostimportantregionfor isentropic
analyses,recall that rawinsondeanalyseswere formed from mixingand S/T exchange
in northernsummer[Trepteet al.,
stationclimatologies,
regardless
of when a particularstation 1993;Chenet al., 1994;Chen,1995].
was in operation,whereasECMWF climatologies
were obOf the stations
shownin Figure3, the largeststratospheric
tainedfrom twice-dailyanalyses
usinga subsetof thesestations meridionalwindsare observed
at Ankara,typicalof the east(andothers)at anyonetime,togetherwith otherinputdata ern Mediterranean. Whether the observed values at other losuch as aircraft reports, satellite radiances,and cloud winds. cationsare significantly
largedependson the contextandrel-
DUNKERTON:
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CIRCULATIONS
IN LOWER STRATOSPHERE
16,677
U
150 mb jul
90
60
30
EQ
30
60
; '----•.
•, -----•---•-;--•--_-.-:-
.ø
:
,
[
•.
:
•
'
'
'
:
!
9O
180
135
90
45
GM
45
90
135
W
180
E
Figure 1. Zonalvelocityat 150mbarfor July,derivedfromECMWF data.Contourinterval5 ms-1, with
easterlies
shaded.
ative magnitudeof u. Any of the local extrema of 70 mbar v
lation,whichwill be discussed
elsewhere).Weak surfacewest-
(approximately
1-3 ms-•) are largecompared
to the zonally erlies appearnorth of the equator,separatedfrom very weak
averaged,seasonallyvarying diabatic circulation at this altitude. They are alsolarge comparedto inducedQBO circulationsin the tropicallower stratosphere.Over North America,
u and v are both smaller than over Asia, so the deflection of
wind from a purely zonal direction is comparablein the two
regions.
3.2.
Asian
and African
Monsoons
Cross sections of zonal and meridional velocity from
ECMWF data, averagedover 16ø-46øE,are shownin Figures
4a and 4b. Among the four regionsof large v in Figure 2, this
sector and its complementover China display the greatest
penetration of meridional motions into the summer lower
stratosphere.The region of maximummeridional flow at 150
mbar is slightlyequatorwardof the local jet maximum.The
meridionalcomponentterminatesabruptlypolewardof 45øN.
Most of the climatologicalnorthward motion at 150 mbar
simplyprovidesmassto the acceleratingjet stream. Conver-
westerlies
in summer
midlatitudes.
The profile of meridionalvelocity(Figure 4b) shouldnot be
confusedwith the Hadley circulation,althoughthe southward
(northward)Hadley flow is evidentat the equatorin the upper
(lower) troposphere.The apparentmeridionalcell at 30øN is
actuallya quasihorizontalrotationalcirculation,sothisfeature
mustbe compensatedon the samepressurelevel by opposing
equatorwardflow at some other longitude.Accordingto Figure 2, the return flow at 150 mbar occursin a broad region
from China
centrated
eastward
to the central
over eastern China.
Pacific. Much
of it is con-
Cross sections of ECMWF
me-
ridionalflow in this regionindicatethat maximumnegativev is
locatedsouthof the jet core,with significantpenetrationinto
the lower stratosphere(not shown).As in westAsia, the zerowind line is close to the latitude
of maximum
meridional
ve-
locity.Southwardflow at the equatoris considerablystronger
in eastAsia,approaching
-7 ms-•. This agrees,for example,
genceof meridional
velocity
in thisregion(-4.5 x 10-6 S-1) with the climatologyof v at Singapore.The jet core and tropoagrees
wellwiththedivergence
of zonalcurrent(-3.9 x 10-6 pausebreak are locatedat approximatelythe samelatitude as
s-•). Thejet is quitenarrowandpushed
wellpolewardof its on the westernsideof the monsoonregion.Temperaturein the
winter position;its maximum coincideswith the tropopause interior of the tropospherehas a local maximumat 30-40øN,
break in this sector. The zero-wind line meanders upward coincidentwith zero verticalshearasrequiredby thermalwind
throughthe region of largestmeridionalvelocitybefore turn- balance.
Crosssectionsof rawinsondemeridional velocity for 1973ing poleward in the lower stratosphere.We observestrong
easterliesover the northerntropicsin upper troposphere,ex- 1993 are shownin Figures5a and 5b for western and eastern
tendinginto the southerntropics(causinga semiannualoscil- sides,respectively,of the Asian summermonsoon.Although
16,678
DUNKERTON:
MONSOON CIRCULATIONS
IN LOWER STRATOSPHERE
V
90
150 mb jul
60
30
EQ
,
o
30
60
90
180
W
135
90
45
GM
45
90
135
180
Figure2. Meridionalvelocityat 150mbarfor July,derivedfromECMWF data.Contourinterval3 ms-•,
withnegative(southward)
valuesshaded,
zerocontourheavy.
the datacoverageat otherlatitudesis incompleteasnotedin
penetratethe lower stratosphereduring northern summer.
ECMWF v at 70-100 mbarare approximately
1-2 ms-• on
firmed,including
thestratospheric
penetration.
A muchlonger both sidesof the continent(not shown);similarvaluesare
data recordwas usedin Figure 5, providinga more stable observed
in rawinsonde
data(seeFigure3d and3e). Southestimate of meridional motion in the summer lower stratoward flow at 150 mbar prevailsover easternNorth America
sphere.It is onlyin thesetwo regionsthat the climatological and westernAtlantic, but is smaller and lessconcentratedthan
meridional velocities at 70 mbar exceed +3 ms-•. Zonal variits polewardcounterpartoverwesternNorth America.Jet veationsof 70 mbar zonal wind are inducedby the rotational locitiesare considerably
weakerthan overAsia,sothe relative
circulationat thislevel:for example,strongertropicaleaster- magnitudeof v is aboutthe samein bothregions.
liesare foundsouthof India (-12 ms-•), compared
to the
Streamlinesof horizontalwind form nearlyclosedcirculawestern
hemisphere
at thislatitude(-6 ms-•).
tionscenteredaboutthe zero-windline at 150and 70 mbar,as
When averagedoverthe entireAsianmonsoonregion,the shownin Figures7 and 8, respectively.
The streamlinesare
meridional
windshownin Figure6 is obtained(fromECMWF displayed
in sucha wayasto approximate
the streamfunction
data).Thissectoris sufficiently
wideto encompass
mostof the in regionswherethe flowis horizontally
nondivergent.
At 150
rotationalcirculationin upper troposphere,
leavinga fairly mbar, streamlines
gentlyspiraloutwarddue to horizontaldiaccuratepictureof the easternhemisphere's
divergent
Hadley vergence. In northern summer, one of these cells is centered
circulation.Maximum divergentflow occursin the 150-200 over northwestern Mexico, associatedwith the Mexican monmbarlayer,decayingmorerapidlywith heightthanthe rota- soon, and a pair of cells are located over Iran and Tibet,
tionalcirculations
thatpenetratethelowerstratosphere.
Weak associated
with the Asianmonsoon.Anticyclonic
circulations
meridionalcellsof reversedsignare visibleon either side.
at 70mbarareslightly
polewardof thoseat 150mbar,byabout
During early summerthere is somemeridionaltransport 8ø-12ø,consistent
with a polewarddisplacement
of zero-wind
overnorthwestern
Africa(seeFigure3c)witha splitzonaljet. linein thelatitudeheightcross
section
(e.g.,Figure4a).Weak
In the northAtlantic,jet entryandexitregionsoccurside-by- convergence
is evidentin the anticyclones.
Zonal variationof
section 2, our main conclusions from ECMWF
data are con-
side.ECMWF andrawinsonde
v at 70-100mbarareapprox- tropical easterliescan alsobe seen.
imately1-2 ms-• butcloser
to theequator,
asis thetropopausebreak.
3.3.
Mexican
Horizontal
4.
Monsoon
circulations associated with the Mexican mon-
Climatology: Southern Summer
Meridionalvelocityat 150mbarin Januaryis largelyantisymmetricabout the equator,as shownin Figure 10. This
soonand quasistationary
anticycloneover North Americaalso situation contrasts with that of northern summer. The ten-
DUNKERTON:
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50
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mb
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,
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0
dency of deep convectionto overlap the equator in southern
summer,rather than being displacedwell off the equator as in
northern summer,causesthe differentcirculationresponsein
the two seasons.There are sixpotentiallyinterestingregionsof
large meridional transport in the southern summer hemisphere. These are related to zonal variations of the zonal
velocitycomponentasshownin Figure 9. Rawinsondeanalyses
agreewell, both with regard to the magnitudeand location of
significantanomalies(not shown).The corresponding
mapsfor
December and February are similar.
Crosssectionswere examinedin each of the sixregions.As
might be inferred from comparisonof Figures 9 and 10, v
extremaare well removedfrom the jet maximaand tropopause
breaks,by as much as 250-40ølatitude, exceptfor the "eastern
Australia" sector.An example of transportby the Australian
monsooncirculationwas discussedby Danielsen[1993]. Lower
stratosphericv are comparableto or lessthan that associated
with the Mexicanmonsoonin northernsummer(not shown).
The rotational
circulations
are also of limited
10
15
T: 17130
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20
30
50
100
200
300
5.
Variability of Monthly Means
over the eastern Mediterranean
5OO
• .................................
i
195
• ..........
I
210
225
• ........
i
240
extent
Monsoon circulations and quasistationarywaves are not
identical to those of the climatology in any particular year
[Websterand Yang, 1992]. Interannualvariationsoccur,some
of which may be attributable to or interact with remote phenomenasuchasthe E1Nifio/SouthernOscillation(ENSO) and
Eurasiansnowcover[Nigam,1994].Nor are thesecirculations
steadyfor an entire season.Intraseasonaloscillations,monsoon breaks, and transient synoptic-scaledisturbancescause
significantday-to-dayvariability.The role of eddieson shorter
timescalesthan seasonalis essentialto isentropicmixing and
the potentialvorticitybalanceof the upper troposphere(section 6).
It was noted in Section 3 that the maximum poleward flow
10
................................
horizontal
.
5
A3
mb
lOOO
18o
16,679
comparedto the massiveAsian monsoon.Although someisentropicmixingof the southerntropicallower stratosphereseems
possiblein December,January,and February (DJF), significant S/T exchangeis lesslikely in this hemisphereand season
[Chen, 1995].
Upward penetrationof v is slightlygreater on the opposite
(winter) side of the equator, but zonal jets near 30øN are
considerablystrongerand more concentratedthan in summer
(Figure 9). Accordingto Chen, S/T exchangein the upper
middle world is insignificantin either winter hemisphere,due
to the strong gradient of potential vorticity associatedwith
enhanced, concentratedzonal jets. Chen's results and ours
highlightthe importanceof monsooncirculationsand quasistationarywavesduring northern summeras discussedin the
previoussection.
ms-'
lO
1000
-20
IN LOWER STRATOSPHERE
255
270
285
300
Figure 3. Climatologicalprofiles of zonal wind, meridional
wind, and temperatureat rawinsondestationsduringnorthern
summer (June or July): (a) Ankara (40.0øN, 32.9øE), (b)
Chengdu (30.7øN, 104.0øE), (c) Santa Cruz de Tenerife
(28.5øN,16.3øW),(d) Nashville(36.1øN,86.7øW),and (e) Oakland (37.8øN,122.2øW).Data are displayedat mandatorypressure levelsonly.
occurs in each northern
sum-
mer, while greatervariabilityis observednorth of India. Figure
11a showsthe variability of ECMWF v at 150 mbar in July,
averagedover20ø-45øN.This bandencompasses
the centersof
maximumIvl overAsia and North America.In someyears,
there is a secondarymaximumv near 75øE(1986, 1987, 1989,
1991)while in other years,broad regionsof positiveand negativev occuradjacentto one another(1985,1988,1992).These
differences
are attributable
to intraseasonal
as well as interan-
nual variability,sinceAugustmonthlymeanssometimesdiffer
from thoseof July in the sameyear.
Variance of monthlymean v has severalindividualmaxima
from Europe eastwardto the Pacific,eachwith rms magnitude
16,680
DUNKERTON:
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50
100
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200
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300
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500
500
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0
10
20
30
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jun
20
0
10
20
30
V: 60020
l0
C2
mb
ms-'
I
1000
-40
ms-'
10
40
ms-'
•/
:20
...........................................
/"..........................................
30
50
50
100
100
200
200
300
300
500
500
lOOO
-20
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-5
jut
,
.
0
5
10
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1000
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-5
jun
øK
10.
20
30
30
50
50
100
100
0
5
10
15
T: 60020
10
C3
mb
20
I
20
øK
I
2OO
200
3OO
300
5OO
500
,
lOOO
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Cl
mb
lOO0
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mb
MONSOON
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300
Figure 3c. SantaCruz de Tenerife (28.5øN,16.3øW)
Figure 3b. Chengdu(30.7øN,104.0øE)
---4ms-• (notshown).
Thisis smallcompared
to theclimato- over centralAsia is part of a well-definedteleconnectionpatlogicalextremaat 150 mbar, but largewithin regionsof smaller
v, as in central Asia. We infer that climatological motions
dominate meridional transport near their extrema, but interannual and intraseasonalvariationsare relativelyimportantin
central Asia and elsewhere,attaininga magnitudecomparable
to the climatology.Anomaly mapssuggestthat the variability
tern extendingover this region with zonal wavelength---50ø60 ø.
Figure 11b showsthe variability of monthly mean ECMWF
v at 70 mbar. Climatologicalextremaare apparent,and there
is evidenceof the samevariation over centralAsia as in Figure
11a (e.g., 1989).The impressiongainedis that thesevariations
DUNKERTON:
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IN LOWER STRATOSPHERE
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10
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30
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[
.
E2
mb
.
20-
20
30
.
50
50
.
lOO
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300
500
500
1000
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-5
0
5
10
15
20
1000
-20
-15
-10
-5
jul
D3
mb
10
E3
mb
20
30
30
50
50
100
100
200
200
300
300
500
500
1000
180
195
210
225
240
255
270
285
Figure 3d. Nashville(36.1øN,86.7øW)
300
1000
180
5
10
15
T: 72493
10
20
0
20
øK
I
.
195
210
225
240
255
270
285
300
Figure 3e. Oakland(37.8øN,122.2øW)
from climatologydecaymore rapidlywith height than the cli- line in the upper troposphereand lower stratosphere.Signifimatologicalfeatures,possiblydue to their shorterzonal scale. cant meridional parcel displacementsoccur in a developing
critical layer becausedisturbanceswith • = c, by definition,
6.
Discussion
are stationarywith respectto the fluid. Adiabatic mixing is
The nearly closedanticycloniccirculationsof northern sum- expectedwithin a Rossbywave critical layer, particularlyif the
mer may be likened to critical layer "cat's-eyes"associated isentropic
potentialvorticity(PV) gradientisweak.An exampleis
with quasistationaryRossbywavesencounteringa zero-wind the stratospheric
"surfzone" [Mcintyreand Palmer,1984].
16,682
DUNKERTON:
a
MONSOON CIRCULATIONS
5 ms-•
IN LOWER STRATOSPHERE
U
16 E -
46 E ul
10
20
30
50
70
150
200
300
500
700
1000
-90
-60
-30
EQ
30
60
S
90
N
LATITUDE
•
1 ms-•
V
16E - 46 E jul
20
30
50
70
100
150
200
300
...
.....
500
.....
:::::::::::::::::::::
:::::::::-...v..,..
:::::::::::::::::::::
....
700
1000
-90
-60
-30
EQ
S
30
60
90
N
LATITUDE
Figure 4. Latitudeheightcrosssectionsof ECMWF (a) zonaland (b) meridionalvelocityfor July,averaged
over16ø-46øE,
fromECMWF data.Contourinterval3 ms-• in (a), 1 ms-• in (b), negative
valuesshaded.
Although important for transport,climatologicalmonsoon
circulationscannotcauseisentropicmixingor S/T exchangeby
themselves.
By definition,thesecirculations(includingthe resolvedclimatologicalPV, and conservedtracersadvectedby
the climatologicalflow) undergoa steadyoscillationwith the
seasonal cycle. Nonconservation of PV, either of coarsegrainedPV due to unresolvedmotions,or of exactPV due to
diabatic heating and moleculardiffusion,is essentialfor irre-
versible mixing and S/T exchange.Transient eddies,even if
resolvedby the data, are an unresolvedpart of the climatology.
Three classesof transientlarge-scalemotion are potentially
important to augmentthe mean transportillustratedin this
paper, leading to irreversiblemixing and S/T exchange.(1)
Fluctuationsin the intensityof quasistationarymonsooncirculations, along with slowlymoving intraseasonaloscillations,
stationaryorographicwavesand low-frequencyvariabilitywill
DUNKERTON:
a
MONSOON
CIRCULATIONS
IN LOWER
1ms -•
STRATOSPHERE
15 E -
16,683
45 E jul
]01
.....I.....I
2O
30
50
70
100
150
200
300
7OO
1000
-90
-60
-30
EQ
30
90
60
S
N
LATITUDE
b
1 ms-•
95
E
125
E
ul
- :::::::::::::::::::::::::::::::::::::::
101
.....I.....I
....................
..............
.....................
...........................................................
20
70
100
150
200
300
5OO
700
lOOO
-90
S
-60
-30
EQ
30
60
N
90
LATITUDE
Figure 5. Crosssectionsof rawinsondemeridionalvelocityfor July, averagedover (a) 15ø-45øE;(b) 95ø125øE. Contour interval 1 ms-•.
bancescrossesthe tropopausebreak. This is unlikely for stationary waves,sincethe zero-windline crossesthe tropical
tropopausebefore turningpoleward.Critical layersneverthelesshavefinite width, somixingcausedby stationaryaswell as
eastwardpropagatingwavesmay be enhancedin the tropopausebreak in relativelyweak zonal flow. Baroclinicsystems
ms-• andmaycontribute
to mixingequatorward
or poleward contributeto S/T exchangein the lower middle world [Chen,
descendbeneaththe jet maximum.(3)
of the jet maximum.It is interestingto note that in the caseof 1995],where O-surfaces
of tropical origin are ima weak jet, the critical level of eastwardpropagatingdistur- Westwardpropagatingdisturbances
contributeto parceldispersionin the vicinityof climatological
anticyclones
that straddlethe zero-windline. The role of largescalecirculationswas emphasizedin the companionpaper by
Chen as the primarymeansof S/T exchangein northernsummer. (2) Eastwardpropagatingbaroclinicwavesoriginating
polewardof the jet axishave criticallayersin the range5-10
16,684
DUNKERTON:
MONSOON
CIRCULATIONS
1 ms-1
IN LOWER
STRATOSPHERE
V
16 E
121 E ul
20
30
50
70
100
150
200
300
::::::::::::::::::::::::::
'700
............
=================================
............
lOOO
-90
S
-60
-30
EQ
30
60
N
90
LATITUDE
Figure 6.
1 ms-•.
Crosssectionof ECMWF meridionalvelocityfor July,averagedover 16ø-121øE.Contourinterval
portantequatorwardof the jet maximum,beingresponsible
for
daily fluctuationsof tropicalmotions,cloudiness,
or precipitation, and contributing to isentropic and diapycnal mixing
within the tropics.Of the three classes,thesewavesseemleast
likely to be involved in S/T exchangein the upper middle
world. Tropical wavespenetratingthe lower stratospheremay
contributeto isentropicmixingas observedwithin the first few
weeksfollowingmajor eruptionsof tropical volcanoes.
A schematicillustrationof transportin the vicinityof the jet
axisand tropopausebreak is shownin Figure 12, representing
the westernside of the monsoonregion. The situationportrayed is typical of northern summer, with large meridional
velocitiesnear the jet core. For the eastern side of the monsoon, the sign of mean meridional velocity (large arrow at
30øN) shouldbe reversed.The zonal wind, tropopause,isentropic slopes,and Hadley cell are typical of the entire Asian
monsoonregion(16øE-121øE,July).In the uppertroposphere,
mean
meridional
motions
associated
with
rotational
circula-
tionstransportair into (out of) the jet streamon the western
(eastern)side.Isentropicmixingis attributableto eddieswith
criticallayersabove,beside,or belowthe jet maximum.Mixingis
enhancedin a relativelyweak jet, when PV gradientsare small
(ineffective
barrier)andcriticallayersrelativelywide.Significant
S/T exchange
ispossibleacrossthe tropopause
break.Bycontrast,
a strongjet suppresses
mixingacrossitscenterdueto the effective
PV barrier[Chen,1995]anddiscourages
criticallayermixingfor
quasistationary
or eastwardpropagatingdisturbances
having
zonalphasespeedsin the observedrange.
Apart from radiative heating,irreversibletransportacross
O-surfaces is attributable
to small-scale
motions such as break-
ing gravity or inertia-gravity waves, Kelvin-Helmholtz instabilities,andlatentheat releasein deepmoistconvection.These
processesact on timescalesthat are rapid comparedto transient large-scalemotions. Radiative heating operates on a
comparabletimescaleandhasbeenimplicatedin S/T exchange
[Lamarqueand Hess,1994].
7.
Conclusion
Twenty-oneyears of rawinsondedata were used together
with 8 yearsof uninitializedECMWF analysesto describethe
climatologicalstructureof large-scalecirculationsadjacentto
monsoonregionsin northern and southernhemispheresummers.In the uppertroposphere
andlowerstratosphere,
equatorward and polewardmotionsoccuron the easternand western sidesof monsoonregions,respectively.It was shownthat
significant
meridional
velocities
(>1 ms-•) penetrate
thelower
stratosphere
up to a maximumheightof 50-30 mbar. Largest
meridional
velocities
are observed
in connection
with
the
Asian monsoonin northernsummer.Although evanescentin
height, thesemotionsare relativelyimportantfor horizontal
transport of constituentsin the summerlower stratosphere,
whenplanetarywavesare otherwisesmall.The role of monsoon
circulations
in the polewarddispersal
of tropicalvolcanicaerosol
in the lowerstratosphere
wasnotedby Trepteet al. [1993].
Asian and Mexicanmonsoonsin this seasonare displaced
sufficientlyfar from the equator, in closeproximityto the
tropopausebreak, to have a significantrole in stratospheretroposphere(S/T) exchange.The companionpaper by Chen
[1995] providesevidenceof irreversibleS/T exchangein the
"upper middle world" during northern summer.
Climatologicalcirculationsare expectedto playa majorrole
in lower stratospherictransport in all seasonsand S/T exchangein northernsummer.Irreversiblemixingis attributable
to eddiesand (ultimately)nonconservation
of potentialvorticity. Considerablymore work mustbe done to interpretisentropic mixing and S/T exchange,as well as their seasonaland
interannualvariability.Calculationssuchasthoseof Hoerlinget
(U,V)*
n.d.
150 mb jul
90
60
30
EQ
30
60
90
180
135
90
45
GM
45
90
135
W
180
E
Figure7. ECMWFstreamlines
at 150mbarforJuly,withzero-wind
linesuperposed,
easterlies
shaded.
90
n.d.
(U,V)*
70mbjul
60
30
60
90
180
135
90
45
GM
45
90
135
180
E
w
Figure 8. As in Figure7, but at 70 mbar.
16,686
DUNKERTON: MONSOONCIRCULATIONSIN LOWER STRATOSPHERE
u
150 mb jan
90
,
,
30
EQ
s
90
180
135
90
45
GM
45
90
135
180
W
E
Figure 9. ECMWF zonalvelocityat 150mbarasin Figure2, but for January.
15o mb jan
,
,
,
,
,
,
60
30
EQ
30
60
90
180
135
90
45
GM
45
90
135
w
180
E
Figure 10. ECMWF meridionalvelocityat 150mbarasin Figure1, but for January.
DUNKERTON:
A
'
20, 45
'
'
I
'
V
'
'
I
'
'
'
I
MONSOON CIRCULATIONS
150mbjul
''
'
I
'
'
'
I
'
'
'
1985
1986
1987
5'
1988
1989
1990
1991
1992
,
,
- 180
,
I,
,
,
- 120
I
,
,
,
-60
I
,
,
,
0
I
,
,
,
60
I
,
,
,
120
180
LONGITUDE
B
'
20, 45
'
'
I
'
V
'
'
I
'
'
'
I
70 mbjul
'
'
'
I
'
'
'
I
'
'
'
1985
1986
1987
'7
1988
IN LOWER STRATOSPHERE
Asian and Mexican monsoonsis a two-wayprocess,with tracers entering the extratropicalstratospherefrom the upper
tropical troposphereand vice versa.The ultimate fate of tracers enteringthe lower stratosphere,to be sure,is alsoaffected
by diabatic descent at higher latitudes. Thus it is unclear
whethermonsoontransportis very importantfor the injection
of CFCs and chemistryof the ozone layer. This transport
seemsmore important in the vicinity of the tropopause:a
regioncomingunder scrutinyfor possibleeffectsof supersonic
and subsonicaviation on stratosphericchemistry.The residence time of trace constituentsand pollutantsin the lower
stratosphere
is determinedby transportprocesses
[Douglass
et
al., 1991]whichare poorlyunderstoodor difficultto quantify.
Hydrationof the stratospheric
middleworldis apparentlypossibledue to S/T exchangein midlatitudes(A. E. Dessleret al.,
Mechanismscontrollingwatervaporin the lower stratosphere:
A tale of two stratospheres,
submittedto Journalof Geophysical Research,1995).Transportof dehydratedstratospheric
air
into the tropicalupper tropospheremay prove importantto
the radiativebalanceof this region and terrestrialclimale.
Of interest to this paper is the Rossbywave excitationand
propagationassociatedwith monsooncirculations.The summer stratosphereis relatively undisturbedbecausequasistationary Rossbywavesrequire westerliesfor vertical propagation [Charney and Drazin, 1961]. Rotational monsoon
circulationsand midlatitudeanticyclonesin summerare therefore unable to significantlyinfluencethe stratosphereabove
--•25 km. The magnitude of evanescentmeridional motion in
the lower stratosphereis neverthelesslarge comparedto the
diabatic circulationin this region. Although such motion is
essentiallyrotational and thereforevanishesidenticallyin the
zonalmean,its local effecton constituenttransportis relatively
importantwithin the summerhemisphere,as observationsof
volcanicaerosol[Trepteet al., 1993]and semi-Lagrangian
calculationsof isentropicmixing[Chenet al., 1994;Chen,1995]attest.
These observationsinvite further theoreticalstudyof monsoon circulationsand Rossbywave propagation.The role of
combinedlatitudinaland verticalshearin Rossbywave critical
layerdynamicsandthe nonlinearadvectionof angularmomentum and PV causedby rotational and divergentcirculations
must be taken
1989
1990
1991
1992
16,687
into
account
in order
to understand
the re-
sponsein the upper troposphereand lower stratosphere.Nonlinear dynamicsis likely to be important for the evolution of
secondaryvortices,suchas that over the Arabian peninsulain
northern summer.This feature is responsiblefor the extreme
valuesof meridional velocity over the easternMediterranean.
Accordingto Figure 7 of Ting [1994], the linear responseto
diabaticheating explainsthe overall structureof Asian monsooncirculationin the upper troposphere,but stationarynonlinearity has a significanteffect in certain regions,such as
centralAsia. Monsooncirculationsover Mexico during northern summer and over South America, Africa, and Australia
- 180
- 120
-60
0
60
120
180
LONGITUDE
Figure 11. Variations of ECMWF monthly mean v in July,
averagedover 20ø-45øN.(a) 150 mbar; (b) 70 mbar.
during southern summer illustrate flow patterns associated
with more localizeddistributionsof diabaticheating,in contrast to those associated with the massive Asian monsoon
and
zonally elongatedintertropicalconvergencezones.
Acknowledgments.The assistance
of Mark Baldwin in unpacking
ECMWF data was appreciated.Discussionswith Ping Chen helped
al. [1993]havehelpedto quantifyS/T exchangein extratropical shapethe ideasof section6, and provideda stimulusfor timely publication of these observations.This researchwas supportedby the
latitudes.Further studyof large-scaleadiabaticprocesses
that National Aeronautics and SpaceAdministration, Contract NASWlead to S/T exchangewould be valuable.
4844, and by the National Science Foundation, Grant ATM-
As noted by Chen [1995], S/T exchangeassociatedwith
9123797.
16,688
DUNKERTON:
MONSOON
CIRCULATIONS
IN LOWER STRATOSPHERE
u
10 ms -•
16 E-
10
121 E jul
20
30
50
70
100
150
200
300
7OO
1000
•
-90
-60
-30
•
EQ
S
30
60
90
N
LATITUDE
Figure 12. Schematicdiagramof meridionaltransportand mixingadjacentto monsoonregionsin northern
summer,
superposed
on contours
of zonalwind(interval10ms-•). Heavycontours
(interval2 ms-•) and
one-waybold arrowsindicateclimatologicalmeridionaltransport;two-wayarrowsillustratemixingalong
isentropicsurfaces.The large bold arrow at 30øN representsthe western side of the Asian monsoon.Its
directionshouldbe reversedfor the easternside,wherev is oppositeandslightlysmaller.Tropopauseisshown
(heavydotted line, "T") and zero-windline is labeled"0".
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