GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 16, PAGES 1999-2002,AUGUST 15, 1997
Rossbywavepropagationintothetropicalstratosphere
observedby the High ResolutionDopplerImager
D. A. Ortland
Space
Physics
Research
Laboratory,
Department
ofAtmospheric,
Oceanic
andSpace
Sciences,
University
of Michigan,AnnArbor,Michigan
Abstract. Stratosphericwind observations by the High
ResolutionDoppler Imager on the Upper AtmosphereResearch
Satelliteshowthat quasi-stationary
Rossbywavespropagateinto
the tropics. Rossby wave breaking and dissipationcauses
meridionaltransportanddirectdepositionof momentum,andhas
a significantinfluence on the evolution of the mean tropical
winds in the upper stratosphere.Deeper penetrationinto the
tropics occurs when the quasi-biennialoscillation is in its
westerlyphaseabove30 km. Duringnorthernwinter,subtropical
momentumdepositionby Rossbywaves occursonly in late
NovemberthroughDecember,while duringsouthernwinter this
occursprimarilyin AugustandSeptember.
HRDI is viewing on a given day. These maps were then
averagedover eithera two-weekor month-longtime intervalto
reducerandommeasurementerrorsand to bring out the signalof
the quasi-stationary
Rossbywave. Suchmapscanbe produced
at
the HRDI altitudesamplinggrid from 10 to 40 km at 2.5-km
intervals.
Thesemapswere usedto calculatethe Eliassen-Palm
(EP)
Fluxvectors
F = (F•o
,Fz), where
(see
Andrews
etal.[1987]
for
the standardnotation):
=
_ -v'u'
F•o
P0acos•p
•z Oz
(1)
1. Introduction
Fz
=pøacøs•P
f' WJ
(2)
Rossby waves propagate equatorward from the winter
acosq
'
hemisphere,and providea link betweenthe midlatitudeand the
tropicaldynamics. There has been somequestionas to how
In Equation (2) it is assumedthat the vertical flux of zonal
much of a role Rossbywavesplay in tropicaldynamics. It is momentumis negligiblefor Rossbywaves. The HRDI wind field
uncertainwhether dissipationor breaking of Rossby waves maps are used to calculate the meridional flux of zonal
suppliesa significantamountof easterlyforcingfor the quasi- momentumin Equation(1). The heat flux termswere calculated
biennialoscillation(QBO) [Dunkerton,1983]or the stratopause as follows. First, the wind maps were used to calculatethe
semiannualoscillation[Delisi and Dunkerton,1988; Hamilton verticalcomponent
of vorticityandthenthePoissonequationwas
and Mahlman, 1988]. Globalmeasurements
of the stratosphere solvedfor the horizontalperturbationstreamfunction•l/'. The
are requiredto answerthis question. In the past, only global stream function is related to the perturbation potential
measurements
of temperature
havebeenavailable,buttheRossby temperatureby incorporatingthe geostrophicand hydrostatic
wave signal in temperaturesis proportionalto the Coriolis approximations:
parameter,and thereforeweak in the tropics. Global wind
0' / •'z= f•. / N2.
(3)
measurements
in the stratosphere
up to 40 km are now available
frequency
squared
was
from the high resolutionDopplerimager(HRDI) on the Upper A valueof N2 = 5xl0-4forthebuoyancy
AtmosphereResearchSatellite(UARS), whichhasbeenmaking used.
observationsfrom November 1991 to the present. These
The EP flux vectorsareparallelto themeridionalprojectionof
measurementsshow that Rossby wave propagationinto the the group velocity vectors for Rossby waves if the WKB
tropicsdoes indeedoccur,primarilywhen the QBO westerly approximation
is valid [Edmonet al., 1980],andthusareuseful
windsresideat 30 km over the equator. This paperpresentsa for showinghowtheRossbywavespropagate.Thedivergence
of
descriptionof one of these events in HRDI data and some the EP flux vectorsgivestheRossbywavecontribution
to forcing
discussionof the role Rossby waves play in the tropical of the zonal meanzonal momentum(referredto hereas the body
momentumbudget.
force)'
1
DF =•V.F.
2. Data Analysis
P0acos•p
(5)
A completedescriptionof the HRDI wind measurement It shouldbe notedthat the time averagingperformedon the data
techniquemay be foundin Ortlandet al. [ 1996]. HRDI measures filters out the traveling waves and may resuilt in an
windvectorsaboutevery6øalonga pathto onesideof the UARS underestimationof the total planetarywave body force in the
orbittrack. Mapsof the horizontalwind field measured
on each tropics.
day were madeby interpolating
to a regular10øx5
ø longitudelatitudegrid. Latitudinalcoverage
variesfrom dayto dayasthe 3. Results
orbitprecesses
anddepends
on the sideof the orbittrackthat
Copyright1997by theAmericanGeophysical
Union.
Quasi-stationary
Rossbywavespropagate
upwardfromtheir
sourceregionin the troposphere
and equatorw•xd
throughthe
Papernumber97GL02001.
westerliesof the winterhemisphere.Penetration
intothe tropics
and into the summer hemisphereis observedto occur in
0094-8534/97/97GL-02001
Novemberand Decemberof 1992 and 1994. It was duringthese
$05.00
1999
2000
ORTLAND:
ROSSBY WAVE PROPAGATION
INTO THE TROPICAL STRATOSPHERE
60
ß
()
I
I
I
I
•
'"t
[
40
ß
'"'#
Nk
'"'
20
0
-80
315
360
'--• 25m/s
0
,
45
*
,
•
90
I 5
•
I 0
-_
'
225
,
270
315
360
Longitude
Figure1. Windfieldat35 kmandcontours
oftheperturbation
stream
function
from(a)November
16-30,1994,and(b)January
1-15,
1995.
propagation. Since the perturbationamplitudeis so large, the
dynamicsmay no longerbe convenientlydescribable
in termsof
wave propagationon a zonalmeanbasicstate.
The mean winds evolve rapidly throughoutNovember and
Decemberof 1994. In early November(not shown),westerly
winds in the tropics extend from 22.5 to 40 km and are
by easterliesin the subtropics.From mid-November
November 1994 at 35 km, with contoursof the perturbation surrounded
stream function overlaid. A strong anticycloneover the (Fig. 2a) to mid-December westerliesform in the northern
northwesternPacific is embeddedin the northernhemisphere subtropics,which allows Rossby waves to penetrateto the
westerlies,
andmay be viewedasa predominantly
wavenumber equator. Strong easterliesbegin to extend from the southern
yearsthat QBO westerlieswerecenteredabove25 km overthe
equator.Beginningin November,seasonal
westerlies
formin the
northernwinter subtropicsand providea meansfor stationary
wavesto propagatefrom the midlatitudesinto the tropics. A
particularlystrikingexample,shownin Fig. 1, occurredlate in
November 1994. Figure l a shows the wind field in late
1 disturbance to zonal mean flow. Note that the zonal mean
hemisphere
into the tropicsabove35 km, formingan easterly
windsat theequatorarewesterly,buta regionof easterlies
occurs shearzonethat causesthe upperboundaryof the QBO westerlies
in theupper
to the west of South America. The zero wind line, or critical to descend.In lateDecember(Fig. 2b) theeasterlies
extendwell intothe northernsubtropics,
markingthe
latitudefor stationaryRossbywaves,is about15ø southof the stratosphere
SAO, and the onsetof the next
equator(see also Fig. 2). The perturbation
streamfunction easterlyphaseof the stratopause
contours show the characteristicpattern of equatorward easterlyphaseof the QBO. Note also that the QBO westerlies
propagation,
withphaselinestiltingfromsouthwest
to northeast. have beenslightlyadvectedinto the northernhemisphere.The
Figure lb showshow the wind field has evolvedin early samesequenceof eventswas playedout in 1992 as well (not
of the
January1995at 35 km. The anticyclone
hasgrownin strength shown),indicatingthattheremay be somesynchronization
and moved eastward and poleward from its position in startof the nexteasterlyphaseof theQBO withthe annualcycle
November,knockingthe centerof the polarvortexconsiderably [DunkertonandDelisi, 1997].
A time latitudesectionof the daily meanzonalwind at 35 km
off of the pole. The zonalmeanwindshaveevolvedsothatthe
criticallatitudeliesjust northof the equator,confiningthe wave (Fig. 3) showsthe evolutionof the winds in a little more detail.
activity to the northernhemisphere.
The perturbation
stream Westerlies are strongly advected northward at 35 km and
function contoursno 1ohgershow a pattern of equatorward disappearrapidlyin early winter. The subtropicalwindsundergo
-60
-30
0
Latitude
30
60
-60
-30
0
30
60
Latitude
Figure 2. Latitude-heightcrosssectionsof HRDI meanzonalwindsfor (a) November15-30, 1994, and (b) December15-31, 1994.
The contour interval is 5 m/sec.
ORTLAND:
ROSSBY WAVE PROPAGATION
INTO THE TROPICAL
STRATOSPHERE
2001
The body force DF due to stationaryRossby waves was
computed
from monthly averagedHRDI winds throughoutthe
40
observationperiod. A time-latitudecross-section
(Fig. 5) at 35
km showsthatdeceleration
in thenorthernsubtropics
occursonly
in
early
winter,
and
that
this
deceleration
occurs
closer
to the
< -20
equatorduringthe westerlyQBO years1992 and 1994. Later in
the winter, easterlybody forcing is generallyconfinedto the
midlatitudes.The patternin the southernhemisphereseemsless
15-Sep
15-Oct
15-Nov
15-Dec
15-Jan
15-Feb 15-Mar
1994
regular. Subtropicalpenetrationof Rossbywavesfrom the south
Figure3. Time-latitude
cross
sections
ofmeanzonalwindsat35 occursin October1992, August 1993, June1994 and September
km. The contourintervalis 5 m/sec,easterlies
are darklyshaded 1994. Note that the only apparentRossby wave drag in the
andwesterliesbelow 10 m/secarelightlyshaded.
tropicsarisingfrom the southernhemispherein June 1994 also
corresponds
to the only time that westerliesextendedfrom the
southernhemisphereinto
periodsof accelerationand deceleration,
presumablydue to
fluctuationsof the strengthof the meridionalcirculationand 4. Discussion
intermittent
periodsof waveforcing.Notein particular
thebrief
HRDI wind observations
showthat equatorwardpropagation
subtropicaldecelerations
that coincidewith the time of the
Rossbywaveeventsshownin Fig. 1. In contrast,
QBOwesterlies of Rossbywavesis modulatedby the QBO. This hasalsobeen
at 30 km (see Fig. 2) are only shiftedslightlyinto the northern
subtropics
throughout
thewinteranddisappear
in thespring.
Figure4 showsthe Eliassen-Palm
crosssectionfor the quasistationarywave numberI Rossbywavethat waspresentin the
notedin the modelingstudiesof O'Sullivanand Chen[1996] and
O'Sullivan [1997]. This modulation of the propagation
characteristics
is thoughtto be responsiblefor the QBO-like
variations in midlatitude dynamics [Holton and Tan, 1980;
wind field at the end of November 1994 and the contours of the
Dunkertonand Baldwin, 1991; O 'Sullivanand Dunkerton, 1994].
bodyforceDe. This crosssectionindicates
thatsignificant
mean
flow decelerationcan occurin the tropicsandsuggests
thatthere
is some cross-equatorialpropagationof the wave. Strong
radiationinto the tropicsoccursbetween30 and 40 km. The
subtropics
below 30 km have easterlyor weak westerlywinds
(Fig. 2b) and thusplanetarywave propagation
into the tropics
Rossbywave propagationinto the tropicsalso affectstracer
distributionsthere, and an understanding
of how Rossbywaves
behaveastheyenterthe tropicsis essential
for understanding
the
completepictureof how suchtracersaretransported
betweenthe
tropicsandmidlatitudes.
Themodelingstudies
of O'Sullivanand
Chen[1996]andChen[1996]showthatthistransport
behavioris
does not occur.
alsodependent
onthephaseof theQBO,withmoremixingoutof
The momentum budget of the tropical and subtropical thetropicsintothewinterhemisphere
duringeasterlyQBO phase
stratosphere
is a delicatebalanceof contributions
from four and with wavespassingthroughthe tropicsand causingmixing
sources:meridionalvorticity flux, body forcesfrom vertically into the summer hemispherewhen the tropical winds are
evidencefor this cross-equatorial
propagatingequatorialand gravity waves,body forcesfrom westerly.Otherobservational
in N20 distributions
is discussed
by Dunkertonand
laterallypropagating
Rossbywaves,and verticaladvectionof propagation
momentum. This balance is describedby the transformed 0 'Sullivan[1996] andO 'Sullivan[1997].
Eulefianmeanmomentumequation[Andrewset al., 1987]
ut
(f- •y)+w*•z =DFß
(6)
40
Themeanzonalwindacceleration
•t, thezonalmeanabsolute
vorticity
f-•y, andtheRossby
wave
contribution
toDewere
computedfor November1994 from HRDI wind measurements.
The residualmeanmeridionalvelocityv* wascomputedfromthe
United Kingdom MeteorologicalOffice assimilationmodel
[Swinbank
and 07•eill, 1994]datafor the sameperiod. This is
30
probablyonly qualitativelycorrect,but doesshownorthward
cross-equatorial
flow of about0.5 m/secat 35 km, in agreement • 25
with calculationsby Eluszkiewiczet al. [1996]. An attemptto
deducethe equatorialand gravitywave contribution
to De as a
residualof the othertermsrequiresgreateraccuracythancanbe
20
achieved at this point. These computationsshow that
decelerationin the southernsubtropicsis due to equatorward
momentum advection.
Noah of the equator, momentum
advection alone would cause an acceleration that increases with
I
i
-20
0
20
latitude. This is largelyoffsetby theRossbywavebodyforce.It
is also possiblethat gravitywaveswith easterlyphasespeed
contributeto a net easterlybody force as well, sincewesterly
gravitywaveswill be partiallyfilteredout by the underlying
QBO westerliesat the equatorandby the subtropical
westerlyjet
at the tropopause.It can be seenin Fig. 3 that after the late
NovemberRossbywaveevent,acceleration
of the zonalwind in
the northernsubtropics
doesoccurasthe westefiiesareadvected
Figure4. Eliassen-Palm
crosssections
for the quasi-stationary
wavenumber1 Rossbywavecalculated
from HRDI wind data
averaged
fromNovember15 to 30, 1994. Eliassen-Palm
flux
vectors
F aredividedby thedensityandscaledsothattheypoint
in the directionof wave propagation.Their lengthsare thus
proportional
to thewaveactivityperunitmass.Contours
of the
Rossbywave body forceDF are overlaid,at 0.5 m/sec/day
noahward.
intervals,with valuesover0.25 m/sec/dayshaded.
Latitude
2002
ORTLAND:
ROSSBY WAVE PROPAGATION
INTO THE TROPICAL STRATOSPHERE
20
I
:::...-..::j::½:::
/
.: .......
'""-";:"?
t i:;-'i:•;i;!;'i•'"'
•:...,.....
......
•-r.,
-....-.
.-,-...-•.-...
,--.....-.......•:•/
-2o
""'"'"-'-:...:½:":....:.....'.':..':!.:...,
.,:.::,,
.. ::::- :::':
• ! .......
......
z"i'-:..:!!::
'""*"'-":"--"?::.•::i
z;:..--•.
ii::::
::?:
:::::::::::::::::::
i::::'
::•--:::•
•:•'::'
'"'::"':::'"'"'*
•
..
-40
::'ii',::
...........
J•
Jul
J•
1993
Jul
Jan
::::::::::::::::::::::::::::::::::::::::
.......
Jul
1995
1994
. .......
Jan
1996
Figure5. Time-latitude
crosssectionof thestationary
wavebodyforce(DF) at 35 km, calculated
frommonthlyandzonallyaveraged
HRDI zonalwinds. The contourintervalis 0.5 m/see/day,andeasterlyforcingis above0.25 m/see/dayis shaded.The zerocontouris
omitted. The zero wind line is shown as a dashed contour.
The main effect of the Rossbywave body forcein the tropics Dunkerton,
T. J., Laterallypropagating
Rossbywavesin the easterly
accelerationphase of the quasi-biennialoscillation,Atmosphereat 35 km is to causea more rapid onsetof easterliesthan would
occur from a mean meridional
circulation
alone.
A meridional
Ocean, 21, 55-68, 1983.
Dunkerton,T. J., and M.P. Baldwin, Quasi-biennialmodulationof
circulationinto the winter hemisphereinitially produceswesterly
planetarywave fluxesin the northernhemisphere
winter,0r.Atmos.
accelerationin the winter subtropics,which is offset by the
planetary wave drag. This contributesto the formation of a
vertical easterly shear zone uniformly acrossthe tropics that
becomesthe next easterlyphaseof the QBO. This appearsto be
the courseof events for the QBO cycles observedby HRDI,
which fall into the "two-year"cycle classificationof Dunkerton
and Delisi [1997]. The "three-year"cyclesin their classification
are characterized
by a slowerrate of easterlyshearzonedescent,
and one might speculatethat this is the resultof weakertropical
Rossbywavedragat the beginningof thesecycles.
Meridional advectionis also presentat 30 km, as is evident
from the northwardshift of the QBO westerlies(Fig. 2), but it
doesnot eliminatethe westerliesaltogether,ashappensat 35 km.
The rapiddemiseof westerliesat 35 km whenthey are presentin
early northernwinter, and not at 30 km, may be due to the
strongermeridionalwind. This papershows,however,thatthere
is evidencethat direct Rossbywave body forcingcan also be
responsible,
sincethesewavesdo penetrateintothetropicalupper
stratosphere.
$ci., 48, 1043-1061, 1991.
Dunkerton,T. J., and D. O'Sullivan, Mixing zone in the tropical
stratosphere
above10 mb, Geophys.Res.Letts.,23, 2497-2500,1996.
Dunkerton,T. J., and D. P. Delisi, Interactionof the quasi-biennial
oscillationand the stratopause
semiannualoscillation,J. Geophys.
Res., in press, 1997.
Edmon,H. J., Jr., B. J. Hoskins,andM. E. Mcintyre,Eliassen-Palm
cross
sectionsfor the troposphere,
J. Atmos.Sci.,37, 2600-2616,1980.
Eluszkiewicz,J. et al., Residualcirculationin the stratosphere
andlower
mesosphereas diagnosedfrom microwavelimb sounderdata, J.
Atmos.Sci., 53, 217-240, 1996.
Hamilton, K., and J. D. Mahlman, General circulationmodel simulation
of the semiannualoscillationof the tropicalmiddle atmosphere,
J.
Atmos.Sci., 45, 3212-3235, 1988.
Holton, J. R., and H.-C. Tan, The influenceof the equatorialquasibiennialoscillationon the globalcirculationat 50 mb, J. Atmos.Sci,
37, 2200-2208, 1980.
Ortland, D. A., W. R. Skinner, P. B. Hays, M.D.
Burrage,R. S.
Lieberman, A. R. Marshall, and D. A. Gell, Measurementsof
stratospheric
winds by the High ResolutionDoppler Imager, J.
Geophys.Res., 101, 10,251-10,363,1996.
O'Sullivan,D., Cross-equatorially
radiatingstratospheric
Rossbywaves,
GeophysRes.Lett., in press,1997.
O'Sullivan, D., and T. J. Dunkerton, Seasonaldevelopmentof the
extratropical
QBO in a numericalmodelof the middleatmosphere,
J.
Acknowledgments. The author thanks Donal O'Sullivan, Tim
Dunkertonand an anonymousreviewer for many helpful comments.
Atmos.$ci., 51, 3706-3721, 1994.
Deborah Eddy provided expert assistancewith preparationof the O'Sullivan,D., and P. Chen, Modeling the quasi-biennial
oscillation's
manuscript.This work wassupported
by NASA contractNAS 5-27751.
influenceon isentropic
tracertransportin thetropics,J. Geophys.
Res.,
101, 6811-6822, 1996.
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(ReceivedMay 5, 1997;
acceptedJune19, 1997.)