Variations of atmospheric angular momentum associated with ...

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. D6, PAGES 12,981-12,998, JUNE 20, 1994

Variations of atmospheric angular momentum associated with intraseasonal oscillations forced

by zonally moving prescribed heating Hisanori

Itoh

Department of Earth Sciences,Wakayama University, Wakayama, Japan

Abstract. IntraseasonMvariationsof atmosphericangularmomentum(AAM) are numeric'Mlystudied. With prescribed40-day-period external heating which moves eastward from 60øE to the dateline in the tropics, intraseasonM oscillations are forced in a numerical model. In accordance with the oscillations, the variations of AAM also show 40-day periods. The phase relationship between the model intraseasonMoscillationsand AAM variations is similar to that observed. Further,

AAM exhibitsthelargestvariationin the equatorialregion,anda phasepropagation fromequatorialregionsto the subtropics, andfromhighlatitudesto the subtropics, as found in the real atmosphere.After carefulinspectionof thesefeatures,it was clarified that the sourceregion for intraseasonMvariations of AAM is not the equatorial region, but the subtropics. Also, the equatorial region is an apparent or superficialsourceregion, since AAM is transported from the subtropics,in accordancewith the propagationof Rossbywave trains to the subtropics. The phase propagation of AAM from high latitudes to the subtropicscan aJsobe explainedfrorn the behaviorof Rossbywave trains in the midlatitudesradiating from.the equatorialregion. Thus, Rossbywavetrains associatedwith intraseasonal oscillationsplay an important role in intraseasonalvariations of AAM. Introduction

tor is the primary sourceof changesin AAM. That is, as cumulus clusters move from the Indian

Ocean to the

western Pacific, the easterliesare intensifiedin the eastVariationsof atmospheric angularmomentum(AAM) ern Pacific, which import the angular momentum to the on intraseasonaltimescaleshaverecentlyreceivedmuch atmosphere. Madden also showeda phase relationship attention, particularly since the discoverythat the robetween the MJO and AAM variations; the strongest tation rate of the Earth (i.e., lengthof day) has the AAM occurswhen tropical convectionassociatedwith same timescale variations. Many studies have shown the MJO beginsto weakennear or east of the date line. that these two phenomenaappear to be linked via the conservationof total angular momentum of the Earth Lau et al. [1989],Kang and Lau [1990],and Maga•a

[e.g., Langleyet al., 1981; Rosenand $alstein, 1983; [1993]confirmedMadden'sfindings. However,there are severalquestionsconcerningthese Barnesel al., 1983].Importantproblemsconcerning in-

observationalresults,sincethesedata setslack data at the Earth's surface,which are requiredto calculatefric(1) Betweenthe frictionalandmountaintorques,which tional and mountain torque. The results of AAM varitorqueyieldsdominantAAM variations?(2) Howdoes ations mentioned above are based on several assumpthe atmosphereobtain the angularmomentumfrom the tions. For instance, the results in M87 were determined Earth? (3) I-Ioware AAM variationsrelatedto the at- from the assumption that the surface winds are 0.75

traseasonal

variations

of AAM

can be listed as follows.

mospheric intraseasonal oscillation (ISO)? Several studies have shown that AAM

variations

are

closelylinksfits-the ISO in the tropical atmosphere,i.e.,

timesthe 850-hPalevelwinds. Kang and Lau [1990] also used a similar assumption. When actual surface winds are used, the results could possiblydiffer.

Madden-Julianoscillation(MJO [Madden and Julian, In this respect,the resultsof Madden[1988](here1971, 1972]). Andersonand Rosen[1983]determined after referredto as M88), in whichsurfacewind data

that the tropical contribution to AAM variations is pre-

wereutilized, are of interest. The main area of frictional dominantlylarge, and the AAM phasepropagatesfrom torque influenceis not near the equator, but poleward the equatorialarea to the subtropics.Madden[1987] of 10 ø latitude. (hereafterreferredto as M87) first proposedthat the This raisestwo questions.First, which is right, M87 frictional torque over the eastern Pacific near the equa- or M887 Madden himself consideredthe inconsistency Copyright 1994 by the American GeophysicalUnion.

of these two results, and does not positively support the results in M88. Second, if M88 is true, how can

Paper number 94JD00245.

the results in M88 be consistent with the result of An-

0148-0227/94/94JD-00245505.00

dersonand Rosen[1983],in whichthe contributionto 12,981

12,982

ITOH: INTRASEASONAL ATMOSPHERIC ANGULAR MOMENTUM VARIATIONS

AAM variationsis large in the equatorial region? These subjects will be examined in the present paper.

0.890, 0.800, 0.690, 0.565, 0.430, 0.290, 0.160, 0.075, and 0.025.

The ISO hasalsobeenobserved in extratropical reThe dynamicalportion of the model, includingboundgions(Weickmannet al. [1985]amongmany others). ary layer processes,is exactly the same as that of There has also been controversyas to the origin of the extratropical ISO; is it just the responseof the tropical MJO, or does it have an extratropical origin? In

the University of Tokyo version of Japan Meteorolog-

that mountain torque also appearsto play an important role in variations of AAM in the extratropics. They found that the phase propagation of AAM variations having 40-day periodsis equatorwardin the extratropical regions,being oppositeto that in the tropics. They concludedthat this phenomenonis evidenceof an extra-

process. In this model, the radiative effectsare simply

ical Agencygeneralcirculationmodel(jMA-UT GCM) [Hayashi and $umi, 1986; see also Kanamitsu et al., conjunction with this, Dickeyet al. [1991]demonstrated 1983]. The differenceis mainly found in the diabatic

incorporated in the Newtonianheating/cooling scheme, and the latent heat releaseis prescribedas stated in the Introduction. The radiative equilibrium temperature

(RET) and the time constantof the Newtonianheating

used in the present experimentsare illustrated in Figtropicalorigin. Weickmannet al. [1992]alsosuggested ure 1. The RET is assumedto be symmetric about the that mountain torque may help force AAM fluctuations equator. The cumulusheating is prescribedas an external paduring the northern hemispherewinter. The formulation is the same as in Iloh and However,the equatorwardphasepropagationof AAM rameter. in extratropical regionsdoesnot necessarilysupport an Nishi [1990](hereafterreferredto as IN90). That is, extratropicalorigin of the ISO, sincewave trains radi- the external heatingmovesfrom 60øE to the datelinein ating from the tropics can reach the extratropicsand the equatorialatmospherewith a period of 40 days. The amplitude factor is 1.5. This meansthat the amplitude interact with mountains, which exert mountain torque. changes in themanner of I sin1'••rt/T ], By careful inspection of surface pressurepatterns over of theforcing mountains, the causemay be clarified. This is another where T representsthe period of 40 days and t is the time. The half length of a cumuluscluster is 300 lonsubject of the present paper. For these purposes,it is suitable to examine model gitude, with the latitudinal profile also the same as in output, sincecompletesurfacewind and pressuredata IN90. The vertical profile can be expressedas are provided. However,model results have not been er -- era er- era previouslyusedfor thesepurposes.The reasonfor this q(er)-- q0sin(•r )exp(7 ) era< er< erb fib -- ffa fib -- ffa is obvious;the ISO has not been realistically simulated, q(a) --O a • aa or a • av, (1) especiallyover a long term. !ntraseasonalvariations of AAM would have high noiselevelsin the output from where ? is a constant. Coefficients era and erbare assuch models.

Here, a device is adopted in the numerical model to overcomethis difficulty. Namely, the ISO is forced by external heatingwith a prescribedperiod. The ISO will then be ideally excited, and intraseasonalvariations of AAM will have a high signal-to-noiseratio. The idealized relationshipsbetweenthe ISO and AAM variations can be easily determinedfrom the model output. Special attention is paid to the sourceof AAM fluctuations, whichhasbeenwidely recognizedas locatedin the equatorial area, and to the causeof the equatorward phase propagationof AAM in extratropical regions. In section2, the model usedin this study is outlined, while the model ISO is briefly describedin section 3. The problem concerningthe sourceof AAM variations is examined in section 4. The cause of the equatorward phasepropagationof AAM in the midlatitudes is pursued in section 5. Finally, conclusionsare off, red in section 6.

2. Model Description ,

signed values of 0.15 and 0.95, respectively. Further,

? is set to -0.5, and q0 is assumedto be 5 K/d. For observationalsupport and verification by numericalexperiments as to the choice of these parameter values, see IN90.

Surfacetemperaturesare prescribedfor both the sea and land surfaces. Sea surface temperatures are assumed to be zonally uniform, taken from the zonal averageof April climaticdata. Land surfacetemperatures are calculated as the sea level temperature minus 0.6

K/kmxzs, wherezs representsthe land surfaceheight in kilometers

During the courseof time integrationof the model, it was noted that the upper equatorialtroposphericwest-

erliesbecameas strongas 15 m/s or greater.This behavior is commonlyobservedin model with heat sources

overthe equator[e.g.,Ting and Held, 1990;Bladdand Hartmann,1993],andthe reasonfor the strongwesterlies is given in these articles. Since it was felt that this was too unrealistic, a term to damp the zonal mean flow was introduced in the zonal mean vorticity equation.

The model used in this study is a 12-level spectral model, with triangular truncation at wavenumber 21

above sea level.

The formulation

of this term is

_ _a sin (•'erd er--- er-•-• )sin (7Yd ) a erc

(2)

(T21), basedon the systemof primitiveequations.The verticalcoordinateis the ersystem,whereer= PIPs,P

whenere< er< erdand l Y I< Yd,wherec•is a damping

expressesthe pressure,and ps the surfacepressure. The 12 levels are defined at er values of 0.995, 0.980, 0.950,

coefficient, a the radius of the Earth, and y the latitudinal distance from the equator. The values of c•,

ITOH- INTRASEASONALATMOSPHERIC ANGULAR MOMENTUM VARIATIONS

(a)

12,983

(b)

Rodiot

ive

Equi I ibr ium Temperoture

( K )R.E.T. and NewtonionCooling Time Const. /

-

200

n

400 -

4O0

-,-

600 -

600

800

800

200

-

1000

I

-90

I

-60

I

I

I

I

-30

0 Lot

I

lOOO

I

.30

I

200

90

I

I

220

I

I

240

I

260

I

I

I

280

300

Temperature

i tude I

I

I

I

I

0

5

I0

15

20

Time

Const.

(day

I

Figure 1. (a) Radiative equilibrium temperature (unitsof degrees Kelvin)asa function of latitudeandheight,andFigurelb the Newtonian cooling rate(dashed line,unit l/d) usedin the present study.The solidlinein (b) represents the radiativeequilibrium temperature used in the experiment in whichthenorth-south contrast oftheradiativeequilibrium temperature is removed(seeFigure$).

etc,eta,and Ydare takenas 0.5 m/s/d, 0.15, 0.55, and 20ølatitude,respectively.This term dampsthe zonal mean flow between20øSand 20øN, being especiallyeffectivenearthe equator.The resultsfrom this modificationwerecompared with resultsfromthe modelwithout this artificial term. The conclusionsconcerningthe AAM budget were the same. Two mainexperiments wereperformed.The first was an experimentwith the modelhavingan all-oceansur-

The equationfor the momentumbudgetis derived here. The total AAM denoted as M is the sum of the

relativeAAM,ru, andtheplanetary AAM,r•'•, where r is a cosq•,u the eastwardcomponent of the wind, • the rotationrate of the Earth, • the latitude. The total

AAM is governed by the followingequation:

(p' -

r0A

(p, - -r (p, ½os O•

Op,

Or•

0(psM&)ps - RT•+rg (3)

face(experiment A), whilethe second hadrealistictopography(experimentB). Sincethe RET is symmet-

ric about the equator and the sea surfacetemperature where A denotesthe longitude,v the northward comis takenfrom April data, theseexperimentswill simu- ponentofthewind,& = da/dt, andß thegeopotential. the gasconstantfor dry air, T the late springconditions.Althoughseveralsupplementary Also,R represents

experiments werealsocarriedout, includingthe sim- temperature,g the gravitationalacceleration,and ru

the u momentumflux by verticaldiffusion.Other than bodyof conclusions wassimilarfor thetwoexperiments. theseterms, there are dissipationterms resultingfrom Therefore, the resultsof the twoexperiments (A andB) horizontaldiffusionand the artificial dampingover the

ulation for the northern hemispherewinter, the main

are mainly presented.

equatorintroduced in (2), but theyareomittedfor sim-

Time integrationswere performedover 320 model plicity;theeffectsdueto thesetermsarenotsignificant. The verticalandlongitudinalintegrationof (3) gives days,and analysescoverthe periodfrom day 161 to the momentumbudgetoverone latitude belt: 320. In otherwords,four cyclesof the ISO weretreated. This periodof time doesnot appearto be sufficient,but we are confidentthat it is long enoughto extract the • p• MdadA p• Mycos •dadA essentialfeatures of AAM variations, since the model ISO has an exact 40-day period, and almost the same behaviorwas repeatedfor the four cycles.

-

p••dA - rg p•CD Vu•dA,

(4)

12,984

ITOH: INTRASEASONAL

ATMOSPHERIC

ANGULAR

MOMENTUM

Zonal

where p indicates the air density, Cz> the drag coefficient, and the subscript s definessurface values. The first, second,and third terms on the right-hand siderepresent momentum flux convergence,mountain torque and frictional torque, respectively. For the deriva-

VARIATIONS

Mean

F low

( m/sec

)

0.0

0.2

tion of the mountain torque term from p•(O(I)/OA)+

RT(Ops/OA),see,e.g., Arakawaand Lamb[1977]. In

O 014

the calculations below, we will use the equation multi-

plied by a/g, insteadof (4) itself. The momentum

flux terms should be subdivided

into

._

•0.6

the contributions of the mean flow and waves, to seethe relative importance of these components. The follow-

ing equationcan be obtainedfrom (3), approximating p•(A,qb, t) m •7(qb,t) and takingthe zonalmean: 0

10

pw(+

-90

4)-;1• (F• •• cos •) -

o

1,0

-

0

0.8

Oa (•r•'n•)+rgOa'

)

(5)

where the overbar and the prime expressthe zonal mean and waveparts, respectively.This equationgivesa good

approximation of (3) overlatitudebeltswherethereis

-60

-30

0 Lot

30

60

90

i tude

Figure 2. Latitude-height sectionof the zonal mean flow averagedoverday 161 to 320 of experimentA. The units are m/s, with easterliesshaded.

Figure3b is shownfor comparison.In the latter, the latitudinallyconstantRET isemployed(Figurelb); therefore the zonal flow in midlatitudes is weak, implying that tropical-extratropical interactions are prohibited.

no mountain area. The quantities in the parentheses Figure 3b is similar to Figure 2 found in IN90. That is, both U250 and U850 are large near the region of maxof the first (fourth), second(fifth), and third (sixth) imum heating. On the other hand, in Figure 3a, the terms on the right-hand side representthe relative momaximum U250 occursnear the dateline, although that mentum transport by mean circulations, relative momentum transport by waves,and planetary momentum of U850 is still found near the maximum heating. Further, in the westernhemisphere,U250 doesnot gradutransport by mean circulations,respectively.Generally ally decrease,but amplify instead in several regions. speaking, the latitudinal convergenceterms dominate These features have been observed,e.g., by Madden the vertical ones. The seventhterm is negligible,except and Julian[1972]and GutzlerandMadden[1989],with for the planetary boundarylayer. The third term is furthe latter pointing out the influenceof the midlatitudes.

ther dividedinto -rf•0(•7• cosqb)/O•b and•7rv-2f•sin

The latter, which is much larger than the former, is important for the vertical redistribution of AAM because

Severalstudies[e.g., Hsu et al., 1990]showthat the

amplification is attributed to Rossbywave trains which originatenear tropical heat sources,returning again to generallyshowsoppositesignsbetweenupper and lower the tropics, but at different locations from the source layers. That is, wheneverAAM increasesin lower layers region, after goingthrough the midlatitudes. by this term, AAM decreasesin upper layers, and vice In the present study, it will also be shown that the versa. same scenariooccurs. Figure 4 illustrates the horizontal structureof the geopotentialheight and wind at 250 3. Brief Description of the Model hPa for severalcategories.The 40-day period is divided Intraseasonal Oscillation into eight categories.Category I is for the area around the minimum heating, while the maximum heating ocSinceAAM variations are closelyrelated to the model curs at category5. In all the figures,it can be clearly intraseasonal oscillation and tropical-extratropical in- seenthat wave trains propagatefrom the tropics to the teractions, a brief description of these phenomena is midlatitudes, then return to the tropics. The maximum given in this section. amplitudeis attainedin category6 or 7 (not shown). Figure 2 showsa latitude-heightsectionof the zonal The regionwhere the wave trains return (90ø-45øW; mean flow in experiment A averagedfrom day 161 to i.e., 270ø-315ø) exhibitslarge amplitudes.This is the 320. The maximumis lessthan 40 m/s. It can be con- reasonthat the amplification occursin the same region cluded that spring conditionsare simulated, although in Figure 3a. Further, wave trains with large amplithe zonal wind is slightly strongerthan that observed. tudes radiating near the dateline return to 30ø-90øE Figure 3 showsthe amplitudesof the 40-day-period in the tropicsin categories 8 (not shown)and 1. This components of the 250- and 850-hPazonalflows(U250 route of wave train propagationhas been reported by and U850), and the surfacepressureat the equator. Fig- Gao and Stanford[1988]. ure 3a representsthe resultsfrom experiment A, while In contrast, wave trains cannot return to the tropics

ITOH:

INTRASEASONAL

ATMOSPHERIC

(a) 15

ANGULAR

MOMENTUM

VARIATIONS

12,985

4. Source Region for Intraseasonal Variations of Atmospheric Angular Momentum

First,results fromexperiment A willbediscussed. In

this experiment, due to the all-ocean surface, there is no mountain torque.

CompositedAAM variationsand associated frictional

torqueare presented in Figure6. Two cyclesare depicted. A small gap at day 40 reflects the fact that the

initialtrendisnotcompletely eliminated in thefourcycles.Notethat at day 20 (and 60) the heatingreaches its maximum.

I

o

I

go

I

I

I

I

180

I

270

The maximum

in AAM

is found at about

day 35' Before this maximum occurs,frictional torque attains maximum valuesat about day 25, which is just after the occurrenceof maximum heating. These relationshipsall coincidewith those observed. Figure 7 illustrates the latitudinal profile of the vertically integrated variationsof AAM. It can be seenthat the contribution near the equator is large, and the phasepropagatesfrom the equator to the subtropics. Further, the vertical structure shows that the largest variation of AAM occurs in the upper equatorial troposphere, and the phasepropagationis downwardas well as poleward

360

Long i t ude

(b)

in low latitudes(not shown).Thesefeaturesalsocoin-

cidewiththosereported byAnderson andRosen [1983] and others, although it is unrealistic that phase leads

arealsoseennear30ølatitude.

Thefrictional torque is thencalculated in a similar

mannerto M87. Surfacewindsare set to 0.7 timesthe

850-hPalevelwinds,assuming CD andPsas constant values of1.3x 10-a and1.2kg/m3,respectively. There-

sultsareshown in Figure 8. Thesource/sink region can be identifiedas the equatorialregion.This is "consistent" with the resultshownin Figure7, and coincides with that of M87. Thus, the problem concerningthe

source region ofAAMvariations appears tobesolved. However,in this case,it is unnecessaryto use the 850hPa level winds as a substitute for the surface winds, 0

i

i 270

since model surface wind data are available.

i

Frictional

0

torque is then recalculated,utilizing the surfacewind data, with the results shown in Figure 9. These results completely differ from those in Figure 8. Variations have minimum values near the equator, while maxiFigure 3. Amplitudevariationsof the 40-day period mum values are found in the subtropics, poleward of componentsaveragedbetween5øN and 5øS, as a func- 10ø. In other words,the source/sinkregionof intraseation of longitude,for (a) experiment A and (b) an ex- sonal AAM variations is not the equatorial region, but periment in which the north-south contrast of the rais found in the subtropics. This feature is similar to 0

go

180

360

Long i tude

diativeequilibriumtemperatureis removed(seeFigure lb). The 250-hPazonalflow is shownby the line labeled U, the 850-hPa zonal flow by the line labeled L, and the surfacepressureby the line labeled P. Note that the scales for the zonal flow are different between

Figures3a and 3b.

that in M88.

These results lead to two questions. First, how can the relationship between the subtropical source and maximum AAM variations near the equator be inter-

preted? More precisely,the relationshipbetweenthe subtropicalsourceat the Earth's surfaceand maximum AAM variations at the upper equatorial troposphere

at the 850-hPalevel(Figure5). The reasonfor this must be clarified. Second,why are the two results, i.e., is obvious; the wave trains are absorbed between the

midlatitudewesterlies andtropicaleasterlies (Figure2). Therefore, amplificationnever occurs,apart from the sourceregion at this level.

the result from making use of the 850-hPa level winds and that from usingsurfacewinds, so very different? We will considerthe former questionfirst. Figure 10 shows the latitude-time

section of AAM

fluxes.

AAM

Geopotential andWind at 250hPa Cotegory

I

from doy 161 to 320

go 70

5o 3o

I0

-30

-70

-go

go

(b)

go

180

2tO

360

Cotegot, 5 fro-do,161 to320

70

5O 3O

3

I0

...

-5O -70

-go

g0

(C)

180

2r0

360

Cotegot, 6 fro-do, 161 to320

go 70

5O 3O

I0

-30

-5O -70 -go

ß

,

ß

.............

ß

I

ß

o

ß

I

I

go

ß

I

ß

ß

ß

I

.

I

180

,

,

I

ß

ß

ß

I

I

ß

.

,

.

I

2?0

Longitude

. .

,

I

360

:10m/$ec contour shode:'

intervol Om

Figure 4. Horizontalstructureof geopotentialheightanomalies(contours)and wind anomalies

(arrows)at 250 hPa for (a) category1, (b) 5, and (c) 6 composited overday 161 to 320 of experimentA. The contourinterval is 20 m; valuesshadedare greaterthan 0 m. The wind scale is shown at the lower-right-hand side of the figure. Large arrows in the northern hemisphere indicate

wave trains.

ITOH: INTRASEASONAL

ATMOSPHERIC

Geopotent

ial

Category

6

ANGULAR

and Wind from day

MOMENTUM

at

VARIATIONS

12,987

850hPa

161 to 320

9O ß:... 'mF.•,r•.-':' .....

'::•.-'•-'..:• •:•.'-'.-:•

.•

.

ß

ø

ß

''-i!•iiii, .: ..... '::.fiii:-i['" ' :....

ø

'

ß

ß

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"?

'

-.::[ii?: .:::•-:i!:• ....'.... .-".• .... :ii:

50

..!ii[

.... ::i;•!• •: :•":i, .......

4

'

..-- .•:11 !

:.......

ß

"

"'•"...•-.; I ':•

ß

ß

ß

ß

"

'

'

'

'

ß

ß

'1

':. .

50

0

90

'

180

270

360

Long i tude

• I Om/sec contour shade:

interval Om

>

IOm

Figure 5. As in Figure4 exceptfor category6 at the 850-hPalevel. layersand polewardflowin upperlayers(Figure 12). In troposphere, Rossby wavetrainspropagating gion to the equatorialregionof maximum AAM vari- theupper ations, when the atmosphereobtains AAM (between from the equatorial region have maximum amplitudes day 15 and 35). Therefore,it can be concludedthat, at thesecategories (Figure4). Therefore,waveenergy althoughthe subtropicsis the sourceregionof intrasea- is strongly transported from the equatorial region to is transported from the subtropical AAM source re-

sonal AAM variations, it is also the divergenceregion of AAM. Hence, the subtropicsdo not show large variations. On the other hand, the equatorial region is not

the subtropics, which means that relative momentum is 'transported in the opposite direction. Thus, at the

time whenthe atmospheregainsangularmomentumin

the source region,but is the convergence regionwhere the subtropics,AAM is simultaneouslyand necessarily the maximum

variations

occur.

Next, AAM fluxes are subdivided into three com-

transported from the surface in the subtropics to the upper equatorial troposphere. ,

ponents (Figure11). The relativeAAM transportby mean{low is small, and the planetaryAAM transport appearsalmost out of phasewith the total AAM transport. Thus, waves are responsiblefor the latitudinal AAM transport. By examining the vertical section of AAM flux con-

x 10 •8

Global

Mean Momentum

x10 •9

Budget

072

5

071

2

-

vergence (Figure 12), we find that AAM is transported in the following way: In the subtropics,angular momentum given to the lowest atmosphereis simultaneously redistributed to the upper troposphereby plane-

/

070

•.

./

ß

'•

ß

1 •E

,

069

o

tary AAM flux convergence (Figure12a). Then, in the

._

upper troposphere,AAM is transported to the equato-

068

-1

067

-2

rial regionby waves(Figure 12b). This is consistent with the fact that Rossby wave trains are seen only in the upper troposphere in the tropics. ' It is easy to understand these results in terms of the

co

066

20

Hadley circulationand Rossbywavetrains. Let us con-

40

60

80

Boy

sider the time of maximum frictional torque around cat-

i

i

5

egories5, 6, and 7. Sincethe Hadley circulationis intensifiedat thesecategories(Figure 12), anomalous

i

i

7

i

!

I

i

i

3

i

i

5

i

i

7

i

i

1

Category

easterliesare exerted at the surface in the subtropics Figure 6. Time variationsof AAM (solid line) and

(Figure 13). The atmosphere thereforeobtainsangular frictionaltorque(dash-dotted line) composited overday 161 to 320 of experimentA. Two c•cles are depicted.

momentum where the easterliesare strong. The AAM redistribution from lower to upper layersin the subtropics is attributed to anomalousequatorward flow in lower

Note that the maximum heating occursat days 20 and 60.

12,988 Vertical

ITOH- INTRASEASONAL I),

Integrated Time

._

ATMOSPHERIC

Momentum

Mean

( kg-m/sec

ANGULAR

MOMENTUM

)

Fr ict lanai

Subtracted

Time

E 50

VARIATIONS

3 O

Mean

Torque

Subtracted

E 50

3 O

60

5

60

5

70

7

70

7

-90

-60

-30

0

30

60

90

-90

-60

-30

Lot i tude

contour interval5x101rkg.m/sec

Figure 7. Latitude-time sectionof the vertically integrated AAM compositedover day 161 to 320 of experiment A. Time means have been subtracted. The

0

30

60

90

Lot i tude

contourintervalIx1012kg-m/sec •

Figure 9. As in Figure 8 except making use of the model

surface winds.

contourintervalis 5 x 1017kg m/s, with valuesgreater suregradientterm and the frictional(vertical diffusion) than zero shaded.

In order to answer the second question mentioned above, differences between the result at the 850-hPa level and that from the surface will be examined.

Com-

paring Figure 13 with Figure 5, it can be seen that surfacewinds near the equator appear very weak, especially east of the region of heating. Ratios of the absolute value of the surface zonal flow to that

at the

850-hPa level are shown in Figure 14. The ratios are generallysmall in the equatorial region, except west of the heating region. There may be two reasonsfor the weak surface winds

term. Thus, friction is effective near the equator, resulting in small surfacewind variations. The secondreasonthat winds are especiallyweak east of the heating regioncan be understoodfrom Figure 15, in which a vertical sectionalong the equator is plotted for category 6. Upward motions are, of course, strong in the heating region. Downward motions are dominant east of the heating region, with positive temperature anomaliesappearing in the upper levels. On the other hand, near the surfacewhere downwardmotions are weak, the temperature exhibits negative anomalies. This means that the static stability is large in lower layers. Hence, the vertical diffusion of wind is small, resulting in weak surfacewinds. In contrast, the static stability is relatively small west of the heating region.

in the equatorialregion. The first is that the variation of surfacewinds is small over the entire equatorialregion. Consideringthe momentum equation, since timescales Therefore,surfacewinds are not very weak (see Figin question are intraseasonal,the acceleration term is ure 13). The asymmetrybetweenthe areaseast and small, as is the Coriolis term in the equatorial region. west of the heating region was explainedin IN90. The question as to whether or not the above results As a result, the balance is formed between the preshold true in the model including realistic topography will be discussedin the following. Fr i ct i onol Torque Time

Mean

Subtracted

Moment

0

Time

Mean

um

F I ux

Subtracted 1

10 20

5

o $0 ..--

o_

10

o

40

1

50

3

60

5

o 30

o> o

.--

._

40 50

60

70

70

80 -90

3

-60

-30

0

Lot

30

60

90

8O

i tude

-90

contour intervol

lxlO•2kg .m/sec 21

1

-6O

-30

o

Lot

30

60

90

i tude

contour interval

lx10•Skg-m2/sec2

Figure 8. Latitude-time sectionof frictional torque compositedover day 161 to 320 of experiment A, calculated on the assumptionthat the surfacewinds are 0.7

section of the AAM flux Figure 10. Latitude-time compositedover day 161 to 320 of experiment A. The

times the 850-hPa

contour intervalis I x 1018kg m2/sa. Othersareasin

level winds.

The contour interval

1 x 1012kg m/s•'. Othersareasin Figure7.

is

Figure 8.

ITOH:

(a)

Momentum

INTRASEASONAL

Flux

ATMOSPHERIC

(zonal)

!0

7 o

60 fO

80

-60

-90

(b) o

-30

Momentum

0

Flux

30

60

90

(wave) i

io

3

•o

5

-•

40

I

-•

$o

3

6o

5

. •

fo

ANGULAR

MOMENTUM

VARIATIONS

12,989

ern hemisphereis rather obscure.The atmospherealso gainsthe angularmomentumthroughfrictional torque in the subtropics,which is transported to the equator. Figure 18 showsfrictional torque. Although there is strong asymrnetry between the northern and southern hemispheres,the basicfeaturesare the sameas in Figure 9. The AAM flux also convergesnear the equator (not shown).Thus, the conclusion from the modelthat includesrealistic topography is the same as that from the model without topography. Further, two more piecesof evidenceare presentedin the appendix, for the weak surfacewinds. Together with the findings of M88 and the results in the appendix, summarization can be made as follows. Since surfacewindsare very weak near the equator, especially east of the region of heating, where easterliesshould have prevailed, the equatorial region must not be the sourceregion for intraseasonalvariations of AAM. Instead, the sourceis the frictional torque in the subtropics, poleward of 10ø latitude.

5. Propagation of AAM Variations From High Latitudes to the Subtropics

It is of interest to note that in Figure 17, the phase propagates from high latitudes to the subtropicsin the -•o -60 -30 0 30 60 90 extratropical region, while it generally movesin the op(c) Momentum Flux ( planetary ) posite direction in Figure 7. This raises the question as to why the difference is produced. This problem 3 10 will be consideredonly in the northern hemisphere. In 5 the southern hemisphere,the causeof the equatorward propagationis relatively simple. It resultsfrom the fact that frictional torque propagatesequatorward,as seen in Figure 18. Figure 19 illustrates a latitude-time sectionof moun60 tain torque anomalies. Comparing this figure with Figure 17, it can be found that the maxima of AAM in the Northern extratropics appear just after the maxima of i 8O mountain torque. Namely, the AAM in this region is 0 30 60 90 -60 -30 -90 Lat i rude regulated by mountain torque. The time of the maxcontour interval IxlOmSk9.m2/sec2 imum AAM occurs in categories3 through 6. On the other hand, maxima in the tropics which result from Figure 11. As in Figure10 exceptfor (a) the rela- frictional torque and by AAM transport appear at cattive AAM flux by meanflow, (b) relativeAAM flux by egory 6 or 7, and those in the subtropics lag slightly waves,and (c) planetaryAAM flux. behind. Therefore, it can be stated that, since the maxima of AAM anomaliesare attained at earlier stagesin Figure 16 showsa compositeof time variations of the higherlatitudes, the phasesof AAM propagatefrom AAM, frictional torque, and mountain torque from ex- high latitudes to the subtropics. periment B. The variationsof AAM and frictional torque In order to pursue the causeof mountain torque maxare similar to thoseshownin Figure 6. The amplitudes ima, the horizontal structures of surface pressure and of mountain torque are large, but synchronizationwith mountain torque anomalies will be examined. Those the 40-day period is not very good, resultingin the cor- for category 5 are shownin Figures 20 and 21, respec80

I

relationwith (the time derivativeof) AAM variations .tively. It can be seenin Figure 21 that the maxima being low. In short• the basic contribution to AAM variations in this experiment is also frictional torque.

of mountain torque for category 5 are exerted by the torque around the Rocky Mountains. This in turn is Next, a latitude-time sectionof vertically integrated produced by the pressure distribution which exhibits AAM anomalies(Figure 17)is presented.In this fig- positive anomalies east of the Rocky Mountains and ure it can also be seen that the contribution from the negative anomaliesto the west, as seen in Figure 20.

equatorial region is large, and that the phase "in the low-latitudes" propagatesfrom the tropics to the subtropics, although the phasepropagationin the north-

(In other categories,mountaintorquealsoresultsfrom similar pressurepatterns near the Tibetan Plateau and

Greenland.)

12,990

ITOH:INTRASEASONAL ATMOSPHERIC ANGULAR MOMENTU M VARIATIONS

(•)

PIonetony' Momentum Flux Conv.

cot.--

6

Time Meon Subtrocted

0

200

4OO

600

800

IOOO -9o

-60

-30

o

30

Lot i t ude

60 O.002Po/sec

contour intervol Momentum

(b)

Flux Time

Cony. Meon

9o

t[•_o. Zm/,ec

(wove)

5xlOIZkg-m/sec cot.=

Subtrocted

0

2OO

400

600

800

!000 -90

-60

-30

0

30

Lot i t ude

60

90

_•_0.2m/sec O. O0:>Po/sec

contour intervol

5x!OI2kg-m/sec2

Figure12. Height-latitude sections forthefluxconvergence or(a) planetary AAM and(b) relative AAM by waves at category 6. Meridional circulation anomaliesare also plotted by

arrows.The contourintervalis 5 x 10•'kgm/s•', with valuesgreaterthan zeroshaded.The scaleof arrows is indicated at the lower right side of the figure. Since the relative AAM flux convergenceby mean flow is small, the figure is not presented.

The pressureanomaliesaround the Rocky Mountains Finally, an examination is conducted to determine are formed by Rossbywave trains. Figure 22 shows the model dependencyof the equatorward propagation.

experiments in whichthemodelparameters are the horizontal structureof the geopotential heightand Several Windsfor category5 at the 250-hPalevel. It can be changed,including a simulation of the northern hemiclearlyseenthat Rossbywavetrainsreachmidlatitudes, sphere winter, show that the direction of propagation originatingnear the heating regionin the tropics. The is considerablymodel dependent(not shown). Howsurface pressureanomaliesnear the Rocky Mountains seen in Figure 20 are the reflection of the equivalent barotropic structure of midlatitude Rossbywave trains. Note that the pattern of the wave trains in this ex-

ever, at the same time, it is found that the patterns of the wave trains basically determine the occurrenceof mountain torque maxima and hence AAM maxima in

periment is similarto that in experiment A (Figure4),

of propagation changeswith various conditions,Rossby wave trains radiating from the tropics play an essential role in the time variation of mountain torque and AAM in extratropical regions.

although the amplitudes over the Atlantic are much

larger.Therefore, thispatternis nottheresultof in-

the extratropics.To conclude,althoughthe direction

teractionswith the topographyof the extratropics[cf. Dickeyet al., 1991].Thus, it canbe concluded that the 6. Conclusions

equatorwardpropagationof AAM variationsin the midlatitudes in the presentexperimentresultsfrom Rossby Intraseasonalvariationsof atmosphericangular mowave trains propagatingfrom the tropics. mentum(AAM) are studied,usinga numericalmodel

ITOH- INTRASEASONAL

Surface

ATMOSPHERIC

Pressure

Category

6

and

ANGULAR

MOMENTUM

Surface

from do•

VARIATIONS

12,991

Wind

161 to 320

Zonal

Mean

:mR:::::::.;•.:•:: ;:u.•:::::m::•.::::::::::::::::::::::::::::::: :::::::::•.::::':::.

. •""'•••i!i!i?ii•iiii[-•'"'"""'"-':::'"••••jii!".•i!i '"'""-:':'•:'"'"'•••••• . ß • '_._.•;_ • •

......................... •iiii(•:•iiiiiiii:.•iiii!•: •i?•ilk..•iiii!i!ii•:ii!ii•::•::? :'

1'

ii?!•, •.

ß

:'!i:•iii•,

....................................................... :.......... .,....-.-•:...-• ,,,::: •

-'•]::.:•i

i:....,,....:•-...• i-

'"

.....

l -ø

_o ø

o

-s -9

o ,

0

-' 0

90

180

2?0

360

Longi rude

•Sm/sec contour shade:

,

interval O. Omb

2.0hPa

Figure 13. Horizontalstructureof surfacepressure anomalies (contours) andsurfacewindanomalies (arrows) for category6 compositedover day 161 to 320 of experimentA. The contourinterval is 2 hPa, with valuesgreater than zero shaded. The wind scalesare shownat the lower right-hand side of eachfigure. The figureson the

right-handsideare the zonalmeansof the zonalflow (solidline), the meridionalwind (dashedline), and the surfacepressure(dash-dottedline).

Ratio

of

Category

SurFace

6

Wind

to

850hPa

From day

Wind Median

161 to 32o

180

270

360

o.o

0.5

Longitude contour:

0.1,

shade:'

0.2

0.2,

0.4,

0.?

Figure 14. Ratios of absolutevaluesof the zonal flow overthe surfaceto thoseat the 850-hPa level for category6, compositedover day 161 to 320 of experimentA. The contoursare 0.1, 0.2, 0.4, and 0.7, with valueslessthan 0.2 being shaded.

i.o

12,992

ITOH-

INTRASEASONAL

ATMOSPHERIC

Te'mperature

ANGULAR

and (u,w)

Category

6

MOMENTUM

over

from day

the

VARIATIONS

Equator

81 to 320

200

.o

400

ß

•-

600

800

I000

0

90

180

270

360

lOm/sec Longitude contour interval 0.2K

.001mb/sec

shade: ß

O. OK

Figure 15. Verticalcrosssectionof temperature(contours),the zonalflowand the verticalpvelocity(arrows)alongthe equatorfor category6 composited overday 161to 320 of experiment A. The contour interval is 0.2 K, with valuesgreater than zero shaded. The wind scaleis shown at the lower left-hand side of the figure. based on the sphericalprimitive equations. Special attention is paid to the source of AAM fluctuations and to the causefor the directionof propagationof AAM in extratropical regions. The prescribedexternal heating moveseastwardfrom 60øE to the dateline in the tropics with a period of 40 days,forcing intraseasonaloscillations.Associatedwith the model intraseasonaloscillations,AAM also exhibits variations with a period of 40 days. The phaserelationship between the model intraseasonaloscillationsand AAM variations is similar to that observed. Namely, when the heating weakensnear the dateline, AAM attains maxima. Further, AAM exhibits the largest variation in the equatorial region, more specificallyin the upper equatorial troposphere,and a phase propagation from the equatorial region to the subtropics,and from the high latitudes to the subtropics,as observedin the real atmosphere. On careful inspection, the following features have been clarified.

Since surface wind variations

AAM

variations

• 10•"

G I oba I Mean Moment um Budget

must not be there.

x•0',

046

3

045

2

044

I •

045 / \ / \

//\

/\

042

-

\

•

\ -I

ß

041

040

i

0

i

i

20

i

i

40

are weak in

the equatorial region, a frictional torque sourcefor intraseasonal

tum at the surfacein the subtropics,AAM is simultaneouslyredistributedto the upper tropospherethrough planetary momentumflux convergence by the intensified Hadley circulation, and then to the equatorialre-

i

8O

60

Day i

This is

3

5

7

I

3

5

7

I

Category

further supported by observationaldata and GCM output. Instead, the sourceis frictional torque in the sub-

The mountain torque is alsoplotted, representedby the

tropics. When the atmospheregains angular toomen-

dashed line.

Figure 16. As in Figure 6 except for experimentB.

ITOH: INTRASEASONAL Vertically

Integrated Time

0

•......

Mean

ATMOSPHERIC

Momentum

ANGULAR

MOMENTUM

(kg-m/sec)

Frictional

Subtracted

I ............. ii,:ili!,:**i ,....... •

VARIATIONS

Time

Mean

12,993

Torque Subtracted

:::;i;,il;!•i;!!i;!!,!i,!iiiiiiii;;ilili I I

•ii•i•a•!•:aii ..... -•i•' ":i• :• - ............. ._•_..:.• •____• I

--- 40

.

O

I

._

._

60 '-•:*• 80

5

..................

-90

I

-60

-30

0

30

60

80

90

,

,

-90

, -60

-•0

0

Latitude

•0

60

I

90

Latitude

contour interval

5x101?kg-m/sec

Ix101zkg-m/s•c •

Figure18. Asin Figure 8 except forexperiment B.

Figure 17. As in Figure7 exceptfor experimentB. Mount o in Time

contou½ interval

Mean

Torque

Subtracted

I0

5

o 30 .-.

40

._

5o

6O 7'0

8O -90

I

-60

-30

ß

Lot

0

30

60

90

i tude

contour interval

IxlOI2kg-m/sec 2

Figure 19. As in Figure 8 exceptfor mountain torque from experiment B. Surface

Pressure

Category :::.-•

m

:::::::::-.::::: ............... ::::::::::::::::: ..... '-...: ......::::: ......... -....

5

and

from

Surface

day

Wind

161 to

320

Zcpal

Mean

::::::::::::::::::::::: .•:•:J•:::'::•::F•!•!•J!•!•ff•!•i•!•:: ........ :::=:-,::...

70

50

50

30

3o

I0

•0

I

3 •o

-I0

-•0

30

-3o

50

-so

70

-7o

90

-90

0

90

180

270

Longitude

360

•Sm/sec contour

sheder'

intervol

O.Omb

Figure20. Asin Figure13except forcategory 5 ofexperiment B.

2.0mb

-•

12,994

ITOH- INTRASEASONAL

Category

ATMOSPHERIC

ANGULAR MOMENTUM

VARIATIONS

Mountain Torque 5 Fro= day 161 to 320

Zonal

Mean

9O

7o 5o ß

•

•o -30 - ,5.0 -7o

-90

'

I

0

I

I

'

I

I

I

90

I

180

I

I

I

I

I

270

360

Longitude

-I

I

I

I

I

0xl OIs

Figure 21. Horizontal structure of mountain torque anomaliesfor category5 compositedover day 161 to 320 5 2 of experimentB. The contourintervalis 5 x 10 kg/s , with valuesgreaterthan zeroshaded.The figureon the right-handside representsthe zonal mean. Mountain torque is not zero over the oceans,sinceit is calculatedby the spectral method.

Geopo ten t i a I and Wi nd a t Categocy

5

250hP a

foam day 161 to 320

9O 7O 5O 3O

10 -10

-3O -5O

-7O -9O

90

180

Long i t:ude

360

270 ---+10m/sec contou•

shade;

interval

'

Figure 22. As in Figure 4 exceptfor category5 of experimentB.

Om

20rn

ITOH:

INTRASEASONAL

ATMOSPHERIC

ANGULAR

gion, associated withthepropagation ofRossby wave

The

MOMENTUM

VARIATIONS

data used are the rawinsonde

12,995 station

data over

trains to the subtropics. After all, the upper equato- the equatorialregionfor 9 years(1979 through 1987), rial troposphereis where the largest amplitude and the which are an extended versionof the data utilized by phaselead in AAM variations are seen. This processis Nishi [1989]. The 33 stationswere selectedbetween summarizedin Figure 23. 10øN and 10øS (see Nishi for the station locations). Rossbywave trains radiating from the tropics reach First, 5-day means of the zonal flow at the surface and the midlatitudes. When positive(negative)anomalies the 850-hPa level are created. Missing data are treated of the surface pressure associatedwith Rossby wave as follows. When one day is missingout of the 5 days trainsexist east(west)of largemountains,the moun- to be averaged,4-day averagesare regarded as a subtain torque displayspositive anomalies,and hence ex- stitute. When 2 days or more are missing,these data ert positive AAM anomalies. The occurrenceof this are excluded. The sample number for each station is is earlier than that of the positive AAM anomalies in therefore657 (365/5 x 9) for the caseof no excluded the subtropics in the present experiment. Therefore, data. Time means, yearly variations, and half-yearly Ratios of the surface the phaseof AAM propagatesfrom the high latitudes, variations are then subtracted. zonal flow to the 850-hPa zonal flow are calculated for where large mountainsexist, to the subtropics.Taking the 5-day mean data. into account the results from other experiments, the occurrenceof mountain torque maxima and, therefore, The results are shownin Figure A1 for the eight stathe direction of propagationof AAM are considerably tions located in the Pacific area. The medians lie bemodel dependent,but mountain torque variations are tween 0.20 and 0.40 for six of the eight stations. Kota mainly determinedby the patterns of wave trains orig- Kinabaru has a small value, but the distribution is very scattered. Tarawa has a large value, but the sample inating in the tropics. Thus, the role of Rossbywave trains associatedwith number(shownafter the station name) is small. The intraseasonal oscillations is twofold.(1) They causethe other stationsgenerallyhave valueslessthan 0.20, exequatorialarea to appear to be an apparent or superfi- cept for Atuona. These small valuesmay be attributed cial AAM source,by transporting AAM to the tropics. to the large surface drag, since most of these stations (2) They are closelyrelatedto the directionof phase are not located on small islands. Thus, the 5-day averaged surfacezonal flow has a value of 0.2 to 0.4 times propagationof AAM in the extratropical regions. the 850-hPa level zonal flow over the small islands in the Appendix. Results From Observational Pacific area. These valueswould not drastically change Data

and

the

JMA-UT

GCM

over the ocean.

In this appendix, further evidence of weak surface winds near the equator is given by observationaldata and GCM output.

Next, an experiment is performed,usingthe JMA-UT GCM. The horizontal resolutionis T21; i.e., the number of zonal grid points is 64. Kuo's method is used for cuMidlatitude

L AnomalousHadley Circulation(positive)

Lo•v etLev el Hane•,'y AAM

by Anomalous

Easterly

FrictionalTorque

E

Equator

Category5 or 6 Figure 23. Schematicfigurefor the AAM acquisitionand transportas well as circulationfields around category 5 or 6. See text for details.

KOROR

472

Median-

TRUK

0.29

4T5

Median-

0.29

160 -

160 t

120

120 c e

80

o' e

40

80

40

0

I

I

--

'1

I

-3.2

I

I

-0.8

I

•1

0

Ratio

KOTA

I

-0.2

I

I

0.2

•f

I

I

0.8

I

"1

0

'1

3.2

I --

I

I

-3.2

I

u,/u•0

KINABARU

I

-0.8

I

I

-0.2

Rat•o

493

Median-

0.03

I

I

0

I

I

0.2

I

0.8

I

I

I

3.2

oF u,/u850

YAP

507

Medlon-

0.30

160

,60]

120

80

=

40

80

4O

0 i

,

-'

m i

•

•

-3.2-0.8

,

,

•

-0.2

,

0

Ratio

m t 0.2

m , 0.8

,

,

3.2

0

I '

--

-3.2

-0.8

of u,/ue50

KWAJALEIN

ß

-0.2

0

Ratio

553

Medlon=

0.40

0.2

TARAWA

160 -

160

120 -

120

0.8

3.2

of uo/ue5o 241

Median-

0.45

c

1:3-

80

80

40

4O

.

0

0 --

-3.2

-0.8

-0.2

0

Ratio

0.2

0.8

3.2

-

--

-3.2

I' I -0.8

of u,/u85o

PONAPE

I I -0.2

Ratio

515

Medlan-

0.23

I 0

I 'I I I I I I 0.2 0.8 3.2 of u,/u850

MAJURO

457

I

Medlon-

0.33

160 -

,60]

120 -

120 c e :3 o'

80

40

0

80

40

• --

I

I

-3.2

I

I

-0.8

I

I

-0.2

Ratio

I'

I

0

I

I

I

0.2

I

0.8

I'

I

3.2

I

I

-

of u,/u85o

o

I

--

I

I

I

I

I

-:3.2-0.8-0.2

I

I

o

Ratlo

I

I

I

0.2

I

0.8

I

I

I

I

3.2

of u,/u850

Figure A1. Histograms showing thefrequency of occurrence for ratiosof thesurface zonalflow to the 850-hPazonalflowovereightstationsin the Pacificareanearthe equator.The abscissa denoteratios,whiletheordinaterepresents frequency (%). The stationnames,samplenumbers, and mediansare displayedaboveeachpanel.

ITOH: INTRASEASONAL

ATMOSPHERIC

Rot io of u,/u850

ANGULAR

MOMENTUM

VARIATIONS

12,997

References

1.2

Anderson,J. R., and R. D. Rosen,The latitude-height structure of the 40-50 day variationsin the atmosphericangular momentum, J, Atrnos. Sci., .lO, 1584-1591, 1983. Ar•.awa, A., and V. Lamb, Computational designof the basic dynamical processesof the U CLA general circulation model, Meth. Cornput. Phys., 17, 173-265, 1977. Barnes, R. T. H., R. Hide, A. A. White, and C. A. Wilson, Atmospheric angular momentum fluctuations, length of day changesand polar motion, Proc. R. Soc. London A, 387, 31-73, 1983. BlaA•, I., and D. L. Hartmann, Tropical intraseasonaloscillations in a simple nonlinear model, J. Atrnos. Sci., 50,

0,2

2922-2939, 1993. O. 0

i

i

0

i

i

i

30

i

i

60

i

"

90

Lotitude

Figure A2.

Latitudinal distributionof mediansfor

the ratios of the •urface zonal flow to the 850-hPa

zonal

flow, calculated from the GCM output. Medians are

Dickey, J. O., M. Ghil, and S. L. Marcus, Extratropical aspects of the 30-60 day oscillationin length-of-day and atmospheric angular momentum, J. Geophys. Res., 96, 22,643-22,658, 1991.

Gao, X. H., and J. L. Stanford, Possiblefeedback path for low-frequency atmospheric oscillations, J. Atrnos. Sci., •5, 1425-1432, 1988.

takenfromm the data for eachlatitude circle,whichcon- Gutzler, D. S., and R. A. Madden, Seasonalvariations in the spatial structure of intraseasonaltropical wind fluctutains3072data (48 five-daymeansper grid point x 64 ations, J. Atrnos. Sci., •6, 641-660, 1989. grid points). Hayashi, Y.-Y., and A. Sunli, The 30-40 day oscillations

mblusparameterizatlon. Time integrationis carriedout

for320modeldays,andlanalyses areconducted forthe

simulated in an "aqua planet" model, J. Meteorol. Soc. Jpn., 6.1, 451-467, 1986.

Hsu, H.-H., B. J. Hoskins,and F.-F. Jin, The 1985/86 in-

period from day 81 to 320. Five-day mean data, subtraseasonaloscillationand the role of the extratropics, J. Atrnos. Sci., •7, 823-839, 1990. tracting time meansin a similar manner to that above, are madefor eachgrid point. The samplenumberover Itoh, H., and N. Nishi, Considerationsfor the structure of

eachlatitudecircleis therefore 3072((320-80)/5 x 64).

the tropical intraseasonaloscillation,J, Meteorol. Soc. Jpn.

Then, as in the caseof the observeddata, ratios of the

Kanamitsu, M.,K.Tada, T. •udo,•. Sato, andS.Isa,De-

surface zonal flow to the 850-hPa calculated.

level zonal flow are

The medians over each latitude circle, i.e., the medians for the 3072 data are shown in Figure A2 as a function of latitude. While the medians over the midlat-

itudes are near 1.0, the median over the latitude nearest

to the equator(2.77ø) is as small as 0.35. Thus, both of these two data sets showthat the sur-

facewindsnear the equatorare weak. Consideringthe asymmetry betweenthe areaseast and west of the heating region,the weaknessof surfacewinds may be more conspicuous. It is concluded that the -•alues of 0.75

68, 659-675, 1990.

scription of the JMA operational spectral model, J. Me-

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MaAden,R. A., Largeintraseasonal variationsin wind stress over the tropical Pacific, J. Geophys.Res., 93, 5333-5340, his hearty appreciationto A. Numagut i for many approAcknowledgments.

The author would like to express

priate comments on the original manuscript and technical support of the JMA-UT GCM. He is also indebted to

J. R. Anderson, Y. Hayashi, M. Kimoto, Y. Matsuda, and I. Naito, and especially R. A. Madden for comments and

discussion of many points related to this subject. N. Nishi kindly providedthe rawinsondestation data over the equatorial region. Thanks are extended to T. Matsuno, A. Sumi, Y. Miyoshi, D.-H. Wu, and especiallyS. Miyahara, for providing the knowledgeto use the JMA-UT GCM. The computations were performed on the FACOM VP-2000 at the Data ProcessingCenter of Kyoto University and the Data

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Wakayama University. This work was supported by the Grant-in-Aid for Scientific Researchof the Ministry of Education, Science and Culture.

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H. Itoh, Department of Earth Sciences,Wakayama Uni-

versity, 930 Sakaedani,Wakayama640, Japan. (e-mail: traseasonal (30-60 day) fluctuationsof outgoinglongwave hisa-i@wusun. center.wakayamau.ac.jp)

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(ReceivedJune2, 1993;revisedNovember9, 1993; acceptedJanuary21, 1994.)