A CGCM Study on the North ward Prop a ga tion of Trop i cal IntraseasonalOs cil la tion over the Asian Sum mer Mon soon Regions

Shu-Ping Weng1, * and Jin-Yi Yu 2

1 De part ment of Ge og ra phy, Na tional Tai wan Nor mal Uni ver sity, Tai pei, Tai wan, ROC2 De part ment of Earth Sys tem Sci ence, Uni ver sity of Cal i for nia, Irvine, USA

Re ceived 11 November 2008, ac cepted 18 February 2009

AB STRACT

This study per forms nu mer i cal ex per i ments to (1) ex am ine the in flu ences of Pa cific and In dian Ocean cou plings on the

prop a ga tion of trop i cal intraseasonal os cil la tion (ISO) in the ex tended bo real sum mer (May-through-Oc to ber) and (2)

de ter mine the rel a tive con tri bu tions of the ocean cou pling and in ter nal at mo spheric dy nam ics to the ISO prop a ga tion over the

Asian-Pa cific mon soon re gions. For (1), three ba sin-cou pling ex per i ments are per formed with a cou pled at mo sphere-ocean

gen eral cir cu la tion model (CGCM), in which the air-sea cou pling is lim ited re spec tively to the In dian Ocean, the Pa cific Ocean,

and both the In dian and Pa cific oceans. For (2), three forced ex per i ments are per formed with the at mo spheric GCM (AGCM)

com po nent of the CGCM, in which the sea sur face tem per a ture (SST) climatologies are pre scribed from the CGCM

ex per i ments. Us ing ex tended Em pir i cal Or thogo nal Func tion and com pos ite anal y ses, the lead ing ISO modes are iden ti fied

and com pared be tween the ob ser va tion and the model ex per i ments. The CGCM mod el ing re sults show that the In dian ocean

cou pling is more im por tant than the Pa cific Ocean cou pling to pro mot ing both zonal and me rid i o nal propagations of the

sum mer time ISO. In this sea son, the Indo-Pa cific warm pool re tracts west ward and shifts into the North ern Hemi sphere

al low ing the In dian Ocean cou pling to be come more im por tant. The In dian Ocean cou pling is found to pro mote the north ward

prop a ga tion mainly through wind-evap o ra tion feed back, whereas in ob ser va tions the cloud-ra di a tion feed back is found to be

equally im por tant. The AGCM mod el ing re sults in di cate that mon soonal dy nam ics aid the me rid i o nal ISO prop a ga tion mainly

in the low-level winds. With out the ocean cou pling, the north ward ISO con vec tion fea ture is weaker and is lim ited by the

north ern bound ary of cli ma tic east erly ver ti cal shear. The ocean cou pling en ables the sim u lated ISO-re lated con vec tions to

cross the north ern boundary of the shear. This modeling study concludes that the Indian Ocean coupling plays a crucial rather

than a secondary role for the observed northward propagation of summertime ISO.

Key words: Intraseasonal os cil la tion, At mo sphere-ocean cou pling, Mad den-Julian os cil la tion, Indo-Pa cific cli mate vari abil ity, East erlyver ti cal shear

Ci ta tion: Weng, S. P. and J. Y. Yu, 2010: A CGCM study on the north ward prop a ga tion of trop i cal intraseasonal os cil la tion over the Asian sum mermonsoon re gions. Terr. Atmos. Ocean. Sci., 21, 299-312, doi: 10.3319/TAO.2009.02.18.01(A)

1. IN TRO DUC TION

Trop i cal intraseasonal os cil la tion (ISO; Mad den and

Julian 1994; Zhang 2005) is char ac ter ized by plan e tary-

scale prop a ga tion in the moist con vec tion and cir cu la tion

anom a lies. This quasi-pe ri odic os cil la tion oc cu pies a broad

20 - 90-day timescale and has been rec og nized as a phe -

nomenon im por tant to both weather fore cast and cli mate

prediction. Through the in ter ac tions with sur face winds and

sea sur face tem per a tures (SSTs), for ex am ple, the ISO can

af fect trop i cal cy clone gen e sis in the In dian and west ern

Pa cific oceans (Liebmann et al. 1994), Aus tra lian re gion

(Hall et al. 2001), Gulf of Mex ico (Maloney and Hartmann

2000a), east ern North Pa cific (Maloney and Hartmann

2000b), and west ern North Pa cific (Kim et al. 2008). As

another ex am ple, the onsets of sum mer mon soon in In dia,

South China Sea-East Asia, and In do ne sia-North Aus tra lia

are of ten sig ni fied by the ar rival of ISO’s wet phase (e.g.,

Murakami et al. 1986; Chen and Chen 1995; Hung and

Yanai 2004). The wet and dry spells of mon soon life cy cle

over the vast Asian-Aus tra lian mon soon re gions are strongly

Terr. Atmos. Ocean. Sci., Vol. 21, No. 2, 299-312, April 2010 doi: 10.3319/TAO.2009.02.18.01(A)

* Cor re spond ing au thorE-mail: znspw302@ntnu.edu.tw

reg u lated by the pas sage of ISO (e.g., Chen et al. 1988).

The ISO is also con sid ered a ma jor source of sto chas tic

forc ing (Penland 1996; Hen don et al. 1999) im por tant for

the on set and de mise of El Nino South ern Os cil la tion

(ENSO) (McPhaden 1999). Due to its pro found so cio eco -

nomic ben e fits once a suc cess ful ISO pre dic tion can be

made, this phe nom e non has there fore in trigued nu mer ous

re search ers dur ing the past de cades to study its or i gin, dy -

nam i cal char ac ter is tics, and var i ous mech a nisms con trib ut -

ing to its prop a ga tion (see Wang 2005 for a com pre hen sive

re view).

Prop a ga tion char ac ter is tics of the ISO were no ticed to

change with the sea sonal cy cle in the Indo-Pa cific sec tor. In

bo real win ter, ISO prop a ga tion is pre dom i nantly east ward.

Af ter be ing ini ti ated in the west ern In dian Ocean, the ISO

passes through the Mar i time Con ti nent, re de vel ops over the

west ern Pa cific warm pool, and of ten in ter sects the South

Pa cific con ver gent zone where the warm SSTs re side (Wang

and Rui 1990; Jones et al. 2004). In bo real sum mer, the east -

ward ISO prop a ga tion weak ens sub stan tially as it ap pro -

aches the Mar i time Con ti nent (e.g., Kemball-Cook and Wang

2001). A north ward (or northwestward) prop a ga tion branch

be comes prom i nent in the Asian-Pa cific mon soon re gions

(Yasunari 1980; Lau and Chan 1986; Wang and Rui 1990;

Hsu and Weng 2001; Hsu et al. 2004; Li and Wang 2005).

Sev eral mech a nisms have been pro posed to ex plain the

north ward prop a ga tion of sum mer time ISO. Web ster (1983)

sug gested that destabilization of the bound ary layer heat

fluxes over the north ern land sur face can cause a north ward

shift of the equa to rial con vec tion. Hsu et al. (2004) also em -

pha sized the air-land in ter ac tion for the north ward prop a ga -

tion. They sug gested that deep mois ture con ver gence in the

free at mo sphere and bound ary layer aided by the slop ing ter -

rain and stron ger fric tional ef fects over the land mass of

South Asia can con trib ute to the north ward prop a ga tion of

ISO. Goswami and Shukla (1984) pro posed a con vec tion-

ther mal re lax ation feed back mech a nism to ex plain the os cil -

la tion. The oc cur rence of con vec tion sta bi lizes the atmo -

sphere which in-turn sup presses the con vec tion it self, whereas

si mul ta neous dy nam i cal and ra di a tion re lax ation will setup

the at mo sphere to be con vec tively un sta ble again. In ferred

from their nu mer i cal sim u la tions, Lau and Peng (1990) sug -

gested that while the equa to rial moist Kel vin waves weaken

af ter in ter act ing with mean mon soonal cir cu la tions, un sta ble

baroclinic waves can be pro duced over the In dian pen in sula.

Re sults of this in ter ac tion hence sig nify the north ward ad -

vance of the ISO. Wang and Xie (1997) pro posed that the

north ward ISO con vec tion in the In dian Ocean (and west ern

Pa cific) is a man i fes ta tion of the northwestward em a na tion

of Rossby waves from the equa to rial Kel vin-Rossby wave

packet when the lat ter trav els through the Mar i time Con ti -

nent and rap idly de cays in the cen tral Pa cific. Jiang et al.

(2004) fur ther showed that such a north ward com po nent of

the Rossby waves can be at trib uted to sev eral fac tors in clud -

ing the sum mer time off-equa to rial mean east erly ver ti cal

shear (EVS) which char ac ter izes the mon soon cir cu la tions,

mois ture advection in the bound ary layer, and air-sea cou -

plings. More re cently, Wang (2005) sug gested that the north -

ward prop a ga tion of ISO in the In dian Ocean may be an un -

sta ble mode of at mo spheric in ter nal dy nam ics as so ci ated

with the asym met ric mon soonal flow in bo real sum mer.

These stud ies sug gest that the ex is tence of an off-equa tor

EVS ba sic state pro vides an in trin sic mech a nism for the ISO

to prop a gate north ward.

It has been no ticed that there are co her ent fluc tu a tions

be tween ISO-re lated con vec tion and SST anom a lies, which

has prompted the sug ges tion that the at mo sphere-ocean

coupling may be an es sen tial el e ment of the trop i cal ISO

dy nam ics. Field ex per i ments in di cated that ISO-in duced

SST anom a lies can be as large as 2°C in the north ern In dian

Ocean and Bay of Ben gal (Bhat et al. 2001; Web ster et al.

2002). Sim i lar con nec tion be tween the ISO prop a ga tion and

warm SST is also noted in the east ern North Pa cific dur ing

the bo real sum mer (Maloney and Kiehl 2002). The in flu ence

of SST on the ISO is also ev i dent dur ing ENSO events. Ob -

ser va tional (e.g., Woolnough et al. 2000) and mod el ing

(e.g., Watterson and Syktus 2007) stud ies showed that the

east ward extention of the west ern Pa cific warm pool dur ing

El Nino events is ac com pa nied by more east ward in va sions

of ISO-re lated SST, low-level zonal wind, and con vec tion

anom a lies.

This study aims at com par ing the rel a tive im por tance

be tween the ocean cou pling and the in ter nal at mo spheric

dynamics (such as those as so ci ated with the mon soon flow)

to the me rid i o nal prop a ga tion of sum mer time ISO. In par -

ticular, we ex am ine the im por tance of air-sea cou pling in

the Pa cific and In dian oceans sep a rately. These two ocean

bas ins have dif fer ent mean SST dis tri bu tions and op po site

mean sur face wind di rec tions as so ci ated with the Pa cific and

In dian cells of the Walker cir cu la tion. It is ex pected that

air-sea in ter ac tions over two oceans may have dif fer ent in -

flu ences on the sum mer time ISO. A se ries of ba sin-cou pling

CGCM ex per i ments is per formed in this study and their ISO

sim u la tions are com pared to ex am ine this hy poth e sis. The

ba sin-cou pling mod el ing strat egy of Yu et al. (2002) en ables

us to turn on the at mo sphere-ocean cou pling in the model

over a spe cific ocean ba sin while the cou pling is turned off

ev ery where else.

This pa per is or ga nized as fol lows. The ba sin-cou pling

meth od ol ogy and the CGCM used in this study are de scribed

in sec tion 2. The sim u lated sum mer time ISOs are con trasted

with the ob ser va tion in sec tions 3. The role of the ocean cou -

pling is dis cussed in sec tion 4. Sec tion 5 sum ma rizes the ma -

jor findings of this study.

2. MODEL AND BA SIN-COU PLING EX PER I MENTS

The CGCM (de tailed in Yu and Mechoso 2001) used in

300 Shu-Ping Weng & Jin-Yi Yu

this study con sists of the UCLA AGCM (Mechoso et al.

2000) and GFDL Mod u lar Ocean Model (MOM; Bryan

1969; Cox 1984). The AGCM has a global cov er age with a

hor i zon tal res o lu tion of 4° in lat i tude and 5° in lon gi tude and

15 lev els in the ver ti cal with the top at 1-mb. The OGCM has

a lon gi tu di nal res o lu tion of 1° and a lat i tu di nal res o lu tion

vary ing grad u ally from 1/3° be tween 10°S and 10°N to

about 3° at both 30°S and 50°N. It has 27 lay ers in the

vertical with 10-m res o lu tion in the up per 100 me ters. The

AGCM and OGCM are cou pled daily with out flux cor rec -

tion. Three ba sin-cou pling ex per i ments are per formed: the

Pa cific Ocean (PO) Run, the In dian Ocean (IO) Run, and the

Indo-Pa cific (IP) Run. In the PO Run, CGCM in cludes only

the Pa cific Ocean (30°S - 50°N, 130°E - 70°W) in its ocean

model do main. The IO Run in cludes only the In dian Ocean

(30°S - 50°N, 30 - 130°E) whereas the IP Run in cludes both

the In dian and Pa cific oceans (30°S - 50°N, 30°E - 70°W) in

the ocean model do main. Out side the ocean model do mains,

cli ma to log i cal monthly SSTs are pre scribed. In side the

ocean model do mains, SSTs polarward of 20°S and 30°N are

re laxed to ward their cli ma to log i cal val ues. There fore, the

ef fec tive cou pling re gions in these CGCM runs are lim ited

to the trop i cal Pa cific and In dian Oceans. All three runs were

each in te grated for 60 years. Only the last 30-yr in te gra tions

are an a lyzed in this study. The ob served ISO prop er ties in

cir cu la tion and con vec tion fields are de rived re spec tively

from the daily-mean Eu ro pean Re-Anal y sis prod uct (ERA-

40; Uppala et al. 2005) and the pentad-mean CPC merged

anal y sis of pre cip i ta tion (CMAP; Xie and Arkin 1997) in

their over lapped pe riod of 1979 - 2001.

The sim u lated SSTs from the IP Run are com pared with

the ob ser va tion in Fig. 1 for the ex tended sum mer sea son

(May-through-Oc to ber; MJJASO). The ob served climato -

logy (1981 - 2001) (Fig. 1a) is cal cu lated from NOAA

Optimum In ter po la tion SST (OISST_v2; Reynolds et al.

2002). In the ob ser va tion, the equa to rial warm SSTs re treat

to the west of the date line from its win ter time coun ter part

(not shown). The dis tri bu tion of the Indo-Pa cific warm

waters also be come more asym met ric with re spect to the

equator, with the wa ters shift north ward to cover the en tire

north ern In dian Ocean and west ern Pa cific (south of 20°N).

The gen eral pat terns of the ob served Indo-Pa cific SST dis tri -

bu tion are rea son ably cap tured in the sim u la tion. The north -

ward ex ten sion of warm SSTs in the In dian Ocean is re al is ti -

cally sim u lated. A sim i lar north ward ex ten sion is also pro -

duced in the west ern North Pa cific, even though it does not

ex tend as far north as in the ob ser va tion. Al though the gross

fea tures are quite re al is tic, the model SSTs tend to be colder

than ob served in the In dian Ocean. Other model de fi cien cies

in clude a warm bias in the east ern Pa cific, a too zonal and

too east ward ex ten sion of the south ern branch of the west ern

Pa cific warm pool, and a cold bias around the Mar i time Con -

ti nent (Fig. 1c). The two warm bi ases are as so ci ated with the

so-called dou ble ITCZ prob lem that many con tem po rary

CGCMs face (Mechoso et al. 1995). The SST climatologies

pro duced by the IO and PO Runs (not shown) are gen er ally

sim i lar to that of the IP Run.

3. SUM MER TIME ISO IN THE BA SIN-COU PLINGCGCM EX PER I MENTS

To ex tract the ISO sig nals, we adopt the pro ce dure used

by Rui and Wang (1990). The mean an nual cy cle is first re -

moved from the daily data to pro duce daily anom a lies. A

3-month run ning mean anom aly, which es ti mates the sub -

annual- to-interannual vari a tion, is then re moved from the

daily anom aly be fore com put ing the 5-day mean pentad

ano m aly. This band-pass fil ter ing pro ce dure re tains intra -

seasonal vari a tions with timescales rang ing from 10 days to

A CGCM Study on North ward Prop a ga tion of TISO 301

Fig. 1. The SST cli ma tol ogy cal cu lated from: (a) NOAA OISST_v2

dataset (1981 - 2001); and (b) the Indo-Pa cific (IP) CGCM Run dur ing

the bo real sum mer (May - Ocrober; MJJASO). The dif fer ence be tween

them [(b) mi nus (a)] is also shown in (c). Con tour in ter vals are 1°C.

(a)

(b)

(c)

3 months. The CMAP pre cip i ta tion dataset is also pro cessed

in a sim i lar way but tak ing its pentad-res o lu tion into ac -

count. We ex am ined the vari ance of the sim u lated and ob -

served fil tered anom a lies in 850-mb zonal wind (U¢850) av er -

aged be tween 10°S and 10°N dur ing the MJJASO sea son

(not show), and found that the max i mum vari ance pro duced

by the three CGCM ex per i ments (which is lo cated in the

east ern In dian Ocean) is about 50% of the ob served peak

value. To iden tify the dom i nant ISO mode and its pro pa -

gation char ac ter is tics, an Ex tended Em pir i cal Or thogo nal

Func tion (EEOF) anal y sis (Weare and Nasstrom 1982) is

applied to the fil tered east erly ver ti cal shear (EVS) anom a -

lies in the trop i cal east ern hemi sphere (20°S - 20°N, 30 -

180°E) where the intraseasonal vari ance is con cen trated. A

5-pentad lead-lagged win dow is used in the EEOF anal y sis.

Here the EVS is de fined as the dif fer ence be tween the anom -

a lous zonal winds at 200 and at 850-mb (i.e., U¢200 - U¢850).

This quan tity gives a good mea sure ment of the strength of

the grav est baroclinic mode as so ci ated with the large-scale

deep con vec tions (Wang and Fan 1999). We found ap ply ing

the EEOF anal y sis to the EVS anom a lies par tic u larly ef fec -

tive in iso lat ing the ISO from the model ex per i ments where

the intraseasonal vari abil ity is weak. Choice of this quan tity

is also in ac cord with the re cent sug ges tion of Wheeler and

Hen don (2004) that ISO is best iden ti fied when an EOF

anal y sis is ap plied to the com bined U¢200, U¢850, and the out -

go ing longwave ra di a tion anom a lies to gether.

The per cent ages of the 10-to-90 days EVS vari ance ex -

plained by the first EEOF modes for ERA-40 and the IP, PO,

and IO runs are, re spec tively, 11.0%, 8.4%, 8.2%, and 10.3%.

For the sake of dis cus sion, only the half cy cle of the first

EEOF modes from the ob served and sim u lated EVS evo lu -

tions in three CGCM ex per i ments is shown in Figs. 2a - d. In

Fig. 2, a con ver gent (di ver gent) zone in the 850-mb level is

de noted at the zero con tour line of EVS with pos i tive (neg a -

tive) anom a lies to its right and neg a tive (pos i tive) anom a lies

to its left. The ob ser va tion (Fig. 2a) shows that the lead ing

ISO mode is char ac ter ized by a wavenumber-1 pat tern with

its con ver gence/di ver gence cen ter prop a gat ing east ward

along the equa tor. This is sim i lar to the lead ing EEOF modes

ob tained when the anal y sis was ap plied to the bo real win ter

ISO (see Fig. 3 of Weng and Yu 2010). Un like its win ter time

coun ter part, a north west-to-south east tilt ing in the zero con -

tour line of EVS anom aly sig ni fies a north ward prop a ga tion

branch. The zero line in the east ern In dian Ocean moves

from the equa tor in phase 1 to about 20°N in phase 4. The

ob served east ward and north ward propagations over the

Indian Ocean sec tor are rea son ably sim u lated by both IP

(Fig. 2b) and IO (Fig. 2d) runs. Lack ing of the In dian Ocean

cou pling, the PO Run (Fig. 2c) shows a larger stand ing com -

po nent.

The composited ISO lifecycle in var i ous vari ables is

then por trayed by sub tract ing the mean of 20 most ex treme

neg a tive events from that of 20 most ex treme pos i tive events.

These ISO events (each with a 50-day time win dow) were

se lected based on the or dered val ues of the prin ci pal com -

ponent time se ries in the first EEOF mode of EVS. The

same pro ce dure is ap plied to all the CGCM ex per i ments to

con struct the sim u lated ISO lifecycles.

Fig ure 3 shows the Hovmoller di a grams of the com pos -

ite ISO lifecycle in 200-mb ve loc ity po ten tial anom a lies

(c¢200) along the trop i cal band (2°S - 14°N). It should be

noted that the con tour in ter val used for the CGCM re sults

(Figs. 3b - d) is only 1/3 of that used for the ob ser va tion

(Fig. 3a), be cause the sim u lated ISO in ten sity is weaker than

the ob served one. In the ob ser va tion (Fig. 3a), three ac tive

cen ters with en hanced anom a lous am pli tude can be iden ti -

fied when the c¢200 pat tern prop a gates east ward: near the

Ara bian Sea-west ern coasts of the In dian sub con ti nent

(60 - 75°E), the Mar i time Con ti nent-west ern Pa cific (120 -

150°E), and the east ern trop i cal Pa cific-Cen tral Amer ica

(120 - 90°W). When ap proach ing these re gions, the ISO

prop a gat ing speed also slows down. The cen ter in the In dian

Ocean is well cap tured in the IP Run (Fig. 3b) and is best

repro duced in the IO Run (Fig. 3d), but is very weak in the

PO Run (Fig. 3c). The IO Run re pro duces the east ward

prop a ga tion of this cen ter most re al is ti cally. These re sults

sug gest that the In dian Ocean cou pling strength ens the ISO

in ten sity and its east ward prop a ga tion in the In dian Ocean

sec tor. The Mar i time Con ti nent-west ern Pa cific cen ter is

sim u lated by all the three model runs. The PO Run has the

stron gest in ten sity and the IP Run cap tures the east ward

prop a ga tion more re al is ti cally. The PO Run pro duces an er -

ro ne ous stand ing os cil la tion com po nent, which over pow ers

the east ward-prop a ga tion com po nent around the Mar i time

Con ti nent-west ern Pa cific cen ter. All three runs pro duce a

too strong stand ing com po nent at the east ern trop i cal Pa -

cific- Cen tral Amer i can cen ter.

The ISO prop a ga tion is fur ther ex am ined in Fig. 4,

where the ob served and sim u lated ISO lifecycles in 850-mb

wind and rain fall anom a lies are com pared. Only the first half

of the lifecycle is shown in the fig ure. The other half cy cle is

sim i lar but with op po site signs. The color-shaded ar eas and

black vec tors de note re spec tively the rain fall and wind ano -

m a lies at 95% and 90% sig nif i cance lev els. The ob served

ISO (Fig. 4a) shows dis tinct fea tures that ini ti ate near the

coasts of equa to rial East Af rica in phase 1 and prop a gate

east ward along the equa tor be fore ar riv ing at Bor neo in

phase 6 wherein the prop a ga tion slows down and later dis si -

pates near New Guinea. While trav el ing across the In dian

Ocean, a por tion of rain fall anom a lies moves north ward to -

ward the Ara bian Sea, the Bay of Ben gal, and the South

China Sea. This north ward prop a ga tion is ac com pa nied by

off-equa tor Rossby wave re sponse to its north west ern quad -

rant, as ev i dent in the wind anom a lies. The north ward ISO

de cays over the north ern plain of In dia and later over the

East China Sea. Two or three pentads af ter their ar rival at

the Mar i time Con ti nent, rain fall anom a lies re vive over the

302 Shu-Ping Weng & Jin-Yi Yu

A CGCM Study on North ward Prop a ga tion of TISO 303

Fig. 2. Evo lu tions of the first EEOF mode of east erly ver ti cal shear (EVS) anom a lies in the: (a) ob ser va tion, (b) Indo-Pa cific (IP), (c) Pa cific Ocean

(PO) Run, and (d) In dian Ocean (IO) runs. Only the first half of full cy cle from phase 1 (-25 days) to phase 6 (+0 day) is shown.

(a) (b)

(c) (d)

304 Shu-Ping Weng & Jin-Yi Yu

Fig. 3. Lifecycle of sum mer time ISO in 200 mb ve loc ity po ten tial (c¢200) anom a lies along the trop ics berween 2°S and 14°N. Panel (a) is composited

from the ob ser va tion, (b) from the Indo-Pa cific (IP) Run, (c) from the Pa cific Ocean (PO) Run, and (d) from the In dian Ocean (IO) Run. Con tour in ter -

vals are 1.0 ´ 106 m2 sec-1 in (a) and 1/3 ´ 106 m2 sec-1 in (b) - (d).

Fig. 4. The com pos ite ISO life cy cles in rain fall (color-shad ing) and 850-mb wind (vec tor) anom a lies cal cu lated from: (a) ob ser va tion, (b) Indo-Pa -

cific (IP), (c) Pa cific Ocean (PO), and (d) In dian Ocean (IO) runs. Only the first half of life cy cle from phase 1 (-25 day) to phase 6 (+0 day) and the

rain fall (wind) anom a lies that are sig nif i cant at 95% (90%) con fi dence level are shown. The ob served/sim u lated rain fall (wind) anom a lies are scaled

by the val ues (vec tor) be low/above the color bar (to the left/right cor ner).

(a) (b) (c) (d)

(a) (b) (c) (d)

western North Pa cific (phase 3) and move northwestward

to ward Tai wan where the pre vi ous north ward branches still

ex ist. It thus re sults in an intraseasonally fluc tu at ing rain

band along the lo ca tion of the mean mon soon trough ex -

tending from Bay of Ben gal, pass ing through the Indo-

China pen in sula and South China Sea to the east of the Phi -

lippine Is lands (phase 4). In ter act ing with SST anom a lies

built up from pre vi ous sur face flux changes, rain fall ano -

malies flare up in the trop i cal east ern Pa cific (phase 5).

Both the IP (Fig. 4b) and IO (Fig. 4d) Runs cap ture the

zonal and north ward prop a gat ing rain fall anom a lies over the

In dian Ocean sec tor. The zonal prop a ga tion pro duced by the

IO Run in the South Asian mon soon re gions is par tic u larly

re al is tic. The north ward prop a ga tion of rain fall anom a lies

and the as so ci ated wind anom a lies are also rea son ably sim u -

lated over the In dian Pen in sula. In the PO Run (Fig. 4c),

stand ing os cil la tions dom i nate the lifecycle. Both the zonal

and me rid i o nal prop a ga tion of con vec tion anom a lies are

weak. In par tic u lar, the ac tion cen ter shifts to the west ern

North Pa cific, where the as so ci ated con vec tion/cir cu la tion

anom a lies prop a gate west ward, in stead of east ward, from

Phil ip pine Sea to ward Tai wan and south east ern China. In

ad di tion, they are more vi a ble than their coun ter parts in the

IO Run, sug gest ing the im por tance of air-sea in ter ac tions to

es tab lish the prop a gat ing char ac ter is tics of off-equa tor mo -

ist Rossby waves. In ter est ingly, the ISO over the west ern

North Pa cific and East Asia in the IP Run are weaker than in

the PO Run. The In dian Ocean cou pling in the IP Run seems

to help re duce the am pli tude of the un re al is ti cally off-equa -

to rial ISO. It is noted in the ob ser va tion and the IP and IO

Runs that anom a lies in the west ern North Pa cific have an

opposite po lar ity from those in the east ern In dian Ocean. It

is likely that the north ward-prop a gat ing ISO orig i nated from

the In dian Ocean off set the west ward ISO over the west ern

North Pa cific and East Asia, or vice versa. When both the

Pa cific and In dian Ocean cou plings are in cluded, the IP Run

pro duce weaker (but more re al is tic) ISO vari abil ity in the

west ern North Pa cific than the PO Run. Re sults of Figs. 3

and 4 sug gest that the In dian ocean cou pling help pro mot ing

both the zonal and me rid i o nal propagations of the sum mer

ISO in the In dian Ocean sec tor. The Pa cific Ocean cou pling

is less im por tant to the sum mer ISO prop a ga tion.

4. ROLE OF THE OCEAN COU PLING

To un der stand how ocean cou pling af fects the ISO pro -

pagation, we ex am ine in Fig. 5 the evo lu tions of ISO-re -

lated SST and sur face wind stress anom a lies in the ob ser va -

tion and three CGCM ex per i ments. Note the dif fer ent scales

used in the ob served and sim u lated anom a lies. For the sake

of dis cus sion, the cen ters of en hanced rain fall in Fig. 4 are

la beled as open tri an gles in the pan els for the ob ser va tion

and the IP Run. In the ob ser va tion (Fig. 5a), pos i tive SST

anom a lies show a north ward mov ing ten dency which ap -

pears first in the Ara bian Sea and sub se quently in the Bay of

Ben gal, South China Sea, and north ern West Pa cific Ocean.

The SST anom a lies ex hibit a north west-to-south east tilt ing

pat tern. En hanced rain fall anom a lies are preceeded by pos i -

tive SST anom a lies to the north and fol lowed by neg a tive

SST anomaloies to the south, sug gest ing that the cou pling in

the In dian Ocean help pro mot ing the north ward prop a ga tion

of the sum mer time ISO. The IP Run (Fig. 5b) rea son ably

sim u lates the north ward prop a ga tion of SST anom a lies over

the Ara bian Sea and Bay of Ben gal. The SSTA evo lu tion in

the IO Run (Fig. 5d) is sim i lar to that in the IP Run. The

largest mag ni tude of the sim u lated SST anom a lies in the IP

Run is about 0.3 ~ 0.5°C, which is no tice ably com pa ra ble

with the ob served ones. Model runs do not pro duce very

well the ob served east ward-trav el ling com po nent of the SST

anom a lies in the low lat i tudes. How ever, such an east ward

ten dency does ex ist in both the IP and IO Runs.

We fur ther an a lyze the as so ci ated sur face heat fluxes

to ex am ine how the ISO-SST in ter ac tions are pro duced.

Fig ure 6 shows the composited la tent heat fluxes (LHF;

shad ings) and short-wave ra di a tion fluxes (SWR; con tours)

from the ob ser va tion and three CGCM runs. The sign of

these flux terms is pos i tive down ward. In the ob ser va tion

(Fig. 6a), a well-or ga nized pat tern of pos i tive/neg a tive SWR

anom a lies with a north west-to-south east tilt ing is found to

lead the pos i tive/neg a tive SST anom aly (SSTA) cen ters

(marked by +/- sym bols in pan els a and b) about 1/4 cy cles.

Since the pos i tive/neg a tive SSTA also leads the re gions of

en hanced/sup pressed rain fall anom a lies about 1/4 cy cles

(cf. Fig. 5a), the SWR anom a lies, with an op po site sign, thus

nearly co in cide with the rain fall anom a lies due to the changes

of cloud i ness. In re sponse to the in creased (de creased)

south west erly mean flow (see Fig. 5a), which is in turn

caused by the en hanced (sup pressed) rain fall to the north,

the de vel op ment of the neg a tive (pos i tive) val ues of LHF

anom a lies first ap pear ing in the equa to rial west ern Indian

Ocean thus slightly lags that of the neg a tive (pos i tive)

SWR anom a lies. Nev er the less, with a same po lar ity, sub se -

quent de vel op ment of the LHF anom a lies even tu ally catches

the SWR anom a lies while mi grat ing north ward to wards the

lat i tudes of the Ara bian Sea, Bay of Ben gal, and south ern

South China Sea. It is in these re gions that the SST anom aly

max ima ap pear af ter 1-to-2 pentads. Con trast ing to the ano -

m a lous SWR pat tern, which is more or less con fined to the

east ern hemi sphere, a wavenumber 1 (and 2) LHF sig nal that

prop a gates east ward along the equa to rial lat i tudes can be

rec og nized in both hemi spheres as its main body mi grates

north ward. It in di cates that the ob served plan e tary- scale fea -

ture of the bo real sum mer ISO is at trib uted to the un der ly ing

LHF that pro vides the en ergy source to sup port the per tur ba -

tions grow ing in the free at mo sphere aloft.

In the IP Run (Fig. 6b), the mag ni tudes of SWR anom a -

lies are less or ga nized and weaker than the LHF anom a lies in

most ar eas. The un der es ti ma tion of the cloud-ra di a tion-SST

A CGCM Study on North ward Prop a ga tion of TISO 305

feed back is likely due to the way cloud i ness is treated in the

UCLA AGCM, which is ei ther 100% or 0%. There is no par -

tial cloud i ness in this par tic u lar model. As a re sult, model

cloud i ness is not sen si tive to the weak ISO events sim u lated.

The as so ci ated shortwave ra di a tion flux anom a lies are there -

fore very weak. Be sides, coarse ver ti cal res o lu tion in this

em ployed AGCM may not be suf fi cient ei ther to fully re -

solve ob served deep con vec tion and ac com pa ny ing sub si -

dence in key ISO re gions. It is well-known that high clouds,

with a thick op ti cal depth, play an im por tant role in driv ing

the trop i cal diabatic cir cu la tion and the oc cur rence of an

enhanced rain fall anom aly also has a re gion of mid- to-

upper-level straitform cloud i ness as so ci ated with it. The in -

volved ra di a tion pro cess, which is de ter mined by cloud pro -

p er ties, con trib utes to the ob served ver ti cal struc ture of di -

abatic heat ing. Var i ous phys i cal pro cesses by which the

cloud- ra di a tion feed back af fects the ISO de serve fur ther

model study us ing a finer ver ti cal res o lu tion.

How ever, rel a tive phas ings be tween the SWR and LHF

anom a lies, and their re la tion ships with the SST and rain fall

anom a lies in deed show some re sem blances with the ob ser -

va tion. The sim i lar ity also in cludes the plan e tary-scale fea -

ture in the LHF anom a lies cap tured by the IP Run. In con -

trast, the LHF anom a lies in both the IO and PO Runs (Figs. 6c,

d) largely de crease in the re spected ocean ba sin that lacks of

the air-sea cou pling. Other sa lient fea tures in the sim u la tion

are also no ticed. In par tic u lar, the de vel op ment of the north -

ward branch of pos i tive (neg a tive) SWR anom a lies ini ti ated

along the East Af ri can coasts is not in ten sive enough to off -

set the pre dom i nant neg a tive (pos i tive) LHF anom a lies de -

vel op ing in its north east. This can be iden ti fied in both the

IP and IO Runs but not in the PO Run where the as so ci ated

LHF anom a lies show no prop a ga tion. As a re sult, the north -

ward mov ing SSTA in the Ara bian Sea is also largely at -

306 Shu-Ping Weng & Jin-Yi Yu

Fig. 5. The com pos ite ISO life cy cles in SST (col ored shad ing) and sur face wind stress (vec tor) anom a lies cal cu lated from: (a) the ob ser va tion, (b) IP,

(c) PO, and (d) IO runs. Cen ters of en hanced rain fall anom a lies in Fig. 4a (b) are in di cated as open tri an gles in Fig. 5a (b). Only the first half of

lifecycle and stress anom a lies sig nif i cant at 90% con fi dence level are shown. The ob served/sim u lated SST (stress) anom a lies are scaled by the val ues

(vec tor) be low/above the color bar (to the left/right cor ner).

(a) (b) (c) (d)

tributed to the lo cal wind-evap o ra tion feed back rather than

the cloud-ra di a tion feed back.

Ac cord ing to at mo spheric dy namic the o ries of ISO

(e.g., Lau and Peng 1990; Wang and Xie 1997; Jiang et al.

2004), the mean mon soonal cir cu la tion de ter mines the ex -

tent of the me rid i o nal prop a ga tion of sum mer time ISO. To

ex am ine this mech a nism fur ther, we show in Fig. 7 the dis -

tri bu tions of mean EVS in the ob ser va tion and three model

ex per i ments. Here the EVS serves as an in dex to mea sure the

strength of the sum mer mon soon, sim i lar to the mon soon

cir cu la tion in dex de fined by Web ster and Yang (1992). The

larger the neg a tive EVS val ues, the stron ger the sum mer

mon soon. In the ob ser va tion (Fig. 7a), neg a tive EVS val ues

ex tend ing from the Ara bian Sea, the Bay of Ben gal, to the

South China Sea, co in cide rea son ably with the re gions

where the north ward ISO propagations are ob served (cf.

Fig. 4a). The north ern most ex tent of the ISO prop a ga tion

also ap pears to be lim ited by the bound ary of the zero con -

tours (near 25°N). Among all three runs, the PO Run (Fig. 7c)

pro duces the most re al is tic EVS pat tern with ex treme val ues

(dark shaded ar eas) cov er ing a wider me rid i o nal ex tent than

that in both the IP (Fig. 7b) and IO (Fig. 7d) Runs. This in di -

cates that the PO Run pro duces a more re al is tic Asian sum -

mer mon soon cir cu la tion, prob a bly be cause its In dian Ocean

SSTs are pre scribed with the ob ser va tion. How ever, the PO

Run does not pro duce a sig nif i cant ISO prop a gat ing north -

ward. Fur ther more, in the IP and IO Runs, the north ward

ISO propagations go be yond their cor re spond ing neg a tive

EVS do mains. These ba sin-cou pling re sults sug gest that the

north ward ISO prop a ga tion can not be ex plained solely as a

re sult of at mo spheric in ter nal dy nam ics. In stead, the air-sea

cou pling in the In dian Ocean is also im por tant to the sum -

mer time ISO dy nam ics.

We fur ther con trast the ISO sim u la tions pro duced by the

A CGCM Study on North ward Prop a ga tion of TISO 307

Fig. 6. The com pos ite ISO lifecycles in la tent heat (LH; col ored shad ing) and short-wave ra di a tion (SWR; con tour) anom a lies cal cu lated from: (a) the

ob ser va tion, (b) IP, (c) PO, and (d) IO runs. Down ward fluxes are pos i tive. Cen ters of en hanced rain fall (in Figs. 4a, b) and pos i tive/neg a tive SSTA (in

Figs. 5a, b) are re spec tively la beled as open tri an gles and +/- in both Figs. 6a and b. The ob served/sim u lated LH anom a lies are scaled by the val ues be -

low/above the color bar. Con tour in ter vals of SWR anom a lies are 5 W m-2.

(a) (b) (c) (d)

ba sin-cou pling runs with those from their cor re spond ing

forced AGCM runs. Monthly SST climatologies pro duced

by the CGCM ex per i ments are used as bound ary con di tions

in the re spected cou pled ocean bas ins to drive the same

AGCM in an un cou pled con fig u ra tion. Out side the forced

ocean do mains, SSTs are still pre scribed from the ob served

monthly-mean cli ma tol ogy. All AGCM runs are in te grated

for about 13 years, and the last 10 years of the sim u la tions

are an a lyzed. The same EEOF anal y sis and com pos ite pro -

ce dure are ap plied to the AGCM runs to con struct the

lifecycle of the sim u lated ISO. Fig ure 8 shows the evo lu -

tions of the sum mer time c¢200 along the low lat i tudes (av er -

aged be tween 2°S and 14°N). The stand ing c¢200 os cil la tion

pro duced by the forced PO Run (Fig. 8b) is very sim i lar to

that of the cou pled PO Run (Fig. 3c), con firm ing again that

the model SST bi ases in the Pa cific Ocean cause the stand ing

ISO fea ture and that the cou pling in the Pa cific Ocean does

not pro mote the ISO prop a ga tion. For the forced IO Run

(Fig. 8c), the c¢200 shows a stron ger stand ing but weaker

prop a gat ing fea ture in the In dian Ocean than the cou pled IO

Run (Fig. 3d). The dif fer ence be tween them in di cates that

the stand ing fea ture is caused by the CGCM bi ases in the

In dian Ocean SSTs and also that it re quires the In dian Ocean

cou pling to en hance the zonal prop a ga tion of the sum mer -

time ISO. It is noted in Fig. 8c that the In dian Ocean SST

bi ases also en hance the stand ing c¢200 os cil la tion cen tered at

the date line. The dif fer ences in c¢200 be tween the forced

(Fig. 8a) and the cou pled (Fig. 3b) IP Runs are sim i lar to

those be tween the forced (Fig. 8c) and the cou pled IO

(Fig. 3d) Runs. It should be noted over the In dian Ocean

sec tor (60 - 120°E) that the east ward prop a gat ing fea ture can

still be iden ti fied in the forced IP and IO runs (Figs. 8a, c),

al though less ob vi ous than in their cor re spond ing CGCM

runs. It then sug gests that the at mo spheric in ter nal dy nam ics

also con trib ute to the zonal prop a ga tion of sum mer time ISO.

This pos si ble ac tive role of the at mo sphere it self in the ISO

re lated air-sea in ter ac tion is in di cated by com par ing the SST

and heat flux with the sur face wind stress anom a lies (see

Figs. 5, 6). The west erly anom a lies to the west of anom a lous

con vec tion, for ex am ple, will re trieve LHF from ocean (thus

cool the SST) and de liver mois ture to the con vec tion cen ter,

which man i fests the im por tant air-sea cou pling pro cess.

How ever, it is the at mo sphere that de cides the lo ca tion to

ex tract heat from the ocean whereas the ocean re acts pas -

sively. This spatiotemporal re la tion ship can only be sus -

tained in the CGCM. Lack ing of this wind-evap o ra tion ef -

fect in the AGCM, the zonal prop a ga tion ten dency due to the

at mo spheric in ter nal dy nam ics is likely slowed down or

locked by the lo cal pos i tive SST, re sult ing in the sim u lated

os cil la tion be ing more sta tion ary.

To high light the rel a tive roles be tween the ocean cou -

pling and at mo spheric in ter nal dy nam ics upon the north -

ward prop a gat ing ISO in bo real sum mer, we show in Fig. 9

the time-lat i tude plots of the U¢850 (con tours) and rain fall

(color-shaded) com pos ites in the ob ser va tion and var i ous

model ex per i ments around the lon gi tudes of the Bay of

Bengal (av er aged be tween 85 and 95°E). Some sa lient fea -

tures are as fol lows. Firstly, lack of the cou pling not only re -

sults in the re duc tion of anom a lous mag ni tude but also the

weak en ing of the prop a gat ing fea ture in the ISO-re lated

con vec tion (panel b vs. d and panel c vs. e), con sis tent with

the find ings in some re cent stud ies (e.g., Fu and Wang

308 Shu-Ping Weng & Jin-Yi Yu

Fig. 7. The cli ma tic mean EVS (east erly verical shear) cal cu lated from: (a) the ob ser va tion, (b) IP, (c) PO, and (d) IO runs in MJJASO. Only neg a tive

val ues of EVS are shaded and con toured with 5 m sec-1 in ter vals.

(a) (b)

(c) (d)

A CGCM Study on North ward Prop a ga tion of TISO 309

Fig. 8. The Hovmöller di a grams of the compositing sum mer time c¢200 from the: (a) forced IP, (b) forced PO, and (c) forced IO AGCM ex per i ments

averaged be tween 2°S and 14°N. Con tour in ter vals are 1/3 ´ 106 m2 sec-1.

Fig. 9. The time-lat i tude di a grams of the sum mer time U¢850 (con tour lines; solid: west erly) and rain fall (color shad ing) com pos ites in: (a) the ob ser va -

tion, (b) IP, (c) IO, (d) forced IP, and (e) forced IO runs. The anom a lies are lon gi tu di nally av er aged be tween 85 and 95°E roughly around the Bay of

Ben gal. Con tour in ter val of U¢850 is 1.0 m sec-1 in panel (a), and 0.5 m sec-1 else where. The ver ti cal green lines mark the av er aged lo ca tions of zero con -

tours in the sea sonal-mean EVS.

(a) (b) (c)

(a) (b) (c)

(d) (e)

2004). Sec ondly, the evo lu tion of U¢850 some what still owes

a north ward prop a gat ing sig nal even if the In dian Ocean

cou pling is ab sent (panel e), sug gest ing the in creased role of

at mo spheric in ter nal dy nam ics to aid the ISO prop a ga tion

when an asym met ric mon soonal cir cu la tion ap pears in bo -

real sum mer (Wang and Xie 1997). Fi nally, when the air-sea

cou pling is ex cluded in the forced IP Run (panel d), the

north ern most lat i tude (around 15°N; ver ti cal green line) that

the dwin dling zonal winds’ prop a ga tion can reach co in cides

with the north ern bound ary of sea sonal-mean neg a tive EVS

(cf. Fig. 7b). How ever, it is not the case in the cor re spond ing

cou pled run (panel b) that al lows the sim u lated U¢850 to cross

the north ern bound ary of sea sonal-mean neg a tive EVS and

thus leads to a lat i tu di nal ex tent closer to the ob ser va tion.

Al though it could be model de pend ent, it leads us to con -

clude that the In dian Ocean cou pling plays an im por tant

rather than a sec ond ary role in the north ward prop a ga tion of

ISO in bo real sum mer.

6. SUM MARY AND CON CLU SIONS

In this study, we dem on strated with three ba sin-cou -

pling CGCM ex per i ments that the In dian Ocean cou pling is

cru cial to the strength and ex tent of the me rid i o nal prop a ga -

tion of sum mer time ISO. In bo real sum mer, when the warm

SSTs re tract west ward and shift into the North ern Hemi -

sphere, the north ward prop a gat ing com po nent strength ens

while its east ward prop a gat ing com po nent weak ens. The

prop a ga tion mainly oc curs in the north ern In dian Ocean and

the west ern North Pa cific, al low ing the In dian Ocean cou -

pling to be come im por tant. The Pa cific Ocean cou pling is

less im por tant to the sum mer time ISO. The IO Run pro duces

the most re al is tic sim u la tion of the north ward ISO prop a ga -

tion, while the PO Run fails to sim u late the north ward pro -

pagation. Our com pos ite anal y ses re vealed that the ocean

couplings pro mote the ISO propagations by con fig ur ing a

proper tem po ral and spa tial phase re la tion ship be tween the

sur face fluxes, winds, con vec tion, and un der ly ing SST fluc -

tu a tions. In this par tic u lar CGCM, how ever, the ISO-SST

interaction is gen er ated mainly by the wind-evap o ra tion

feedback through which the anom a lous LHF changes the

SST and thereby the anom a lous con vec tion. In the obser -

vations, an in ter ac tion is also pro duced via short wave ra -

diation flux through the con vec tion-cloud feed back. From

the forced AGCM ex per i ments, we found that the me rid i o -

nal ISO prop a ga tion weak ens once the ocean cou plings are

turned off.

The rel a tive roles of the air-sea cou pling in the In dian

Ocean and the off-equa tor mon soon mean flow in shap ing

the sum mer time ISO are fur ther ex am ined by com par ing the

cou pled runs with their forced AGCM coun ter parts. Re sults

in di cate that: 1) the north ward prop a gat ing fea ture in the

ISO-re lated con vec tion is en hanced when an in ter ac tive

Indian Ocean is in cluded; 2) the at mo spheric in ter nal dy -

namics aid the prop a gat ing fea ture of the ISO mainly th -

rough the as so ci ated low-level zonal winds, though with

reduced mag ni tude when the cou pling is ex cluded; and 3)

Indian Ocean cou pling helps the sim u lated ISO to cross the

north ern bound ary of mean neg a tive EVS, re sult ing in a

more re al is tic sim u la tion. This study there fore con cludes

that the In dian Ocean cou pling plays an im por tant rather

than sec ond ary role in the north ward prop a ga tion of the ISO

in bo real sum mer.

Acknowledgements The au thors thank two re view ers for

their valu able com ments on the manu script. The sug ges -

tions from John Farrara are es pe cially ap pre ci ated. Spe cial

thanks are ex tended to Wen-Haung Huang for his help in

fig ure prep a ra tion. SPW was sup ported by the Na tional

Sci ence Coun cil of Tai wan un der Grant NSC 95-2111- M-

003-002. JYY was sup ported by the Na tional Sci ence

Foun da tion of USA un der Grant NSF ATM-0321380. Mo -

del sim u la tions were per formed at the Uni ver sity of Cal i -

for nia Irvine’s Earth Sys tem Mod el ing Fa cil ity and the UC

San Diego Super com puter Cen ter.

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