Contribution of Equatorial Pacific Winds to Southern Tropical Indian Ocean Rossby Waves
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. C2, PAGES 2407-2422, FEBRUARY 15, 2001
Contribution of equatorial Pacific winds to southern tropical Indian Ocean Rossby waves
Jame . PotemrasT 1 International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii
Abstract. Westward propagating features, identified as Rossby waves, have been observe modeled dan southere th n di n tropical Indian Ocean (STIO) betwee° n10 d 30°San . These STIO Rossby waves, which have broad zona d meridionaan l l extents, could interact with the westward flowing South Equatorial Current (SEC) as well as coastal currents on the south shore of Java (the South Java Current) d alon an e westergth n shor f Australieo a (the Leeuwin Current). Previous work has attributed these waves to variations in wind stress along the west coast of Australia, Ekman pumping in the STIO interior, and a combination of both. This study investigate importance sth thira f eo d factor: remotely forced coastal Kelvin waves. Observations show that changes in wind stress curl in the eastern equatorial Pacific create annual upwelling and downwelling Rossby waves. Numerical model results confirm previous studies that demonstrate these waves, upon reaching the western boundar Pacifice th f yo , create poleward propagating coastal Kelvin waves along the western shore of the Irian Jaya/Australia land mass. Direct observations of annual sea level variations along the northwest coast of Australia show a phase frog e northernmosla mth t statiosouthernmose th o nt t texplaine thano s i t y b d direct wind forcing, suggesting that this signa propagatins i l g fro e Indonesiamth n seasshows i t I thin .i n s work that when these waves reac Indiae hth n Ocean, they phasn i e ear wit e locahth l Ekman forcin enhancd an g STIe eth O Rossby waves. In the model the signal from the equatorial Pacific accounts for almost 80% of the energy of the STIO Rossby wave near the coast of Australia and 10% of the energy offshore.
lo Introduction it turns northward to form the SEC. Along the coast, however e flo th s towarw,i e poleth d , agains e preth t- The large-scale circulatio southere th n i n n tropical vailing winds. This current, calle Leeuwie dth n Current Indian Ocean (STIO), shown schematically in Figure 1, / /-<\ • ^ n ± u j • u i i. , ; , '' . , ^, „ /^-r-^x (LC), is though drivealongshore n b a y o t ntb T e pressure is, dommate thy edb South Equatorial Current (SEC) dient th&t ^ established by a density gradient from tnfc ?°r! T rt t0 TSt 7ten the warm, fresh throughflow water [see, e.g., Church 30 S (although the boundaries of this current are not ^ d ing. a
2407 2408 POTEMRA: EQUATORIAL PACIFIC CONTRIBUTIO STIO NT O ROSSBY WAVES
ION
40S E 10090 E E HO80 E E 70 120E E 60 130E E 50 E 40 Figure 1. Schematic representation of surface flows in the southern tropical Indian Ocean (STIO). Abbreviations for the western boundary currents are defined in the text.
stein [1983] climatological wind stress). The model pro- a multilevel z coordinat, e model (forced with Heller- duced annual Rossby waves, forme e easd th jus t o t t man and Rosenstein [1983] climatological wind stress) of 100°E (maximum signal from 20° to 15°S). It wa scompared an resulte dth expendablo st e bathythermo- postulated that variations in the Southern Hemisphere graph (XBT TOPEX/Poseidod an ) n (T/P) data. They trade winds wer e causeth f thes- eo ef ee waveth d an s determined that STIO Rossby waves were largely forced fect f theso s e waves wer e transpor th see e n i nth f o t eastere inth n Indian Ocea Ekmay nb n pumping along SEC and in the currents off the west coast of Australia the wave characteristic but that Ekman pumping af- [Woodberr t al,e y 1989]importans i t I . noto t t e that fected the signal throughout the ocean interior. While e modeth l use thin di t sinclud studno d yPacifiea di c the model they used was global and therefore included basin, and the throughflow was included only as an open remote Pacific effects, remote Pacifict forcinno s wa g boundary cordition. specifically addressed. subsequena n I t study, Perigaud Ddeclused an [1992] e presenTh t study examine possibilitw ne a s y that use a one-and-one-half-layed r reduced gravity model remotely forced Rossby e equatoriawaveth n i s - Pa l (forced by Florida State University (FSU) winds) to cific contribut STIo et O Rossby waves. Annual Rossby investigate Rossby waves in the STIO. Their model, waves are formed in the equatorial Pacific from changes which compared favorably with remote Geosat obser- in Ekman pumping nea eastere th r n boundary [Meyers, vations, showed upwelling Rossby waveeastere th n si n 1979; Lukas Firing,d an 1985; Kessler, 1990]. When part of the basin during March through October (along these Rossby waves reac westere hth n boundare th f yo 90 . Minimu°E) a levemse l variation (deparm c 2 1 -' f so Pacific, they induce poleward propagating Kelvin waves ture from the long-term mean) were noted at 90° E dur- alon e wesgth t sid f Iriao e n Jayd Australiaan a s A . ing July. A downwelling wave was evident during the these Kelvin waves travel along the western coast of e yearth res f ,o t with pea a levekse l variation2 1 f o s Australia, they excite Rossby waves. cm at 90° E during December. Perigaud and Delecluse The idea of energy "leakage" from the Pacific to the pointed out that the wind stress curl is weak in the in- Indian Ocea firss nwa t describe y Clarkedb [1991]n I . e basinterioth f o r, wher e waveth e s reach maximum that study the effects of the gappy western boundary of amplitud t 90°E)e(a suggested an , wavee dth proe sar - equatoriae th l Pacifi large-scaln co e wave reflection were duced from coastal winds alon e westergth n shorf o e theorized. Clarke [1991] determine- en e th f do tha% 10 t Australi thed aan n propagate offshore. Again, similar ergy flux that reache westere sth n equatorial Pacifie cdu to Woodberry et al [1989], the model geometry only to propagating long Rossby waves makes it into the In- included the Indian Ocean. dian Ocean. Reflected equatorial Kelvin waves account Finally, STIO Rossby waves were the focus of a re- for 34% of the energy flux, and the remainder gets dissi- cent study by Masumoto and Meyers [1998]. They used pated in the boundary layers of the various islands that POTEMRA: EQUATORIAL PACIFIC CONTRIBUTION TO STIO ROSSBY WAVES 2409
make up the western boundary of the Pacific [Clarke, 1991]. « 1. (1) Verschell t ale [1995] s parmodelina a ,f o t g study, showed numerically tha first-moda t e Rossby wave (gen- The wave number k is given by ™, where u; is the fre- 1 erated in the eastern equatorial Pacific) would provide quencRossbe th f yo y wave (~ years"intere th r - fo energIndiae th o yt n Ocean modee th n freI a l. e Rossby annual waves describe y Clarkedb e th s i [1991]c d an ) 1 wave impinged on the western boundary, and energy en- first baroclinic mode phase speed s"(givem y b 3 s na tered the Indian Ocean (presumably by a coastal Kelvin Clarke [1991]) representx A . e Pacifie widtth th s f ho c wave) evidens i s a , uppey b t r layer thickness anomalies boundary region, from Kalimanta Solomoe th o nt - nIs alon northwese gth t Australian coas t 10a t. °S s approximatellandsi d an , y 490 0case km)f th e o n I . 8 1 Other studies have also demonstrate importance dth e interannual Rossby waves , s aboui 10~x uj 6 6. t s" , of equatorial Pacific forcing on sea level variations on and the left side of (1) is 0.16. Therefore low-frequency the northwest Australian coast [e.g., Clarke and Liu, Pacific Rossby waves (period f yearso s ) have motions 1994; Wajsowicz, 1996; Murtugudde et a/., 1998]. All of mainly independent of x and leak some energy (10% these studies, however, focuse inten do r annual variabil- by Clarke [1991]) into the Indian Ocean, and in fact, ity., In addition, these studies were involved primarily it has been shown that interannual variations in north- with variations along the northwest coast of Australia west Australian sea level are correlated to variations in t directlno d di y d addresan affece th s t these variations e equatoriath l Pacific [e.g., Clarke Liu,d an 1994; Waj- would have on STIO Rossby waves. sowicz, 1996; Murtugudde et a/., 1998]. Her e Pacifieth Indiao t c n Ocean connection wile b l For higher frequencies, however, the left side of (1) investigated with respect to its effect on STIO Rossby is closer to 1. For example, at semiannual periods (u waves. Since these waves have been observed annually = m day8'1), ^ is 1.13. Therefore it must first be (one upwelline downwellinon d an g g wav r year)pe e , demonstrated that variations in sea level in the equato- presene th t study will focu monthln o s y timescales- un , rial Pacific due to annual Rossby waves will propagate likprevioue eth s studie Pacifie th f Indiaso o t c n Ocean into the Indian Ocean as coastal waves. connection that focuse interannuan do l variability (and Once these waves reac e westh h t coas f Australio t a therefore relatively low frequency Rossby waves). and propagate southward, a high-frequency limit is issuey ke se thaTh t wil addressee b l d includn Ca ) e(1 placed on their ability to supply energy to the STIO levea se l variabilit equatoriae th n yi l Pacific propagate region. Coastal Kelvin waves along an eastern bound- to the Indian Ocean on annual timescales; (2) What is ary will generate offshore Rossby waves into the ocean e phasth e relationshi e forcine equatoriath th f pn o i g l interior only within certain latitudes (so-called turning Pacific and the observed STIO Rossby wave variations latitudes; see, for example, McCreary and Kundu [1987] (Do remotely forced sea level variations act to enhance r diminiso h STIO Rossb mucw yf Ho ho waves)) (3 d an ; the observed variability in STIO sea level is explained by remote Pacific forcing theoreticaA . l discussion aimed 2.6c= s 2m/ at the first issue is given in section 2 followed by some 1.0s = c 7m/ - - observational evidenc somd ean e results from large-scale general circulation models (GCMs addreso t ) sece th s- ond and third issues.
2. Theoretical Considerations Previous studie f STIo s O Rossby waves hav- ne e glected remote effects from the Pacific; however, it has been shown that equatorial Rossby wavePacifie th n si c could provide energy to the Indian Ocean via coastal Kelvin waves. Clarke [1991] showed that when first- mode, long Rossby waves (interannual period) impinge e westerth n - o nen boundare th f e Pacifico th f % yo 10 , 30 60 90 120 150 180 210 240 270 300 330 360 ergy make t ints i India e oth n Ocean. That work focused Forcing Period (days) e interannuaoth n l waves thae createar t d durinl E g Figur . Equatioe2 uses computo wa dt ) n(2 critie eth - Nine-Southern Oscillation (ENSO) events. As part of cal latitude, given a certain periodic forcing and c. The solie firsth dtr baroclinifo lin s ei c 2.6mod— 2m c ( e derivatione th limia , placehorizontas e i t th n do l scale 1 d perioe wavean th f do . Following Clarke [1991]s i t i , s" ), and the dashed line is for the second baroclinic mode (c = 1.07 m s"1). The curve shows, for example, assumed that motio independens ni zonae th f lo t extent for semiannual forcing, first mode coastal Kelvin waves of the western boundary region, Ax, which, through will only generate offshore Rossby waves equatorward expansion, requires ° latitudeo27 f . 2410 POTEMRA: EQUATORIAL PACIFIC CONTRIBUTION TO STIO ROSSBY WAVES
0.20 where
2 A0=0.2Nm ; u = 27T/30 and27r/180d-1;
x0 20° W of the boundary (170°W for this example); 0.05 y0 equator;
LXL =2 160E 180 170W 160W 150W 10N
5N model'e Th s initial m0 thic uppe30 o t k t r se laye s rwa and a density of 1024 kg m~3. The resulting first baro- EQ clinic mode phase spee 2.6s i s"dc modee 2m 1s Th . lwa run with monthly and semiannual periodic forcing. 5S Figur e perioe forcin showth e4 th f w do g sho affects the solution. Periodic forcing was used to generate pole- 0.05 0.10 0.15 0.20 Zonal Wind Stress along 170W (N/m2) ward propagating coastal Kelvin waves. The equatori- ally centered forcing actually creates westward equato- Figure 3. The idealized forcing for the rectangular rial Rossby wave wels s a eastwar s a l d propagating equa- basin model was purely zonal (periodic in time), cen- torial Kelvin waves. It is the latter that impinge on tered at the equator, 170°W: (a) wind stress along the equator and (b) variations with latitude at 170°W. the eastern boundary and generate the coastal Kelvin waves. Energy (per unit volume) was computed at each grid fo derivation)a r turnine Th . g latitud determines ei d as point (i,j) from functioa frequencyf no , givey nb t=T
/ ^ (4) tan(0c) = (2) Earth Again, u) is the frequency of the wave, and c is the with the pressure P computed from the model upper ls tne ra layer thickness h as phase speed. ^Earth Earthe diu Oth d cf so an , is the critical latitude. When the latitude is equator- war e f wavenumbe0do Cth , e waveth e reals i rd ar s an , (5) westward (offshore) propagating Rossby waves. Pole- war f thido s latitude e wavenumbeth , complexs i r d an , e initia- Th pe le uppeth d ran thicknes, m 0 30 s hwa 0 only coastal Kelvin waves are permitted. Figure 2 show firse dayth 0 tr 3 s experimens fo riowa dT t (Figur) e4a the critical latitude for values of cj up to 1 year fosecone rday th 0 tw r 18 osdfo d (Figuran e 4b) eacn I . h case, value c (approximatin f o s e d firssecongth an t d baro- coastal Kelvin waves appear on the eastern boundary. clinic modes). Consistent with Figure 2, however, these coastal waves To demonstrate this limit, a simple reduced gravity only shed energy int basie oth n interioe r withith f o ° n5 model was run with idealized forcing. A rectangular equatoe casth y forcinger da fo witr lower 0 Fo 3 h . - grid (||°y °b ) representin longitud ° lat° g50 - 60 d ean frequency coastal v/aves (180 day forcing), Rossby wave itude (from 30° 30°No St useds generato wa ) T . e pole- energ evidens yi t withi equatore th n f aboue o ° Th . 30 t ward propagating coastal Kelvin waves alon easte gth - experimen y Verschellb t t al.e [1995 forces n a wa ] y b d ern boundary (as would be found along the west coast equatorial Pacific Rossby wave wit hperioa days 0 2 f d,o of Australia) e mode th ,forces wa lzonaly b d , periodic which limited the amount of energy that would be gen- wind stress centere e equatorth e n th ° weso d f 20 o ,t eratemidlatitude th n di e Indian Ocean. eastern boundary (in the STIO case the coastal Kelvin waves would be formed as Pacific Rossby waves interact 3e Applicatio e STIth Oo nt Region with the western boundary of the Pacific). The wind stress magnitude (show s taperen Figuri n wa ) n 3 ei d theore Th y outline applie w sectioe n di no th s o i dt n2 bot zonae hmeridionath d an l l direction followss sa : STIO region. Observations have show existence nth f eo long Rossby waves in the equatorial Pacific, formed by x — Ekman pumping near the eastern boundary [Meyers, .r= x 1979; Lukcis Firing,d an 1985; Kessler, 1990]. Further, (3) Perigaud and Delecluse [1992] showed, using remote ob- POTEMRA: EQUATORIAL PACIFIC CONTRIBUTION TO STIO ROSSBY WAVES 2411
30N
30S 175W 170W 165W 160W 155W 150W
2 4 6 8 10 30N
3QS4 175W 170W 165W 160W 155W 150W
10
Figur . Energe 4 m"" g k n s~~y(i ), computed fro ) usin m(4 e result gth s fromodeo mtw l exper- 2
iments, is shown1 : (a) the energy in the monthly periodic forcing case (u = 27T/30 days""1) and e semiannuae resul(bth th )r fo t l forcing case (u =. 27T/180 days"1). Both runs were made using a rectangular basin, and the wind was prescribed as shown in Figure 3. servations, that an upwelling and a down welling Rossby waves provide energe Indiath o t yn Ocea d conan n- wav formes ei d eac he STIO yeath t 90°En i rA . , 12°S, tribute to the observed STIO Rossby waves? The peak sea level occurs in December (downwelling wave), hypothesi s thai se near-equatoria th t l Pacific Rossby and the minimum sea level signal is in July (upwelling wave interacn sca t wit varioue hth s island weste th n -si wave). ern Pacific (primarily Halmahera, Kalimantan, and Irian The question is, given the amplitude, phase, and pe- Jaya/New Guinea) and create poleward propagating, riod of this forcing, can the equatorial Pacific Rossby coastal Kelvin waves. These waves would then travel 2412 POTEMRA: EQUATORIAL PACIFIC CONTRIBUTION TO STIO ROSSBY WAVES
alon e westergth n shore f Iriao s n Jayd Australian a a (Ekman pumping). The relative phase of the local wind inte Indiaoth n Ocean e resultinTh . a levegse l varia- and the remotely forced waves need to be compared to tions would then contribute to STIO Rossby waves. phasobservee e th th f eo d STIO Rossby waves othen I . r As outlined above, the period of the forcing is critical words, give ncertaia n Pacifie phasth o et c forcind gan to whether equatorial Pacific Rossby waves will provide a certain phase speed of the generated waves, the re- forenerge th f coasta mo n y(i l Kelvi- nIn wavese th o t ) motely forced coastal waves will either enhanc- di r eo dian Ocean and at what latitude these Kelvin waves will minish locally forced sea level variations. generate Rossby waves (if at all). Further, the coast- The approach taken here is to employ the same one- line configuration of the eastern boundary of the Indian and-one-half-layer reduced gravity model described pre- Ocean plays a role. The Malay Archipelago stretches viously. Realistic boundaries are used, and both the from west to east along 10° S (see Figure 1), thus plac- Pacific a:«iu Indian Ocean basins are included. To in- ing a northern boundary on STIO waves formed along vestigate propagation paths e samth , e simplified wind eastere th n boundary discusses A . d previously- pe e th , forcing is used (see (3)). Wind forcing of monthly, semi- riod of the forcing in the eastern Pacific will determine annual, annual yea4 d r an period, s appliewa a s n i d e perioe coastath th f o d l Kelvi ne criti th wave -d an s regioe easterth n ni n equatorial Pacific (see Figur. e5) cal latitude of Rossby wave formation. Thus, foyear5 r r waveremovo t sfo n e initiase moderu th e s Th lwa l with periods less than 60 days, the critical latitude is respons yea4 e r case yearth 5 perio f th e(1 o n si d forc- 10° withi e equatore Lessenth th d r an ,Sund a Islands ing) , and results were computed from the final year of prevent these waves from entering the Indian Ocean. integration. Finally, remotely formed coastal Kelvin waves alon ge modeth e winn th I l d patch excite a westwars d western Australia will be affected by local wind stress Kelvin wave that reflects off the eastern boundary of
100 120 140 160 180 200 220 240 260 230 300 1 2 3 4 5 6 7 8 9 10 11 12
100E E 0 12016018 HO 160EE1 W 9 1400. O6 8 WJ0. 0. 5 0. 120 4 W0. 3 0. 100 2 0. W 1 W -0.0. 60 SO 10 W Figurwine Th d . estres5 s use forceo dt idealizee dth d model experiment purels swa y zonalt A . its peak, the wind stress attains 0.2 N m"2. (a) The meridional modulation of the wind stress amplitud m~N n 2(i e) alon e e equatortime-vargth Th ) (b modulatio,g yin e winth f dno stress amplitud foue th rr experimentsefo : four-year, annual, semiannual monthlyd an , Win) (c , d stress amplitude (the 0.05 and 0.1 N m~2 levels are contoured), (d) The zonal modulation of the wind stress amplitude (in N m~2) along 140° W. POTEMRA: EQUATORIAL PACIFIC CONTRIBUTIO STIO NT O ROSSBY WAVES 2413
e relativelTh y simple reduced gravity model forced with realistic winds [Hellerman and Rosenstein, 1983] arid realistic coastlines produces STIO Rossby waves simila observationo rt mord san e complex global models (this is presented in section 4). The maximum signal in the STIO region near Australia as seen in the T/P data is at about 15°S [Yang et a/., 1998]. For the model in this study the maximum is farther south. However, for the following analysis, comparisons will be made along 15°S in the STIO, unless otherwise noted. Sea level was computed from the model upper layer thickness (long-term mean removed) along 15° S from e easterth n Indian Ocea e westh n to (90t coas ) °f E o t Australia (120°E)e resultth d s an ,fro m three experi- ments wil discussee b l d (see Figure 7). Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec baseline th r (Figurn Fo eru e 7d), when climatological Figure 6. Sea level deviations at 15°S, 115°E are wind forcing is applied, annual STIO Rossby waves are shown for the five model experiments. The legend refers eviden s hig a tn Marc (i h h through mid-Augustd an ) to the period of the wind forcing in the eastern Pacific. loAugusn w(i t through April levea )se l deviations along The wind forcing varies in time as shown in Figure 5. the Australian coast that propagate offshore. The peak values are at 110°E (8 cm in September and -8 cm in March). equatoriae th l Pacific e reflecteTh . d first-mode Rossby Also shown in Figure 7 are sea level perturbations wave (symmetric sea level deviations along 5°N and from two additional experiments. In the first experi- 5°S) takes abou month5 t reaco st westere hth n bound- ment, the model was run with wind stress fixed to the ary of the Pacific. The theoretical phase speed, ap- mean December values everywhere from 10°S to 10°N. proximate s v/cpTi/ dequatora e th r 3 fo 0.87s i , s"3m 1. Seasonally varying winds were used everywhere outside Upon reaching the western boundary, a complex inter- e equatoriath l region. Thiintendes swa eliminato dt e actio f wavno e reflection occurs between islande th n si the equatorial Pacific Rossby waves (i.e., to highlight Celebes Sea. The result is a coastal Kelvin wave that juse locallth t y forced componen STIe th f Oo t waves). propagates south along the west coast of Australia. Th secone th n I d experiment, seasonally varying winds were coastal Kelvin wave travels much faster than the Rossby applied equatoriae onlth n yi l Pacific (10°S-10°N, 135°E wav taked an e s abou weekfro2 o t e equatog mth o t s r to the eastern boundary of the Pacific). December mean to 25°S along the west coast of Australia, so the signal winds were applied everywhere else. In this way the reache STIe sth O abou day0 18 ts afte disturbance th r e model-produced STIO Rossby waves were purely a re- was forced in the eastern Pacific. sul f equatoriao t l Pacific winds. Both result come ar s - Figur showe6 modee sth levea lse l deviation t 15°Ssa , pared wit baseline hth e model results (seasonally vary- eastere 115°th n E(i n Indian Ocean) experimene th n I . t wing in d everywhere Figurn i ) . e7 whe winde nth s vary wit annuan ha l period (again, only t shoulI e noteb d d tha mora t e traditional method imposed in the eastern equatorial Pacific), STIO Rossby of closin e throughfiogth eliminato wt e remote Pacific waves have their largest amplitud cm)9 e( . Semiannual effects was not used here. By closing the throughflow forcing generate somewhaa s t smaller amplitude wave e coastlinth e Malath f eo y Archipelago gets connected (6 cm), and the winds with a 4 year period produce to Australia/New Guinea. Coastal Kelvin waves gener- STIO Rossby waves with a 3 cm amplitude. When the ated in the Indian Ocean can propagate from Indonesia mode forces wa lwindy b d s with monthly, semiannual, Australio t thed aan n south alon coaste gth realitn I . y and annual variability, the result is smaller-amplitude the gap between Indonesia and Australia is too large for Rossby waves in the first part of the year (about 8 cm coastal waves to cross. Therefore a regional wind ap- amplitude) whe semiannuae nth annuad an l l signale sar proach was chosen to isolate the effects of the equatorial largephasf a o d t ean rou amplitude late) (aboucm r 6 1 t Pacific from the higher-latitude STIO. Similarly, effects yeae inth r whe annuae nth semiannuad an l l signale sar fro equatoriae mth l Indian t considere Oceano e nar r dfo in phase. this study and are assumed to be small [see Potemra, It appears from these model results that annual forced 1999]. equatorial Rossby waves in the Pacific can produce, via The two experiments demonstrate that the model coastal Kelvin waves, Rossby waves in the STIO region. STIO signa superpositioa s i l f remotelno y force- Pa d The next issue is the phase of these Rossby waves gen- cific wave localld san y forced Hellermane wavesth r Fo . erated in the Pacific compared to the phase of those and Rosenstein [1983] wind stress modee forcinth d lgan generated localle STIth On i y region investigato T . e vertical parameterization (which results in a first baro- this, the simple model was run with realistic seasonal clinic phase speed of 2.62 m s"1), the equatorial Pa- wind stress of Hellerman and Rosenstein [1983]. cific forcing produces high sea level along the model Off-Eq Wind Eq-Pac Wind
E 99 102 E E96 105E 93 E 108E 90 E E H 1111 117E E 120E 90E 93E t6E 99E 102E 105E 108E 111E 114E 117E 120E
-6 -4 -2 0 Off-Eq Wind plus Eq-Pac Wind All Wind (Baseline)
90E 93E 96E 99E 102E 105E 108E 11 IE 1 HE 117E 120E 90E 93E 96E 99E 102E 105E, 108E 111E 114E 117E 120E
2 - 4 - -6
Figure 7. Model sea level deviations (in centimeters) in the Indian Ocean along 15°S are contoured (negative values shaded) e mode forces Th . wa l d with time-varying win specifin di c regions: (a) the result when only off-equator winds (outside 10°S-10°N) are allowed to vary, (b) resule th t when only equatorial Pacific winds (eas f 135°Eo t , 10°S-10°N allowee ar ) varyo dt ) (c , e baseline resultth th ) f f theso (d o scases o e m d tw emode su an , n (winde ru th l s time-varg yin everywhere). POTEMRA: EQUATORIAL PACIFIC CONTRIBUTION TO STIO ROSSBY WAVES 2415
100 of reflected waves canceling the forced waves (because of —— Indian Ocean forcing the location and period of the forcing). This, however, 80 —— Pacific Ocean forcing is beyond the scope of the present investigation. 60
40 4. Observational and Numerical Evidence
105E 110E 115E 120E Sectio demonstrate3 n reducea n di d gravity model that equatorially forced waves in the Pacific have a con- Figur . Energe8 computes ywa d fro modee mth l veloc- ity and upper layer thickness as the integral of (4) from structive effect on STIO Rossby waves. The question 20° to 10° S. The solid line is the ratio (in percentage) of remains whether this connectio observee b e n th n ca n di the off-equatorial wind case to the total, and the dashed real ocean. line is the ratio of the equatorial Pacific wind case to Direct observations of the connection between the Pa- the total. cific and Indian Oceans is challenging, however, since e propagatioth n spee s relativeli d y fast whil dise th e - tance is short. First-mode coastal Kelvin waves that eastern boundar t 15°Sy(a ) from early March through start near the equator (along the west coast of Irian August. The maximum signal (7 cm) occurs right at Jaya) would take less tha weekn2 propagato st 25°o et S e boundarth Mayn yi . Lower than averag a leveese s i l (assuming a phase speed of 3 m s"1). from September through February, with a peak along repeae P satellitTh T/ t sufficien t e no orbi th s ei f o t t the boundary in mid-November of -6 cm. The effects to discern this signal, but the apparent continuation of of local wind forcing (no equatorial winds) result in a a signal from the Pacific to the Indian Ocean can be maximum along boundare th y from earlthrougy yMa h seen. Sea level deviations (long-term mean removed) October, almos month2 t s afte e Pacifith r c signald an , from T/P are plotted along 15° S in the Indian Ocean the maximum signal is offshore at 110°E. This maxi- (from 100° to 120°E) and then along 5°N in the Pacific mum (5 cm) is slightly less than that formed by the from 120 °o 160°t E (see Figure 10) r comparison.Fo , equatorial t windsi doe t occud no s an ,re th righ n o t a levese l deviations fro e modemth l use thin di s study boundary. Negativ levea ese l perturbations prevaie th l (referred to as the University of Hawaii Layered Model e yeath res rf o talon e coastgth , m witc 5 peaha - f ko (UHLM) fro d moro an )m tw e complex GCMs (the Navy at 110°E. Layered Ocean Model (NLOM) [Wallcraft, 1991]d an , When these result addede sar resule th , vers i t y simi- the Parallel Ocean Climate Model (POCM) [Semtner lar to the baseline model run, with positive sea level per- and Chervin, 1992] alse ar o shown alon same gth e lines). turbation coase th t ta s from mid-March through mid- e modelTh s have different grids (specifically defe th , - September and the maxima occurring offshore at 110°E. inition of land points is different in each), and sea level experimento tw e th f o sm doe t su exactl e sno Th y match can only be computed from the models to a certain baseline severar th fo n elru reasons, including nonlinear- point offshore fro reae mth l coastlineP datT/ ae Th . e modelitieth n i s , equatorial Indian Ocean locad an , l and the UHLM show high sea level along the Australian Indonesian winds and coastal waves that may propagate coast (at 15° S) during the first half of the year and low entirely around Australia. sea level during the second half, suggesting that the Figure 8 shows the relative contribution of each sig- more simple UHLM is getting the correct phase of the nal. Energy (per unit area computes wa )intee th -s da STIO waves. 10°o t fro) ° graS(4 m 20 f usin lo modee gth l results from Propagation fro Pacifie mth alss ci o suggestee th n di e experimenth t with only Indian Ocean winds (locally remote observation moded san l results. Rossby waven si forced waves) and from the experiment with equatorial the Pacific are evident along 5°N, and the signal at the Pacific winds relative Th . e energy f .cm each (compared western boundary of the Pacific is in phase with the to the sum of the two experiments) is given in. Figure 8. e signaeasterth t t 15a a l n ° S boundar e Indiath f yo n As stated, the Pacific effects have the largest effect at Ocean. the coast, while the local forcing has a large effect in While the GCMs and altimeter agree and seem to interioe th t 108°E)(a r . sugges lina t k betwee e Pacifinth c arid Indian Oceans, As a measure of energy, the model velocity and upper e onlth y direc sitn i t u measurement comes from tide layer thickness fields were integrated over one forcing gauges. Daily measurements of sea level are available period (se foue e th (4) rr experimentfo ) s (monthly, semi- t fiva e stations alon e northwesgth t shor f Australieo a annual, annual yea4 d r an forcing), e resultsTh . , given gives i p Figurn ni ( ama ) froe11 m 1984-1996 (although in Figure maximu9e , th sho w who m STIO R,ossby wave some dat missine firse aar th year 2 tr gt fo certai sa - nlo energy is produced by the annual forcing. In the case cations). Annua semiannuad an l l harmonics were com- of monthly forcing energe , almosth f o alons y i l e al t gth yea0 1 re puteperioth r dfo d 1987-199 t eac6a h loca- eastern boundar e Pacifie forth th f coastaf m o yo n i c l tion. The sums of the two harmonics at each location Kelvin waves. Interestingly, in the semiannualy forced givee ar Figurn ni e 11. case, little energ eastere sees th yi n ni n Pacific because Ther apparenn a s ei t phas betweeg ela n each station, 2416 POTEMRA: EQUATORIAL PACIFIC CONTRIBUTION TO STIO ROSSBY WAVES
4 Year forcing 30N- 20N- 10N- EQ- 10S- 20S- 30S' 100E 120 E HO E160 0 W 18 EHO 160 120W W 100W 60 SOW 365 Day forcing 30N
30S 100E 120 E HO E160 0 18 E160 W HO W120 W 100WW 60 SOW forciny Da 0 g 18 30N
30S 100E 120 E 0 W HO E18 HO 160160 120EW W 100WW 60 SOW 30 Day forcing 30N 20N- 10N- EQ- 10S- 20S- 30S 100E 120 E 0 W HO E18 HO 160 160120EW W 100W 60 SOW
16 20
Figure 9. Energy (in kg m"1 s~2; computed from (4) from four model experiments with different period forcing. TOPEX UHLM
100E 110E 120E 130 E 150HO E E 160E 100E 110E 120E 130E HOE 150E 160E NLOM POCM
OOE 110E 120E 130E HOE 150E 160E OOE 110E 120E E 130150HO E E 160E
0 Figure 10. Sea level (long-term mean removed) is given (in centimeters) along 5°N in the Pacific alond an Indiae gth 15°n i nS Ocean from TCPEX/Poseido) (a : n (compute climatologa s da y from 1993-1998); (b) the model used in this study; (c) the NLOM; and (d) the POCM. Negative values are shaded. 2418 POTEMRA: EQUATORIAL PACIFIC CONTRIBUTIO STIO NT O ROSSBY WAVES
90°E 100°E 110°E 120°E 130°E 140°E 5°N 5°N
25°S
30°S 30°S 100°E 110°E 120°E 130°E 140°E
20
Darwin 15 ———— Wyndham — Broo —— m — — Pt. Hedland 10 - — Carnarvo— n
-5
-10
-15
O 3O 6O 9O 12O 15O 18O 210 24O 27O 3OO 33O 36O yeae th rf o y Da Figur . Annua11 e l plus semiannual harmonics were computed from dail sitn yi u tide gauge records (for 1987-1996). The map shows the relative locations along the western coast of Aus- tralia.
again suggestive of a southward propagating feature. ity of the STIO Rossby waves. To determine if this e annuaTh l cycl t Darwia e n shows maximum (mini- is actually a propagating signal, or merely local wind mum) sea level in late February (late July). At the forcing, local Ekman pumping was also examined. next location (traveling south), Wyndham e peath , k Ekman pumping driven by alongshore winds will also earln i s i y March. Continuing south e annuath , l cycle create changes in sea level along the coast, and it is maximum is early April for Broome and Port Hedland importan determino t t e wha locae th t l wind effecte ar s earld an y Jun r Carnarvonefo annuae e peath Th n .ki l to separate them from the freely propagating wave (e.g., cycle then appears to propagate at about 0.37 m s"1. the apparent propagation of the signal from Darwin to Interestingly, the minima in the annual cycle propagates Carnarvo explainee b y nma propagatioa parn di y b t n at a slightly faster speed, about 0.61 m s"1. Both are of the alongshore winds). considerably slower than the predicted speed of a first- investigato T locae eth l wind effects, daily wind data mode Kelvin wave but could be continental shelf waves were obtaine t eacda h tide gauge locatio 199r nfo 3 (from [Robinson, 1964; Mysak, 1967; Smith, 1972]. the NCEP reanalysis). The annual cycle from the tide Nevertheless, ther epropagatioa seeme b o - st an f no gauges during 1993 is similar to the annual cycle from nua levea lse l variability southward alon wese gth t coast 1984 through 1996 (compare Figured - 12)an Ap .1 1 s of Australia that is in phase with the sea level variabil- proximating the slope of the shoreline, the NCEP wind POTEMRA: EQUATORIAL PACIFIC CONTRIBUTION TO STIO ROSSBY WAVES 2419
10
o
o
o
o
O
-40 -•'0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 12. (left) The 1993 sea level deviations (in centimeters) at five locations and (right) alongshore wind speed (in m s"1) from the sarie locations (from the National Centers for Evi- ronmental Prediction (NCEP)) e winTh . d spee plottes di d such that positiv alongshors ei e with e lef coase th t th n (downwellino t g favorable) e annua Th semiannua.d an l l harmonic showe ar s n wit heave hth y line.
speeds were converted to alongshore and offshor upwelline eth r com0.6s i fo s l t 1i g- m d signals l 0.3,an m 7 . ponents e alongshorTh . e component, along wite th h It is perhaps interesting that the alongshore winds are tide gauge data, is shown in Figure 12. The annuasame th e n i tid e l senst th e s no gauga e levela e ese Th . cycle (annua semiannuad an l l harmonics alss i ) o given. tide gauges record positiv levea ese l deviations earln yi There is an apparent propagation of the alongshor yeare e th yeae f negativd .o th an rd middle en th n d ei ean wind speed (excludin e recorgth t Broome)da . Down- e alongshorTh e wind speed indicate e beginninth s f go welling favorable winds occur at Darwin and Wynd- yeae th r (wit exceptioe hth Broomef no upwelline b o )t g ham almost simultaneously, wit hpeaa earln ki y June. favorable (low sea level) and the middle of the year to Three weeks late peae rth k occur t Porsa t Hedlaridd an , downwelline b g favorable (hig a level)hse . 2 weeks after that (mid-July peae th ) k downwellin- goc curs at Carnarvon. The propagation of the annual cy- 5, Discussion clalongshorn ei e wind speed (tim f maximueo m down- welling) is about 1.0 m s"1 when the data at Broome Theoretical constraints plac uppen e a lowed an r r fre- are not included (0.93 otherwise). Similar to the tide quency limi n waveo t s than propagatca t e froe mth gauge data, the apparent propagation of the time of equatorial Pacific throug Indonesiae hth n seaintd san o maximum upwelling is faster at about 3.7 m s"1. interioe th r Indian Ocea Rossbs na y waves. Inter annual Figur show3 1 e e extrem th se annua th n ai l cyclf o e Rossby waves in the Pacific (such as those generated sea level and alongshore wind speed (from Figure 12). during ENSO events) mostl t reflecte yge weste th t -a d e apparenTh t phase downwelline speeth f do g signas i l n boundaryer . Higher-frequency Rossby waves form 2420 POTEMRA: EQUATORIAL PACIFIC CONTRIBUTIO STIO NT O ROSSBY WAVES
Darwin
500 - Wyndham
1000
CD O 1500 OJ S2 Broome ID 2000 0 O JC I 2500 - O) c PT Hedland _o < 3000
BI Carnarvon levea se lx Ma O 3500 levea se l n nMi • Max along-shore wind (downwelling) 4000 '•••••'• PiMin,along-shore wiad.dipweHina). i . 1 p Se 1 g Au 1 i Ju 1 n Ju 1 y Ma 1 r Ap 1 r Ma 1 b Fe Ja n1
Figure 13. The maxima in the annual cycle of sea level deviations and alongshore winds (from e showne 1993Figur ar e alongshorse ) ;Th 12 .e e locatio e rights e givetimni th f Th t eo a .n maximu a levemse l deviations (downwelling wi.ids s givei ) n wit e squarese timhth th f eo d an , minimu levea mse l deviations (upwelling winds gives i ) n wit circlee hth s (the symbol solie r sar dfo wind speed). Note that the winds at Broome are out of phase with the other locations and were not taken into account for the linear regression fit. Also, the winds at Carnarvon are upwelling favorabl solie l yearth eal d o s ,circl t thiea s location represent time sth e whe upwelline nth g winds weakeste ar .
coastal Kelvin e Indonesiawaveth n i s e n th seas n I . tha equatoriae th t l Pacific enhancwindo t t ac s e STIO model geometry these coastal Kelvin waves propagate Rossby waves and that this adds a third factor (along counterclockwise around Australia (until dissipated by with local coastal wind fluctuations and Ekman pump- friction). If the frequency is too high, offshore Rossby ing in the STIO interior) to be considered in the forcing wave solutions are not permitted. For typical stratifica- of STIO Rossby waves. tion, Rossby wave limitee sar equa e th withido f t -o ° n27 Forcing in the Pacific, in a climatological sense, pro- tor when the forcing is semiannual and to within 10° of duces sea level changes along the northwest coast of the equator when the forcing has a 60 day period. The Australi aphasn i tha e ear t wit locae hth l STIO Ekman Indonesian archipelago stretches roughly west to east pumping. It is therefore hypothesized that the equato- along 10°S, thus blockin e Rossbgth y waves generated rial Pacific windenhanco t t ac s e STIO Rossby waves. by higher frequencies. Forcing within thes rangeso etw , Observational evidence of wave propagation from the i.e., of order several months to a year, will leak en- Pacific to the Indian Ocean is not available. The typical ergy int Indiae oth n Ocea generatd nan e offshore Rossby sampling frequenc capturo t f altimeterw o y lo eo to s si waves. e coastath l waves (which take abou week2 t propo t s - Numerical model experiments show that STIO R,ossby agate from the equator to 25°S off the west coast of wave solution possible sar annuar efo - l Pa forcin e th n gi Australia). Tide gauge data, available at five locations modee cificth n I l, this remote forcing account siga r -sfo alon e wesgth t coas f Australiao t , does sho wpropaa - nificant fraction (up to 80%) of the STIO Rossby wave, gating feature that is not fully explained by changes in particularly near the coast. Since forcing in the Pacific, alongshore winds. in a climatological sense, produces sea level changes Phase differences in the tide gauge records along the alon northwese gth t coas Australif o t aphasn i tha e ear t northwest coas Australif o t a suppor idee th ta tha siga t - with the local STIO Ekman pumping, it is hypothesized nal is propagating from north to south along the coast. POTEMRA: EQUATORIAL PACIFIC CONTRIBUTIO STIO NT O ROSSBY WAVES 2421
This phase difference in the sea level record was dis- by Frontier Research Syste r Globamfo l Change. SOEST cussed by Godfrey and Golding [1981]. Godfrey and contribution number 526d IPR9an C contribution number Ridgway [1985] also discuss this propagating signas a l IPRC-60. originating in the Indonesian seas. They explain that the signal at Darwin could originate from upwelling in References the Banda and Arafura Seas during the northwest mon- Church, J. A., G. R. Cresswell, and J. S. Godfrey, The soon, which is maximum in February and March. Leeuwin Current, in Poleward Flows Along Eastern Ocean The observed phase speed is lower than a first-mode Boundaries, edite. NeshybaS . y Mooersd. b K R . d N , an , coastal Kelvin wave. While the simple reduced grav- L. Smith . 230-252pp , , Springer Verlag Yorkw Ne , , 1989. modey it onln ca ly support this typ f waveeo , thi- re s e reflectioClarketh . n J.d transmissioA O , an n f lowo n - frequency energe irregulath t ya r western Pacific Ocean gion could support a continental shelf wave [e.g., Mysak, boundary . Geophys.J , Res., 96, 3289-3305, 1991. 1967; Mysak, 1968a; 1968b]. These were first observed . LiuClarkeX , d . J.Iiiterannuaan A ,, nor:ne levea th se ln i l- thin i s regio Haraony nb [1966] e phasTh . e speea r dfo ern and eastern Indian Ocean, J. Phys. Oceanogr., 24? shelf wave depends, among other things, on the slope 1224-1235, 1994. Cresswell Leeuwie . R.G Th , n Current, . Soc./R . West. widtd shelfe th an f h.o Haraon [1966 phasa t ]go e speed Aust, 74, 1-14, 1991. close to the first-mode Kelvin wave but used data from Cresswell. PetersonL . J Leeuwie d . R.Th G ,an ,, n Current southwestere th easterd nan n coasts wher shele th es i f sout f westerho n Australia . Aust.J , Mar. Fresh. Res., much more narrow. On the northwestern coast the shelf 44, 285-303, 1993. phase th d e speean , shelextendr dkm fo ovea f0 r 10 rsfo . GoldingJ Godfrey . T sverdrue d . S.Th J ,, an , p relation ni the Indian Ocean and the effect of Pacific-Indian Ocean wave inversels si y relate shele th fo dt width modele Th . , throughflo Indian wo n Ocean Ease circulatioth tn o d nan however, does not resolve the shelf, and the only allow- Australian Current . Phys.J , Oceanogr., , 771-77911 , able solutio Kelvie th s ni n wave exace Th . t mechanism, 1981. however, is for future studies. In any case the observed . RidgwayR Godfrey . K e large-scal d . Th S.,J an , , e envi- annua a levese l l variability propagaten i s s i sout d han ronmen e poleward-flowinth f o t g Leeuwin Current, west- ern Australia: Longshore steric height gradients, wind phase wit STIe hth O Rossby waves. 1 stresses and geostrophic flow, /. Phys. Oceanogr., 15, e changTh phasn i e e speee th d s"r m fro fo 4 m0. 481-495, 1985. downwelling wav n Februar(i e y throug6 h0. Mayo t ) Godfrey, J. S., and A. J. Weaver, Is the Leeuwin Current m s"1 for the upwelling wave (in mid-July through driven by Pacific heating and winds?, Prog. Oceanogr., September) is also interesting. This could be explained , 225-27221 , 1991. Hamon, B. V., Continental shelf waves and the effects of at- b yseasonaa l chang stratificatioe th n ei n ofnorthe fth - mospheric pressur d winean d levela stres se . Geo-J n , so west Australian shelf. The slow downwelling signal phys. Res., 71, 2883-2893, 1966. occur n mid-Februari s t Darwint (a y (a y o latt ) Ma e Hellerman, S., and M. Rosenstein, Normal monthly wind Carnarvon). During this tim e densitth e y gradient stress over the world ocean with error estimates, J. Phys. e flosetuth w y b pfro e Indonesiamth n sea s weai s k Oceanogr., 13, 1093-1104, 1983. (maximum throughflow occur n northeri s n summer). Kessler, W. S., Observations of long Rossby waves in the northern tropical Pacific, /. Geophys. Res., 5183-5217, 95 , Levitus data [Levitus, 1982] shog wgradiena k 5 1. f o t 1990. 3 m~ betwee surface nth e water t Darwisa Carnard nan - Levitus, S., Climatologica l worle atlath f dso ocean, NO AA vo Novemben ni m~g d abouk Marchn an ri 6 3 0. t n I . Prof.pp.3 ,17 , NatlPap.13 . Oceani d Atmoscan - Ad . addition, the mixed layer deepens from a minimum in min., Silver Spring, Md., 1982. January through Marc maximua o ht mJunen e i Th . Lukas . FiringE , e annua d R.Th an ,, l Rossby e wavth n ei relatively shallow mixed layer early in the year could central equatorial Pacific Ocean, J. Phys. Oceanogr., 15, 55-67, 1985. also caus wave eth slowo et . Masumoto . MeyersG , d Y.an , , Forced Rossby wavee th n si More observations, particularly higher temporal res- southern tropical Indian Ocean . Geophys../ , Res., 103, olution data, are required to explain fully the coastal 27,589-27,602, 1998. signal off northwest Australia. Nevertheless- ob e th , McCreary . KunduP , d J.an , , Thermohalme forcin eastf go - servations and model experiments suggest that energy boundarn er y currents with applicatio circulatioe th o nt n off the west coast of Australia, J. Mar. Res., 46, 25-58, does propagate fro e Pacifie Indiamth th o t nc Ocean 1987. thad an t this represent importann sa t consideratior nfo Meyers, G., On the annual Rossby wave in the tropical the formation of STIO Rossby waves. North Pacific, /. Phys. Oceanogr., 663-674, 9 , 1979. Morrow, R., and F. Birol, Variability in the southeast In- Acknowledgments e authoTh . r most gratefull- ac y dian Ocean from altimetry: Forcing mechanisms for the knowledge assistance sth Dennif eo s Moore, Mark Merrifield, Leeuwin Current Geophys.J. , Res., 103, 18,529-18,544, r manfo u y helpfuQi o B ld discussionan commentd an s o t s 1998, the draft. Jay McCreary and the rest of the IPRC staff Murtugudde, R., A. J. Busalacchi, and J. Beauchamp, Sea- were instrumental throughout this research effort tide eTh . sona interannuao t l l effectIndonesiae th f o s n Throughflow gauge data were provide Pacifie th Leve a y db Se c l Centet ra tropicae onth l Indo-PacTiC basin . Geophys.J , Res., 103, e Universitth f Hawaiio y . Fundin provides gwa JAMy db - 21,425-21,441, 1998. STEC gran 9819511granE F t 43495e NS OC t Th d . an 7 Mysak e theorth . A.n f continentaL yo .O , l shelf waves. J , International Pacific Research Cente s partli r y sponsored Mar. 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Mysak . A.L , , Edgewave a gentl n o s y sloping continental Wajsowicz response . C.R ,Th , Indo-Pacifie th f eo c through- shelf of finite width, J. Mar. Res., 26, 24-33, 1968a. flow to interannual variations in the Pacific wind stress, Mysak, L. A., Effects of deep-sea stratification and current par , RealistiII t c geometr d ECMV/an y F wind stress on edgewaves, J. Mar. Res., 26, 34-42, 1968b. anomalie 1985-89r sfo . Phys.J , Oceanogr., , 2589-261026 , Perigaud, C., and P. Delecluse, Annual sea level variations 1996. in the southern tropical Indian Ocean from GEOSAT and Wallcraft, A. J., The navy layered ocean model user's guide, shallow-water simulations . Geophys.J , Res., , 20,16997 - NOARL Rep. 35, 21 pp., Stennis Space Center, Miss., 20,178, 1992. 1991. Potemra, J. T., Seasonal variations of the Pacific to Indian Woodberry, K. E., M. E. Luther, and J. J. O'Brien, The Ocean throughflow, /. Phys. Oceanogr., 29, 2930-2944, wind-driven circulation in the southern tropical Indian 1999. Ocean . Geophys.J , Res., , 17,985-18,00294 , 1989. Robinson , ContinentaR. . A , l shel fresponse waveth d an se . KoblinskyYangC . , AdamecD Yu , d . J.L an ,, , Dynam- of sea level to weather systems, J. Geophys. Res., 69, ics of the seasonal variations in the Indian Ocean from 367-368, 1964. TOPEX/Poseidon sea surface height and an ocean model, Semtner, A. J., and R. M. Chervin, Ocean general circu- Geophys. Res. Lett., , 1915-191825 , 1998. lation fro ma globa l eddy-resolving model . Geophys.J , Res., , 5493-555091 , 1992. Smith, R., Nonlinear Kelvin and continental-shelf waves, J. . T Potemra . J , Schoo f o Oceanographyl , Univer- Fluid Mech., 52, 379-391, 1972. sity of Washington, Box 357940, Seattle, WA 98195. Verschell . O'BrienJ . . KindleJ C . A. . d J M ,, ,an , Effects ([email protected]) of Indo-Pacific throughflow on the upper tropical Pacific Indiad an n Oceans, /, Geophys. Res., 100, 18,409-18,420, (Received Augus , 19992 t ; revised Septembe , 200019 r ; 1995. accepted Octobe , 2000.13 r )