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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. C12, PAGES 12,941-12,946, NOVEMBER 15, 1987

Indonesian Through Flow and the Associated PressureGradient

KLAUS WYRTKI

Department of Oceanographyand Hawaii Institute of Geophysics,University of Hawaii, Honolulu

The flow of water from the western Pacific to the eastern Indian Ocean through the Indonesian

archipelago is governed by a strong pressure gradient. Dynamic height computations determine the

average sea level difference as 16 cm and show that most of the pressuregradient is contained in the

upper 200 m. Sea level data from Davao in the Philippines and from Darwin in Australia are used to

determine he annual signal and the interannual variations of the pressuregradient for the years 1966 to

1985. The annual signal has a maximum during the southeast monsoon in July and August and a

minimum in January and February. Interannual variations are not related to the Southern Oscillation

becausesea level is low at both stations during E1 Nifio events,and thus there is little influence on the

sea evel dift•rence. The mechanismof the through flow is discussed, ut a determination of its numerical

value will have to await direct measurements.A comparison of the sea evel difference with results rom a

numerical model by Kindle shows satisfactory agreement. t is concluded that the variability of the

through flow can be monitored by sea evel measurements.

INTRODUCTION

The prevailing trade winds over the tropical oceans cause

an increase of sea level on the western sides of the oceans and

a lowering on its easternsides.Consequently,a pressuregradi-

ent must exist between the western Pacific and the eastern

Indian Ocean across he Indonesian waters, as was recognized

by Wyrtki'[1961]. This pressuregradient should drive a flow

of water through the Indonesian archipelago, which connects

the two oceans.An estimate of the magnitude of this flow by

Wyrtki gave a low value of only 1.5 Sv and was based on the

assumption that the many narrow passages and channels

through which the water has to pass would seriously estrict

the flow. More recent estimates of the flow, which are summa-

rized by Gordon [1986], give substantially higher values for

this through flow. The neededdirect measurements f the flow

are planned for the forthcoming ndonesian SeasThroughflow

Experiment (INSTEP). These measurementsare designed o

provide a quantitative determination of the flow and of its

variation during 18 months and to relate the water transports

to measured sea level differences. The sea level observations in

turn will then allow us to monitor the flow in the future and

to draw conclusionsbased on existing data about fluctuations

of the through flow during the past 2 decades.

This study is an attempt to shed some light on the annual

and interannual variations of the through flow and on its

vertical structure. Observations of sea level will be used to

form and analyze time seriesof the pressuregradient between

the Pacific Ocean and the Indian Ocean. Observations of dy-

namic height will give the absolute value of the pressuregradi-

ent and give insight into its vertical structure.

SEA LEVEL OBSERVATIONS

In the western Pacific Ocean, long time series of sea level

exist at Davao and Jolo in the Philippines and at Guam and

Truk. Records start in 1948. The record from Jolo is interrup-

ted in 1959 and 1979 but otherwise resembles that of Davao

very well. The records of all four stations are dominated by

large drops of sea evel associatedwith E1 Nifio events.These

Copyright 1987 by the American GeophysicalUnion.

Paper number 7C0706.

0148-0227/87/007C-0706505.00

drops are slightly more pronounced at Guam and Truk than

at Davao. The annual cycle is weak and is often over-

shadowedby both the high- and the low-frequencyvariations.

Its amplitude is less than 6 cm at all stations. Sea level at

Davao risesslowly at a rate of 6.4 mm per year. This trend has

been removed from the record shown in Figure 1. The other

three stations do not show a trend significantlydifferent from

zero. The detrended sea evel record at Davao is used o repre-

sent the pressure head for the through flow in the western

Pacific Ocean.

Along the northwestern coast of Australia, sea evel stations

exist at Dampier, Port Hedland, Broome, Wyndham, and

Darwin. The annual cycle and the low-frequency variations at

Port Hedland and Darwin agree very well. They also agree

with shorter and interrupted records at Dampier and Wynd-

ham but not with Broome, which probably has a record of

low quality. Sea level along this coast is dominated by the

annual cycle, which has an amplitude of about 10 cm. Super-

imposed are low-frequency signalsof similar amplitude, which

coincide with E1 Nifio events. No long-term trend is obvious

in these short records. Because the record from Port Hedland

has several short gaps and because it is further away from

Indonesia, the time series rom Darwin is used to represent sea

level along this coast, which is considered representative for

the lower side of the pressure gradient between the two

oceans. Unfortunately, no sea level records have been taken in

Indonesia since 1940, but a 7-year-long record is available

from Cilacap (previously Tjilatjap) at the south coast of Java

for the years 1925 to 1931.

The sea level record at Davao in the Philippines with the

linear trend removed and the record at Darwin, Australia, are

now used to determine the pressure difference between the

Pacific Ocean and the Indian Ocean across the Indonesian

waters relative to an unknown mean difference. The corre-

sponding time seriesshown in Figure 1 exhibit a large annual

signal and interannual fluctuations. The annual signal is very

regular and has a mean amplitude of 15 cm. The interannual

variations have an amplitude of about 5 cm, but they are no

longer obviously related to El Nifio events as are the records

of the two stations forming the difference. Maximum devi-

ations of the sea level difference from its mean value are q-28

cm.

12,941

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12,942 WYRTKI'NDONESIANHROUGHLOW NDPRESSURERADIENT

-10

-20

66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 30

i i i i i , i , , , i i i i , i ,

DAVAO

-2O

-10

-2O

30

20

10

o

-10

-20

66 67 68 69 70 71 72 73 7• 75 76 77 78 79 80 81 82 83 8• 85 30

i i • , , , , [ , , i , . , i , ,

66 67 68 69 70 71 72 73 7't 75

_ ....

DAVA

-30 66 ' 67 ' 68 ' 69 ' 70 ' 71 ' 72 ' 73 ' 7• ' 75

-10

-2O

76 77 78 79 80 81 82 83 8• 85

......... 30

20

10

-- -•- .-- •-- * ..... 0

-10

-20

-30

76 77 78 79 80 81 82 83 8• 85

Fig.1. Seaevel tDavao,hilippines,ndDarwin,ustralia,ndhe eaevel ifferenceetweenavaondDarwin

duringheperiod 966o 1985n centimeters.he hincurve iveshemonthly eans'heheavyurve iveshe

12-month unningmean.A linear rendhasbeen emovedrom hedataat Davao.

DYNAMIC HEIGHT DIFFERENCES

A determination of the absolutevalue of the mean sea evel

differences possibley theuseof dynamicopographies.he

circulation n the westernPacific Ocean is dominated by the

Mindanao Current in which large parts of the North Equa-

torial Current are recirculated nto the North Equatorial

Countercurrent.Whereas the Mindanao Current and the as-

sociatedyclonic indanao ddyare permanenteatures f

the circulation n this region,during he period rom May to

October he SouthEquatorialCurrent lowsalong he coast

of New Guinea and suppliessome water to the counter-

current.The circulation n this region s such that high dy-

namic opographys locatedbetweenMindanaoand New

Guinea Wyrtki, 1974].Typicalvalues f dynamic eight ela-

tive to 1000 dbar are 180 dyn cm, and they change ittle

during the year.

In the regionbetweenndonesia nd Australia, irculation

is anticyclonicnd nvolveshe formation f the SouthEqua-

torial Current in the Indian Ocean. A ridge of high dynamic

topographytretchesrom hecenter f thesubtropicalyreof

the Indian Ocean toward the Timor Sea, as is shown in the

Indian Oceanatlas [Wyrtki, 1971]. Dynamic opographys

low along the periphery f the anticycloniclow, namely,

along he coasts f Australia nd Java.Lowest alues f dy-

namic opographyelative o 1000dbar are near 150dyn cm

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WYRTKI' INDONESIAN HROUGH FLOW AND PRESSURE RADIENT 12,943

and occur from July to October, when the southeastmonsoon

blows in full strength over this region. At this time the sea

level difference between the two oceans has a maximum of

about 30 cm.

It is difficult to determine the mean value or the annual

variation of dynamic height from the maps quoted above, and

therefore hydrographic data have been used to compute dy-

namic heights for two selectedareas thought to represent the

head and the bottom of the pressure gradient governing the

flow through the Indonesian waters. The data prepared by

Levitus [1982] have been used to compute dynamic heights

relative to 1000 dbar for 20 one-degreesquaresbetween Min-

dano and New Guinea for each of the four seasons for which

data are given. The results are shown in Figure 2 together

with the mean annual variation of sea level at Davao. The sea

level curve lies well within the scatter of dynamic height,

which exhibits a similar annual cycle. The high values of dy-

namic height near 190 dyn cm are all found north of New

Guinea and east of Halmahera. We will use sea level at Davao

relative to a mean dynamic height of 183 dyn cm as repre-

sentative or the pressurehead in the area where the through

flow originates.

Corresponding omputationsof dynamic height have been

made for the area south of Java, between 10øS and the coast

of Java and between 104øE and 120øE. The data of Levitus

[1982] give a mean dynamic height of 174 dyn cm and show

no values below 167 dyn cm for any 1ø square or season.This

is in stark contrast with the maps of dynamic height shown in

the Indian Ocean atlas [Wyrtki, 1971], where several maps

show values below 150 dyn cm. Consequently,we have used

the data from the Indian Ocean atlas and have computed

dynamic heights or all stations n the region south of Java.

The results are shown in Figure 3 and indicate that dynamic

height doesdrop below 150 dyn cm during the period July to

October, n particular n the easternpart of the area. The data

with low valuesof dynamic height come from Australian, Jap-

anese,and Soviet researchvessels, nd we have no explanation

why similar valuesdo not appear n the Levitusdata.

SEA LEVEL DIFFERENCES

The mean annual cyclesof sea evel at Cilacap and Darwin

are shown n Figure 3 togetherwith the dynamicheights ela-

190

180-

170

.•. .... DAVAO

/

ß-• -\

x•.,

I 2 3 4 5 6 ? 8 9 I0 II 12

TIME (months)

+10

-I0

Fig. 2. Monthly mean sea evel at Davao, Philippines, uring the

period 1966 to 1985 n centimeters right scale),and quarterly mean

and extremedynamicheights n the area betweenMindanao and New

Guinea in dynamic centimeters left scale).

190

180

170

160

150

140

130

1 2 3 4 5 6 7 8 9101112

+1o

o

-10

DARWIN

+

CILACAP +

1 2 3 4 5 6 7 8 9101112

Fig. 3. Monthly mean sea evel at Cilacap,Java, and at Darwin,

Australia, in centimeters right scale),and dynamic height at individ-

ual stations in the area south of Java in dynamic centimeters (left

scale).Dynamic heights are shown by dots west of 110øE and by

crosses east of 110øE.

tive to 1000 dbar in the area south of Java. Sea level is plotted

relative to a mean dynamic height of 168 dyn cm. It is appar-

ent that the annual signals of dynamic height and sea level

agreeand that the low valuesof dynamicheight n the second

part of the year are real. It is at that time of the year that the

sea level difference between the Pacific and Indian oceans has

a maximum. The low values of sea evel and of dynamic height

have two different causesof origin. During the southeastmon-

soon between June and October, low sea level and dynamic

height are located close o the coast of Java, where upwelling

exists [Wyrtki, 1962]. During the period from December to

April, low dynamicheight s found further offshoreand relates

to the Java Coastal Current [Soeriaatmadja, 1957], but sea

level along the coast is comparatively high. In view of the

general agreementbetween the annual cyclesof sea level at

Cilacap and Darwin and of dynamic height in the region

south of Java, we consider sea evel at Darwin to represent the

pressure t the bottom of the sea evel gradient rom the Pacif-

ic to the Indian Ocean. It would, of course, be better to have

data from a sea level station at the south coast of Java.

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12,944 WYRTKI' NDONESIANHROUGHLOWANDPRESSURERADIENT

The annual cyclesof sea evel at Davao and Darwin are out

of phase,as can be seen rom Figures 2 and 3, and conse-

quently he sea eveldifference etween he two locations x-

hibits a very large amplitude. The annual cyclesat the two

locations are also rather regular (Figure 1), and therefore he

differenceof sea evel has a very regular annual cycle,which is

due to the regularityof the monsoons. he existence f this

strongand regularannual cycle n the pressure radientbe-

tween the two oceans indicates that the Indonesian through

flow will be governedby a large annual cyclehaving a maxi-

mum during July and Augustand a minimum during January

and February.

The absolute value of the sea level difference between the

western Pacific and the eastern Indian Ocean can be deter-

mined from the differenceof dynamic height between the two

areas Figure 4). This pressure ifference ecreasesrom the

sea surface to about 500 m, where it reaches a minimum.

Below the minimuma weak pressure ifferencerom the Pacif-

ic to the Indian Ocean is again presentat the 1000-m level,

indicating a deeper flow of water from the Pacific to the

Indian Ocean.The mean pressure ifference t the sea surface

relative to the minimum near 500 m is 16.3 dyn cm. Strong

gradients re concentratedn a rather thin layer of only 250

m. From February to April the pressure radienthas a mini-

mum of about 9 dyn cm, whereasduring August o October t

is as arge as 23 dyn cm. This annualvariation s concentrated

in the uppermost 150 m.

MEAN ANNUAL VARIATION

The mean annual variation of the through flow can now be

assessed y using he sea evel record from Davao to represent

the pressurehead in the western Pacific and the sea level

record from Darwin to represent he pressure n the eastern

Indian Ocean and plotting them relative to a mean sea level

difference of 16.3 cm (Figure 5). Accordingly, the sea level

gradient governing he through low varies rom about zero

during Januaryand February to a maximumof about 33 cm

during July and August.

Monthly meansof sea evel at Davao and Darwin do not

correlate at all, despitean inversemean annual cycle,but the

sea level difference between the two stations is correlated

better with Darwin (--0.80) than with Davao (+ 0.60), indicat-

ing that the sea evel differences governed hieflyby the sea

level in the eastern Indian Ocean. In contrast, the 12-month

24

DYNAMIC HEIGHT DIFFERENCE

20 16 12 8 4 0

•• 200•

'•400 Ld

1-600

-800 •

I t I t ', ', I000

MEAN

FMA

ASO

Fig. 4. Difference f dynamicheightbetweenhe area southof

Mindanao and the area south of Java from the sea surface to 1000 m

in dynamic entimeters.he solidheavycurvegives he mean; he

short-dashedine gives he averageduring February,March, and

April; and the long-dashedine gives he averageduring August,

September, nd October.

+8

+4

-4

-8

1 2 3 4

DAVAO

DARWIN

5 6 7 8 9 10 11 12

,,

,,

i

i

ß

ß

ß

s

is

/

/

s

s

s

I I I I I I I I I I I

1 2 3 4 5 6 7 8 9 10 11 12

+8

+4

-4

-8

-12

Fig. 5. Monthly mean sea evel at Davao and Darwin plotted

relative o the mean pressure ifference f 16.3 dyn cm, representing

the annual variation of the pressure radient rom the Pacific to the

Indian Ocean.

running means of sea level at the two stationscorrelate at

+0.70, indicating that low-frequencysignals are coherent

across he Indonesian waters, he importanceof which will be

discussed later.

THROUGH FLOW MECHANISM

The mechanism for the transfer of water from the Pacific to

the Indian Ocean has been outlined by Wyrtki [1961]. It is

linked to the monsoonsand to the developmentof upwelling

in the Banda Sea and along the south coast of Java. During

the north monsoon from November to March, surface water

flows both from the Java Sea and from the Pacific Ocean into

the BandaSea,causing n accumulation f warm, ow-salinity

water and a depression f the thermocline. he intertropical

convergenceies southof Javaand stretchesoward northern

Australia. Winds are not favorable for a removal of the water

from the Banda Sea into the Indian Ocean, and consequently

water accumulates in the Banda Sea and depresses he ther-

mocline.The pressure radient rom the Pacific o the Indian

Ocean s alsoweak (Figure4), and thus he through low from

the Pacific to the Indian Ocean is rather weak during this

season.

During the southeastmonsoon rom May to September,

strongsoutheastwindsblow over the entire region,and the

South Equatorial Current of the Indian Ocean ormsbetween

Java and Australia. Much of its water is apparently supplied

from the Banda Sea, where strong upwelling occurs.Within

the Indonesian waters, flow patterns seem to be more com-

plex.There s a strongsouthwardlow n the MacassarStrait,

as measured on an anchor station during the Snellius Ex-

pedition [Lek, 1938]. The flow is concentratedn the upper

200 m and reaches speedof 84 cm s-x at a depth of 50 m

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WYRTKI' INDONESIAN HROUGH FLOW AND PRESSURE RADIENT 12,945

I-Wyrtki, 1961]. Water is also flowing into the Banda Sea

between Halmahera and New Guinea as an extension of the

New Guinea Coastal Current, which flows stronglywestward

during this season. Some of this water seems o return to the

Pacific between Halmahera and Celebes. Some water leaves

the Banda Sea to the west and enters the Java Sea, but it is

most difficult to give a water balance of the area without the

help of extensive urrentmeasurements.ea evel during the

southeastmonsoon season s high on the Pacific side of the

through flow and very low at Darwin and in the Indian Ocean

south of Java. The sea level difference has a maximum of

about 30 cm during this season, nd a very strong hrough

flow can be expected.The lowering of sea level is directly

related to the strong southeast winds and to the formation of

upwellingand of the South Equatorial Current along the

south coast of Java.

INTERANNUAL VARIATIONS

Interannual variations of the sea level difference across the

Indonesian waters are apparent from Figure 1. The low-

frequency variations of sea level at Davao and Darwin corre-

late and show low sea level at both stations during E1 Nifio

events. This is most pronounced in 1969, 1972-1973 and

1982-1983 but also can be seen in 1976 and 1980. In contrast,

the sea level difference does not show a pattern related to E1

Nifio. This phenomenon can be explained by the behavior of

the wind field over the Indonesian region. According to Bar-

nett [1983] the convergenceof surface winds over Indonesia is

subject to strong interannual variations in its intensity and

location becauseof the coupling of the trade winds over the

Pacific with the monsoons over the Indian Ocean. The prin-

cipal time and space scalesof this coupling lead either to an

intensification or to a weakening of the convergenceof winds.

During strong convergence of the wind field, which is associ-

ated with a high state of the Southern Oscillation, sea level is

raised both in the western Pacific and in the eastern Indian

Ocean nd s high n the •donesian aters. uringE1Nifo,

winds are divergent over Indonesia, and sea evel drops within

as well as on both sidesof the Indonesian archipelago.These

characteristics f the wind field imply that the differenceof sea

level between the western Pacific and the eastern Indian

Ocean is only weakly affected by the principal wind patterns

associated with E1 Nifo and the Southern Oscillation, and

this is reflected in the sea level record.

The slow interannual variations of the sea level difference

between Davao and Darwin seen in Figure 1 are probably

related to fluctuations of the wind field which are not associ-

ated with the Southern Oscillation, or they may be due to

uncertainties in the sea level records themselves.

COMPARISON WITH MODEL RESULTS

A global numerical model has recently been used by J. C.

Kindle et al. (manuscript n preparation, 1987) to compute

variations of the Indonesian through flow for the period 1977

to 1984. The computations show the development of a very

strong western boundary current which separates from the

Mindanao Current, flows through the Celebes Sea and the

Macassar Strait, continues through the Flores Sea and turns

around Timor into the Indian Ocean. The model givesa mean

annual cycle or the through flow with a maximum in August

and a minimum in February in agreement with the sea level

data. The interannual variations computed from this wind-

driven model for the years 1977 to 1984 are shown n Figure

6. They compare very well with the sea level difference Davao

minus Darwin. The high maximum of through flow in July

1979 and the low minima in January 1977 and January 1980

are well represented, s is the small maximum of through flow

in July 1981.Not properly represented re the high maxima of

through flow in July 1977 and July 1982. The steady ncrease

of the minimum of through low from 1980 to 1983 s appar-

ent in both the model and the sea level differences. The dis-

agreement between the low pass-filtered curves of the sea evel

differenceand of the model shouldnot be surprising n view of

the relative crudeness f the model, of the winds driving it,

and of the possibilityof trends n the sea evel data.

DISCUSSION AND CONCLUSIONS

Dynamic height differencesbetween the areas south of Min-

danao and south of Java show that a strong pressuregradient

exists from the Pacific to the Indian Ocean. This pressure

gradient is concentrated n the upper 200 m. The mean annual

variations of the pressure gradient are reflected in the differ-

ence of sea level between Davao in the Philippines and

Darwin, Australia. The mean annual cycle and the interannual

variations of the sea evel differenceagree well with the results

of a wind-driven numerical model, which gives confidence n

both the validity of the data and of the model. This agreement

indicates hat the wind-driven changes n the structure of mass

in the two adjoining oceans govern the variability of the

through flow. The agreement also implies that the low-

frequency ariability of the through low can be monitoredby

means of sea level observations. Whereas sea level observa-

tions can monitor the variability of the through flow, its mag-

nitude will have to be measureddirectly by means of current

30 77 78 79 80 81 82 83 84

20

_

-I0

-20

cm

14

12

I0

.........

Fig. 6. Variation of the sea level difference (in centimeters) be-

tween Davao and Darwin from 1977 to 1984 and (bottom) transports

(in sverdrups) hrough the Indonesian waters from a numerical model

by Kindle et al. (manuscript n preparation, 1987).

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12,946 WYRTKI: NDONESIANHROUGH LOWAND PRESSURERADIENT

meter moorings, nd suchmeasurementsre planned n the

forthcoming INSTEP project.

The details of the sea evel gradient within the Indonesian

waters will have to be determinedby a network of sea level

gauges, hich s plannedor NSTEP. Data from hissea evel

network will also contribute mportant information to the

studyof the dynamics f the flow n this complex egionand

in particular o thedynamics f cross-equatoriallow.

Acknowledgments.upport or this researchwasprovidedby the

National ScienceFoundationunder grant NSF OCE85-15404.This

support s gratefully cknowledged.also ike to thank Gary Mit-

chum and Roger Lukas or valuable omments. awaii Instituteof

Geophysics ontribution 1917.

REFERENCES

Barnett,T. P., Interactionof the monsoon nd Pacific rade wind

system t interannual ime scales,, The equatorial one, Mon.

Weather Rev., 111,756-773, 1983.

Gordon, A. L., Interocean xchange f thermoclinewater,J. Geophys.

Res., 91, 5037-5046, 1986.

Lek, L., Die Ergebnisse er Strom- und Serienmessungen,nellius

Exped.East. Part Neth. East ndies1929-30,2, 169 pp., 1938.

Levitus,S., Climatological tlasof the world ocean,NOAA Prof. Pap.,

13, 173 pp., 1982.

Soeriaatmadja,R. E., The coastalcurrent south of Java, Mar. Res.

Indonesia, 3, 41-55, 1957.

Wyrtki, K., Physical ceanographyf the southeast sianwaters, ol.

2, NAGA report, 195 pp., Univ. of Calif., San Diego, 1961.

Wyrtki, K., The upwellingn the regionbetween ava and Australia

during the south-eastmonsoon,Aust.J. Mar. FreshwaterRes.,13,

217-225, 1962.

Wyrtki, K., Oceanographic tlas of the International ndian Ocean

Expedition, 31 pp., National Science oundation,Washington,D.

C., 1971.

Wyrtki, K., The dynamic opographyof the Pacific Ocean and its

fluctuations,Ref. HIG 74-5, 19 pp., Univ. of Hawaii, Honolulu,

1974.

K. Wyrtki, Departmentof Oceanography, ivision of Natural Sci-

ences,University of Hawaii at Manoa, 1000 Pope Road, Honolulu,

HI 96822.

(ReceivedMay 11, 1987;

acceptedAugust 6, 1987.)