Tides in the Indonesian Seas

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or collective redistirbutionor collective of this by of any article portion Th THE INDONESIAN SEAS published in been has is article

A BRIEF OVERVIEW OF Oceanography , Volume 4, a quarterly journal of Th 18, Number

Tides in the or other means reposting, photocopy machine, is only w permitted Indonesian Seas

BY RICHARD D. RAY, GARY D. EGBERT, AND SVETLANA Y. EROFEEVA Society. Copyright e Oceanography 2005 by Th

Tidal phenomena in the Indonesian Gross characteristics of the Indone- portance of tidally generated residual

seas are among the most complex in the sian tides were worked out during the circulation, especially east of Sulawesi, ith the approval of Th world. Complicated coastal geometries colonial period in the early twentieth for horizontal mixing. with narrow straits and myriad small is- century. Summary descriptions of coast- Ffi eld and Gordon (1996) and Hata- lands, rugged bottom topography next to al tides were published by both Krüm- yama (2004) noted the important role Permission reserved. Society. All rights is e Oceanography gran wide shelves of shallow water, and large mel and Van der Stok in 1911, and tidal of tides in vertical mixing, a fact that all correspondence to: Society. Send e Oceanography [email protected] or Th quantities of tidal power input from the phase charts were published by Dietrich calls for development of high-resolution, adjoining Indian and Pacifi c Oceans—all during the war in 1944. Given the dif- three-dimensional tidal models in order combine to form a complex system of fi culties of collecting in situ data, it is to investigate the sources and variability interfering three-dimensional waves. worth noting that those early measure- of internal tides. Some initial steps in The seas feature multiple amphidromes ments still form the bulk of the avail- that direction are presented in this issue (points in the sea where there is zero able station data in the International by Robertson and Ffi eld. See also related tidal amplitude due to canceling of tidal Hydrographic Bureau’s tidal archives. work by Schiller (2004). waves), strong tidal currents, residual Much of that early work was subse- The charts we show here are based on circulations, internal waves, and solitons. quently recompiled and synthesized by the data-assimilation work of Egbert and ted in teaching to copy thisand research. for use Repu article Diurnal tides are unusually strong and Wyrtki (1961), who constructed diurnal Erofeeva (2002). Data assimilation is es- are dominant along some coastlines. and semidiurnal cotidal charts based pecially attractive in regions such as the e Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. 1931, Rockville, Box PO Society, e Oceanography The tides are known to affect local mix- on all available coastal and island infor- Indonesian Archipelago where numeri- ing and circulation, but the tidal energy mation. Owing to lack of data, Wyrtki available for these processes is not yet re- could draw no contours in the adjoining Richard D. Ray ([email protected]) is liably determined. And while mapping of Indian and Pacifi c waters, but his (sub- Geophysicist, Space Geodesy Laboratory, the Indonesian tides has benefi ted mark- jective) interpolations over the region of NASA Goddard Space Flight Center, Green- edly by assimilating satellite altimeter enclosed seas were—and still are—emi- belt, MD, USA. Gary D. Egbert is Professor, measurements into numerical models, nently reasonable. College of Oceanic & Atmospheric Sciences, improvements to the energy budget will More recently, Hatayama et al. (1996) Oregon State University, Corvallis, OR, USA. blication, systemmatic reproduction, likely require higher-resolution analyses computed two major diurnal and semi- Svetlana Y. Erofeeva is Research Associate, and a densifi ed network of satellite tidal diurnal tides with a numerical baro- College of Oceanic & Atmospheric Sciences, measurements. tropic model; they emphasize the im- Oregon State University, Corvallis, OR, USA.

74 Oceanography Vol. 18, No. 4, Dec. 2005 Figure 1. Topographic chart, 1BDJmD with isobaths drawn at 100, 200, 4PVUI$IJOB 4VMV 1000, 2000, 4000, 6000 m. Red 0DFBO lines denote the groundtracks of the Topex/Poseidon satellite. Although these tracks are too widely spaced to support a pure- $FMFCFT ly empirical mapping of the tides in this region, the satellite data provide critically valuable data for assimilation. , #PS O F P B .PMVDDBD S S J N B T B T U B B L B

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cal modeling efforts are hindered by in- inated by the large tide from the Indian the tide from the Indian Ocean, with adequate knowledge of bathymetry and Ocean, where amplitudes are well over increased amplitudes in the Flores Sea stratifi cation and by uncertainties con- a meter off northwest Australia. This and with markedly increased amplitudes cerning dissipation. Data assimilation wave is delayed slightly (about 2 hours) westward in the Java Sea. The diurnal can compensate for some of these mod- as it passes into the Banda and Flores tide propagates westward around both eling diffi culties. Our charts are based on Seas. Those seas are suffi ciently deep that the northern and southern coasts of Bor- ten years of sea-level measurements from high tide occurs almost simultaneously neo, intersecting to form a complicated the Topex/Poseidon satellite altimeter. throughout both basins. From the Banda system of large-amplitude, nearly amp- The altimeter data are used to compute Sea, the semidiurnal tide passes slowly hidromic systems west of Borneo. linearized inverse adjustments to a prior, northwards through the Molucca Sea From Figures 2 and 3 it is evident that multi-constituent, nonlinear, barotropic, region. From the Flores, it propagates the tides throughout the eastern archi- time-stepping model (for details, see slowly northwards into Makassar Strait pelago, as well as in the adjoining Pacifi c, Egbert and Erofeeva, 2002). The satellite and also westwards, but more weakly, are predominantly semidiurnal but with track pattern is shown in Figure 1. across the Java Sea. signifi cant diurnal inequality, whereas Cotidal charts of the largest semidi- The diurnal tide, in contrast to the the tides west of about 118°E are mixed urnal tide M2 and diurnal tide K1 are semidiurnal, passes southwards through diurnal, and west of Borneo almost shown in Figures 2 and 3, respectively. Makassar Strait and Molucca Sea. In the wholly diurnal. The large M2 amplitude The semidiurnal response is clearly dom- Banda and Flores Seas this wave meets on the coast of west-northwest Borneo is

Oceanography Vol. 18, No. 4, Dec. 2005 75 a localized shelf resonance, but not well most part, with shallow water giving Timor and in the Molucca Sea and its determined because it falls almost per- rise to rapid currents. A few exceptions boundaries. These are regions of intense fectly between two satellite tracks. where relatively strong currents appear barotropic energy ﬂ uxes, as can be seen

The M2 tidal current velocities, shown in deeper water are along the southern explicitly in Figure 5. Figure 6 shows in Figure 4, reﬂ ect bathymetry for the and northern coasts of the island of similar ﬂ uxes for K1. These diagrams re-

Figure 2. Amplitude (top) and Green-

wich phase lags (bottom) of the M2 tide, based on assimilation of ten years of Topex/Poseidon satellite altimetry into a nonlinear hydrodynamic model (Egbert and Erofeeva, 2002). Phase-lag contour interval is 30°, equivalent to . 1 lunar hour. Inverse theoretic error estimates suggest that these charts are accurate to a few centimeters, except in a few high-amplitude locations that fall between satellite tracks (e.g., the large tides on the west coast of Borneo and oﬀ Australia).

76 Oceanography Vol. 18, No. 4, Dec. 2005 inforce the notion that the Indian Ocean What becomes of all this tidal energy the estimated tidal dissipation in the tide governs the regional energetics of in the Indonesian seas is a topic of some Indonesian region was the most poorly

M2, and the Paciﬁ c governs K1. The baro- importance, but dissipation estimates are determined of any region in the world tropic M2 ﬂ uxes either side of Timor are not well determined. In fact, in the global oceans, with estimates for M2 ranging very large, exceeding 500 kW m-1. compilation by Egbert and Ray (2001), from 20 GW to 250 GW. A somewhat de-

Figure 3. Amplitude (top) and Green- wich phase lags (bottom) of the diur-

nal K1 tide. Phase-lag contour interval is 30°, equivalent to 2 sidereal hours. Th ese diurnal tides are unusually large, and they dominate the semidiurnal tides in some regions, such as the Java , Sea and much of the South China Sea (extending also into the Gulf of Th ai- land, just oﬀ the chart).

Oceanography Vol. 18, No. 4, Dec. 2005 77 Figure 4. Maximum barotropic current velocity of M tide 2 (equivalent to length of semi- major axes of tidal current el- lipses). Note nonlinear color bar. Units are cm s-1. In general, these velocities reﬂ ect bathymetry, with shallow water giving rise to rapid currents. Currents in the Banda, Timor, and part of the Flores Seas tend to rotate anti- clockwise; currents elsewhere are generally close to rectilinear, although the deep Paciﬁ c currents rotate clockwise.

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fendable upper bound is 150 GW, which budget for the energetics is of consider- REFERENCES is about 6 percent of the global total. able importance to understanding the Egbert, G.D., and S.Y. Erofeeva. 2002. Effi cient inverse modeling of barotropic ocean tides. Journal of At- The uncertainty is a refl ection of several effects of tides on circulation and mix- mospheric and Oceanic Technology 19:183-204. complications: the large energy fl uxes ing in the region. To our minds, the Egbert, G.D., and R.D. Ray. 2001. Estimates of M2 tidal energy dissipation from Topex/Poseidon altimeter entering the region through some very most fruitful approach to such problems data. Journal of Geophysical Research 106:22,475- narrow channels, limited data constraints will likely be through development of 22,502. from relatively widely spaced satellite generalized-inverse, three-dimensional Ffi eld, A., and A.L. Gordon. 1996. Tidal mixing sig- natures in the Indonesian seas. Journal of Physical tracks, and uncertainties in bathymetry tidal models, which can provide a frame- Oceanography 26:1,924-1,937. and implied currents; the latter must be work for exploring errors in dynamical Hatayama, T. 2004. Transformation of the Indonesian well determined to compute the balance assumptions and in individual terms of throughfl ow water by vertical mixing and its rela- tion to tidally generated internal waves. Journal of between energy fl ux divergence and the an energy balance. Such work is in its Oceanography 60:569-585. direct rates of working by the astronomi- infancy. These problems also call for ad- Hatayama, T., T. Awaji, and K. Akitomo. 1996. Tidal currents in the Indonesian seas and their effect on cal tidal forces and self-attraction forces. ditional high-quality tidal measurements transport and mixing. Journal of Geophysical Re- How much of the barotropic dissipa- over a much denser network. The recent search 101:12,353-12,373. tion is attributable to frictional stresses shift of the Topex/Poseidon satellite to fl y Schiller, A. 2004. Effects of explicit tidal forcing in an OGCM on the water-mass structure and circula- in bottom boundary layers and how tracks interleaving those shown in Fig- tion in the Indonesian throughfl ow region. Ocean much to conversion of energy into inter- ure 1 is a valuable step toward improved Modeling 6:31-49. Wyrtki, K. 1961. Physical Oceanography of the South- nal waves and turbulence is even more data coverage. east Asian Waters. Naga Report 2. Scripps Institu- of a mystery. Yet, developing a reliable tion of Oceanography, La Jolla, California, 195 pp.

78 Oceanography Vol. 18, No. 4, Dec. 2005 Figure 5. Mean barotropic energy fl ux vectors for the M tide. Fluxes smaller than . 2 20 kW m-1 are not drawn. Th e very large integrated fl ux from the Indian Ocean and the relatively small fl ux into the Pacifi c suggests, at fi rst glance, large tidal dissipa- L8N tion throughout the region, but part of the incoming energy works against Earth’s body tide (Egbert and Ray, 2001, Plate 1). An accurate accounting of the local tidal energy budget remains an outstanding issue, with published estimates varying widely.

Figure 6. Mean barotropic

energy fl ux vectors for the K1 tide. Fluxes smaller than 20 kW m-1 are not drawn. Note scale diff erences between Figures 5 and 6. Un- like the semidiurnal tide, the diurnal tide in the Indonesian seas is powered primarily L8N by energy from the Pacifi c Ocean.

Oceanography Vol. 18, No. 4, Dec. 2005 79