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  • View in gallery

    Spatial distribution of (a) temperature and (b) salinity profiles (asterisk) used for this study.

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    Annual mean dynamic height (dyn cm) and geostrophic flow at the sea surface relative to 1200 dbar. Light shading denotes the region where water depth is shallower than 100 m.

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    As in Fig. 2 but at 100, 200, 400, and 600 m.

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    Annual mean (a) geostrophic velocity (cm s−1) and (b) salinity (psu) against depth (m) along 105°E. Positive values indicate eastward flow.

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    Annual mean (a) geostrophic velocity (cm s−1) and (b) alongshore pressure gradient against depth (m) at 25°S. Here, the alongshore pressure gradient is measured as dynamic height difference (dyn cm) between 23.25° and 26.75°S along the western Australian coast.

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    Wind stress (10−1 Pa) and its curl (10−8 Pa m−1) from Hellerman and Rosenstein (1983) in (a) Aug and (b) Feb. Light shading denotes the region where water depth is shallower than 100 m.

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    Deviation from annual mean dynamic height (dyn cm) and geostrophic flow (cm s−1) at the sea surface in (a) Aug and (b) Feb.

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    Deviation from annual mean dynamic height (dyn cm) along 110°E in (a) Aug and (b) Feb.

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    Seasonal variation of zonal velocity (cm s−1) at (a) 9°S, 110°E and (b) 12°S, 110°E. Annual mean values have been subtracted before plotting.

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    As in Fig. 9 but at 8°S, 105°E.

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    Amplitude (cm) of (a) annual and (b) semiannual harmonics of the T/P data. The phase (shaded) corresponds to the day of the year when sea surface height is maximum.

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    As in Fig. 11 but for dynamic height at 0, 100, 200, and 400 m.

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    Seasonal variation of dynamic height (dyn cm) at the sea surface along (a) 12.5° and (b) 15°S. Annual mean values have been subtracted before plotting.

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    Amplitude of annual and semiannual harmonics of dynamic height along 100° and 105°E.

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    Seasonal variation of dynamic height (heavy solid) and its annual (light solid) and semiannual (light dotted) components at 9°S, 110°E off the coast of Java. The unit is dyn cm.

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Seasonal Characteristics of Circulation in the Southeastern Tropical Indian Ocean

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  • 1 International Pacific Research Center, SOEST, University of Hawaii, Honolulu, Hawaii
  • 2 Division of Marine Research, CSIRO, Hobart, Tasmania, Australia
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Abstract

The circulation in the southeastern tropical Indian Ocean is studied using historical temperature and salinity data. A southward shift of the subtropical gyre at increasing depth dominates the structure of the annual mean circulation. Near the southern Indonesian coast the westward South Equatorial Current (SEC) is at the sea surface and strongest near 10°–11°S, reflecting strong influence of the Indonesian Throughflow (ITF). In latitudes 13°–25°S the SEC is a subsurface flow and its velocity core deepens toward the south, falling below 500 m at 25°S. The eastern gyral current (EGC) is a surface flow overlying the SEC, associated with the meridional gradients of near-surface temperature and salinity. The ITF supplies water to the SEC mainly in the upper 400 m, and below that depth the flow is reversed along the coast of Sumatra and Java. Monsoon winds strongly force the annual variation in circulation. Dynamic height at the sea surface has a maximum amplitude at 10°–13°S, and the maximum at deeper levels is located farther south. Annual variation is also strong in the coastal waveguides, but is mainly confined to the near-surface layer. Although the South Java Current at the sea surface is not well resolved in the present dataset, semiannual variation is markedly evident at depth and tends to extend much deeper than the annual variation along the coast of Sumatra and Java.

Corresponding author address: Dr. Tangdong Qu, IPRC/SOEST, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822. tangdong@hawaii.edu

Abstract

The circulation in the southeastern tropical Indian Ocean is studied using historical temperature and salinity data. A southward shift of the subtropical gyre at increasing depth dominates the structure of the annual mean circulation. Near the southern Indonesian coast the westward South Equatorial Current (SEC) is at the sea surface and strongest near 10°–11°S, reflecting strong influence of the Indonesian Throughflow (ITF). In latitudes 13°–25°S the SEC is a subsurface flow and its velocity core deepens toward the south, falling below 500 m at 25°S. The eastern gyral current (EGC) is a surface flow overlying the SEC, associated with the meridional gradients of near-surface temperature and salinity. The ITF supplies water to the SEC mainly in the upper 400 m, and below that depth the flow is reversed along the coast of Sumatra and Java. Monsoon winds strongly force the annual variation in circulation. Dynamic height at the sea surface has a maximum amplitude at 10°–13°S, and the maximum at deeper levels is located farther south. Annual variation is also strong in the coastal waveguides, but is mainly confined to the near-surface layer. Although the South Java Current at the sea surface is not well resolved in the present dataset, semiannual variation is markedly evident at depth and tends to extend much deeper than the annual variation along the coast of Sumatra and Java.

Corresponding author address: Dr. Tangdong Qu, IPRC/SOEST, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822. tangdong@hawaii.edu

1. Introduction

The circulation in the southeastern tropical Indian Ocean (STIO) is important in the global climate system for several reasons. The region has a high heat content, transferred from the Pacific Ocean by the Indonesian Throughflow (ITF) and accumulated initially in the region between northwest Australia and Indonesia (Meyers et al. 1995; Sprintall et al. 2002). Large changes in the depth of the thermocline, regional upwelling, strong tidal mixing, and strong eddy currents permit a connection between subsurface dynamical processes that affect heat storage and sea surface temperature (SST) (Wyrtki 1961; Meyers 1996; Feng and Wijffels 2002). The energetic, dynamical variability occurs within and near the so-called Maritime Continent, the most convectively active zone of the tropical atmosphere, and appears to locally affect the SST and rainfall regime (Ashok et al. 2003). This study is intended to document the mean seasonal cycle of ocean circulation in the region as a background to future studies of ocean–atmosphere interaction at interannual time scales.

The Indian Ocean Dipole Mode (IODM; e.g., Saji et al. 1999; Rao et al. 2002) has a maximum of SST and dynamic height variability in the ITF region. However, the role that ocean dynamical processes play in controlling the mode has only begun to be investigated (Ansell 2002). Also, a further complex factor is that the regional thermocline is strongly forced by remote winds over wide areas in the equatorial Pacific and Indian Oceans (Wijffels and Meyers 2004) and this remote forcing effect also appears in the SST. The IODM has become a very active area of research, primarily using atmospheric data, and this needs to be backed up by oceanographic studies. This study represents the initial attempts to assemble the required oceanographic datasets.

The large-scale circulation in the STIO is dominated by the South Equatorial Current (SEC). As a direct consequence of basin-scale wind forcing combined with the Indonesian Throughflow (Godfrey and Golding 1981), the SEC flows westward between about 10° and 20°S, forming the boundary between Southern Hemisphere subtropical and tropical gyres (Wyrtki, 1961; Donguy and Meyers 1995). South of the SEC is the weak, broad eastward-flowing eastern gyral current (EGC), also referred to as the South Indian Ocean Current (Potemra 2001), which is formed by the surface fluxes that generate meridional, near-surface temperature and salinity gradients along the southern edge of the tropical Indian Ocean. Part of the EGC recirculates into the SEC, closing the near-surface anticyclonic circulation in the STIO, while the rest turns southward to feed the Leeuwin Current (LC) along the western Australian coast (e.g., Cresswell and Golding 1980).

The ITF enters the eastern Indian Ocean through a series of straits in the Indonesian seas. Its contribution to the SEC became an issue when Godfrey and Golding (1981) noted that the mean transport of the Indian Ocean SEC was substantially larger than the Sverdrup transport calculated from wind stress curl. Studies of ITF have arrived at transport estimates ranging from 1.7 to >20 Sv (Sv ≡ 106 m3 s−1; see a review by Godfrey 1996), while recent observations tend toward the middle of the range (Meyers et al. 1995; Meyers 1996; Gordon et al. 1999; Sprintall et al. 2002; Susanto and Gordon 2005).

The South Java Current (SJC) is an eastward flow with strong semiannual and intraseasonal variability near the coast of Sumatra and Java. The surface drifter data clearly demonstrated the existence of SJC with peak eastward flow in May–June and October–November (Quadfasel and Cresswell 1992; Bray et al. 1996) and a reversal in the other seasons when the SEC extends northward to the coast of Indonesia. Bottom mounted pressure gauges demonstrated strong intraseasonal variability propagating along the Indonesian archipelago from the equatorial region as far as Timor (Sprintall et al. 1999) and spreading into the internal Indonesian seas (Flores Sea, Banda Sea) (Wijffels and Meyers 2004). The reversal of SJC is also reflected in sea level data, with westward flow corresponding with a low water level at the coast and eastward flow with a high water level (e.g., Clarke and Liu 1993). In addition to the changing monsoon winds and the variations of freshwater flux, the seasonal variation of the SJC is also modulated by remote forcing from the central equatorial Indian Ocean as a result of eastward-propagating Kelvin waves generated in the central equatorial Indian Ocean during the transition seasons of monsoon (Wyrtki 1973; Clarke and Liu 1993, Qu et al. 1994; Potemra and Lukas 1999; Wijffels and Meyers 2004).

Another important contribution to the SEC comes from the upwelling off Java and Sumatra (Wyrtki 1962). The traditional view before Wyrtki was that, as in the other oceans, the formation of the SEC in the Indian Ocean should be accompanied by the development of an upwelling along its eastern boundary, that is, the northwest coast of Australia (e.g., Schott 1935). The presence of upwelling along the south coast of Java rather than the northwest coast of Australia has changed this traditional view. According to Wyrtki’s calculation, the upwelling south of Java may contribute as much as 2.4 Sv to the SEC.

Although these previous studies have provided a comprehensive description of the circulation in the STIO, they all focused on the surface layer. So far, our understanding of the subsurface circulation is mainly based on burst sampling by occasional research campaigns (e.g., Molcard et al. 1994; Fieux et al. 1996; Wijffels et al. 2002; Sprintall et al. 2002) and analysis of frequently repeated XBT lines (Wijffels and Meyers 2004). The only climatoligical interpretation of the subsurface circulation is the atlases recently prepared by Reid (2003), but his analysis failed to provide enough resolution for the seasonal variation of circulation in the STIO. To the best of our knowledge, a three-dimensional picture of the seasonal variation of circulation is still lacking, in spite of the fact that the STIO changes with season as a result of strongly monsoonal winds. Analyses of sea level observations from both tide gauge (e.g., Clarke and Liu 1993) and satellite altimeter (e.g., Potemra and Lukas 1999; Susanto et al. 2001) have clearly demonstrated this seasonal variation at the sea surface, but not much is known for the subsurface.

This study is intended to describe the vertical distribution of the circulation and its seasonal variation, using all existing temperature and salinity data. The results of this analysis are presented in the following sections. In section 2, we describe the data and methods of analysis. In section 3, we show a three-dimensional picture of mean circulation, and, in section 4, we describe the seasonal variation of circulation in the STIO. Results are summarized in section 5.

2. Data and method of analysis

Data used for this study are temperature and salinity profiles at observed levels recorded on the CD-ROMs of the World Ocean Database 2001 of the National Oceanic and Atmospheric Administration (NOAA)/National Environmental Satellite, Data, and Information Service (NESDIS)/National Oceanographic Data Center (NODC) for the region 0°–30°S, 90°–140°E. After all necessary editing processes (cf. Qu et al. 1999; Qu and Lindstrom 2002), these data consist of 47 420 profiles of temperature and 6473 profiles of salinity, spanning almost the whole period of the twentieth century (Fig. 1). To make full use of the temperature profiles, which were more than 7 times as many as salinity profiles, we processed the data in two steps. First, we made an initial estimate of mean temperature–salinity (TS) relationship using hydrographic data. Second, we combined the TS relationship with all temperature profiles to produce the climatology of circulation.

The individual temperature/salinity profiles were first interpolated onto a set of density surfaces with a 0.05 kg m−3 vertical resolution and then averaged in 0.5° × 0.5° boxes for each month regardless of the year of observation. Given that the hydrographic data coverage is relatively poor over the region studied (Fig. 1b), a 3-month running average was applied for each box. Then a spatial average was produced by combining boxes so that each average was based on at least five samples. This roughly formed a three-dimensional ellipse in the data-source region, with a fixed scan radius of 1.5 months in time. The scan radius was zonally stretched, with a radius in latitude only one-half of that in longitude, except along the western Australian coast where this ratio was reversed to better resolve the LC.

The gridded temperature and salinity fields were further smoothed using a two-dimensional Gaussian filter, with e-folding scales of 1.5° longitude and latitude and 1.5 month. Then, these processes were repeated for temperature profiles combined with the TS relationship created from hydrographic data. The smoothed temperature and salinity were finally interpolated back to a 10-dbar uniform pressure series and converted to dynamic height from which geostrophic velocity was determined.

While deep observations are available at some locations and seasons, the number of observations decreases rather rapidly with depth below 1200 dbar. To optimize uniformity of the dataset, the reference level for estimating geostrophic velocity was chosen at 1200 dbar or at the bottom of the ocean where water depth is shallower. Different reference levels have been chosen in literature, but mostly shallower than 500 dbar due to the lack of deep-sea observations (e.g., Meyers et al. 1995; Quadfasel et al. 1996). Reid (2003) recently presented a useful picture of middepth circulation using historical hydrographic data. According to his results, a typical dynamic height difference at depths around 1200 dbar across a distance of 500 km is 1 dyn cm in the STIO (his Figs. 5e,f), which can be translated into a velocity of 0.5 cm s−1 at a latitude of 15°S. This velocity, though important to transport estimates, does not seem to have a significant impact on the pattern of the upper-layer circulation in the STIO.

Gridded altimetric sea level data of the Ocean Topography Experiment (TOPEX)/Poseidon prepared by the Center for Space Research, University of Texas, were also used to provide an overview of the entire south Indian Ocean. The original data had a resolution of 1° latitude and longitude and 1 month in time and spanned from January 1993 through October 1999. They were averaged to construct the climatological monthly mean sea level. Data in regions where water depth is shallower than 1000 m are removed from this analysis to avoid tidal alias.

3. Long-term annual mean circulation

a. Surface flow

The geostrophic flow at the sea surface (Fig. 2) is consistent with previous studies (e.g., Meyers et al. 1995; Quadfasel et al. 1996). The dominant feature in the flow field is an anticyclonic circulation centered at 13°–15°S. The SEC at the sea surface is narrow, confined primarily to the north of 13°S, and extends to the continental slope of Java. In this regard, our result differs from the early work of Wyrtki (1961), who suggested that the SEC flows westward in a broader latitude band between 10° and 20°S, probably due to the inclusion of a large amount of new data in our study. Though water from the Pacific does enter the Indian Ocean through Lombok Strait (Murray and Arief 1988), the SEC at the sea surface seems to be mainly fed by the ITF from the Timor Sea. This flow path of the ITF shows remarkable agreement with the results from both recent observations (e.g., Cresswell et al. 1993; Meyers et al. 1995) and the earliest maps based on tracer patterns (Postma 1958).

On the southern side of the anticyclonic gyre, the EGC generated by the meridional gradients of sea surface temperature and salinity flows eastward in a broad latitude band from about 15° to 25°S. Part of the EGC turns southeastward to feed the LC as it approaches the western Australian coast. Unlike the other eastern boundary current regions, the surface flow (i.e., the LC) along the western Australian coast is poleward and against the surface wind stress (Cresswell and Golding 1980).

b. Subsurface flow

The most striking feature of the subsurface flow is related to the southward shift of the SEC at increasing depth. On a basin scale, the southward shift of the SEC is well represented in Reid’s (2003) figures. The present dataset provides better resolution of the phenomenon in the STIO. At 100 m, the high dynamic height on the southern side of the SEC moves to about 15°S (Fig. 3a). The SEC at this depth is chiefly supplied by water from the ITF and to some extent by water from the upwelling off the southern Indonesian coast. There are no significant supplies from the south. The EGC at this depth becomes less pronounced and can be seen only in a small region west of Australia.

At 200 m, the EGC has completely disappeared, and the circulation is dominantly westward (Fig. 3b). The westward flow appears to consist of two components. One is supplied by the ITF, and its axis remains almost unchanged from the shallower waters at 10°–12°S. The second component is mainly the westward flow associated with the subtropical gyre centered farther southwestward (Scott and McCreary 2001; Reid 2003). Both of these two components are referred to as the SEC in this study. We note, however, that under this definition the Indian Ocean SEC differs from those in the other oceans. Because of the additional contribution from the ITF (e.g., Fig. 3b), the transport of the Indian Ocean SEC is considerably larger that what can be accounted for by Sverdrup theory (Godfrey and Golding 1981).

At 400 m, the ITF becomes considerably weaker than that shown in the shallower waters (Fig. 3c) with only a narrow westward flow at 12°–13°S, and consequently the westward flow associated with the subtropical gyre is now the only significant component of the SEC. At 600 m, the ITF loses its signature as a narrow westward flow, leaving a broad area of almost no motion in the northern half of the region studied (Fig. 3d). The westward flow associated with the subtropical gyre continues to move southward, extending completely south of 18°S at 600 m (Fig. 3d) and south of 20°S below 800 m. This trend is particularly evident in the vertical section of zonal velocity (Fig. 4a).

Careful examination of the flow field indicates that there is a subsurface flow extending southeastward along the coast of Sumatra and Java (Figs. 3c,d). The subsurface flow has been observed in occasional synoptic hydrographic sections (Sprintall et al. 2002; Wijffels et al. 2002), and the present result suggests that it is a year-round phenomenon (Fig. 4a). Consonant with its equatorial supply, the subsurface flow is characterized by high salinity, forming a sharp contrast with the overlying ITF water (Fig. 4b). Off the southern coast of Java, the subsurface flow stands out as a high- salinity core (>34.80 psu) and can be traced continuously to the equator at about 100°E. The subsurface flow weakens as it flows eastward along the coast of Java, presumably due to the strong mixing with the ITF.

The LC resolved by the present dataset is weaker compared with earlier synoptic observations. At 25°S it is confined above 200 m, with a velocity about 2 cm s−1 near the sea surface (Fig. 5a). A poleward-directed alongshore pressure gradient is established by the intrusion of warm and freshwater from the Pacific, which drives the LC to flow against the surface wind stress and friction (Fig. 5b). The alongshore pressure gradient changes sign at depth, and as a consequence the northward-flowing Leeuwin Undercurrent is dominant in the subsurface flow (cf. Thompson 1987).

4. Seasonal variations

a. General characteristics

The circulation in the STIO is strongly forced by local and remote monsoon winds. The monsoon has its strongest signature near the coast of Sumatra and Java, blowing northwestward in the southern winter (June–August) and southeastward in the southern summer (December–February) (Fig. 6). The monsoon signature is relatively weak in the subtropics where southeasterly wind prevails all year round. In the southern winter (August), negative wind stress curl (or upward Ekman pumping) is confined roughly to the north of 15°S (Fig. 6a), while positive wind stress curl (or downward Ekman pumping) extends farther to the south. Ekman pumping and the westward propagation of Rossby waves (Masumoto and Meyers 1998) generate maximum strength of the anticyclonic circulation (Fig. 7a). The largest deviation from mean westward flow is seen near the coast of Sumatra and Java, and this contributes to a maximum transport in the SEC and ITF at this season and longitude (Meyers et al. 1995). In the southern summer (February), the negative wind stress curl (or upward Ekman pumping) moves farther southward and covers a large part of the region studied (Fig. 6b). In response, a cyclonic deviation from mean circulation occurs; its dynamic height at the sea surface is up to 6 dyn cm lower than its annual average (Fig. 7b).

Circulation at depth, though not presented here, has a similar annual cycle as discussed for the surface layer, suggesting that the influence of monsoon forcing extends below the sea surface. At 110°E, for example, the annual cycle of dynamic height in the upper thermocline is in some places even higher than at the sea surface (Fig. 8), reflecting the direct influence of surface heat flux. In the southern winter (August), positive variations are present at all depths from the sea surface down to 1000 m south of about 12°S, with a maximum exceeding 4 dyn at 60–80 m (Fig. 8a). Negative variations are narrowly confined to the coast of Java and occur at the sea surface, in contrast to farther south. In the southern summer (February), the situation is reversed, with positive variations near the coast of Java and negative variations south of about 12°S (Fig. 8b).

In addition to the annual cycle described above, circulation near the Indonesian coast also contains a strong signature of semiannual variation, but its relative importance to annual variation varies with depth. At 9°S, 110°E, for example, the westward flow at depth reaches its maximum strength in February/March and July/August and its minimum strength in May/June and October/November (Fig. 9a), while the variation at the sea surface is essentially annual. The semiannual variation at depth is apparently associated with the eastward propagation of equatorial Kelvin waves. This result is in contrast to farther south (say, at 12°S, 110°E), where the annual variation is a dominant signal for all depths from the sea surface down to 1000 m (Fig. 9b), with a maximum westward flow in August/September and a minimum in February/March. The depth dependence of relative importance between annual and semiannual variations is also evident around the western tip of Java at 8°S, 105°E, where the subsurface southeastward flow underlying the ITF occurs all year round (Fig. 10).

b. Harmonic analysis

To further document the mean seasonal cycle, we estimated the annual and semiannual harmonics of sea level from TOPEX/Poseidon (T/P) and dynamic height from the present climatological data. The T/P measurements provide an overview of the Indonesian region set in the entire south Indian Ocean (Fig. 11). The annual variation shows large amplitudes (>4 cm) across the basin in the latitude band roughly between 8° and 20°S (Fig. 11a) with its maximum exceeding 10 cm at about 11°S, 90°E, similar to earlier analyses (Potemra and Lukas 1999) and the pattern derived from the XBT observations (Masumoto and Meyers 1998). The phase of annual harmonic clearly demonstrates the westward propagation of Rossby waves in the STIO, generated south of Indonesia. Weak waves emanate from the eastern boundary in June/July and grow in response to Ekman pumping along Rossby wave characteristics to reach the location of maximum amplitude near 90°E in October/November (Fig. 11a; Masumoto and Meyers 1998). The semiannual variation has a maximum amplitude (>4 cm) near the coast of Sumatra (Fig. 11b), reflecting the influence of eastward-propagating equatorial Kelvin waves (Wyrtki 1973; Clarke and Liu 1993).

The amplitude of annual harmonic of dynamic height compares favorably at the sea surface with that from altimeter data. The maximum amplitude in dynamic height extending from about 10°S at 95°E to about 13°S at 105°E in the open ocean is located at nearly the same place as the maximum in altimeter sea level (Fig. 12a). The largest discrepancy in amplitude occurs in the continental shelf region off the southern Indonesian and western Australian coast, presumably due to the failure of the altimeter to accurately observe sea level near coastlines. The large amplitudes in dynamic height observed near the coast are consistent with earlier observations of sea level (e.g., Clarke and Liu 1993) and can be interpreted as a result of annually reversing monsoons. In the continental shelf region off the southern Indonesian coast, for example, surface water is forced to pile up against the coast in the southern summer when the northwest monsoon prevails. The summer is also a rainy season with an excess of precipitation over evaporation of about 150 mm month−1 near the coast of Java and Sumatra (Wyrtki 1961). The reduced salinity, together with the piling up of surface water forced by monsoon wind, may account for the dynamic height maximum at this season. The situation is reversed in the southern winter when monsoon wind is from the southeast and precipitation is toward its seasonal minimum in the region.

In an earlier analysis of XBT data, Masumoto and Meyers (1998) noticed that local Ekman pumping alone cannot account for the large annual amplitude in sea level near 10°S, 95°E. They explained the maximum as a forced Rossby wave generated by Ekman pumping along a Rossby wave characteristic. Consistent with their early study, a downwelling Rossby wave can be seen in surface dynamic height (Fig. 13). The Rossby wave has two components. One is narrowly confined to the coast of Australia and appears to originate from the equatorial Pacific as a free Rossby wave (Potemra 2001; Wijffels and Meyers 2004) reaching the latitudes of 12°–15°S in April/May. The signal from the equatorial Pacific dies out near 115°–120°E. From there a forced Rossby wave emanates in June/July, which propagates westward and approaches 105°E in August/September and 95°E in October/November.

The semiannual signal is relatively weak at the sea surface (Fig. 12b); its amplitude exceeds 1.5 cm only in a narrow region off the southern Indonesian coast. As altimeter data have indicated, the semiannual signal has a maximum amplitude along the west coast of Sumatra as a result of eastward propagation of equatorial Kelvin waves, but these waves appear to be damped as they reach eastern Java (∼115°E). Dispersal of the waves through passages between the Indonesian islands into the Flores and Banda Seas is noted in analysis of XBT data (Wijffels and Meyers 2004).

The annual signal varies rapidly with depth. At 100 m, its amplitude is dominated by the high values at about 13°S (Fig. 12c). The strong annual signal near the coast shown at the sea surface disappears at this depth and, as a consequence, the seasonal variation of dynamic height is mostly semiannual along the coast of Sumatra and Java (Fig. 12d). As we progress to the deeper levels, the maximum amplitude shifts slightly southward (Figs. 12e, g). Large amplitude is also seen in the southeastern corner extending from the continental shelf region off west Australia, probably associated with strong surface cooling in winter and the LC variability as well (Masumoto and Meyers 1998).

The maximum of semiannual amplitude continues to hug the Sumatra/Java coast in the deeper levels. At 200 m, the contour of 0.6 cm extends from the equator all the way to the West Timor Sea (Fig. 12f). Its maximum exceeds 1.2 dyn cm around the southern tip of Sumatra, while the amplitude of annual harmonic falls below 0.6 dyn cm in most parts of the continental shelf region off the southern Indonesian coast. This vertical structure was also observed along a repeated XBT line between Java and southwest Australia (Wijffels and Meyers 2004), and could be interpreted using McCreary’s (1984) equatorial ray theory, which suggests that inviscid, equatorially trapped Kelvin waves disperse eastward along vertical ray paths with slopes σ/N, where σ is the wave frequency and N is the Brunt–Väisälä frequency. As a consequence, semiannual energy dives more steeply than annual energy. At 400 m, the semiannual signal is still evident but somewhat weaker than it is in the shallower waters (Fig. 12h). A local maximum (>0.6 cm) is seen in the southwest corner. Its generation is not understood at this time.

Further inspection of vertical sections indicates that the large amplitude of semiannual harmonic extends down to 600 m along the coast of Sumatra and Java, while the large amplitude of annual harmonic does not have a strong signature at depths below 400 m (Fig. 14). Using McCreary’s (1984) theory to interpret this vertical structure implies that the annual signal is more locally forced than the semiannual signal (see Wijffels and Meyers 2004). At 100°E, the large semiannual amplitude (>2 dyn cm) near the sea surface extends from the equator to about 5°S (Fig. 14a). As this signal propagates southeastward along the Indonesian coast, it becomes narrower confined (Fig. 14b).

The phase both of annual and semiannual harmonics of dynamic height (Fig. 12) shows essentially the same pattern at the sea surface as that of altimeter sea level. The largest discrepancies occur in a narrow region off the southern coast of Java and Sumatra, where dynamic height reaches its annual maximum in February/March (Fig. 12a), while T/P sea level in June/July (Fig. 11a). Again, this result suggests that the altimeter cannot accurately observe sea level near coastlines. At 9°S, 110°E, for example, the annual variation of dynamic height at the sea surface has a maximum in February and a minimum in August (Fig. 15a), showing a remarkable agreement with tide-gauge sea level observations (Clarke and Liu 1993). The semiannual variation at the sea surface is weaker than the annual variation by a factor of about 3, but it becomes increasingly important with depth, being a dominant signal at 200 m and below (Figs. 15b–d).

5. Conclusions

Using all available historical and recent temperature and salinity data, this study provides a three-dimensional picture of circulation in the STIO. Although some of the results resemble the previous studies, the present dataset reveals the circulation in greater detail and thus permits a better interpretation of the circulation and its seasonal variation in the STIO. Most strikingly, we found that the SEC moves southward from about 10°–11°S at the sea surface to south of 20°S at 800 m. The eastward EGC is a surface flow above 200 m, supplying water to the westward SEC in the north and to the southward LC along the western Australian coast. Below the EGC, the flow is westward and can be identified as a component of the subtropical gyre at the subsurface.

The LC is barely resolved by the present dataset and consequently weaker compared with earlier synoptic observations. On the inshore side of the LC, there is a southward-directed alongshore pressure gradient, driving the LC to flow against the surface wind stress and friction. This alongshore pressure gradient changes sign at depth, and thus the flow underlying the LC is predominantly northward.

As a major contributor to the SEC, the ITF is confined primarily in the upper 400 m as a narrow westward flow centered at 10°–11°S. Below the ITF, there is a weak southeastward flow along the coast of Sumatra. This subsurface flow appears to originate from the equator and its high-salinity signature can be traced to the coast of Java. This result suggests that the subsurface flow previously reported by occasional synoptic observations along the coast of Sumatra/Java is a robust phenomenon. Circulation of the subtropical gyre is dominant below 400 m.

The circulation in the STIO tends toward its seasonal maximum in the southern winter and its seasonal minimum in the southern summer. Relative to its annual mean field, dynamic height in the southern winter has negative anomalies in the north and positive anomalies in the south, with its zero line at 12°–13°S, and the situation is just reversed in the southern summer.

Although the SJC is not well resolved by the present dataset, presumably as a result of smoothing, the semiannual signal is markedly evident at depths of the thermocline along the southern Indonesian coast. In particular, the southeastward subsurface flow off Java reaches its maximum strength twice a year: one in May–June and the other in October–November, reflecting strong influence of the equatorial Kelvin waves on the subsurface thermal structure of the region.

Annual variation is a dominant signal in most parts of the region studied. Its maximum amplitude occurs at 10°–13°S near the surface and shifts slightly to 13°–15°S at depths of the thermocline. Both the Ekman pumping and the Rossby wave propagation are believed to play an important role in generating this large annual variation. The annual variation also has a strong signature in the continental shelf region in response to the monsoonal winds and precipitation, but is essentially confined in the upper 100 m. Below that depth, the semiannual variation becomes a dominant signal. This perhaps reflects the fact that waves propagating from far away penetrate deeper than those locally generated.

Acknowledgments

This research was supported by the National Aeronautics and Space Administration through Grant NAG5-12756. Author TQ was also supported by the Frontier Research System for Global Change through its sponsorship of the International Pacific Research Center (IPRC), and GM was also supported by the Commonwealth Scientific and Industrial Research Organization (CSIRO). Both authors are grateful to J. Potemra and H. Mitsudera for valuable discussions. Author TQ is also grateful to J. Toole for a useful conversation on the methods of analysis and to T. Yamagata and E. Lindstrom for useful communication on the present topic.

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Fig. 1.
Fig. 1.

Spatial distribution of (a) temperature and (b) salinity profiles (asterisk) used for this study.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 2.
Fig. 2.

Annual mean dynamic height (dyn cm) and geostrophic flow at the sea surface relative to 1200 dbar. Light shading denotes the region where water depth is shallower than 100 m.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 3.
Fig. 3.

As in Fig. 2 but at 100, 200, 400, and 600 m.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 4.
Fig. 4.

Annual mean (a) geostrophic velocity (cm s−1) and (b) salinity (psu) against depth (m) along 105°E. Positive values indicate eastward flow.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 5.
Fig. 5.

Annual mean (a) geostrophic velocity (cm s−1) and (b) alongshore pressure gradient against depth (m) at 25°S. Here, the alongshore pressure gradient is measured as dynamic height difference (dyn cm) between 23.25° and 26.75°S along the western Australian coast.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 6.
Fig. 6.

Wind stress (10−1 Pa) and its curl (10−8 Pa m−1) from Hellerman and Rosenstein (1983) in (a) Aug and (b) Feb. Light shading denotes the region where water depth is shallower than 100 m.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 7.
Fig. 7.

Deviation from annual mean dynamic height (dyn cm) and geostrophic flow (cm s−1) at the sea surface in (a) Aug and (b) Feb.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 8.
Fig. 8.

Deviation from annual mean dynamic height (dyn cm) along 110°E in (a) Aug and (b) Feb.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 9.
Fig. 9.

Seasonal variation of zonal velocity (cm s−1) at (a) 9°S, 110°E and (b) 12°S, 110°E. Annual mean values have been subtracted before plotting.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 10.
Fig. 10.

As in Fig. 9 but at 8°S, 105°E.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 11.
Fig. 11.

Amplitude (cm) of (a) annual and (b) semiannual harmonics of the T/P data. The phase (shaded) corresponds to the day of the year when sea surface height is maximum.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 12.
Fig. 12.

As in Fig. 11 but for dynamic height at 0, 100, 200, and 400 m.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 13.
Fig. 13.

Seasonal variation of dynamic height (dyn cm) at the sea surface along (a) 12.5° and (b) 15°S. Annual mean values have been subtracted before plotting.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 14.
Fig. 14.

Amplitude of annual and semiannual harmonics of dynamic height along 100° and 105°E.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

Fig. 15.
Fig. 15.

Seasonal variation of dynamic height (heavy solid) and its annual (light solid) and semiannual (light dotted) components at 9°S, 110°E off the coast of Java. The unit is dyn cm.

Citation: Journal of Physical Oceanography 35, 2; 10.1175/JPO-2682.1

* School of Ocean and Earth Science and Technology Contribution Number 6444, and International Pacific Research Center Contribution Number IPRC-287.

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