Observed Transports through Major Straits of the Indonesian Throughflow

Large-scale observation-based studies (including inverse solutions) reveal significant Pacific export of mass, heat, and freshwater into the Indian Ocean (Piola and Gordon, 1984, 1986; Toole and Raymer, 1985; Wijffels et al., 1992; Toole and Warren, 1993; MacDonald and Wunsch, 1996; Ganachaud et al., 2000; Ganachaud and Wunsch, 2000). The uncertainty in the size of such a warm, fresh throughflow is the dominant source of error in analyses of the basin-wide budgets of heat and freshwater for the Pacific (Wijffels et al., 2001) and Indian Oceans (Robbins and Toole, 1997). Model research reveals dependence of Pacific and Indian Ocean sea-surface temperature (SST) and upper layer heat storage on the throughflow (Hirst and Godfrey, 1993; Verschell et al., 1995; Murtugudde et al., 1998). The Indian and Pacific Oceans would be very different if the ITF were zero (Hirst and Godfrey, 1993, 1994;Maes, 1998; Schneider, 1998; Wajsowicz and Schneider, 2001). Oceanic heat and freshwater fluxes into the Indian Ocean — at the expense of the Pacific — affect atmosphere—ocean coupling with potential impacts on the El Nino-Southern Oscillation (ENSO) and monsoon phenomena (Webster et al. 1998). Saji et al. (1999) found that the cool SST anomalies that lead to a dipole mode of variability in the tropical Indian Ocean first appeared in the vicinity of Lombok Strait in Indonesia. The cold SST anomaly shifts westward as a Rossby wave from the Java and Sumatra Indian Ocean coasts and varies in strength with the phase of ENSO (Susanto et al., 2001).

The ITF source water (North Pacific vs. South Pacific) depends upon land geometry and the tropical Pacific wind fields (Godfrey, 1989; Nof, 1996; Morey et al., 1999; Wajsowicz, 1999; Cane and Molnar, 2001). Observations show that the ITF is composed mostly of North Pacific water flowing through Makassar Strait (Fig. 2; Fine, 1985; Ffield and Gordon, 1992; Gordon, 1995;

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Figure 2: (a) Time series (above) of the average temperature (red) between 150 and 400 db at the MAK-1 mooring (Ffield et al., 2000). The SOI (green) and the Makassar Strait volume transport (blue dashed) are also shown. The data are smoothed by 30-day running averages. (b) Temperature time section constructed from 15 years of Makassar Strait and Flores Sea XBT profiles (Ffield, 2000, personal communication). In the upper panel, the depth of the 22°C XBT isotherm (red) is shown with the SOI (black) highlighting the clear ENSO variability in the XBT temperature data. The data are smoothed by a 1-year running average (For colour version, see Colour Plate Section).

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Figure 2: (a) Time series (above) of the average temperature (red) between 150 and 400 db at the MAK-1 mooring (Ffield et al., 2000). The SOI (green) and the Makassar Strait volume transport (blue dashed) are also shown. The data are smoothed by 30-day running averages. (b) Temperature time section constructed from 15 years of Makassar Strait and Flores Sea XBT profiles (Ffield, 2000, personal communication). In the upper panel, the depth of the 22°C XBT isotherm (red) is shown with the SOI (black) highlighting the clear ENSO variability in the XBT temperature data. The data are smoothed by a 1-year running average (For colour version, see Colour Plate Section).

Gordon and Fine, 1996), which is consistent with theory (Wajsowicz, 1996). The 1997 average southward transport in Makassar Strait is 9.3 7 2.5 Sv, with the highest southward flow occurring at mid to lower thermocline depths (Gordon and Susanto, 1999; Gordon et al., 1999). Warming between the inflow at Makassar and the export into the Indian Ocean is likely a consequence of the impact of mixing and air—sea fluxes within the internal Indonesian seas. While some of the Makassar throughflow exits the Indonesian sea within Lombok Channel (Murray and Arief, 1988), most turns eastward within the Flores Sea to enter the Banda Sea before entering the Indian Ocean via the deeper Ombai Strait and Timor Passage (Meyers et al., 1995; Gordon and Fine, 1996).

The flow in Lombok Strait is well documented, largely due to the Lombok Strait experiment in 1985 (Murray and Arief, 1988; Murray et al., 1990). Flow through the 35-km wide Lombok Strait is restricted by a sill depth of about 300 m at the southern end. From February through May,

Makassar Strait and Flores Sea XBTs

Makassar Strait and Flores Sea XBTs

Figure 2 Continued (For colour version, see Colour Plate Section).

Figure 2 Continued (For colour version, see Colour Plate Section).

transport through Lombok Strait was only 1 Sv southward due to discrete occasions of shallow northward flow, compared to the 4 Sv observed during the southeast monsoon from July through September (Murray and Arief, 1988). Similar periods of northward transport are inferred from changes in the cross-strait pressure gradient measured by the shallow pressure gauges deployed across Lombok Strait from 1996 to 1999 (Hautala et al., 2001). The flow reversals observed in Lombok Strait are likely to be related to the passing of Kelvin waves, forced in the equatorial Indian Ocean (Sprintall et al., 1999, 2000).

Ombai Strait lies between Alor and Timor Islands, and connects the Banda Sea with the Savu Sea (Fig. 1). The strait is 30-km wide, and a recent bathymetric survey by Molcard et al. (2001) suggests a sill depth of 3,250 m. From a single mooring in Ombai Strait, Molcard et al. (2001) suggest a transport range between 4.3 Sv and 5.8 Sv, depending on the assumed cross-strait shear, with 0.9-1.2 Sv average transport in the upper 100 m.

The Timor Passage is a long, narrow trench that lies between the southeast-oriented coast of Timor and the northern edge of the wide, shallow northwest Australian coastal shelf (Fig. 1). The easternmost sill connecting the Timor Passage with the Banda Sea is 1,250 m, shallower than the 1,890 m western sill connecting the trench with the southeast Indian Ocean. Deployment of two year-long moorings across the western sill have been undertaken on two occasions by the French: a total westward transport of 4.5 71.5 Sv from 120 to 1,050 m in August 1989-September 1990 (Molcard et al., 1994); and 4.3 71 Sv westward from 0 to 1,250 m in March 1992-April 1993 (Molcard et al., 1996). An ADCP repeat survey of Timor Passage in December 1995 (Hautala et al., 2001) suggests that the high-velocity surface core of the throughflow may extend further south than what the Molcard et al. (1996) extrapolation allowed for, which would lead to a much higher total transport through Timor Passage.

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