Australia: The Land Where Time Began

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Indonesian Throughflow

The Indonesian Archipelago is the connection at low latitudes between the Pacific Oceana and the Indian Ocean, the flow through the archipelago being referred to as the ITF. This is a unidirectional current flowing from the Pacific Ocean to the Indian Ocean as the sea-surface pressure on the Pacific side is higher than on the Indian Ocean side. The geography of the Indonesian Archipelago is very complicated, and through this complex of islands more than 10 Sv of Pacific water, that is fresher and has high levels of nutrition, threads its way through the the island complex. The global overturning circulation has transports  of this order. The ITF is one of the major upper ocean elements of this global circulation, as it is part of the 10-15 Sv movement from the Pacific Ocean to the Indian Ocean that eventually reaches the Atlantic Ocean.

The Makassar and Lifamatola Straits are the main routes by which the water from the Pacific enters that Indonesian Archipelago. Of the 2 straits the Makassar Strait is the shallower, at 680 m at Dewakang Sill, though it carries most of the transport, at least 9 Sv, which is from the North Pacific (Gordon, Susanto & Ffield, 1999). At 1940 m the Lifamatola Strait is the deeper, and is the route for the water from the South Pacific into the Indonesian Archipelago, and for the deeper part of the throughflow into the Indian Ocean. Water flows through this strait at a rate of at least 2-3 Sv. Gordon, Ciulivi & Ilahude, 2003; Talley & Sprintall, 2005), and some water from the upper layer of the South Pacific also flows through the Halmahera Sea. The waters are mixed, both horizontally and verticallly, within the Indonesian Archipelago, and locally there is some modification of the water as a result of heating and a slight degree of freshening.

There are 3 main routes by which the throughflow water leaves the Indonesian Archipelago: Lombok Strait, Ombai Strait, which connects to Savu Strait and Sumba Strait, and Leti Strait that connects to Timor Passage. The deepest sill for the outflow is Leti Strait, that is situated 1250 m to the northeast of Timor. All of these outflow straits have been instrumented and the transport through each has been found to be 2-5 Sv. Simultaneous observations of the flow through the straits has been made possible by the placement a pair of shallow pressure gauges across the straits in 1995-1999, as well as the deployment of various current gauges in the principal straits. Variation has been found to be large and includes the signature of ENSO (Hautala, et al., 2001). An international array of current metres and pressure gauges in now in place to monitor the inflows and outflows.

After leaving the Archipelago the ITF they form a narrow flow to the west centred at 12oS, within the SEC. The fresh waters of the upper ocean are easily seen on any meridional salinity section in the eastern Indian Ocean. It is also possible to observe the deeper part of the throughflow as a salinity minimum at about the same depth and density as the low salinity of the Antarctic Intermediate Water, AAIW, that extends to almost this latitude from the south. The deeper expression of the ITF unambiguously originates in the Indonesian Archipelago, based on its higher nutrient levels, especially in silica (Talley & Sprintall, 2005). In early treatments the salinity minimum was called the Indonesian Intermediate Water, or Banda Sea Intermediate Water.

A zonal current, the South Equatorial Current, SEC, carries the throughflow waters to the west across the Indian Ocean. It is indicated by mass balances within the Agulhas Current that the waters from Indonesia must join this current following which they exit the Indian Ocean. It is suggested by the baroclinic structure of the Agulhas Current that the excess transport from the throughflow is in the upper ocean. The waters entering the Agulhas Current matching the transport through the Indonesian Archipelago are greatly modified after they enter the Indian Ocean and are not likely to be the same water parcels.

Dramatic global changes in upper ocean circulation, temperature, salinity, winds and precipitation have been suggested by model studies to occur if the ITF were cut off for a period (e.g., Schneider et al., 1998; Song, Vecchi & Rosati, 2007). It has been shown (Song et al., 2007) that eastern part of the tropical Pacific warms and the tropical Indian Ocean cools, which reduces the strength of the trade winds and reinforces the SST changes and also shifts the precipitation. The variability of the ENSO would change, the Pacific Ocean would become fresher and the Indian Ocean would become more saline as the fresher water from the Pacific Ocean would no longer be exported to the Indian Ocean. The flow that at present goes through the Indonesian Archipelago would instead be diverted to flow down the eastern coast of Australia which would result in a marked surface warming to the southeast of Australia.

Sources & Further reading

  1. Tally, Lynne, Pickard, George L., , William J.,  Emery, D., & Swift, James H., 2011, Descriptive Physical Oceanography, an Introduction, Elsevier Ltd.


Author: M. H. Monroe
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