The Indonesian Throughflow (ITF; Indonesian : Arus Lintas Indonesia) is an ocean current with importance for global climate as is the low-latitude movement of warm, relative freshwater from the north Pacific to the Indian Ocean. It thus serves as a main upper branch of the global heat/salt conveyor belt.
The ocean surface of this part of the far western Pacific is on average each day higher than that in the adjacent part of the Indian Ocean. The difference drives upper thermocline water "downhill" through the deep, straight, westerly, north–south Makassar Strait then to meet the in reality combined Java Sea-Banda Sea. About 15% of this then exits directly through the very narrow Lombok Strait. Weaker flows of saltier and denser South Pacific slightly augment the Banda Sea via the Lifamatola Passage, both inflows mixing there due to its bounds and tides, Ekman pumping, and heat and freshwater flux. From this sea 85% of the ITF uses the broad Timor and narrow Ombai passage.
The location and topography of the channels that make up the ITF are shown inset. Lombok Strait is 300m deep and roughly 35 km wide and the currents vary between 0.286 m/s (0.6 mi/hr) eastward to 0.67 m/s westward and average 0.25 m/s westward. Currents in Ombai vary between 0.12 m/s eastward to 0.16 m/s westward, averaging 0.11 m/s westward and are funneled within the 1250m deep and 35 km wide passage. Timor passage, which is 1890 m deep by 160 km wide, is the widest of the exit pathways and averages only 0.02 m/s. From 2004 to 2006, 11 moorings were deployed across the entrance and exit regions of the ITF and were positioned to accurately measure each passage's contribution as part of the International Nusantara Stratification and Transport (INSTANT) program. A study using Princeton Ocean Model has observed that the ITF has maximum volume transport from the Pacific Ocean to Indian Ocean through Savu strait (~6/5 Sv, 1 Sv = 106 m³/s), followed by Timor passage (~3.5/2 Sv) and Lombook strait (~2/1.75 Sv) thus the gross volume transport of ITF is ~10/9 Sv and also it is observed that the ITF increases the temperature of the Southern Indian Ocean while it has no significant effect on the sea surface salinity of Indian Ocean. [1] Flow in through Makassar (11.6 Sv) and Lifamatola (1.1 Sv) sums to 12.7 Sv. Total outflow transport corresponds to 15.0 Sv (varying from 10.7 to 18.7 Sv) and is made up of Lombok (2.6 Sv), Ombai (4.9 Sv) and Timor (7.5 Sv) contributions. [2] Heat Transport of the Indonesian Throughflow is 1.087 PW (1 PW=1015 Watt). [3] Turbulence Kinetic Energy (TKE) of the ITF is of the order of 10−3 m2s−2 in the upper layer whereas it is 10−4 m2s−2 in the middle layer. Corresponding values of ITF TKE dissipation rate are of the order of 10−6 m2s−3 and 10−8 m2s−3 which indicate that this ITF archipelagoes region is a highly turbulent and a high heat dissipative in nature. [4]
Circulation and transport within the Indonesian Seas vary along with large-scale monsoon flow. During June to August, southeasterlies of the southwest monsoon predominate over Indonesia and drive strong Ekman divergence (southwestward flow in the Southern Hemisphere thus increasing ITF to 15 Sv) whereas from December to February, Northwest Monsoon westerlies serve to directly reduce the ITF. During monsoon transitions, strong westerly winds in the eastern Indian Ocean force equatorial downwelling Kelvin waves (eastward moving, eastward flow) that propagate through the Indonesian passages as coastally trapped Kelvin waves and serve to reduce the ITF flow with a minimum in April of 9 Sv. Another way to think about it is that downwelling on the Indian Ocean side increases sea level and so reduces the normal Pacific-to-Indian pressure head reducing the flow.
Global-scale, ocean waves such as equatorial/coastal Kelvin and Rossby waves drive interannual variation of the ITF with an amplitude of roughly +/-3 Sv. [5] Western-central Pacific westerly winds from El Nino force westward moving-equatorial Rossby waves and eastward currents that hit eastern New Guinea and propagate around the west coast as coastal Kelvin waves and down through the ITF along the west Australia Shelf coast serving to reduce the ITF. Upwelling (i.e. reduced sea level) associated with Rossby waves on the Pacific side reduces the Pacific-to-Indian pressure gradient and reduces the ITF. Interannual variability of Indian Ocean westerlies acts in the same manner as the seasonal equatorial Kelvin waves to reduce the normal westward ITF flow as well.
An important feature of the Indonesian Throughflow is that because the water in the western equatorial Pacific Ocean has a higher temperature and lower salinity than the water in the Indian Ocean, the Throughflow transports large amounts of relatively warm and fresh water to the Indian Ocean. When the Indonesian Throughflow (through Lombok Strait, Ombai and the Timor Passages) enters the Indian Ocean it is advected towards Africa within the Indian South Equatorial Current. There it eventually exits the Indian Ocean with the Agulhas Current around South Africa into the Atlantic Ocean. So the Indonesian Throughflow transports a significant amount of Pacific Ocean heat into the southwest Indian Ocean, which is approximately 10,000 km (6,200 mi) away from the Lombok Strait. [6]
The Lombok Strait, is a strait of the Bali Sea connecting to the Indian Ocean, and is located between the islands of Bali and Lombok in Indonesia. The Gili Islands are on the Lombok side.
The Drake Passage is the body of water between South America's Cape Horn, Chile, Argentina and the South Shetland Islands of Antarctica. It connects the southwestern part of the Atlantic Ocean with the southeastern part of the Pacific Ocean and extends into the Southern Ocean. The passage is named after the 16th-century English explorer and privateer Sir Francis Drake.
An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents are primarily horizontal water movements.
Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.
A Kelvin wave is a wave in the ocean or atmosphere that balances the Earth's Coriolis force against a topographic boundary such as a coastline, or a waveguide such as the equator. A feature of a Kelvin wave is that it is non-dispersive, i.e., the phase speed of the wave crests is equal to the group speed of the wave energy for all frequencies. This means that it retains its shape as it moves in the alongshore direction over time.
The Madden–Julian oscillation (MJO) is the largest element of the intraseasonal variability in the tropical atmosphere. It was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric Research (NCAR). It is a large-scale coupling between atmospheric circulation and tropical deep atmospheric convection. Unlike a standing pattern like the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation is a traveling pattern that propagates eastward, at approximately 4 to 8 m/s, through the atmosphere above the warm parts of the Indian and Pacific oceans. This overall circulation pattern manifests itself most clearly as anomalous rainfall.
The North Equatorial Current (NEC) is a westward wind-driven current mostly located near the equator, but the location varies from different oceans. The NEC in the Pacific and the Atlantic is about 5°-20°N, while the NEC in the Indian Ocean is very close to the equator. It ranges from the sea surface down to 400 m in the western Pacific.
The Equatorial Counter Current is an eastward flowing, wind-driven current which extends to depths of 100–150 metres (330–490 ft) in the Atlantic, Indian, and Pacific Oceans. More often called the North Equatorial Countercurrent (NECC), this current flows west-to-east at about 3-10°N in the Atlantic, Indian Ocean and Pacific basins, between the North Equatorial Current (NEC) and the South Equatorial Current (SEC). The NECC is not to be confused with the Equatorial Undercurrent (EUC) that flows eastward along the equator at depths around 200 metres (660 ft) in the western Pacific rising to 100 metres (330 ft) in the eastern Pacific.
Ombai Strait is an international strait in Southeast Asia. It separates the Alor Archipelago from the islands of Wetar, Atauro, and Timor in the Lesser Sunda Islands. The strait is also the western portion of a pair of international straits, the other one being Wetar Strait; the two straits combine to link the Pacific Ocean with the Indian Ocean.
Wetar Strait is an international strait in Southeast Asia. It separates the island of Wetar from the eastern part of the island of Timor. The strait is also the eastern portion of a pair of international straits, the other one being Ombai Strait; the two straits combine to link the Indian Ocean with the Pacific Ocean.
The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporation or sea ice formation which result in an increase in salinity. The atmospheric mixed layer is a zone having nearly constant potential temperature and specific humidity with height. The depth of the atmospheric mixed layer is known as the mixing height. Turbulence typically plays a role in the formation of fluid mixed layers.
The Tasman Outflow is a water pathway connecting water from the Pacific Ocean and the Indian Ocean. The existence of the outflow was published by scientists of the Australian CSIRO's Division of Marine and Atmospheric Research team in August 2007, interpreting salinity and temperature data captured from 1950 to 2002. The Tasman Outflow is seen as the missing link in the supergyre of the Southern Hemisphere and an important part of the thermohaline circulation.
The Coral Triangle (CT) is a roughly triangular area in the tropical waters around the Philippines, Indonesia, Malaysia, Papua New Guinea, the Solomon Islands and Timor-Leste. This area contains at least 500 species of reef-building corals in each ecoregion. The Coral Triangle is located between the Pacific and Indian oceans and encompasses portions of two biogeographic regions: the Indonesian-Philippines Region, and the Far Southwestern Pacific Region. As one of eight major coral reef zones in the world, the Coral Triangle is recognized as a global centre of marine biodiversity and a global priority for conservation. Its biological resources make it a global hotspot of marine biodiversity. Known as the "Amazon of the seas" (by analogy to the Amazon rainforest in South America), it covers 5.7 million square kilometres (2,200,000 sq mi) of ocean waters. It contains more than 76% of the world's shallow-water reef-building coral species, 37% of its reef fish species, 50% of its razor clam species, six out of seven of the world's sea turtle species, and the world's largest mangrove forest. In 2014, the Asian Development Bank (ADB) reported that the gross domestic product of the marine ecosystem in the Coral Triangle is roughly $1.2 trillion per year and provides food to over 120 million people. According to the Coral Triangle Knowledge Network, the region annually brings in about $3 billion in foreign exchange income from fisheries exports, and another $3 billion from coastal tourism revenues.
The Indian Monsoon Current refers to the seasonally varying ocean current regime found in the tropical regions of the northern Indian Ocean. During winter, the flow of the upper ocean is directed westward from near the Indonesian Archipelago to the Arabian Sea. During the summer, the direction reverses, with eastward flow extending from Somalia into the Bay of Bengal. These variations are due to changes in the wind stress associated with the Indian monsoon. The seasonally reversing open ocean currents that pass south of India are referred to as the Winter Monsoon Current and the Summer Monsoon Current. The Somali Current, which is strongly linked to the Indian monsoon, is also discussed in this article.
Equatorial waves are oceanic and atmospheric waves trapped close to the equator, meaning that they decay rapidly away from the equator, but can propagate in the longitudinal and vertical directions. Wave trapping is the result of the Earth's rotation and its spherical shape which combine to cause the magnitude of the Coriolis force to increase rapidly away from the equator. Equatorial waves are present in both the tropical atmosphere and ocean and play an important role in the evolution of many climate phenomena such as El Niño. Many physical processes may excite equatorial waves including, in the case of the atmosphere, diabatic heat release associated with cloud formation, and in the case of the ocean, anomalous changes in the strength or direction of the trade winds.
Rossby-gravity waves are equatorially trapped waves, meaning that they rapidly decay as their distance increases away from the equator. These waves have the same trapping scale as Kelvin waves, more commonly known as the equatorial Rossby deformation radius. They always carry energy eastward, but their 'crests' and 'troughs' may propagate westward if their periods are long enough.
The Mindanao Current (MC) is a southward current in the western Pacific Ocean that transports mass and freshwater between ocean basins. It is a low-latitude western boundary current that follows the eastern coast of the Philippine island group and its namesake, Mindanao. The MC forms from the North Equatorial Current (NEC) that flows from east to west between 10-20°N. As it travels west, the NEC reaches its western limit: the coast of the Philippines. Once it encounters shallower waters near land, it “splits” into two branches: one moves northward and becomes the Kuroshio current and one moves southward and becomes the Mindanao Current. The process of splitting is called a bifurcation.
The Barrier layer in the ocean is a layer of water separating the well-mixed surface layer from the thermocline.
The Australasian Mediterranean Sea is a mediterranean sea located in the area between Southeast Asia and Australasia. It connects the Indian and Pacific oceans. It has a maximum depth of 7,440 m and a surface area of 9.08 mil. km².
Low-latitude western boundary currents (LLWBC) are western boundary currents located between the subtropical gyres, within 20° of the equator. They are important for closing the tropical circulation driven by the equatorial zonal flow, and facilitate inter-ocean transport between the subtropical gyres. They occur in regions of negative (positive) wind stress curl in the southern (northern) hemisphere, and originate at the western bifurcation point of the South or North Equatorial Current. They are typically equatorward (cyclonic) as opposed to sub-tropical western boundary currents, which tend to be poleward (anticyclonic). Some well-known examples include the Mindanao Current (MC) and the East African Coastal Current (EACC).
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