Indonesian throughflow
Encyclopedia
The Indonesian throughflow (ITF) is important to global climate since it provides a low-latitude pathway for warm, fresh water to move from the Pacific to the Indian Ocean and this serves as the upper branch of the Pacific-Indian overturning circulation global heat conveyor belt
. Higher ocean surface topography
in the western Pacific than in the Indian Ocean drives upper thermocline
water from the North Pacific through the western route of the Makassar Strait to either directly exit through the Lombok Strait or flow eastward into the Banda Sea
. Weaker flow of saltier and denser South Pacific water passes over the Lifamatola Strait into the Banda Sea, where these water masses are mixed due to tidal effects, Ekman pumping
, and heat and fresh water flux at the ocean surface. From the Banda Sea the ITF exits Timor, Ombai, and Lombok passages.
From 2004–2006, 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. Flow in through Makassar (11.6 Sv, 1 Sv
= 106 m³/s) 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. Circulation and transport within the Indonesian Seas varies 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) whereas from December to February, Northwest Monsoon westerlies serve to 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.
Using a combination of INSTANT mooring observations and model results, it was shown that remote wind forcing of equatorial/coastal Kelvin and Rossby waves plays a major role in interannual variability of ITF. 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. Interannual variability of Indian Ocean westerlies act in the same manner as the seasonal equatorial Kelvin waves to reduce the normal westward ITF flow.
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 10000 km (6,213.7 mi) away from the Lombok Strait.
Thermohaline circulation
The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....
. Higher ocean surface topography
Ocean surface topography
The ocean surface has highs and lows, similar to the hills and valleys of Earth's land surface depicted on a topographic map. These variations, called "ocean surface topography" or "dynamic sea surface topography" are mapped using measurements of sea surface height relative to Earth's geoid...
in the western Pacific than in the Indian Ocean drives upper thermocline
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
water from the North Pacific through the western route of the Makassar Strait to either directly exit through the Lombok Strait or flow eastward into the Banda Sea
Banda Sea
The Banda Sea is a sea in the Maluku Islands of Indonesia, technically part of the Pacific Ocean but separated from it by hundreds of islands, as well as the Halmahera and Ceram Seas...
. Weaker flow of saltier and denser South Pacific water passes over the Lifamatola Strait into the Banda Sea, where these water masses are mixed due to tidal effects, Ekman pumping
Ekman spiral
The Ekman spiral refers to a structure of currents or winds near a horizontal boundary in which the flow direction rotates as one moves away from the boundary. It derives its name from the Swedish oceanographer Vagn Walfrid Ekman...
, and heat and fresh water flux at the ocean surface. From the Banda Sea the ITF exits Timor, Ombai, and Lombok passages.
From 2004–2006, 11 moorings
Mooring (oceanography)
thumb|right|Sketch of a mooring with traps and current metersA mooring in oceanography is a collection of devices, connected to a wire and temporarily anchored on the sea floor. The devices are current meters to measure the direction and speed of ocean currents, sediment traps to catch settling...
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. Flow in through Makassar (11.6 Sv, 1 Sv
Sverdrup
The sverdrup, named in honour of the pioneering oceanographer Harald Sverdrup, is a unit of measure of volume transport. It is used almost exclusively in oceanography, to measure the transport of ocean currents. Its symbol is Sv. Note that the sverdrup is not an SI unit, and that its symbol...
= 106 m³/s) 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. Circulation and transport within the Indonesian Seas varies 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) whereas from December to February, Northwest Monsoon westerlies serve to 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.
Using a combination of INSTANT mooring observations and model results, it was shown that remote wind forcing of equatorial/coastal Kelvin and Rossby waves plays a major role in interannual variability of ITF. 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. Interannual variability of Indian Ocean westerlies act in the same manner as the seasonal equatorial Kelvin waves to reduce the normal westward ITF flow.
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
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
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
Advection
Advection, in chemistry, engineering and earth sciences, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries...
towards Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
within the Indian South Equatorial Current
South Equatorial Current
The South Equatorial Current is a significant Pacific, Atlantic, and Indian Ocean current that flows east-to-west between the equator and about 20 degrees south. In the Pacific and Atlantic Oceans, it extends across the equator to about 5 degrees north....
. There it eventually exits the Indian Ocean with the Agulhas Current
Agulhas Current
The Agulhas Current is the Western Boundary Current of the southwest Indian Ocean. It flows down the east coast of Africa from 27°S to 40°S. It is narrow, swift and strong...
around South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
into the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
. So the Indonesian Throughflow transports a significant amount of Pacific Ocean heat into the southwest Indian Ocean, which is approximately 10000 km (6,213.7 mi) away from the Lombok Strait.