TheTasman Outflow is a water pathway connecting water from thePacific Ocean and theIndian Ocean. The existence of the outflow was published by scientists of the AustralianCSIRO's Division of Marine and Atmospheric Research team in August 2007, interpreting salinity and temperature data captured from 1950 to 2002.[1] The Tasman Outflow is seen as the missing link in the supergyre of theSouthern Hemisphere and an important part of thethermohaline circulation.
The source of the water of the Tasman Outflow is theEast Australian Current. Until 2007, it was assumed that the water of this current moved in a southeastern direction towardsNew Zealand. However, this eastward turn towardNew Zealand only occurred close to the surface, as was confirmed by the use ofArgo floats at the sea surface and at a depth of 1000 dbar.[3] At intermediate depth -around 300 to 1000 meter- the water actually turns south and westward, moving around the south ofTasmania. This water, which escapes from theEast Australian Current and moves pastTasmania, is called the Tasman Outflow. The current moves further westward past theGreat Australian Bight and into theIndian Ocean. In this way, the Tasman Outflow links theSouth Pacific Ocean to theIndian Ocean. Due to its depth, the current mainly transportsSubantarctic Mode Water andAntarctic Intermediate Water with a volume transport of 4.2 ± 4.3 Sv.[4] Here Sv stands forSverdrup, a measure for volumetransport in the ocean. The current is limited to a narrow path betweenTasmania and theAntarctic circumpolar current, due to the strong eastwardAntarctic Circumpolar Current to the south ofTasmania.
Before the discovery of the Tasman Outflow, research on thethermohaline circulation in theSouthern Hemisphere was mainly focused on two other routes. One of them is known as the cold route, which moves through theDrake Passage and transports cold water deep in the ocean aroundAntarctica into the Pacific andIndian Ocean. The other is known as the warm route, which moves through theIndonesian Throughflow and transports warm water into theIndian Ocean. With the Tasman Outflow there is a third route of thethermohaline circulation withSubantarctic Mode Water andAntarctic Intermediate Water transport from the Pacific to theNorth Atlantic. Furthermore, the Tasman Outflow functions as the second gateway for Pacific waters to reach theIndian Ocean, besides theIndonesian Throughflow.
At the equatorial Atlantic the Tasman Outflow's contribution is even comparable to that of the other two better known routes with a volume transport of approximately 3 Sv. The Tasman Outflow is seen as a third route since the water flow does not come into contact with the other two routes as it underrides both of them in depth.[5] It is colder, less saline and denser than the other two routes, which is caused by the fresh input from theAntarctic Intermediate Water in the South Pacific. The waterflow to which the Tasman Outflow contributes, is almost entirely situated below a depth of 300 meter. Influences from outside stay limited because of its situation well below themixed layer, causing its salinity and temperature to vary little.[6]
The Tasman Outflow was the missing link in research on the Southern Hemispheric supergyre. This supergyre is hypothesized to connect all three southern basingyres, namely theSouth Pacific Gyre, theIndian Ocean Gyre and theSouth Atlantic Gyre.[8] The water in this supergyre originates from the Antarctic zone asSubantarctic Mode Water. It moves in an eastward direction aroundAntarctica within theAntarctic Circumpolar Current. Within this current, theSubantarctic Mode Water is partially converted toAntarctic Intermediate Water. When the water reaches the South Pacific, the water is included in theSouth Pacific Gyre System close toNew Zealand. Here, thegyre is provided with fresh water below thethermocline. Before moving on to the Tasman Outflow, the water can flow through large parts of the Pacific basin. Eventually, theEast Australian Current picks up the water and moves it further southwards, where it rounds the south ofTasmania to the west and through the Tasman Outflow ends up in the Indian Ocean. In the east of the Indian Ocean, the Tasman flow stays below 15S and between 300 and 1100 meters deep. After reaching the west of theIndian Ocean, the flow meets theAgulhas current where it is partly inverted towards the east and partly passing through to theSouth Atlantic Ocean, closing the circle of the supergyre.[9]
Thethermohaline circulation is important for our climate system; this is equally true for the Tasman Outflow addition to the thermohaline circulation. When compared to theDrake Passage andIndonesian Throughflow routes, the Tasman outflow endures fewer influences from outside. Its exposition to air, just as other sea interactions, is limited since it rarely comes into contact with the oceanicmixed layer. As a result, its temperature and salinity stay mostly conserved throughout its way to theNorth Atlantic where it comes to the surface. It thus functions as a stable and constant supply of fresh water, which could work to counteract the changing heat transport in thethermohaline circulation.[10]
The wind also seems to play an important role in the size of the contribution of the Tasman Outflow. Before being injected into theSouth Pacific subtropical gyre system and subsequently into the Tasman Outflow, the water has travelled many times aroundAntarctica. The wind forcing driving this circulation therefore has an outsize influence on the freshwater transport into the Atlantic.[5] Besides, it is thought to control the stability and functioning of thethermohaline circulation. The Tasman Outflow is also directly influenced by wind forcing, especially by winds in the Southern and Pacific Ocean. These winds have an effect on the extent of the outflow, since it reduces in size when theSubtropical Front shifts towards the north. However, no evidence of any seasonality has been found. Although measurements show large variations in the size of the outflow, from 1 Sv to more than 25 Sv on both sub-weekly and inter-annual scales, no long-term trends were found over the period of 1983 to 1997.[11]
