The Tasman Outflow is a water pathway connecting water from the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...

and the

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by t ...

. 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 Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to temp ...

Features

Data of Argo floats at the seasurface and at 1000 dbar

The source of the water of the Tasman Outflow is the

East Australian Current The East Australian Current (EAC) is a warm, southward, western boundary current that is formed from the South Equatorial Current (SEC) crossing the Coral Sea and reaching the eastern coast of Australia. At around 15° S near the Australian co ...

. Until 2007, it was assumed that the water of this current moved in a southeastern direction towards

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

. However, this eastward turn toward

only occurred close to the surface, as was confirmed by the use of Argo floats at the sea surface and at a depth of 1000 dbar. At intermediate depth -around 300 to 1000 meter- the water actually turns south and westward, moving around the south of

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. This water, which escapes from the

and moves past

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

, is called the Tasman Outflow. The current moves further westward past the

Great Australian Bight The Great Australian Bight is a large oceanic bight, or open bay, off the central and western portions of the southern coastline of mainland Australia. Extent Two definitions of the extent are in use – one used by the International Hydrog ...

and into the

. In this way, the Tasman Outflow links the

South Pacific Ocean South is one of the cardinal directions or compass points. The direction is the opposite of north and is perpendicular to both east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Proto-Germanic ''*sunþaz ...

to the

. Due to its depth, the current mainly transports Subantarctic Mode Water and Antarctic Intermediate Water with a volume transport of 4.2 ± 4.3 Sv. Here Sv stands for

Sverdrup In oceanography, the sverdrup (symbol: Sv) is a non- SI metric unit of volumetric flow rate, with equal to . It is equivalent to the SI derived unit cubic hectometer per second (symbol: hm3/s or hm3⋅s−1): 1 Sv is equal to 1 hm3/s. It is u ...

, a measure for volumetransport in the ocean. The current is limited to a narrow path between

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

and the Antarctic circumpolar current, due to the strong eastward Antarctic Circumpolar Current to the south of

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

Role in the thermohaline circulation

Before the discovery of the Tasman Outflow, research on the

in the Southern Hemisphere was mainly focused on two other routes. One of them is known as the cold route, which moves through the

Drake Passage The Drake Passage (referred to as Mar de Hoces Hoces Sea"in Spanish-speaking countries) is the body of water between South America's Cape Horn, Chile and the South Shetland Islands of Antarctica. It connects the southwestern part of the Atla ...

and transports cold water deep in the ocean around

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

into the Pacific and

. The other is known as the warm route, which moves through the

Indonesian Throughflow The Indonesian Throughflow (ITF; ) 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 glob ...

and transports warm water into the

. With the Tasman Outflow there is a third route of the

with Subantarctic Mode Water and Antarctic Intermediate Water transport from the Pacific to the North Atlantic. Furthermore, the Tasman Outflow functions as the second gateway for Pacific waters to reach the

, besides the

. 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. It is colder, less saline and denser than the other two routes, which is caused by the fresh input from the Antarctic 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 the

mixed layer 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 evaporat ...

, causing its salinity and temperature to vary little.

Role in the Southern Hemisphere supergyre

The Tasman Outflow was the missing link in research on the Southern Hemispheric supergyre. This supergyre is hypothesized to connect all three southern basin gyres, namely the

South Pacific Gyre __NOTOC__ The Southern Pacific Gyre is part of the Earth's system of rotating ocean currents, bounded by the Equator to the north, Australia to the west, the Antarctic Circumpolar Current to the south, and South America to the east. The center ...

, the

Indian Ocean Gyre The Indian Ocean gyre, located in the Indian Ocean, is one of the five major oceanic gyres, large systems of rotating ocean currents, which together form the backbone of the global conveyor belt. The Indian Ocean gyre is composed of two major curr ...

and the

South Atlantic Gyre The South Atlantic Gyre is the subtropical gyre in the south Atlantic Ocean. In the southern portion of the gyre, northwesterly (or southeastward-flowing) winds drive eastward-flowing currents that are difficult to distinguish from the northern ...

. The water in this supergyre originates from the Antarctic zone as Subantarctic Mode Water. It moves in an eastward direction around

within the Antarctic Circumpolar Current. Within this current, the Subantarctic Mode Water is partially converted to Antarctic Intermediate Water. When the water reaches the South Pacific, the water is included in the South Pacific Gyre System close to

. Here, the

gyre In oceanography, a gyre () is any large system of circulating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction dete ...

is provided with fresh water below the

thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...

. Before moving on to the Tasman Outflow, the water can flow through large parts of the Pacific basin. Eventually, the

picks up the water and moves it further southwards, where it rounds the south of

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

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 the

, the flow meets the

Agulhas current The Agulhas Current () is the western boundary current of the southwest Indian Ocean. It flows south along the east coast of Africa from 27°S to 40°S. It is narrow, swift and strong. It is suggested that it is the largest western boundary curren ...

where it is partly inverted towards the east and partly passing through to the

South Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe an ...

, closing the circle of the supergyre.

Role in the climate system

The

is important for our climate system; this is equally true for the Tasman Outflow addition to the thermohaline circulation. When compared to the

and

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 oceanic

. As a result, its temperature and salinity stay mostly conserved throughout its way to the North 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 the

. The wind also seems to play an important role in the size of the contribution of the Tasman Outflow. Before being injected into the South Pacific subtropical gyre system and subsequently into the Tasman Outflow, the water has travelled many times around

. The wind forcing driving this circulation therefore has an outsize influence on the freshwater transport into the Atlantic. Besides, it is thought to control the stability and functioning of the

. 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 the

Subtropical Front A subtropical front is a surface water mass boundary or front, which is a narrow zone of transition between air masses of contrasting density, air masses of different temperatures or different water vapour concentrates. It is also characterized by ...

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.

References

{{ocean Currents of the Pacific Ocean Physical oceanography Southern Ocean Currents of the Southern Ocean