Tasman Outflow

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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. [1] The Tasman Outflow is seen as the missing link in the supergyre of the Southern Hemisphere and an important part of the thermohaline circulation.

Contents

Features

Mean current speed (color-coded, m s-1) and current velocity vectors near Australia, both at (a) the sea surface and (b) 1000 dbar. The inset illustrates the available number of data point for each 0.5deg x 0.5deg cell element. Data of Argo floats at the seasurface and at 1000 dbar.jpg
Mean current speed (color‐coded, m s−1) and current velocity vectors near Australia, both at (a) the sea surface and (b) 1000 dbar. The inset illustrates the available number of data point for each 0.5° × 0.5° cell element.

The source of the water of the Tasman Outflow is the East Australian Current. Until 2007, it was assumed that the water of this current moved in a southeastern direction towards New Zealand. However, this eastward turn toward New Zealand 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. [3] At intermediate depth -around 300 to 1000 meter- the water actually turns south and westward, moving around the south of Tasmania. This water, which escapes from the East Australian Current and moves past Tasmania, is called the Tasman Outflow. The current moves further westward past the Great Australian Bight and into the Indian Ocean. In this way, the Tasman Outflow links the South Pacific Ocean to the Indian Ocean. 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. [4] Here Sv stands for Sverdrup, a measure for volumetransport in the ocean. The current is limited to a narrow path between Tasmania and the Antarctic circumpolar current, due to the strong eastward Antarctic Circumpolar Current to the south of Tasmania.

Role in the thermohaline circulation

Before the discovery of the Tasman Outflow, research on the thermohaline circulation 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 and transports cold water deep in the ocean around Antarctica into the Pacific and Indian Ocean. The other is known as the warm route, which moves through the Indonesian Throughflow and transports warm water into the Indian Ocean. With the Tasman Outflow there is a third route of the thermohaline circulation 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 Indian Ocean, besides the Indonesian 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 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, causing its salinity and temperature to vary little. [6]

Role in the Southern Hemisphere supergyre

Horizontal streamfunction displaying the complete Atlantic-to-Atlantic Tasman water roundtrip, shown here for ORCA. Contour interval is 1 Sv, the stream function value has been set to zero in Tasmania. The patterns reveal a horizontal view of the quasi-total THC cell. Southern Hemisphere supergyre.png
Horizontal streamfunction displaying the complete Atlantic‐to‐Atlantic Tasman water roundtrip, shown here for ORCA. Contour interval is 1 Sv, the stream function value has been set to zero in Tasmania. The patterns reveal a horizontal view of the quasi‐total THC cell.

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, the Indian Ocean Gyre and the South Atlantic Gyre. [8] The water in this supergyre originates from the Antarctic zone as Subantarctic Mode Water. It moves in an eastward direction around Antarctica 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 New Zealand. Here, the gyre is provided with fresh water below the thermocline. Before moving on to the Tasman Outflow, the water can flow through large parts of the Pacific basin. Eventually, the East Australian Current picks up the water and moves it further southwards, where it rounds the south of Tasmania 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 Indian Ocean, the flow meets the Agulhas current where it is partly inverted towards the east and partly passing through to the South Atlantic Ocean, closing the circle of the supergyre. [9]

Role in the climate system

The thermohaline circulation is important for our climate system; this is equally true for the Tasman Outflow addition to the thermohaline circulation. When compared to the Drake Passage and Indonesian 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 oceanic mixed layer. 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 thermohaline 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 the South Pacific subtropical gyre system and subsequently into the Tasman Outflow, the water has travelled many times around Antarctica. 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 the thermohaline 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 the Subtropical 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]

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The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and has a mean transport estimated at 100–150 Sverdrups, or possibly even higher, making it the largest ocean current. The current is circumpolar due to the lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.

<span class="mw-page-title-main">North Atlantic Deep Water</span> Deep water mass formed in the North Atlantic Ocean

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<span class="mw-page-title-main">Weddell Sea</span> Part of the Southern Ocean between Coats Land and the Antarctic Peninsula

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<span class="mw-page-title-main">Drake Passage</span> Body of water between South America and the South Shetland Islands of Antarctica

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<span class="mw-page-title-main">Ocean current</span> Directional mass flow of oceanic water generated by external or internal forces

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<span class="mw-page-title-main">Thermohaline circulation</span> Part of large-scale ocean circulation

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<span class="mw-page-title-main">Subantarctic</span> Term describing the parts of the three largest oceans nearest the Southern Ocean

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<span class="mw-page-title-main">Weddell Gyre</span> One of two gyres within the Southern Ocean

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<span class="mw-page-title-main">Mindanao Current</span> Narrow, southward-flowing ocean current along the southeastern coast of the Philippines

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<span class="mw-page-title-main">Mode water</span> Type of water mass which is nearly vertically homogeneous

Mode water is defined as a particular type of water mass, which is nearly vertically homogeneous. Its vertical homogeneity is caused by the deep vertical convection in winter. The first term to describe this phenomenon is 18° water, which was used by Valentine Worthington to describe the isothermal layer in the northern Sargasso Sea cool to a temperature of about 18 °C each winter. Then Masuzawa introduced the subtropical mode water concept to describe the thick layer of temperature 16–18 °C in the northwestern North Pacific subtropical gyre, on the southern side of the Kuroshio Extension. The terminology mode water was extended to the thick near-surface layer north of the Subantarctic Front by McCartney, who identified and mapped the properties of the Subantarctic mode water (SAMW). After that, McCartney and Talley then applied the term subpolar mode water (SPMW) to the thick near-surface mixed layers in the North Atlantic’s subpolar gyre.

A Wind generated current is a flow in a body of water that is generated by wind friction on its surface. Wind can generate surface currents on water bodies of any size. The depth and strength of the current depend on the wind strength and duration, and on friction and viscosity losses, but are limited to about 400 m depth by the mechanism, and to lesser depths where the water is shallower. The direction of flow is influenced by the Coriolis effect, and is offset to the right of the wind direction in the Northern Hemisphere, and to the left in the Southern Hemisphere. A wind current can induce secondary water flow in the form of upwelling and downwelling, geostrophic flow, and western boundary currents.

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The Agulhas Leakage is an inflow of anomalously warm and saline water from the Indian Ocean into the South Atlantic due to the limited latitudinal extent of the African continent compared to the southern extension of the subtropical super gyre in the Indian Ocean. The process occurs during the retroflection of the Agulhas Current via shedding of anticyclonic Agulhas Rings, cyclonic eddies and direct inflow. The leakage contributes to the Atlantic Meridional Overturning Circulation (AMOC) by supplying its upper limb, which has direct climate implications.

Oceanic freshwater fluxes are defined as the transport of non saline water between the oceans and the other components of the Earth's system. These fluxes have an impact on the local ocean properties, as well as on the large scale circulation patterns.

References

  1. ↑ Ridgway, K. R. (2007). "Observational evidence for a Southern Hemisphere oceanic supergyre". Geophys. Res. Lett. 34 (L13612). doi:10.1029/2007GL030392.
  2. ↑ Rosell-Fieschi, Miquel (2013). "Tasman Leakage of intermediate waters as inferred from Argo floats". Geophys. Res. Lett. 40 (20): 5456–5460. doi:10.1002/2013GL057797. hdl: 10261/90093 . S2CID   20016530.
  3. ↑ Rosell-Fieschi, M. (2013). "Tasman Leakage of intermediate waters as inferred from Argo floats". Geophys. Res. Lett. 40 (20): 5456–5460. doi: 10.1002/2013GL057797 .
  4. ↑ van Sebille, E. (2012). "Tasman leakage in a fine‐resolution ocean model". Geophys. Res. Lett. 39 (L06601). doi: 10.1029/2012GL051004 .
  5. 1 2 Speich, S (2002). "Tasman leakage: A new route in the global ocean conveyor belt". Geophys Res Lett. 29 (10). doi: 10.1029/2001GL014586 .
  6. ↑ van Sebille, E. (2014). "Pacific‐to‐Indian Ocean connectivity: Tasman leakage, Indonesian Throughflow, and the role of ENSO". J. Geophys. Res. Oceans. 119 (2): 1365–1382. doi: 10.1002/2013JC009525 .
  7. ↑ Speich, Sabrina (2002). "Tasman leakage: A new route in the global ocean conveyor belt" (PDF). Geophys. Res. Lett. 29 (10). doi:10.1029/2001GL014586. S2CID   13464732.
  8. ↑ Speich, S. (2007). "Atlantic meridional overturning circulation and the Southern Hemisphere supergyre". Geophys. Res. Lett. 34 (L23614). doi: 10.1029/2007GL031583 .
  9. ↑ Speich, S (2002). "Tasman leakage: A new route in the global ocean conveyor belt". Geophys Res Lett. 29 (10). doi: 10.1029/2001GL014586 .
  10. ↑ Ridgway, K. R. (2007). "Observational evidence for a Southern Hemisphere oceanic supergyre". Geophys. Res. Lett. 34 (L13612). doi:10.1029/2007GL030392.
  11. ↑ van Sebille, E. (2014). "Pacific‐to‐Indian Ocean connectivity: Tasman leakage, Indonesian Throughflow, and the role of ENSO". J. Geophys. Res. Oceans. 119 (2): 1365–1382. doi: 10.1002/2013JC009525 .
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