Australia: The Land Where Time Began
The Southern Ocean
The Southern Ocean formed when the gap between Antarctica and the southernmost margins of the southern continents opened as they began drifting north at the break up of Gondwana. The water of the Southern Ocean is dominated by the Antarctic Circum Polar Current. Local currents and conditions lead to an uneven distribution of marine life around the continent.
Prior to the separation of South America and Australia from Antarctica, cold currents flowing along the Antarctic coast were diverted north to the tropics when they struck the west coasts of South America and Australia, returning south to Antarctica after they had been heated by their passage through the equatorial regions, taking that heat south to warm Antarctica. These warming currents were disrupted by the opening of the ocean between Antarctica and South America and Australia, allowing the polar regions to become a progressively colder closed climatic system. The southern parts of Australia became cooler, and the latitudinal temperature gradients steepened, and the climatic zones became more pronounced. The movement of Australia north gradually moved the central and northern parts of the continent away from the moist westerly winds, and into the region of the drier, warmer subtropical high pressure systems (Bowler, 1982; Bowman, 2000).
As the ocean to the south of the drifting southern continents was widening, allowing the Southern Ocean to form the strengthening circumpolar current, the equatorial oceans were being progressively blocked by the northward movement of the continents of the Southern Hemisphere. As Africa slammed into Laurasia it closed the remaining part of the Tethys Sea, isolating the Mediterranean Sea Basin. The western end of the Mediterranean Basin was closed by the closing of the gap between Spain and Morocco.
The movement of Australia into the north, ploughing into a previously continuous string of islands, making the flow of water between the Pacific and Indian Oceans more restricted.
The final gap to be blocked was that between North and South America, as the Isthmus of Panama rose about 5 million years ago. Throughout the Cainozoic Era the equatorial flow was being progressively weakened as each gap was closed by the drifting continents.
Permanently ice covered regions are present close to the coast of Antarctica. Around this is a zone of seasonal ice, where ice covers the sea for most of the year, and between this zone and the Polar Front is a zone ice-free sea.
The Polar Front, the point where the cold circumpolar water meets the warm water from the north, had formed by 22 million years ago. This is a major biological barrier in the circumpolar zone, the water immediately south of the boundary of the northern water, is 2o C colder.
The Permanent Open Ocean Zone has high winds and rough seas as a result of the strong current and strong winds. The result is that there is deep mixing of the surface waters. It is a region of relatively high nutrients but the primary production is kept below its potential because the deep mixing pushes much of the phytoplankton below the photic zone. The phytoplankton in this zone are mostly very small nannoplankton, such as diatoms and flagellates.
The ecosystems of these zones are connected but are 3 separate ecosystems, not a single ecosystem as is sometimes believed.
The Southern Ocean produces deep and bottom waters that are very dense, mostly along the coast of Antarctica, as a result of its high southern latitude and its production of sea ice, and these dense waters fill the deepest part of the oceans to the north of the ACC.
On an annual basis wind forcing is dominated by the westerlies in the Southern Ocean in the band of latitudes from 40-60°S and closer to Antarctica, south of 60°S, by easterlies. The westerlies are not zonally uniform, being at a maximum in the section of the Southern Ocean to the south of the Indian Ocean that centres on about 50°S. There is also a significant southward component associated with the westerlies to the south of Australia in the Indian Ocean. In the latitudes of the Subantarctic Front (SAF) and Polar Front (PF), part of the ACC, the westerlies in a northern direction. The significant northward transport across the (circumpolar) SAF, on the order of 30 Sv, must be fed from the south by upwelling water. Associated with Ekman upwelling and downwelling is the wind stress curl, that is zero when associated with zero Ekman upwelling, occurs at maximum wind stress at around 50°S. South of this, and closer to the coast of Antarctica, positive values of upwelling occur. To the north of the maximum of westerly wind Ekman downwelling occurs which is strongest in the eastern Atlantic Ocean and throughout the sector of the Southern Ocean to the south of the Indian Ocean.
The easterly winds that occur closet the coast of Antarctica can be very strong because of the continental forcing of the katabatic winds, which include a northward component. Downwelling is induced at the boundary as Ekman transport is driven by the easterly winds, resulting in the sea surface mounding and the pycnocline deepening adjacent to the continent. The geostrophic flow found near the continent at most locations is a result of this.
The sum of air-sea fluxes of heat and freshwater is the surface buoyancy forcing. The authors² say that south of about 45°S the net buoyancy flux is positive, meaning the surface waters become less dense. The only large region of the world ocean where a significant contribution is made by freshwater fluxes to the air-sea flux is in this region, though similar magnitude heat fluxes here warm the ocean. According to the authors² the upwelling of very cold water , subsequently associated with Ekman transport to the north appear to control the air-sea fluxes with the result that the cooler water is equilibrated by the maritime air that is slightly warmer. In the ACC the buoyancy/heat gain occurs along the SAF and PF, places of high westerly winds resulting in high levels of Ekman transport to the north.
The western boundary current regions, Agulhas Current, East Australian Current and Brazil Current, and the Leeuwin Current along the west coast of Australia, are the regions of the Southern Ocean where buoyancy loss that is due almost entirely to heat loss is highest. The authors² say in these regions the annual mean heat losses are more than 100 W/m². Along the Agulhas Return Current, extending to the southeastward from Africa to a point more than half way across the Indian Ocean is a zonal band of buoyancy (heat) loss with values more than 25 - 50 W/m². In a similar position to the north of the SAF there is also an quazi-zonal band of loss of buoyancy in the Pacific Ocean. The southward mean flows bringing warmer waters into cooler regimes are associated with the regions of highest buoyancy loss.
According to the authors² In the Southern Ocean isopleths of all properties are almost zonal to great depths in an east-west direction as a result of the open zonal passage and the ACCA being nearly zonal. Especially in the zonal range of Drake Passage this zonal nature is illustrated by the near-surface potential temperature, salinity and geopotential anomaly. The cyclic gyres, that are clockwise in the Southern Hemisphere, in the Weddell Sea and the Ross Sea, and that are not zonal in the ACC zonal band, organise the surface properties and circulation to the south of the ACC.
In the ACC the near-zonal isopleths properties are organised into 3 major fronts that separate 4 broad zones in which the isopleths are more widely separated. The currents to the east are strong within the fronts, and in the zones between the fronts the flow can be in any direction as eddies dominate it.
Within the ACC the fronts that are part of it that encircle Antarctica are the SAF and PF and the southern ACC Front (SACCF). The Southern Boundary (SB), to the south of the SACCF has been defined as the southern edge of the low oxygen layer of the Upper Circumpolar Deep Water (UCDW), a non-dynamical front (Orsi, Whitworth & Nowlin, 1995). The Antarctic Slope Frons (ASF) is present along the continental slope in most locations, flowing westwards, and it separates very dense water of the shelf from the offshore water (Jacobs, 1991; Whitworth, Orsi, Kim & Nowlin, 1998). The Antarctic Coastal Current (ACoC) is found on shelves close to the coast, especially where they are broad.
The Subantarctic Zone (SAZ; north of the SAF), the Polar Front Zone (PFZ; between the SAF and the PF), the Antarctic Zone (AZ; between the PF and the SACCF), the Southern Zone (SZ; between the SACCF and the SB), and the subpolar region (south of the SB; Orsi et al., 1995). The Weddell Gyre and the Ross Sea Gyre are included in the Subpolar Region (Subpolar Zone; SPZ). Dense shelf water is present south of the ASF on the continental shelf; the authors suggesting it can be considered to be the Continental Zone.
There are 3 major fronts in the near-zonal isopleths in the ACC separate 4 broad zones where the isopleths are spaced more widely. The strong currents flow eastwards within these fronts. The flow in the zones between the fronts can be in any direction because they are dominated by eddies.
Prior to the classification of Orsi et al (1995) the commonly used older scheme that it superseded didn't include the SACCF or the SB, though it identified a Continental Water Boundary and a Continental Zone (CZ). The Drake Passage region was the only part of the Southern Ocean that these older schemes were appropriate for.
Zones of the Southern Ocean
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|