Australia: The Land Where Time Began
Antarctic Circumpolar Current
The water of the Southern Ocean is dominated by the Antarctic Circum Polar Current, a current of surface waters, that flows from west to east. It formed in response to the spinning of the earth when the way became clear for it to travel unhindered around Antarctica. By 27 million years ago it had become fully established, as indicated by fossil evidence. It is the largest of the Earth's ocean currents. The water near the coast of Antarctica flows in the opposite direction, from east to west. It is believed that the shape of the Antarctic continent causes localised eddies. The features believed to cause this effect by changing the width and speed of the current flowing to the east, are the large indentations of the Ross Sea and the Weddell Sea, the Antarctic Peninsula that projects north towards the southern tip of South America, and the varying contour of the seafloor. The wind shear from the southern edge of the subantarctic cyclonic winds, as well as geotropic currents of fresher coastal water flowing north, are believed to contribute to the coastal countercurrent.
The strong Antarctic Circumpolar Circulation (ACC) continuously from the west to the east around the coast of Antarctica without wrapping back to the west, dominating the large-scale circulation of the Southern Ocean. According to Talley et al. at Drake Passage latitudes there is no western boundary, hence no support for western boundary currents and wind gyres in the open ocean, though in the deep and abyssal waters barriers exist in the form of deep topography. Talley et al. say the closest analogue in the oceans to the major wind systems, the westerly's and the easterlies, as there are also no boundaries to the atmosphere. The strongest in the ACC lie to the north or south of the Drake Passage, where there are western boundaries, South America to the north and to the south the Antarctic peninsula, spending a short time in the Drake Passage. There are 2 major indentations among those along the coast of Antarctica, the Weddell Sea and the Ross Sea, to the south of the ACC. There are gyres that are wind-driven that are supported in these 2 seas with western boundary currents.
The northernmost excursion of the ACC is about 38°S. Talley et al. say the greatest latitudinal blockage to the flow of the ACC is in the Drake Passage and the Scotia Arc to its east. As well as Drake Passages there are 2 other wider regions where the flow of the ACC is constricted, to the south of Africa and Australia, all 3 constrictions being areas where the continents of the former Gondwana are still moving away from Antarctica. All these constricted regions western boundary currents, the Brazil Current, the Agulhas Current and the East Australian Current, all warm water from the tropics that is flowing to the south and interact with the circulation of the Southern Ocean.
There are a number of subsurface plateaus, the Kerguelen Plateau, Campbell Plateau and the Falkland Plateau, that deflect the ACC. Strong steering of the ACC also occurs where it crosses through gaps in the mid-ocean ridges in the Southern Ocean. Deep topography at the latitudes of Drake Passage can allow meridional geostrophic flow in the region of Drake Passage-Scotia Arc, the Kerguelen Plateau, and to the south of New Zealand, the Macquarie Plateau (Warren, 1990: Section 13.5).
The ACC carries most flow between the major oceans as it connects all 3 major ocean basins, and it is a deep-reaching current, though there is also some minor flows between the major oceans in the north, as from the Pacific Ocean to the Indian Ocean through gaps in the Indonesian islands, 10-15 Sv and a flow of about 1 Sv from the Pacific to the Atlantic via the Arctic Ocean. Both of these minor flows are small compared with the more than 100 Sv of the ACC. Present in the ACC are the unique character of the deep-water masses from each of the oceans. According to Talley et al. waters reemerge and enter the ocean basins to the north of the ACC after mingling, upwelling and being transformed to both denser and lighter waters.
In the latitudes of the The Subantarctic Front (SAF) and the Polar Front (PF), both part of the ACC, Ekman transport to the north is driven by the westerlies. Upwelling water in the south feeds the net transport across the (circumpolar) SAF which is about 30 Sv, a figure that Talley et al. consider to be significant.
The SAF, the PF, and the Southern ACC Front (SACCF) are fronts that surround Antarctica as part of the ACC
|Author: M.H.Monroe Email: email@example.com Sources & Further reading