Current Systems In The Southern Ocean

CURRENT SYSTEMS IN THE SOUTHERN OCEAN

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CURRENT SYSTEMS IN THE SOUTHERN OCEAN A. L. Gordon, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA Copyright

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2001 Academic Press

doi:10.1006/rwos.2001.0369

Summary The Southern Ocean, encircling Antarctica, plays a major role in shaping the characteristics of the global ocean. It provides the most signiRcant interocean conduit by which waters of the three major oceans, the Atlantic, PaciRc and Indian, can intermingle, acting to diminish their differences in temperature, salinity, and chemical properties. The most prominent current is the Antarctic Circumpolar Current, which transfers about 134 million m3 s\1 of sea water from west to east within a latitudinal range from 503 to 603S. North of the Antarctic Circumpolar Current are the poleward limbs of the large subtropical gyres of the southern hemisphere, referred to as the South Atlantic, South Indian, and South PaciRc Currents. Within W 20°

large embayments of Antarctica, notably the Weddell and Ross Seas, south of the Antarctic Circumpolar Current are large clockwise Sowing gyres. The Weddell Gyre carries about 30}50 million m3 s\1 of water. Along the continental margin of Antarctica is the coastal current that advects water from east to west. The coastal current is directed towards the north along the east coast of Antarctic Peninsula, forming the western boundary of the Weddell Gyre. At the northern tip of the Antarctic Peninsula the coastal current is directed into the open ocean. The coastal waters injected into the open ocean separate the Antarctica Circumpolar Current from the Weddell Gyre, in what is called the Weddell-Scotia ConSuence. Besides ocean currents Sowing on nearly horizontal planes, the Southern Ocean experiences major overturning of ocean water. Overturning is forced by the production of dense surface water along the margins of Antarctica, leading to the formation of Antarctic Bottom Water. Within the Antarctic Circumpolar Current, at the Polar Front, 0° E 20°

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surface waters sink under the more buoyant surface water to the north, forming Antarctic Intermediate Water, a low salinity intrusion spreading at a depth of nearly 1000 m under the main thermocline of subtropical ocean. The horizontal and vertical circulation inSuences the distribution of sea ice, which in turn modiRes the heat, freshwater, and gas exchange between the Southern Ocean and polar atmosphere.

Introduction Ocean currents are the product for the most part of the stress exerted on the sea surface by the wind. The winds are strong over the Southern Ocean, particular within the Indian Ocean and Australian sectors (Figure 1) and therefore drive a vigorous circulation (Figure 2). Strong westerlies (wind directed from west to east) extend from the subtropical high atmospheric pressure near 303S, a latitude often used to deRne the northern limits of the Southern Ocean, to a belt of low atmospheric pressure at

653S. South of 653S the winds are easterlies, marking the northern edges of the polar high pressure over Antarctica. Density changes of surface water induced by sea}air Suxes of heat and fresh water, often involving sea ice within the Southern Ocean, also produce circulation. However, buoyancy (sometimes referred to as thermohaline), circulation is sluggish and mainly occurs within the meridional vertical plane, as dense water sinking forces slow, compensatory upwelling of less dense resident water. Sinking of dense waters along the continental margins of Antarctica results in Antarctic Bottom Water. Ocean currents are for the most part in equilibrium with the distribution of density within the ocean (Figure 3) satisfying approximately the socalled ‘thermal wind equation’ (see Elemental Distribution: Overview; Ocean Circulation.) Along the Greenwich meridian the surface of equal density, or isopycnals, rise up towards the sea surface as latitude increases to the south. Strongly sloped isopycnals are coupled to strong ocean current, more

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precisely to strong geostrophic ocean currents, relative to the seaSoor. As a rule of thumb, in the southern hemisphere higher density water occurs to the right of the direction of the ocean current. Hence the increasing density as latitude increases is linked to west to east Sow of water. Along the margin of Antarctica the descent of isopycnals marks a Sow towards the east. Regions of rapid changes in temperature, salinity or density mark the positions of ocean fronts, often coinciding with a strong ocean current. Maximum westerlies in the wind occur near 553S, which roughly coincides with the axis of the Antarctic Circumpolar Current. To the south of this latitude, wind-induced northward Ekman transport of surface water results in a wide region of upwelling. North of 553S the Ekman transport diminishes. This causes a region of surface water convergence. Near 553S surface water sinks under the more buoyant surface water to the north, producing Antarctic Intermediate Water. Antarctic Intermediate Water forms a low salinity layer found at the base of the thermocline of the subtropical southern hemisphere regions. Upwelling poleward of the maximum westerlies brings deeper water to the sea surface to compensate for the sinking of Antarctic Bottom Water and Antarctic Intermediate Water. The up-

welling also drives two large clock-wise-Sowing, cyclonic Gyres within the large embayments of Antarctica, marking the Weddell and Ross Seas (Figure 2) and a smaller one east of Kerguelen Plateau.

Antarctic Circumpolar Current The most prominent current of the Southern Ocean is the west to east Sowing Antarctic Circumpolar Current lying within a latitudinal range from 503 to 603S. It is the greatest ocean current on the Earth, covering a distance of 21 000 km, with an average transport through the Drake Passage (between South America and Antarctic Peninsula) of 134 million m3 s\1 (134 Sv). The transport varies with time mirroring variations in the circumpolar wind Reld, from about 100 to 150 million m3 s\1. Transport is enhanced south of Australia by return of water to the PaciRc Ocean lost to the Indian Ocean within the Indonesian Seas, by about 10 million m3 s\1. The Antarctic Circumpolar Current transport passing between Tasmania and Antarctica is estimated as 143 million m3 s\1, with a range from 131 to 158 million m3 s\1. The Antarctic Circumpolar Current is a deep reaching or barotropic current, meaning that it

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extends to the seaSoor. Because of this it is said that the Antarctic Circumpolar Current ‘feels’ the shape of the sea Soor and hence its path is steered by the seaSoor topography (Figure 4). The Sow following the southern deSection in the mid-ocean ridge reaches its southern-most position in the southwest PaciRc Ocean, near 603S. Upon passing through Drake Passage it turns sharply to the north, transversing the Atlantic Ocean near 503S. As the ocean surface temperature pattern responds to the circulation pattern the surprising result is that the bottom topography is ‘projected’ in the sea surface temperature pattern. Rather than a broad diffuse Sow, the Antarctic Circumpolar Current is composed of a number of high speed Rlaments, separated by zones of low Sow, or even reversed Sow (towards the west). The jets are typically 40}50 km wide. Surface currents average about 30}40 cm s\1 within the axes, and speeds of over 100 cm s\1 are common. The high

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speed Rlaments are marked by ocean fronts, where the temperature and salinity stratiRcation changes rapidly with latitude (Figure 5). Between these fronts are zones of similar stratiRcation. The primary axis occurs at the polar front (Figure 6). Meanders of the Sow axes and associated fronts displace these features by at least 100 km to either side of their mean position. Meanders occasionally produce detached eddies, in which pools of water ringed by a high speed current from one zone invade an adjacent zone. A characteristic of the Antarctic Circumpolar Current is its high degree of eddy activity (Figure 7). The most active eddy Relds are observed where the Antarctic Circumpolar Current crosses submarine ridges or plateaus, as south of Australia, in the southwest Atlantic and south of Africa. The correlation of the eddy currents and of the ocean temperature leads to signiRcant poleward Sux of ocean heat by the eddy processes. The poleward

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Figure 5 Vertical averaged geostrophic speeds of the upper 2500 m directed normal to a line across the Drake Passage. The positions of the front and stratification zones are shown. The highest flows defining the axes of the Antarctic Circumpolar Current coincide with the position of the ocean fronts. (Reproduced with permission from Clifford, 1983.)

heat Sux measured in the Drake Passage, if extrapolated all around Antarctica, is 0.3 PW, which can account for most of the meridional heat Sux across 603S. However caution is suggested as the Drake Passage eddy Reld may not be typical of the full circumpolar belt. Meanders and eddies of the Antarctic Circumpolar Current also act to carry wind-delivered momentum downward to the seaSoor, where pressure forces (often referred to as form drag) acting on the slopes of bottom topo-

graphic features act to compensate the force of the wind. The downward transfer of momentum is integrally linked to the meridional Suxes of heat and fresh water by the eddy Reld, by baroclinic instability.

Weddell Gyre The Weddell Gyre is the largest of the cyclonic Gyres occupying the region between the Antarctic

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Figure 6 Position of the Antarctic polar front for the years 1987}1993 as revealed by satellite images of sea surface temperature. (Reproduced with permission from Moore et al., 1999.)

Circumpolar Current and Antarctica, stretching from the Antarctic Peninsula to 303E. The clockwise Sow pattern is linked to doming of isopycnals and upwelling of deep water within its central axis (Figure 3). As the deep water is warmer than the surface layer the Gyre injects heat into the surface layer, which limits the winter sea ice cover to a thickness of only 0.5 m. Along the Antarctic margins intense cooling of surface water lead to the formation of the coldest, densest ocean water masses of global importance: Antarctic Bottom Water. A branch of the Antarctic Circumpolar Current turns southward near 303E forming the eastern limb of the Weddell Gyre. Some of this water turns westward along the coast of Antarctica (the rest continues to Sow eastward, but at a more southern latitude than the axis of the Antarctic Circumpolar Current). Westward Sowing coastal current is characteristic all around Antarctica, with a surface speed of about 10 cm s\1 as detected by satellite tracking of the drift of icebergs calved from Antarctica. Within the Weddell Gyre the coastal current westward Sow is blocked by the Antarctic Peninsula. Upon encountering the southern base of the penin-

sula, the coastal current turns northward, forming the western boundary current of the Weddell Gyre. At the northern tip of the Antarctic Peninsula, the western boundary current composed of the cold, low salinity stratiRcation characteristic of Antarctic continental margin, is injected into the open ocean. This feature, called the Weddell-Scotia ConSuence (Figure 2) separates the Antarctic Circumpolar Current from the interior of the Weddell Gyre. It can be traced as a low salinity band to the Greenwich Meridian. Along the sea Soor Antarctic Bottom Water escapes from the Gyre, Sowing northward within deep crevices in the seaSoor morphology, into the Scotia Sea, South Sandwich Trench and south of Africa. Export of Bottom Water is compensated by import of circumpolar water along the eastern boundary. Surface currents of the Weddell Gyre are weak, usually 10 cm s\1, but the Sow extends to the seaSoor, as a strongly barotropic current. There is some evidence that the current increases along the seaSoor of the continental slope, with speeds of up to 20 cm s\1, associated with plumes of dense shelf water descending into the deep ocean as Antarctic

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Figure 7 Variability of sea surface height from mean sea level, as revealed by Topex Poseidon satellite altimetric measurements of sea level for the period 1992}1999. Values are in meters. Variability of sea level is caused by meanders and eddies of the geostrophic flow field, or changes in its strength. (Provided by Donna Witter, Associated Research Scientist at Lamont-Doherty Earth Observatory.)

Bottom Water. Observations of ocean currents during the period from 1989 to 1992 across the mouth of the Weddell Sea, stretching from Kapp Norvegia (71320S; 11340W) to the northern tip of the Antarctic Peninsula, Rnd Gyre transport of about 30 million m3 s\1 (30 Sv), most of which is contained in narrow jets following along the continental slope. An additional 10 million m3 s\1 (10 Sv) of transport is likely around the central axis of the Gyre, making a total recirculation transport around the Gyre of 40 million m3 s\1 (40 Sv). Export from the Gyre is not known exactly, but can be estimated as 5 million m3 s\1 (5 Sv) within the bottom layer.

See also Agulhas Current. Antarctic Circumpolar Current. Atlantic Ocean Equatorial Currents. Icebergs. Indian Ocean Equatorial Currents. Indonesian Through]ow and Leeuwin Current.

Further Reading Belkin IM and Gordon AL (1996) Southern ocean fronts from the Greenwich Meridian to Tasmania. Journal of Geophysical Research 101(C2): 3675}3696. Clifford MA (1983). A Descriptive Study of the Zonation of the Antarctic Circumpolar Current and its Relation to Wind Stress and Ice Cover. MS thesis, Texas A & M University. Gille S (1994) Mean sea surface height of the Antarctic Circumpolar Current from Geosat data: methods and application. Journal of Geophysical Research 99: 18 255}18 273. Gille S and Kelly K (1996) Scales of spatial and temporal variability in the Southern Ocean. Journal of Geophysical Research 101: 8759}8773. Gordon AL, Molinelli E and Baker T (1978) Large-scale relative dynamic topography of the Southern Ocean. Journal of Geophysical Research 83: 3023}3032. Hofmann EE (1985) The large-scale horizontal structure of the Antarctic Circumpolar Current from FGGE drifters. Journal of Geophysical Research 90: 7087}7097.

CURRENT SYSTEMS IN THE SOUTHERN OCEAN Nowlin W and Klinck J (1986) The physics of the Antarctic Circumpolar Current. Reviews of Geophysics 24(3): 469}491. Moore JK, Abbott M and Richman J (1999) Location and dynamics of the Antarctic Polar Front from satellite sea surface temperature data. Journal of Geophysical Research 104(C2): 3059}3073. Orsi AH, Nowlin WD and Whitworth T (1993) On the circulation and stratiRcation of the Weddell Gyre. Deep-Sea Research 40: 169}203.

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Orsi AH, Whitworth T and Nowlin WD (1995) On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep-Sea Research 42(5): 641}673. Whitworth T (1980) Zonation and geostrophic Sow of the Antarctic Circumpolar Current at Drake Passage. Deep-Sea Research 27: 497}507. Whitworth T and Nowlin WD (1987) Water masses and currents of the Southern Ocean at the Greenwich Meridian. Journal of Geophysical Research 92: 6462}6476.