Water Masses and Circulation Patterns

The area covered by the Southern Ocean (about 77x 106 km2) extends from the Antarctic coasts to a northern limit established by water mass characteristics. The isopleth lines in this area are approximately parallel to the lines of latitude, although gradients are more concentrated in frontal zones. The Antarctic Polar Front (APF) is the zone where cold Antarctic Surface Water sinks below warmer Sub-Antarctic Water and continues to flow northwards at intermediate depths as Antarctic Intermediate Water. The latter water mass can be traced through its low salinity and temperature of 4-5 °C; it can be detected in coastal waters emerging both near the continents of the Southern Hemisphere and north of the equator. Antarctic Intermediate Water thus plays a crucial role in the interchange of heat and minerals between the Southern Ocean and other oceans. Antarctic Circumpolar Deep Water, a warmer (about 1-2 °C), more saline (about 34.75 parts per thousand), nutrient-rich water

Fig. 21. Map of principal deep basins and submarine canyons around Antarctica

mass which lies below Antarctic Surface Water (Fig. 22), is the largest water mass in the Southern Ocean; it flows southwards from the world's warmer oceans at a depth of some thousand metres, rises at the APF and upwells at the Antarctic Divergence. Antarctic Bottom Water, another important water mass with slightly lower salinity and temperature (about -1 °C) and higher density, forms near the continent; it flows down along the continental slopes and

Fig. 22. Major water masses in the Southern Ocean

spreads northwards into the lowermost oceanic basins, cooling and ventilating a large proportion of deep sea (Whitworth et al. 1998). Antarctic Bottom Water mainly forms in the Weddell and Ross Seas and other coastal areas of East Antarctica as a result of winter freezing and particular regional conditions, such as the presence of deep ice shelves and polynyas. According to Godfrey and Rintoul (1998), the volume of dense water spreading northwards from near Antarctica is equal to or a little higher than the volume of dense water produced in the North Atlantic and, as a whole, dense water masses formed in the Southern Ocean account for more than 50 % of the volume of the world ocean.

The APF is actually a broad zone of transition with meanders and eddies, rather than the narrow band schematised in Fig. 20. Crossing the Polar Front in a southward direction, surface water temperature sharply decreases (about 3-8 °C in summer and 1-5 °C in winter). In the Weddell Sea-Drake Passage region, the APF has a seasonal variability of 1-2° in longitude and long-term variability of up to 4° in latitude (King and Turner 1997). Despite the variability in its position, width and temperature gradient, the APF divides the Southern Ocean into a sub-Antarctic region to the north and an Antarctic one to the south, with great differences in weather conditions and in physico-chemical and biological features. To the north of the APF, an increase in water salinity (about 0.5%o) marks the Sub-Tropical Convergence or Sub-Antarctic Front. This front encircles Sub-Antarctic Water and is regarded as the boundary between the Southern Ocean and three other oceans to the north. Its position varies widely, especially off the coast of Chile (Fig. 20). Antarctic Surface Water to the south of the APF and up to a depth of about 100-250 m is cold (from 1 to -1.9 °C in winter), with small seasonal variations and salinity usually less than 34.5%. However, salinity (and therefore density) increases when sea ice forms, and decreases when it melts.

Ocean circulation is driven not only by temperature and salinity (thermohaline circulation) but also by winds. The two forcing mechanisms coexist and, although we consider them separately, they cannot be separated geographically or dynamically. The winds in the Southern Ocean are among the most intense and constant and are characterised by an easterly flow from the continental margin to 65° S and by a wide zone of westerlies, which extends northwards to more than 40-35° S. In the latter zone the strong westerlies drive the largest current system of the world, the Antarctic Circumpolar Current (ACC) or West Wind Drift. The ACC flows clockwise around the continent at a relatively slow velocity (about 20 cm s-1, compared to more than 200 cm s-1 for the Gulf Stream), but it transports a water mass larger than that of any other oceanic current system - more than 100x106 m3s-1 (i.e. 100 Sv, Sverdrup; Nowlin and Klinck 1986). The flow is strongly affected by bottom topography, because the current may reach depths of 3,000 m. The Antarctic Peninsula and southern part of South America constrain the flow of the ACC, determining a convergent flow in the Drake Passage, with an increase in speed (up to 1m s-1) and water volume (up to 150 Sv).

In marine areas south of 65° S and close to the coast of Antarctica, there is a westward-flowing coastal current (the Antarctic Coastal Current, water volume of about 10 Sv) driven by easterly winds (i.e. katabatic drainage winds from Antarctic ice sheets which are deflected leftwards by the Earth's rotation). The westward coastal current in the Weddell and Ross Seas is diverted northwards by the Antarctic Peninsula and Victoria Land, and this results in a cyclonic circulation of the sea (Fig. 20). In addition to these two large, permanent-flowing gyres, another one occurs east of the Kerguelen Plateau (Gordon and Molinelli 1982; Deacon 1984), and several eddies, current rings and meanders have been reported in the Drake Passage and other regions of the Southern Ocean (e.g. Joyce and Patterson 1977; Gordon 1988).

In the Southern Hemisphere, the Coriolis force deflects ocean currents to the left, thereby driving the ACC to the north and the Antarctic Coastal Current to the south. This divergent flow (the Antarctic Divergence; Fig. 22) promotes the upwelling of warm and nutrient-rich subsurface Circumpolar Deep Water in the region between the ACC and the Antarctic Coastal Current, except east of the Drake Passage (Deacon 1984).

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