The Southern Ocean

The more or less zonal distribution of the ice cover around Antarctica, shown in Figure 5.27(d). reflects the predominantly zonal current flow. As shown in Figure 5.30. there are cyclonic subpolar gyres in the Weddell and Ross Seas but the predominant feature of the circulation of this region is the Antarctic Circumpolar Current - the only current to flow around the globe without encountering any continuous land barrier.

Given that the overlying winds are essentially westerly, in which direction would you expect the sea-surface to slope in the region of ihe Antarctic Circumpolar ("urrent?

As this is in the Southern Hemisphere. Ekman transport is to the left of the wind, and the sea-surface slopes down towards the Antarctic continent (see Frontispiece and Figure 3.21). This sea-surface slope generates a geostrophic slope current to the east. i.e. flow in the same direction as the wind but extending to greater depths than the surface wind-driven layer (cf. Figure 3.25 for a similar situation in the subtropical gyres). Below the wind-driven layer, the density distribution is such that, in general, the horizontal pressure gradient force and the Coriolis force balance, and geostrophic equilibrium is maintained.

Figure 5.30 Schematic map showing the mean path of the Antarctic Circumpolar Current (blue tone); the two dark blue lines represent the average positions of the Antarctic Front and the Sub-Antarctic Front, and the jets which flow along them (discussed in the text). Note that a significant part of the current branches northward and flows up the west coast of South America as the Peru Current; there is also a branch of the current flowing northwards below the surface between Australia and New Zealand. The approximate positions of the gyres in the Weddell Sea and Ross Sea are also shown, as is the path of the Polar Current. The Antarctic Divergence is between the Polar Current and the Antarctic Circumpolar Current. Blue-grey shading indicates water depths less than 3000 m.

South America

Antarctica

New Zealand

Figure 5.31 Sections of (a) temperature (°C) and (b) salinity across the Drake Passage. The isotherms and isohalines sloping up to the south from a depth of about 3000 m delineate a 'wedge' of warmer, less saline water flowing over colder, more saline water. The sections were made during the southern summer. The blue toned regions are fronts. At the surface, the sharpest changes in salinity and, particularly, temperature occur at the Antarctic Front; for this reason, the Antarctic Front was the first to be observed and was identified as the Antarctic Convergence.

In surface layers, the direct effect of the wind stress, combined with the Coriolis force, leads to a northward component of flow, and a region of convergences forms within the strongest part of the Antarctic Circumpolar Current. This region of convergences was originally thought to be a single convergence, and was named the Antarctic Convergence. It is now known to consist of a series of convergences, or fronts, and has been renamed the Antarctic Polar Frontal Zone (APFZ). The fronts in the APFZ are associated with strong zonal current jets, with velocities reaching 0.5-1.0ms"'. Two major jets occur in association with the northern and southern boundaries of the APFZ - the Sub-Antarctic Front and the Antarctic Front (also known as the Polar Front); average positions of these jets are shown in Figure 5.30.

Despite its great length - about 24000 km - the Antarctic Circumpolar Current has remarkably consistent characteristics wherever it is observed; furthermore, the Sub-Antarctic Front and the Antarctic Front persist throughout the extent of the current, although the distance between them is very variable (Figure 5.30). Figure 5.31 shows temperature and salinity sections across the Antarctic Circumpolar Current in the Drake Passage, between South America and the islands that lie to the north of the Antarctic Peninsula (cf. Figure 5.30). The blue bands indicate the positions of the fronts. The narrowest front is a boundary between oceanic waters and colder, fresher water that originated in the Weddell Sea; the other two fronts are the Antarctic Front and the Sub-Antarctic Front, mentioned above.

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As mentioned above, flow in the Antarctic Circumpolar Current is generally in geostrophic equilibrium. The isopyemc surfaces slope up towards the south, and the steeper their slope the greater the velocity of the eastward aeostrophic current. The fronts on Figure 5.3) are characterized by steeper slopes in the isotherms and isohalines. and hence in the ijiopytrials: they are therefore also characterized by fastergeostrophie currents (Section 4,3.2). Figure 5,32 is a more detailed picture of How through the Drake Passage. This shows not only three eastward current jeis (as in Figure 5.31) hut also two weaker eastward filaments separated by areas of westerly How.

latitude

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