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Generally, the path of the Antarctic Circumpolar Current does not depart greatly from the average path shown in Figure 5.30. but in the central and wesiern South Pacific, it can shift by more than 10" of latitude between summer and winter. Flow within the APFZ is complex, w ilh many eddies and meanders. The current jet associated w ith ihe Antarctic (Polar| Front has been observed to meander southwards and capiure colder water, and there are numerous eddies w¡thin the frontal /one. both cold-core and warm-core, You may remember from Sections 3.5.2 and 4.3,6 that such eddies are a \ery important heat-transfer mechanism, in (his case enabling cold water lo move norih across the Antarctic Circumpolar Current, and w arm water to move south.

Although ii show s up clearly on maps of dynamic topography because ii is so broad, away front the frontal jets the Antarctic Circumpolar Current is not particularly fasl: south of the Antarctic From, surface speeds are about 0.04 nis-1. while in the faster region, to ihe north of the Antarctic Front, they may reach 0,15-0,2 m s_1. However, the current is very deep, extending to the sea-floor at about 4000 m depth, and iis volume transport is therefore enormous. Estimates suggest lhal ihe average transport through the Drake Passage is about 130 X urm's-1. so in terms of volume transport, the Antarctic Circumpolar Current is certainly the mightiest current in ihe oceans.

For a long time, oceanographers were puzzled that the Antarctic Circumpolar Current was not faster than it is, given that it travels uninterrupted around the globe under the cumulative influence of the westerlies. The answer seems to be that the eastward wind stress is more or less balanced by frictional forces resulting from interaction of the flow with the sea-floor topography (which, as indicated by Figure 5.30. may also account for some of the north-south variation in the path of the current). Friction generated by turbulence adjacent to and within the Antarctic Circumpolar Current itself may also play a part, along with that acting on the current as it flows through the restricted Drake Passage.

So far, we have only been considering horizontal motion. However, as discussed above, the APFZ is a region of convergence of surface waters; and between the eastward-flowing Antarctic Circumpolar Current and the westward-flowing Polar Current is a divergence of surface water - the Antarctic Divergence.

What significance do such divergences and convergences have for the three-dimensional circulation of the ocean

They must lead to vertical motion: upwelling at the divergences and sinking at the convergences. The Antarctic Divergence is biologically one of the most productive regions of open ocean in the world. The nutrient-rich water upwelled there leads to high primary productivity which supports large populations of zooplankton, and bigger organisms ranging from krill to whales.

The convergences in the Antarctic Polar Frontal Zone are an important source of cold deep sub-thermocline water for the world ocean. Even colder 'bottom water' is formed off the Antarctic continent. These deep and bottom waters will be discussed further in Chapter 6.

The Antarctic Circumpolar Wave

By now. you will be becoming familiar with the idea that the ocean and atmosphere are not separate but closely coupled one to another, and that the ocean-atmosphere system has natural oscillations with a wide variety of periods. It is clear that the oscillations in the different oceans are often (but not always) inter-related, generally via the atmosphere, particularly by upper level winds. There is. however, another way in which the three oceans are linked, namely by the Southern Ocean, which encircles the globe, and this is the setting for one of the most fascinating of the natural oscillations. This wave-like disturbance involves not only the ocean and atmosphere, but also the cryosphere - the ice-cover.

The Antarctic Circumpolar Wave has been detected through analyses of data relating to atmospheric pressure at sea-level, meridional wind stress, sea-surface temperature (monthly averages) and sea-ice extent, collected over 13 years. When the normal seasonal changes in these four parameters are subtracted from the variability actually observed, a wave-like pattern of highs and lows is revealed. The wave has two wavelengths end-to-end, so there are two high-pressure anomalies, and two low-pressure anomalies, two maxima and two minima in meridional (north-south) wind stress, two areas of anomalously high sea-surface temperature and two areas of anomalously low sea-surface temperature, and two 'bulges' in ice-extent, all travelling eastward around the Antarctic continent (Figure 5.33).

The wave has a period of 4-5 years, and therefore takes 8-10 years to travel all the way round the Antarctic continent. The maxima and minima of the different components do not all coincide (i.e. they are out of phase with one another), but they all travel around the continent with the same speed, suggesting that they are closely linked together by feedback loops acting over the time-scales in question. The speed falls within the range of current velocities found in the Antarctic Circumpolar Current, which is perhaps not surprising given that the ACC can transport unusually warm or unusually cold surface waters around the globe.

The wave is not a purely Antarctic phenomenon. It is believed to be generated by ENSO signals propagating from the equatorial Pacific, probably via the atmosphere. The anomalies in sea-surface temperature travel northwards in the Pacific with the Peru (Humboldt) Current and northwards in the Atlantic with the Benguela Current, and generally spread equatorwards in all three oceans, probably as a result of Ekman transport in response to the westerly winds over the Southern Ocean.

Figure 5.33 Simplified schematic map of the Antarctic region and the mean path of the Antarctic Circumpolar Current, to show the characteristics of the Antarctic Circumpolar Wave, which involves sea-surface temperature (red, warm; blue, cold), atmospheric pressure at sea-level (H = anomalously high pressure, L = anomalously low pressure), meridional wind stress (open arrows), and sea-ice extent. The heavy arrows indicate the general eastward motion of the Wave, and the blue and red arrows indicate the direction of travel of anomalous conditions within the ocean, southward from the western Pacific, eastward around the Antarctic continent (with the ACC), and northward into the eastern Pacific, Atlantic and Indian Oceans.

Figure 5.33 Simplified schematic map of the Antarctic region and the mean path of the Antarctic Circumpolar Current, to show the characteristics of the Antarctic Circumpolar Wave, which involves sea-surface temperature (red, warm; blue, cold), atmospheric pressure at sea-level (H = anomalously high pressure, L = anomalously low pressure), meridional wind stress (open arrows), and sea-ice extent. The heavy arrows indicate the general eastward motion of the Wave, and the blue and red arrows indicate the direction of travel of anomalous conditions within the ocean, southward from the western Pacific, eastward around the Antarctic continent (with the ACC), and northward into the eastern Pacific, Atlantic and Indian Oceans.

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