re la! i very waier coW Dense water winds (rom Antarclie continent newiy formed iCe pusnéo aw a y from toast y
Orme formation cold, nigh salinity water u
Figure 6.24 The different roles played by coastal and 'open ocean' polynyas in the production of Antarctic Bottom Water.
\Miat effect will the continued production of sea-ice have on the salinity ol surface w ater?
As discussed in connection with deep water in the Greenland and Norwegian Seas, the formation of ice results in an increase in the salinity of surface water. When ice forms, some salt is trapped amongst the ice crystals, but sea-ice is generally less saline than the water from which it forms and so the remaining water is correspondingly more saline. Sea-ice formation results in 'brine rejection" (as it is known) all round the Antarctic continent, but the effect is especially marked in coastal polynyas where ice is continually forming and being removed from the area.
In 'open-ocean' polynyas. heat is lost from the sea-surface mainly by conduction/convection (Qh). Water cooled at the surface sinks and is replaced by warmer subsurface water which in turn is cooled and sinks, forming deep convection cells (Figure 6.24). Measurements made after the Weddell Polynya had formed showed that the temperature of deep water had changed dramatically, decreasing by 0.8 °C all the way down to 2500 m depth. It is reasonable to assume that the 'missing heat' had been carried to the surface by convection, and it has been estimated that the rate of overturning of water necessary to transport this much heat may have been as much as 6 x 106m3 s_l during the winter months, when the polynya was active.
QUESTION 6 8 It is I hough l thai one ol the factors that determine w hetlier an open-ocean polynya will persist or freeze over is the production ol meltwater from the surrounding ice. Why mighl the production ol melt water cause an open-ocean polynya to die", i.e. Mop convccimg .'
vapour condenses to form liquid water. Coastal polynyas 'manufacture ice on an enormous scale, perhaps producing much of the ice in the adjacent ocean. It has been calculated that the heat flux to the atmosphere from a coastal polynya is more than 300 W m~2, enough to supply a ten-centimetre-thick layer of ice to the adjacent sea each day.
Figure 6.25 Schematic map to show the flow paths of the different varieties of Antarctic Bottom Water. Dark blue arrows show different components of the densest AABW, which originates on the shelf. The broken arrows show the AABW formed in the Weddell Sea, some of which eventually flows north in the western Atlantic and, to some extent, in the Indian Ocean. The thick blue-green arrows indicate production of the less dense component of AABW which forms from Antarctic Circumpolar Deep Water and flows north from the northern edge of the Antarctic Circumpolar Current. Blue-grey shading indicates water depths less than 3000 m.
As mentioned earlier, there seem to be two main types of Antarctic Bottom Water. The densest varieties originate at various locations over the Antarctic continental shelf, where water becomes sufficiently dense to sink as a result of winter ice formation and cooling, particularly in coastal polynyas. Having sunk from the surface, this water circulates for some time over the shelf. As a result, shelf waters have salinities of 34.4-34.8 and can be as cold as —2 °C - this exceptionally low temperature can be attained because the freezing point of seawater decreases with increasing pressure. While flowing near the shelf break, these dense shelf waters mix with tongues of water from the Antarctic Circumpolar Current (Circumpolar Deep Water, discussed shortly) which have been carried southwards in cyclonic subpolar flows. The resulting mixtures flow westwards down the continental slope into the deep ocean (dark blue arrows on Figure 6.25).
These extremely cold varieties of Antarctic Bottom Water are all very dense but their temperature-salinity characteristics vary greatly, depending where they originate - for example, the dense water that forms in the southwestern Weddell Sea is the coldest and the freshest, while that which forms in the north-western Ross Sea is the warmest and saltiest. Most of these water masses are so dense that they never escape from the Antarctic region, because having flowed down into one of the deep basins to the north of Antarctica, they remain trapped there (see unbroken dark blue arrows in Figure 6.25). However, some does manage to flow north into subtropical regions. This is the mixture of shelf water and Circumpolar Deep Water that, having circulated at depth around the Weddell Gyre for some time, overflows into the southern Scotia Sea, mostly through deep channels in the
South Scot i a Ridge, and then flows northwards in the Atlantic western boundary curren! along the continental slope of eastern South America (broken blue arrows in Figure 6.25). beneath the Antarctic Circumpolar Current Some ol this water flows westward and then north into the southern basins ol the Indian Ocean, and the remainder travels around the Antarctic continent until blocked al the Drake Passage, where us remnants fill the South Shetland Trench. The total transport of this "true* Antarctic Bottom Water, with temperature-Salinity characteristics of aboui -0.4 "C and 34,66, is probably about 10 x 10" mV 1 .
Most of the cold, dense Antarctic water that flows north to occupy the deepest parts of the three main ocean basins lef Figure 6.22) is exported from lower levels in the Antarctic Circumpolar Current. Water in the Antarctic Circumpolar Current is well mixed by w ind and turbulence, and consists of North Atlantic Deep Water to which has been added Antarctic Bottom Water formed to the south, over the corn mental shelf (discussed above). This relatively warm 1-0.25 to 2.0°C). relatively saline (>34,6-34.72) water, known as Antarctic Circumpolar Water, may be clearly seen on Figure 5.31
Poleward of about 50' S. there is only a small difference m density between i he surface layer of cold, fresh water and I he underlying (warmer, more saline) Antarctic Circumpolar Water, and so ihe stratification is easily destabilized and the layers mixed together by strong winds aided by ihe turbulence generated around ice-floes extending down into the water. Winter cooling of surface water, especially in open ocean polynyas. produces dense water that sinks While circulating eastwards around the Antarctic continent, perhaps a number of times. The resulting water mass is the 'Circumpolar Deep Water' mentioned earlier. An enormous volume of well mixed and homogeneous Antarctic water eventually flow s equatorwards at depth from the northern edge of the Antarctic Circumpolar Current. This is labelled "Circumpolar AABW' on Figure 6.25. and has a temperature close to 0 :C and salinity of -34,6-34.7.
Because of the Coriolts force. Ihe IIow of Antarctic Bottom Water is concentrated along the western boundaries of the basins, but it is found in all the deepest parts of the basms of the Southern Hemisphere. In the western trough of the Atlantic Ocean, it flows northwards below south ward-flowing North Atlantic Deep Water (readily distinguishable by its higher salinity), but on ihe eastern side of the Mid-Allantic Ridge ils northward passage is restricted by the Walvis Ridge, which extends south-westwards from southwest Africa to the Mid-Atlantic Ridge at depths of less than 3500 m
Antarctic Bottom Water is the densest w ater mass in the open ocean. The components of North Atlantic Deep Water - new ly formed Deep Waters of the Norwegian and Greenland Seas, and those of the Mediterranean - arc denser, hut they entrain such large volumes of less dense water in their turbulent passage through lo the open ocean. I hat NADW ends up less dense than even the 'Circumpolar' type of Antarctic Bottom Water. This is not quite ihe whole story, however, as w ill be discussed in the next Section
Before moving on. we should briefly return to Antarctic Circumpolar Water/ Circumpolar Deep Water, from which the circumpolar type of Antarctic Bottom Water forms. As mentioned above, its combination of relatively high salinity and relatively high temperature is due to the presence of North Atlantic Deep Water, which in flowing southwards has entrained Central Waters (themselves warm and saline) formed in the subtropical gyres, and has been influenced by the warm saline outflow from the Mediterranean.
Figure 6.26 Block diagram showing the surface currents and vertical motion of water masses in the Atlantic poleward of about 40° S. North Atlantic Deep Water (NADW) becomes Antarctic Circumpolar Water (ACW) and rises to the surface at the Antarctic Divergence. Surface water flowing northwards from the Antarctic Divergence sinks at the Antarctic Polar Frontal Zone (as AAIW), while that flowing southwards may become AABW. CDW = Circumpolar Deep Water. Contours show isotherms in °C; this schematic diagram should be compared with Figure 5.31 which shows an actual temperature and salinity distribution measured in the Drake Passage between 56° and 62° S. Note: Here, we have not distinguished between the different types of AABW.
Figure 6.26 shows how the modified North Atlantic Deep Water/Antarctic Circumpolar Water/Circumpolar Deep Water flows up between Antarctic Intermediate Water and the very dense Antarctic Bottom Water, and rises towards the Antarctic Divergence (Section 5.5.2). Thus, warm water carried downwards in the subtropical gyres is transported polewards and then upwards until eventually it reaches the surface of the ocean around Antarctica, where a large amount of heat is given up to the atmosphere. Cooled water flowing northwards from the Antarctic Divergence will sink at convergences in the Antarctic Polar Frontal Zone in the form of Antarctic Intermediate Water, while that flowing polewards from the Antarctic Divergence may eventually be converted to Antarctic Bottom Water (Figure 6.26). The cycle of water mass formation is therefore an intrinsic part of the thermohaline circulation which, along with the surface current system, redistributes heat around the globe.
The deep waters of the Pacific and Indian Oceans are very similar and are sometimes considered as making up one water mass, known as Pacific and Indian Ocean Common Water. Pacific and Indian Ocean Common Water is then the largest water mass in the ocean, accounting for about 40% of the total volume (cf. Figure 6.22).
There are no source regions of Deep Water in the northern Pacific, and Red Sea/Persian Gulf Water - a relatively warm, saline water mass - is the only dense water mass formed in the vicinity of the Indian Ocean (cf. Figure 6.16).
How. then, is the Pacific and Indian Ocean Common Water formed?
It must be formed through the mixing together of other water masses, specifically, Antarctic Intermediate Water, North Atlantic Deep Water and Antarctic Bottom Water. Of these, Antarctic Bottom Water is by far the most voluminous contributor to Pacific and Indian Ocean Common Water. It contributes about half the water, and North Atlantic Deep Water and Antarctic Intermediate Water together contribute the rest. As described earlier. North
Antarctic Circumpolar Current
Was this article helpful?