680 The Stratospheric Circulation 



6. The Polar Bottom Current 



The second water type originating at the sea surface of the Antarctic ocean is the 

 Antarctic Bottom Water. It is formed all along the Antarctic continental shelf and 

 especially in the area of the Weddel Sea which is the place of formation for this coldest 

 and thus heaviest water type; it sinks along the continental slope down to the greatest 

 depths and extends northward following the bottom topography of the ocean as an 

 Antarctic Bottom Current. As it spreads it is subject to continuous mixing with the 

 water masses above. Its spread is hindered by transverse ridges which the current 

 must pass and limits are set to spread by meridionally oriented rises; the deep passages 

 through these zonally and meridionally oriented ridges thus form important guiding 

 channels for the bottom currents. The extension of antarctic bottom water in the 

 individual oceans as deduced from the thermo-haline structure has been described in 

 detail during the discussion of the temperature distribution in the bottom layers of the 

 ocean, so that the reader is only referred to this here (see Pt. I, p. 149). The spread of 

 the bottom water is shown in Plate 4 by lines of equal potential temperature and from 

 these the course of the bottom currents can be readily followed. 



The generation of bottom water in the Antarctic is so enormous that the same 

 process in the Arctic is by comparison quite insignificant. In the Atlantic one can 

 hardly speak of any proper Arctic bottom current, since the high upward extending 

 ridges between North America, Greenland, Spitzbergen and further to the south 

 between Greenland, Iceland, the Faeroes and Scotland almost completely block the 

 outflow of bottom water from the Arctic Basin. Bottom water with a characteristic 

 potential temperature of between —0-2° and — 1-5°C passes over the above mentioned 

 submarine rises into the open ocean in only very small amounts. 



Recent investigations of the flow near the bottom across the Iceland-Faeroes Ridge 

 have been made by Dietrich (1956, 1957). All the five cross-sections over these rises 

 have shown that the warm North Atlantic Water and the cold sub-Arctic water 

 are in contact over the ridge forming a narrow frontal zone. The heavy sub-Arctic 

 water lying underneath the lighter north-east Atlantic water always covers a large part 

 of the summit plateau of the Iceland-Faeroes Ridge, and sinks down immediately on 

 its western side because of its higher density keeping thereby close to the slope. In 

 spite of mixing with warmer water of smaller density its density remains still higher 

 than that of the surroundings, and consequently it sinks to form the bottom water in 

 the north-east Atlantic at depths below 3000 m. The velocity of this downward 

 directed bottom current on the western side of this ridge can be determined using a 

 formula given by Defant (1955) and results to about 35 cm sec"^ For a thickness of 

 the sinking water of 50 m and with a total width of the passage of 150 nautical miles 

 the water transport will amount to about 50 x 10^ m^ sec"^ Like a waterfall these 

 waters flow out in individual bursts and may be observed at any time of the year at 

 the Iceland-Faeroes Ridge. Oceanographically they have a greater importance than 

 the sinking movements caused by winter cooling over the shelf of the Bay of Biscay 

 and elsewhere along the continental shelf and slope which can occasionally be observed 

 (see Cooper and Vaux, 1949 and Cooper, 1952). 



The main mass of North Atlantic Bottom Water thus originates outside the Arctic. 

 WiJST (1943) termed this water type with a potential temperature of between 1° and 

 2°C as the sub-Arctic bottom water and the current fed by it the ^'sub-Arctic 



