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The Water Masses and Currents of the Oceans Within Anygiven Region, the Character of the Water Massesdepends Mainly Upon Three F

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The Water Masses and Currents of the Oceans Within Anygiven Region, the Character of the Water Massesdepends Mainly Upon Three F . CHAPTERXV The Water Masses and Currents of the Oceans `.m. mm.. ms .......m .......m* ....m..*.. m.....* ....................mm... *. .. ........ .......m..m. ..... ........ Within anygiven region, the character of the water masses depends mainly upon three fac$ors—the latitude of the region, the degree of isolation, and the types of currents-the relative importance of which will be brought out in the course of the discussion. It is rational to deal first with the simpler conditions and gradually to enter upon the more complicated. This can be done by first dis- cussing the Antarctic Ocean, where the latitude effect is easily explained, where the waters are in free communication with those of the major oceans, and where the system of prevailing currents is unusually clear- cut. After a description of conditions within the Antarctic Ocean we shall consider the Atlantic, Indian, and Pacific Oceans, the adjacent seas of these oceans, and in conclusion we shall discuss the deep-water circulation between the large ocean basins. The Antarctic Circumpolar Ocean BOUNDARIESANDGENERALCHARACTERISTICSOF WATER MASSES. It is difficult to assign a northern boundary to the Antarctic Ocean because it is in open communication with the three major oceans: the Atlantic, the Indian, and the Pacific Oceans. In some instances the antarctic waters are dealt with as parts of the adjacent oceans and are designated the Atlantic Antarctic Ocean, or the Indian or Pacific Ant- arctic Ocean, whereas in other instances the antarctic waters must be considered an integral part of all oceans. The latter case, for instance, applies when dealing with the deep-water circulation, which is of such a nature that the intercommunication between dif?erent regions must be taken into account. The antarctic waters, on the other hand, can be discussed without entering upon details of the conditions in neighboring oceans, in which case one has to establish oceanographic boundaries between the Antarctic Ocean and the neighboring areas. On the basis of observations of surface temperatures only, it is possible to divide the Antarctic Ocean into two separate regions and to establish its approximate northern boundary. Near the Antarctic Continent the surface temperature is low, but with increasing distance from the coasts it increasesslowly until a region is reached within which an increase of two or three degrees takes place in a very short distance. 605 606 THE WATER MASSES AND CURRENTSOF THE OCEANS This region of sudden increase of surface temperature is one at which part of the surface water sinks, and is called the Antarctic Convergence, which has been found to exist all around the Antarctic Continent. Proceeding further to the north, the surface temperature again rises Fig. 158. Locationsof theAntarcticandSubtropicalConvergence. Locations of theverticalsectionsshownin figures159and160,andlimitbetweenwesterlyand easterlywinds(dashedline). Lightshading:depthslessthan3000m. slowly until a second region of rapid increase is encountered at the Sub- tropical Convergence,which also has been traced all around the earth except in the eastern South Pacific, where its location is not established, The area extending from the Antarctic Continent to the Antarctic Convergence is called the antarcticregion and the area between the two convergence is called the subantarcticmglon. From the oceanographic point of view it is logical to consider an Antarctic Ocean extending from the Antarctic Continent to the Subtropical Convergence except in the eastern South Pacific, where an arbitrary limit has to be selected. THE WATER MASSES AND CURRENTSOF1’HE OCEANS 607 Figure 158shows the location of the two convergences according to the results of the D&xweryExpedition (Deacon, 1937a). Thel~pation ~ of the Antarctic Convergence is, in part, dependent upon the distribution of land and sea and upon the bottom topography. The southward displacement of the Antarctic Convergence to the south of Australia and New Zealand can probably be ascribed to the relative narrowness of the passage between these regions and Antarctica, and the similar displacement off South America can be attributed to the southerly location of Drake’s Passage separating South America from Graham Land. The bends of the Antarctic Convergence are to a great extent related to the bottom topography. On the western side of the submarine ridges, the more conspicuous of which are indicated in the figure by the 3000-m contour, the convergence is deflected to the north, and on the eastern side to the south. The reason for this relation to the bottom topography was explained when dealing with the currents (p. 466). The location of the Subtropical Convergence is similarly related to the distribution of land and sea, but appears to be less dependent upon the bottom configuration. It is less well defined than the Antarctic Convergence, and instead of referringto a Me of convergence it is, accord- ing to Defant (1938), more correct to refer to a region of convergence. Within the antarctic and subantarctic regions the water masses can be classified according to their temperature-salinity characteristics (p. 141). Such a classification is here conveniently based on a study of the distribution of temperature and salinity in a vertical section running at right angles to the Antarctic Continent. F@ure 159 is taken from Deacon’s discussion (1937a) and shows the distribution of temperature and salinity along a section running south-southwest from Cape Leeuwin, Western Australia, to the ice edge south of Australia in 63041’S. The two convergence are indicated which divide the area into the antarctic and subantarctic regions. Within the antarctic region a surface layer of low temperature and low salinity is present. Below this surface layer one recognizes a transition layer which increases in thickness towards the north and within which the temperature rapidly increases to values higher than 2°, and the salinity gradually increases to values higher than 34.5 O/OO. Below the transition layer one encounters the Antarctic Ciircumpdur Water, the greater mass of which has a salinity a little above 34.70 O/OO and a temperature between 2° and OO. Water of similar characteristics is found within the subantarctic region below a depth of about 2000 m, That is, the deep water within the subantarctic region has the same characteristics as the Antarctic Circumpolar Water which rises towards the surface near the Antarctic Continent. At station 887, close to the continent, a salinity of 34.67 0/00 k met with at a depth Of 200 m, but at station 879 south of the Subtropical Convergence the 608 THE WATER MASSES AND CURRENTSOF THE OCEANS same salinity k found in the deep water below 2000 m. (lose to the Antarctic Continent the temperature of the deepest water is less than O“ and the salinity is less than 34.7 O/OO.Thk cold water represents the Antarctic BottomWater. STC STATION 877 878 1000-“ M. ~ ,5.’:” , /<” . /“ -------- ~—.~--/“ , >.- ,,”- /’ ..-& H- / ../ . .-- __ &------- - . 0 2000 ,,“,~” /’ : ,!:;{:: .1.s ./. /“ a :~fi, ‘,/ ,“ . 1,0/j . ,,~” wo ::~~; ., ,./ . ~w 1,/y,,.,,#,,_ :f.:fi,- - ~~ :;:;:?:: ----~ ---- a- PI:!.:.;.,,.,,,7, “ --------- Fig.159. Distributionof temperatureandsalinityinaverticalsectionfromCape Leeuwin,Australia,to theAntarcticContinent(afterDeacon). Locationof section shownby lineA in fig.158. Within the subantarctic region one can conveniently distinguish between two water massesabove the deep water, the SubantarcticUpper Waterof a uniform temperaturebetween 8° and 9° and of a relatively high salinity, appearing in the salinity section as a tongue extending towards the south, and the AntarcticIntermediateWaterof a temperature between THE WATER MASSES AND CURRENTSOF THE OCEANS 609 3° and 7° and of low salinity, extending towards the north like a tongue that appears to form a continuation of the water in the antarctic layer of transition. The deep water, which is encountered below a depth of about 1500 m, consists of two water masses,the upper deep water which has salinities up to 34.80 O/OO and the lower deep water which is of the same character as the bulk of the Antarctic Circumpolar Water. These different water masses, which will be discussed in greater detail, can be recognized in the southern part of the vertical section of the Atlantic Ocean (fig. 210, p. 748) in which, however, the bottom water extends further north and in which the upper deep water in the subantarctic region has a higher salinity and a higher temperature, THE ANTARCTICSURFACEWATER. In winter the southern half of the antarctic region is covered by a thin layer of nearly homogeneous surface water of a thickness of 100 m or less which has a temperature about freezing point and a salinity that varies between 34.0 and 34.5 O/OOj the highest value being found in the Weddell Sea. At a station occupied by the Deutschtandin the Weddell Sea (lat. 65032’S, long. 43”00’W) on August 26, 1912 (Brennecke, 1921), a nearly homogeneous layer was found between the surface and a depth of 100 m having a temperature of – 1.83° and a salinity of 34.47 O/OO. With increasing distance from the Antarctic Continent the temperature rises to 0° where the Antarctic Convergence is far south, and to about 1° where this convergence is far north. In summerthe salinity at the surface is decreasedowing to the melting of ice. By far the greater amount of the radiation surplus of the season is used for melting ice, but a small amount is used for raising the tem- perature of the water above freezing point. In the vicinity of the pack ice the surface temperature and salinity vary greatly from one locality to another, because the nearly fresh water produced by melting of ice is not immediately mixed with the surrounding or deeper water. In ice- free areas a more thorough mixing takes place, particularly where strong winds are frequent, and there a well-mixed layer may extend to depths of 55 to 85 m. The temperature of this water is a few degrees above freezing, owing to absorption of heat, but below this warmed layer colder water is found.
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