The Water Masses of the Oceans: A Summary
In figs. 209A and 209B are shown the characters of the water masses that have been discussed, their regions of formation, and their distribution. The chart in fig. 209A and the T-S curves in fig. 209B should together illustrate the concepts that water masses are formed at the sea surface and sink and spread in a manner that depends upon their density in relation to the general distribution of density in the oceans This applies to all water masses except the equatorial water masses of the Indian and Pacific Oceans, which are formed by subsurface processes of mixing.
Approximate boundaries of the upper water masses of the ocean. Squares indicate the regions in which the central water masses are formed; crosses indicate the lines along which the antarctic and arctic intermediate waters sink.
Temperature-salinity relations of the principal water masses of the oceans.
With the exception of two water masses found at intermediate depths and to which we shall return, no water mass is formed at the sea surface in low latitudes, but in all oceans the regions of the subtropical convergences (between 35°S and 40°S and between 35°N and 40°N) are regions where the central water masses originate. This concept is based on the fact that in certain seasons of the year the horizontal T-S relations in these regions are similar to the vertical T-S relations of the different central water masses. These relations are all expressed on a T-S diagram by nearly straight bands, but the slopes vary from one ocean region to another. Table 89 contains the average salinities of the central water masses at different temperatures and the maximum deviations from the averages, according to the curves in fig. 209B. It is seen from the table and the figure that the central water masses of the South Atlantic, the Indian, and the western South Pacific Oceans are very similar, as should be expected, because they are formed in regions in which the external influences, that is, the atmospheric circulation and the processes of heating and cooling, are similar. The corresponding water mass of the eastern South Pacific is of lower salinity, probably because of admixture of the low-salinity Subantarctic Water of the Peru Current. Such admixture may also be responsible for the fact that the Central Water of the western South Pacific has a slightly lower salinity than the Central Waters of the Indian and South Atlantic Oceans.
The Central Waters of the North Atlantic and the North Pacific Oceans are quite different, the former having a very high and the latter a very low salinity. The contrast probably results from the different character of the ocean circulation and from the differences in the amounts of evaporation and precipitation, especially in high latitudes, which are intimately related to the distribution of land and sea.
The central water masses are all of small vertical extension, particularly in the North Pacific Ocean where their thickness over large areas is only 200 to 300 m. In all oceans the greatest thickness of the central water masses is found along the western boundaries; it reaches 900 m in the Sargasso Sea region of the North Atlantic.
In the equatorial part of the Atlantic Ocean the two Central Water masses are separated by a region of transition where the T-S relation is intermediate, but in the Pacific the Central Water masses are separated by a well-defined water mass, the Pacific Equatorial Water. From the T-S curves in fig. 209B it is evident that this water mass is formed in the South Pacific because it is similar to the water masses of that ocean, but it has a higher salinity than any of the water masses of the North Pacific. In the northern part of the Indian Ocean a corresponding Equatorial Water mass is present, which at a temperature of 15° or higher is of the same salinity as that of the Pacific, but at lower temperatures it is of a higher salinity. The higher salinities indicate admixture of Red Sea water, but in general the manner in which the water mass is formed is not clear.
|South Atlantic||34.64 ± 0.08||34.86 ± 0.08||35.11 ± 0.08||35.37 ± 0.09||35.64 ± 0.10|
|Indian Ocean||34.65 ± 0.07||34.89 ± 0.06||35.13 ± 0.07||35.37 ± 0.08||35.62 ± 0.09|
|Western South Pacific||34.58 ± 0.07||34.80 ± 0.06||35.04 ± 0.08||35.30 ± 0.08||35.55 ± 0.09|
|Eastern South Pacific||34.54 ± 0.07||34.70 ± 0.08||34.88 ± 0.08||35.08 ± 0.08|
|North Atlantic||35.12 ± 0.09||35.37 ± 0.09||35.63 ± 0.09||35.88 ± 0.09||36.12 ± 0.09|
|Western North Pacific||34.24 ± 0.07||34.38 ± 0.06||34.52 ± 0.06||34.67 ± 0.07|
|Eastern North Pacific||34.11 ± 0.09||34.32 ± 0.08||34.62 ± 0.08|
The central and equatorial water masses are covered by a surface layer 100 to 200 m thick, within which the temperature and the salinity of the water vary greatly from one locality to another, depending upon the character of the currents and the exchange with the atmosphere, and within which great seasonal variations occur in middle latitudes. A discussion of the surface layer is not included in this summary. The surface layer, the central water masses, and the upper portions of the equatorial water masses together form the oceanic troposphere (p. 141).
The Subantarctic Water occurs between the central water masses of the southern oceans and the Antarctic Convergence. This water has nearly the same character all around the earth, and is therefore considered as belonging to the waters of the Antarctic Ocean (p. 606). The Subantarctic Water is of low salinity and is probably formed by mixing and vertical circulation in the region between the Subtropical and the Antarctic Convergences. In the North Atlantic the corresponding Subarctic Water is found in a small region only and is of relatively high salinity, but in the North Pacific it is of wide extension and of low salinity. The Subarctic Water must be formed by processes which differ from those that maintain the Subantarctic Water. In the southern oceans the Antarctic Convergence represents a continuous and well-defined southern boundary of the Subantarctic Water, but in the northern oceans the corresponding Arctic Convergence is found in the western parts of the oceans only, and in large areas there exists no marked northern boundary of Subarctic Waters. This contrast between south and north must be related to the differences in the distribution of land and sea and is reflected in the character of the waters. The Subarctic Waters are similar to the corresponding Arctic Intermediate Waters, but the Subantarctic Water is distinctly different in character from the Antarctic Intermediate Water.
Below the central water masses the intermediate waters are found in all oceans. The Antarctic Intermediate Water is the most widespread. This water, in contrast to the central waters, sinks along a well-defined line, and the water which leaves the surface is not a water mass but a water type, which, all around the Antarctic Continent, is characterized by a salinity of 33.8 ‰ and a temperature of 2.2°. After sinking, the water spreads to the north, mainly between the σt surfaces σt = 27.2 and σt = 27.4, and mixes with the over- and underlying waters. In this
In the North Atlantic the corresponding Arctic Intermediate Water is formed to the east of the Grand Banks of Newfoundland, but probably in small quantities because it appears only in a limited area of the north-west Atlantic. In the North Pacific Ocean the Arctic Intermediate Water, on the other hand, is present between lat. 20°N and 43°N, except off the American west coast where the Subarctic Water flows south. This Intermediate Water is probably formed mainly to the northeast of Japan, but it is added to off the American west coast, where at a depth of 500 to 600 m Subarctic Water spreads below the intermediate water that originates further to the west. Correspondingly, two salinity minima are found in that region (fig. 200, p. 715, and fig. 202, p. 717).
Two other intermediate water masses are of importance, namely those formed in the Atlantic and the Indian Oceans by addition of Mediterranean and Red Sea water, respectively. The Mediterranean water that flows out along the bottom of the Strait of Gibraltar has a salinity of 38.1 ‰ and a temperature of 13.0°, but it is rapidly mixed with surrounding Atlantic water and spreads mainly between the σt surfaces σt = 27.6 and σt = 27.8, that is, below the Antarctic Intermediate Water. It can be traced over wide areas by an intermediate salinity maximum. The spreading of the Red Sea water is not so well-defined, but over large parts of the equatorial and western regions of the Indian Ocean the Red Sea water is recognized by a salinity maximum at a σt value of about 27.4.
Below the intermediate water the deep ocean basins are filled by deep and bottom water, the maximum densities of which vary from σt = 27.90 in the North Atlantic to 27.75 in the North Pacific. These water masses are formed in high northerly latitudes in the Atlantic Ocean and in high southerly latitudes close to the Antarctic Continent in the Weddell Sea area, and to the south of the Indian Ocean. The spreading of these water masses will be dealt with in the following discussion of the deep-water circulation of the oceans.