The Deep-water Circulation of the Oceans

In the preceding sections reference has frequently been made to the deep and bottom waters of the different oceans, the character of which is illustrated in figs. 161, 168, 183, 189, 195, 196, and 199.

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The manner in which the deep and bottom water is formed has been discussed (p. 138), and certain statements as to the deep-water circulation have been made, but so far no general review of the deep-water circulation has been presented. Table 90 has been prepared in order to facilitate such a review. It contains temperatures, salinities and oxygen values of the deep and bottom water at fifteen selected stations, six in the Atlantic, three in the Indian and six in the Pacific Ocean, including the adjacent parts of the Antarctic Ocean. The content of this table will not be dealt with separately but must be examined as the discussion proceeds.

In order to understand the deep-water circulation, one has to bear in mind that deep and bottom waters represent water the density of which became greatly increased when the water was in contact with the atmosphere, and that this water, by sinking and subsequent spreading, fills all deeper portions of the oceans. The most conspicuous formation of water of high density takes place in the subarctic and in the antarctic regions of the Atlantic Ocean. The deep and bottom water in all oceans is derived mainly from these two sources, but is to some extent modified by addition of high-salinity water flowing out across the sills of basins in lower latitudes, particularly from the Mediterranean and the Red Sea.

In the North Atlantic Ocean, North Atlantic Deep and Bottom Waters flow to the south, the flow being reinforced, and the upper deep water being modified, by the high-salinity water flowing out through the Strait of Gibraltar. The newly formed deep and bottom water has a high oxygen content which decreases in the direction of flow. Figure 210 and the values given in table 90 demonstrate the character of these waters and show particularly the increase in salinity at moderate depth caused by addition of Mediterranean water. Antarctic Bottom Water flows in the opposite direction, from south to north, and has been traced beyond the Equator to lat. 35°N (Wüst, 1935). The spreading to the north of the Antarctic Bottom Water is illustrated in fig. 211, showing the potential temperatures below a depth of 4000 m. Owing to admixture of this water the salinity of the bottom water of the North Atlantic decreases toward the south.

The North Atlantic Deep Water crosses the Equator and continues toward the south above the Antarctic Bottom Water. On the other hand, it sinks below the Antarctic Intermediate Water and therefore, in the South Atlantic Ocean, it becomes sandwiched between the Antarctic Intermediate Water and the Antarctic Bottom Water, both of which are of lower salinity. In a vertical section the deep water of the South Atlantic Ocean is therefore characterized by a salinity maximum, but, owing to mixing with the overlying and the underlying water, the absolute value of the salinity at the maximum decreases toward the south.

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The bottom water of antarctic origin is colder than the deep water and to the south of about latitude 20°S the Antarctic Intermediate Water is also colder. In a vertical section the deep water therefore shows a temperature maximum and also a decreasing temperature to the south, owing to admixture from above and from below.

In the South Atlantic Ocean a large amount of water of antarctic origin, bottom water or intermediate water, returns to the Antarctic after having been mixed with the south-moving deep water. According to the computations which were discussed on pp. 465 and 629, the transport across the Equator of North Atlantic Deep Water amounts to about 9 million m³/sec, whereas between 20° and 30°S the corresponding transport toward the south of deep water is about 18 million m³/sec. If these figures are approximately correct they indicate that 9 million m³ of water of antarctic origin return toward the Antarctic every second. An examination of the salinity in the South Atlantic Ocean at depths below 1600 m confirms this conclusion. At the Equator the average salinity of the North Atlantic Deep Water between 2000 and 3500 m is approximately 34.93 ‰. If this water is mixed with an equal amount of intermediate and bottom water of salinity approximately 34.7 ‰, the salinity will decrease to 34.81 ‰, which approximates the salinity of the deep water in lat. 40°S.

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In sum, the deep-water circulation of the Atlantic appears to represent a superposition of two types of circulation: (1) an exchange of water between the North Atlantic and the South Atlantic Ocean which is of such a nature that North Atlantic Deep Water flows south across the Equator, whereas Antarctic Bottom and Intermediate Waters flow north; and (2) a circulation within the South Atlantic Ocean where large quantities of Antarctic Bottom and Intermediate Water mix with the south-flowing deep water and return to the Antarctic. The final result of these processes is that the deep water reaching the Antarctic Ocean from the north is diluted as compared to the deep water of the North Atlantic and is of a lower temperature. This is the water that contributes to the formation of the large body of Antarctic Circumpolar Water flowing around the Antarctic Continent. The oxygen content of Circumpolar Water is lower than that of the North Atlantic Deep Water and the Weddell Sea water, and decreases somewhat toward the east from Weddell Sea toward Drake Passage (see p. 621). The circulation that has been described is present in the western part of the South Atlantic Ocean, but in the eastern part it is impeded by the Walfish Ridge.

In the Indian Ocean there is no large southward transport of deep water across the Equator. The T-S diagram in fig. 189 and the data in table 90 show that to the north of the Equator the deep water contains an admixture of Red Sea Water that maintains a relatively high salinity down to depths exceeding 3000 m, but to the south of the Equator the T-S curves indicate only a slight effect of the Red Sea Water. In the southern part of the Indian Ocean an independent circulation must be present. The deep water from the South Atlantic Ocean continues into the Indian Ocean and is particularly conspicuous in the western part, where maximum salinities of 34.80 ‰ have been observed. This water flows mainly toward the east, being somewhat diluted by admixtures of intermediate and bottom waters. On the other hand, Antarctic Intermediate Water flows north and the bottom temperatures demonstrate that bottom water also moves north (fig. 211); these water masses must return again to the south. It is probable that the intermediate water

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and the bottom water mix with the deep water and that the return flow takes place within the latter. A slight admixture of deep water from the region to the north of the Equator, which is compensated for by bottom water penetrating across the Equator, appears to maintain the salinity of the Indian Ocean Deep Water at a higher level than would be the case if no addition of saline water took place. Thus the influence of the Red Sea can probably be traced to the Antarctic, but by a mechanism somewhat different from the one which has been suggested by Lotte Möller (1929, 1933) and which has been discussed particularly by Thomsen (1933, 1935).

From the Indian Ocean the Antarctic Circumpolar Water with its components of Atlantic and Indian Ocean origin enters the Pacific Ocean. The Discovery and Dana observations in the Tasman Sea between Australia and New Zealand, and in the Pacific to the east of New Zealand, show that the salinity of the deep and bottom water has been reduced so much that the maximum values lie between 34.72 ‰ and 34.74 ‰. These maximum salinities are found at depths between 2500 and 4000 m, the salinity of the water close to the bottom being slightly lower. From the region where the deep water enters the Pacific Ocean the salinity decreases both toward the north and toward the east. The Discovery data indicate that below the Antarctic Convergence a core of water of salinity higher than 34.72 ‰ is found, which represents water of the Circumpolar Current; but to the north of this region of maximum salinity, values below 34.70 ‰ prevail, increasing uniformly toward the bottom. The Carnegie and the Dana data similarly show that north of 40°S the highest salinities are found near the bottom. The structure of the water masses of the Pacific differs completely, therefore, from that found in the other oceans, where the highest salinities are encountered in the deep water and not in the bottom water.

This feature can be explained if one assumes that in the South Pacific Ocean there also exists a circulation which is similar to that of the South Atlantic and Indian Oceans, namely, that intermediate and bottom water flow to the north and that a flow of deep water to the south takes place. This north-south circulation is superimposed upon a general flow from west to east. The Pacific Deep Water is, therefore, of Atlantic and Indian origin but has become so much diluted by admixture of intermediate and bottom water that the salinity maximum has disappeared. These conclusions as to the character of the deep-water circulation of the South Pacific are in agreement with the concept of Deacon (1937a), who has shown that the deep water of the Pacific moves toward the south and rises within the Antarctic region in a similar manner to that of the deep water of the Atlantic and Indian Oceans.

The more or less closed systems of the deep-water circulation in the Southern Hemisphere between the Antarctic Ocean and the Equator are

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related to the fact that near the Equator all ocean currents tend to flow in east-west directions and that transport of water across the Equator takes place only when it is required to maintain the same sea level in both hemispheres. Such is the case in the Atlantic Ocean, where deep water is formed in the Northern Hemisphere. The water that sinks in high latitudes or flows out of the Mediterranean Sea spreads at great depths, continues across the Equator, and is replaced by horizontal flow from the South Atlantic Ocean. In the Indian Ocean a similar mechanism operates, but on a very small scale, because sinking takes place only in the Red Sea and the amounts of deep water formed there are small and exercise a significant influence only upon conditions to the north of the Equator. In the Pacific Ocean no mechanism exists that will give rise to a considerable exchange of water across the Equator, because conditions are such that no deep water is formed in the North Pacific.

On the other hand, continuity must exist between the South and the North Pacific, for which reason the depths of the North Pacific Ocean are filled by water of the same character as that found in the northern portion of the South Pacific (fig. 212). The most accurate data available indicate that a very small exchange of deep water takes place between the two hemispheres and that a sluggish motion to the north may occur on the western side of the Pacific Ocean, whereas a sluggish motion to the south may occur on the eastern side. A comparison of salinities at the depths of 2500, 3000, and 3500 m in the eastern and western parts of the North Pacific is found in table 91, from which it is seen that the salinity in the eastern part is about 0.02 ‰ lower than that in the western. This

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difference lies at the limit of the accuracy of the observations, but in the table the ranges of the observed values have also been entered and these ranges are not completely overlapping, for which reason the difference probably is significant. It can be explained if the deep water of the North Pacific circulates slowly in a clockwise direction and if a slight renewal takes place on the western side, while on the eastern side small amounts are transported to the Southern Hemisphere. The lower salinity of the deep water on the eastern side would then be due to vertical admixture of low-salinity water where the deep water crosses the Pacific Ocean. The slightly lower temperatures on the eastern side indicate that this admixture takes place in high latitudes. The oxygen content of the water substantiates the conclusion that a slow clockwise circulation of the deep water is present in the North Pacific, because between 2500 and 3500 m the average oxygen content at the Dana stations on the western side was 3.35 ml/L, whereas at the Dana, E. W. Scripps, and Bushnell stations on the eastern side the corresponding oxygen content was 2.62 ml/L. This difference can be interpreted to show that the oxygen content of the deep water has decreased during the long time taken to move from the western around to the eastern side of the ocean.

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In the preceding discussion the term “flow of water” has been used freely, but one has to deal actually with such slow and sluggish motion that the term “flow” can hardly be used, since the average velocities must often be measured in fractions of a centimeter per second.

In conclusion it can be stated that appreciable exchange of deep and bottom water across the Equator takes place in the Atlantic Ocean only. It is rudimentarily present in the Indian Ocean and practically absent in the Pacific. Superimposed upon such an exchange between the hemispheres, independent circulations exist in the three southern oceans, because Antarctic Intermediate and Bottom Water return to the Antarctic as Deep Water. This circulation is well established in the Atlantic Ocean, and in the Indian and Pacific Oceans the existence of such a circulation is derived partly by analogy with the Atlantic Ocean and partly by an examination of the few available precise salinity observations.

The general distributions of oxygen, phosphates, nitrates and silicates discussed in chapter VII are in good agreement with the deep-water circulation that has been outlined. The generally high and uniform content of phosphates, nitrates, and silicates around the Antarctic Continent is consistent with the uniform character of the Circumpolar Water. The low concentration in the North Atlantic is directly related to the exchange of water between the South Atlantic and the North Atlantic. The water that flows into the North Atlantic is mainly central and intermediate water, and the former is low in phosphates, nitrates, and silicates. The net transport of these salts across the Equator must be nearly zero, and the south-flowing deep water of the North Atlantic must therefore have low concentrations. In the South Atlantic the deep water becomes rapidly mixed with Antarctic Intermediate Water and Antarctic Bottom Water, both of which contain so much phosphate, nitrate, and silicate that the contents in the deep water increase as it flows south. The North Pacific Deep Water, on the other hand, contains great amounts of phosphates, nitrates and silicates, in agreement with the concepts that a small exchange of deep water takes place between the South and North Pacific and that in the North Pacific the circulation of

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the deep water is so slow that there the concentrations of these salts are increased by further decomposition of organic matter. In the Indian Ocean conditions appear to be more or less similar to those in the South Pacific. Some of the major features, of the distributions, therefore, are related to the deep-water circulation, but others are intimately related to the biological economy of the sea as explained in chapter VII.