The Adjacent Seas of the North Pacific Ocean

None of the adjacent seas of the North Pacific Ocean exercise any appreciable influence on the water masses of the ocean and it has therefore not been necessary to discuss them until now.

Gulf of California. The Gulf of California lies in a climatic region which is comparable to that in which the Red Sea is located, but hydrographically the Gulf of California is entirely different from the Red Sea, the reason being that the Gulf of California is not separated from the adjacent ocean by a submarine ridge. To 28°30″N the bottom of the Gulf is trough-shaped, with the greatest depth at the opening to the south. The soundings of the E. W. Scripps in 1939 and 1940 have

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shown, however, that the bottom is very irregular with several deeper basins, although at the entrance free communication with the waters of the Pacific exists above depths of nearly 3,000 m. A ridge running nearly north-south in long. 112°40″ separates the inner portion of the Gulf from the outer, the sill depth of the ridge being about 200 m. Inside of the ridge lies a deep trench between the peninsula of Lower California and the island of Angel de la Guardia.

The character of the waters in the Gulf is shown in table 83, containing temperature, salinity, and oxygen at three stations: station B 305 in 20°00″N off the entrance to the Gulf, station EWS VII-27 in the middle portion of the Gulf, and station EWS VII-53 in the trench south of Angel de la Guardia. In March, when the stations were occupied, the surface temperatures in the Gulf were lower than those off the entrance, the difference being due to cooling by northwesterly winds and to local upwelling. The salinity, on the other hand, was higher in the Gulf owing to excessive evaporation. Between depths of 100 and 1500 m the waters in the middle portion of the Gulf were nearly identical with those off the entrance. Some differences were found at 2000 m and 2400 m because

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station EWS VII-27 was located in one of the basins, the sill depth of which was probably about 1500 m. In the trench to the north of the transverse ridge, basin conditions existed from a depth of about 200 m and to the bottom. The bottom water, which had a temperature of about 11° and a salinity of 34.76 ‰, was probably formed by mixing of water flowing in across the sill with water sinking from the surface during periods of excessive cooling and evaporation. The admixture of surface water is indicated by the higher oxygen content as compared to the oxygen content of the water in the middle portion of the Gulf.

During the cruise of the E. W. Scripps in 1939 a number of stations were occupied in the shallow northern portion of the Gulf. The most northern station was located at a distance of about 70 km from the mouth of the Colorado River where the depth to the bottom was 60 m. In this locality the influence of fresh water from the Colorado River was not perceptible, the surface salinity being 35.31 ‰, whereas the highest surface salinity, 35.50 ‰, was observed at the neighboring station only about 35 km to the southeast.

No well-defined exchange of water between the Gulf and the adjacent parts of the Pacific Ocean could be established. It is probable that such an exchange takes place by irregular currents in the upper layers and by a slow inflow of deep water and outflow of surface water.

Bering Sea. In the deeper portions of the Bering Sea the water masses are similar to those in the subarctic region of the North Pacific, as is evident from table 84, showing temperatures, salinities, and oxygen values at stations on the south and the north sides of the Aleutian Islands and at a third station about 240 km to the north of the Aleutian Islands. Below a depth of about 200 m the water masses at these three stations were practically identical. At 200 m the temperature shows a minimum at all stations but, according to the more detailed original observations, the lowest temperatures were found somewhat above 200 m. At the two northern stations an oxygen maximum appeared at the depth of the temperature minimum and at the southern station the oxygen content was high. This situation suggests that the depth of the intermediate temperature minimum represents the depth to which convection currents reach in winter. Such a layer of minimum temperature has been established at a number of stations in the subarctic region and lower temperature values have been observed further toward the west, where winter cooling is more intense.

The shallow shelf areas in the eastern and northern part of the Bering Sea are covered by water of a considerably lowered salinity owing to dilution by runoff from the large Alaskan rivers. The influence of the waters of the Yukon River is particularly conspicuous. In summer warm water of low salinity is found along the northeastern coast of the Bering Sea and this water continues through Bering Strait into the Polar

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Sea, where it can be traced for some distance along the coast of north-west Alaska.

Some of the water that flows into the Bering Sea around the Aleutian Islands contributes in summer to the current which flows through Bering Strait (see p. 655), but the major amount of water turns around and flows south along the coast of Kamchatka, reaching the northern of the Japanese Islands as the cold Oyashio (Barnes and Thompson, 1938).

Okhotsk Sea. The few data available from the Okhotsk Sea (Krümmel, 1911) show that in winter excessive cooling of the waters takes place such that even in summer the temperature at a depth of 100 to 200 m is as low as −1.4°. The salinity of the water is also low, being about 33.1 ‰. It is probable that the cold intermediate layer of the Okhotsk Sea flows out between the Kurile Islands and contributes to the maintenance of the layer of intermediate temperature minimum in the northwestern part of the Pacific and in the Bering Sea; but there is no evidence that the salinity of the water in the Okhotsk Sea in winter is increased so much by freezing that bottom water is formed, nor does any such formation take place in Bering Sea. One finds, therefore, no region

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in the North Pacific in which processes go on similar to those which lead to the formation of bottom water in the Antarctic (p. 611), or similar to those which produce bottom water in the North Atlantic (p. 664).

Japan Sea. The Japan Sea is a basin in which the greatest depth is about 3700 m. The deepest sill is found in the Tsushima Strait between Korea and Japan, where the maximum depth is about 150 m. In the Japan Sea (Suda and Hidaka, 1932, Uda, 1934) above 400 m there exists a striking contrast between the waters along the west coast of Japan and those along the east coast of Korea. A branch of the Kuroshio, the Tsushima Current, flows into the Japan Sea and carries water of high temperature and high salinity toward the north. Branches of the current flow out through the straits between the northern Japanese islands and part of the water continues along the west side of Sakhalin Island, turns around, and flows south after having been cooled and diluted. The contrast between the waters on the east and the west sides of the Japan Sea is shown by the data in table 85. Schott (1935) points out that the cold water along the mainland side cannot come from the Okhotsk Sea through the narrow strait between the Asiatic coast and Sakhalin Island, because the strait at its narrowest is only 6.7 km wide and 12 m deep. The cold water must therefore have been formed in the Japan Sea by excessive cooling in winter and must have been diluted by river water. The water below a depth of about 400 m is of a temperature slightly above 0° and a salinity a little above 34.0 ‰.

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On a small scale the Japan Sea is comparable to the Arctic Mediterranean, into which flows a branch of the North Atlantic Current carrying warm water of high salinity which is cooled and diluted so that the outflowing current carries cold water of low salinity. The contrast between the eastern and western sides of the Japan Sea corresponds to the contrast between the eastern and western sides of the Norwegian Sea and also to the contrast between the eastern and western sides of the Labrador Sea. The main difference is that no great outflow of cold water takes place from the Japan Sea; the cold water on the western side is mainly part of an eddy.

Yellow Sea and East China Sea. In both of these the surface salinity is greatly reduced by runoff from rivers, and temperature and salinity alike are subjected to great annual variations. The waters are shallow and the processes that take place have small bearing on conditions at greater distances from the coast.

South China Sea. In the South China Sea, between the Philippine Islands and the Asiatic mainland, a basin is found within which the greatest depths exceed 4600 m and which is in communication with the adjacent part of the Pacific Ocean through the passage between the Philippine Islands and Formosa, where the sill depth is between 2500 and 3000 m. Table 86 contains observations of temperature, salinity, oxygen, and computed potential temperatures at a Dana station located

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in the central portion of the South China Sea. In the upper layers the salinity is lower than it is further east, owing to admixture of river water, but at depths between 200 and 3000 m the water is of the same character as that of the adjacent parts of the West Pacific, whereas below 3000 m basin conditions exist, the temperature increases toward the bottom, and salinity remains constant. The increase in the temperature is not sufficiently great to cause instability because the potential temperature decreases slightly. The oxygen content is nearly constant below 3000 m, whereas in the open ocean it increases below that depth.

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The Waters of the East Indian Archipelago. Detailed examination of the waters of the East Indian Archipelago has been conducted on the Snellius expedition, and the following summary is based mainly on van Riel's discussions (1932b, 1934, 1938). In fig. 208 are shown the numerous basins which are found in this region of highly complicated bottom topography, also the direction from which the renewal of water in these basins takes place. The names of the basins and their probable sill depths and maximum depths are listed in table 87. The flow of the surface water, according to van Riel (1932b), is also approximately in the direction of the arrows. The high-salinity water, which in the western Pacific is found at depths between 100 and 200 m, flows into the seas between the East Indian Islands, where the thickness of the high-salinity layer decreases in the direction of flow and the maximum salinities are reduced. Between Borneo and Celebes the layer of maximum salinity disappears in about lat. 2°S, but to the west of New Guinea it can be followed to about 8°S. In the southwestern portion of the areas under consideration, then, a region exists without an intermediate salinity maximum and with low-salinity water in the upper layers.

From the arrows in fig. 208 it is evident that the water in the deep basin in the Sulu Sea is renewed from the north by inflow of water from the South China Sea. The sill depth between the South China Sea and the Sulu Basin is probably about 400 m, and the water passing the sill has a potential temperature of about 9.9° and a salinity of about 34.50 ‰. These conditions are illustrated by the observations at Dana station 3685 in the Sulu Sea (table 86), from which it is seen that in the Sulu Sea the temperature increases with depth below 1200 m but the potential temperature decreases slightly except in the lower 1000 m. The oxygen content is somewhat lower than that of the South China Sea, and remains practically constant between 500 and 5000 m.

In all of the other basins shown in fig. 208 except the Timor Trench and the Sunda Trench, renewal of the deep water in the basins takes place from the Pacific Ocean. The potential temperature and the salinity of the water in the different basins is directly related to the manner of renewal and to the type of communication that exists. At the bottom of the figure is shown schematically how the renewal takes place in the series of basins joined by the heavy line in the upper part of the figure. The potential temperatures and the salinities at or directly below the sill depths are entered.

In all basins examined on the Snellius Expedition, a layer of minimum temperature was found a little below the sill depth across which renewal takes place. Below the layer of minimum temperature a small increase toward the bottom was observed, but this increase was in all instances

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somewhat less than the adiabatic one, so that the potential temperature decreased toward the bottom. The salinity of the water in the basins appears to be so constant that the greatest observed differences, amounting to 0.02 ‰, lie inside the experimental error of the determinations. Stable stratification prevailed, then, and no evidence was found of instability which might be caused by heating from the interior of the earth. In most basins the bottom water contained appreciable amounts of oxygen, but in some small basins of shallow sill depth the bottom water contained no oxygen but considerable quantities of hydrogen sulphide.

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The water in the Timor and Sunda Trenches originates from the Indian Ocean, as is evident from the high salinity of the water, 34.71 ‰ in contrast to values between 34.60 ‰ and 34.66 ‰ in all the other basins of the Archipelago.

The classical example on adiabatic increase of the temperature toward the bottom is found in the Mindanao Trench (the Philippines Trench), on the east side of Mindanao, in which a depth in excess of 10,000 m has been recorded. From the somewhat uncertain observations of the Planet in 1907–1908, Schott (1914) concluded that the adiabatic temperature increased toward the bottom and that the stratification was unstable, but Wüst (1929) showed that the observations could be interpreted differently and that indifferent equilibrium probably exists. His conclusion has been confirmed by observations on the Snellius Expedition, according to which the potential temperature actually decreases slightly with depth, whereas the observed differences in salinity are within the limits of the experimental errors. An extract from the Snellius observations in the Mindanao Trench is given in table 88. Thus, stable stratification appears to exist even in the deepest troughs, but a state of indifferent equilibrium is closely approached.