PACIFIC OCEAN 119 a continent attracted the isothermals, making them con verge towards the equator. It has already been pointed out that these effects are due to the winds and the cold currents which strike the western continental shores and run along the coasts. The surface temperature of the Pacific, between the latitudes of 45 N. and 45 S., no where at any season falls below 50. In August the southern isotherm of 50 remains close to the 50th parallel, not diverging more than a degree or two on either side. Between the 45th parallels and the northern and southern limits of the ocean the temperature is almost always below 50. The southern isotherm of 40 is remarkable for its constant position all the year round, between latitudes 55 and 58, a result brought about by the gigantic antarctic icebergs which prevent the surface temperature of the water from rising during the southern summer. The northern and southern " isocryrnes " of 68, that is the lines which pass over water which has a mean temperature of 68 during the coldest months of the year, lie, according to Dana (Corals and Coral Islands, 1872), between the latitudes of 20 and 30 on each side of the equator, except in the neighbourhood of the South- American coast, where the isocryme runs north in a loop beyond the equator, a consequence of the cooling effect of the Peruvian current. These isocrymes mark out an area of great importance ; for the reef-building corals are con fined within it. The highest temperature which sea water has been observed to attain is 90 F., and water of this temperature is only met with in the Red Sea. The maximum in the Pacific in the month of August is reached in the boundary between it and the Indian Ocean (in the Malay Archi pelago) and in a narrow strip along the Mexican coast ; in both these regions the thermometer immersed in the surface water registers 85 as a mean. There is a con siderable area which in August stretches between New Guinea and Japan, from 10 S. to nearly 30 N., where the surface temperature reaches 84, but these are excep tional temperatures. When the "Challenger" was cruising in the South Pacific in 1874 and 1875 the water was found to be uniformly warmer than the air, the difference in tem perature between the two averaging 1 0< 5 to 2 Fahr. In the North Pacific, between the latitudes of 30 and 40, on the other hand, the atmospheric temperature is about half a degree higher than that of the surface water. Such differences may be explained by considering the effect of warm and cold currents, which alter the temperature of the water much more rapidly than that of the air, and of warm and cold winds, which affect the atmosphere more quickly than the ocean. eep-sea Deep-Sea Temperature. The serial temperature sound- mpera- ings of the "Challenger" in the Pacific give a very good ire- idea of the distribution of temperature in the deeper waters. There seems to be a slow massive movement of water from the Antarctic Ocean into the Pacific, which is not confined to the surface currents, but affects the whole mass of water down to the bottom. The rate of this motion is quite unknown. In the open sea, far from coasts and barriers, the temperature of the water con tinually decreases as the depth increases. This is only true for the open ocean, fully exposed to the effects of the mass movement of the water ; there is a very different distri bution of temperature in enclosed seas such as those of the Western Pacific, or even in the ocean when a barrier pre sents itself to the moving water. The difference, which is late II. brought out by the diagram (Plate II. fig. 1), is due to the = ! fact that when a barrier exists it retards the motion of the lower portion of the water, which has the lowest tempera ture, while the higher passes on over it, and fills up the area beyond with water ait the uniform temperature of the great ocean at the point to which the top of the ridge or obstruction reaches. In the Sulu Sea, for instance, the diagram shows 1 that the temperature falls steadily and rapidly from 80 at the surface to 50*5 at 400 fathoms, and then continues at 50 5 right down to the bottom in 2500 fathoms, instead of sinking to somewhere about 35, as it is observed to do in the open ocean at that depth. The inference is that the Sulu Sea is surrounded by a ridge rising to at least about 400 fathoms from the surface, which prevents the great ocean circulation from having its, cooling effect, and soundings indicate that this is really the case. A study of the temperature phenomena, such as those just referred to, points out with considerable certainty the existence and height of barriers and ridges in many parts of the ocean, where their presence has not been detected by actual soundings. 2 During the cruise of the "Challenger" the bottom temperature over the North Pacific was found to be 35 l; south of the Sandwich Islands it fell to 35; in the Low Archipelago it again rose to 35 l ; on the 40th parallel it fell to 34 "7 in the deep water, but rose to 35 - 4 and 35 5 in the shallow water of the Patagonian elevation. The thermometer registered 34 5 at the bottom between Australia and New Zealand ; while in that part of the ocean to the north-east of Australia known as the Coral Sea, although the depth was the same (about 2500 fathoms), the bottom temperature was as high as 35 9. The variations of temperature in the enclosed seas of the Eastern Archipelago were found to be considerable, and nearly all those seas show the phenomenon of constant temperature from an intermediate point to the bottom, consequent on the existence of barriers. The chief details of the thermal conditions of these seas are represented graphically in the diagram (Plate II. fig. 1). Between the Caroline Islands and Japan the bottom temperature was 35 "3. The bottom temperature in the Pacific is on the average about 1 F. lower than that in the Atlantic. The temperature of the water at the depth of 300 fathoms is nearly the same (40 to 45) over the whole of the North Pacific, but above 300 fathoms the water is warmer in the western than in the central portion, while below that depth it is colder in the former than in the latter. The same phenomenon is noticed between the latitudes of 34 S. and 40 S., but here 700 fathoms marks the plane of constant temperature. Between 33 N. and 40 S. the temperature of the water above 200 fathoms is higher in the North than in the South Pacific, whilst from 200 to 1500 fathoms it is lower in the North, and below the latter depth the condition reverts to what it was above 200 fathoms. The diagram (Plate II. fig. 2) exhibits the bathy metrical Plate II. distribution of temperature in a section of the Pacific from a fig. 2. position in 38 9 N. lat. and 156 25 W. long, to one in 40 3 S. lat. and 132 58 W. long, as determined by H.M.S. "Chal lenger" in 1875, and may be compared with similar diagrams of the ATLANTIC (see vol. iii. p. 23). In order to separate the iso thermals in the first 200 fathoms sufficiently the scale of depths required to be made large, while in order that the length of the diagram might be kept within reasonable bounds the scale of latitude was made very much smaller. The result of this is to exaggerate the inequalities of the sea bottom, making the slopes very much steeper than they are ; this effect is best seen in the way in which islands are represented. The rapid falling off of temperature in the first few hundred fathoms, and then its very slow but steady decrease to the bottom are to be observed, and the fact that latitude has a great effect on the surface temperature, but 1 The encircled numbers in the diagrams (Plate II. figs. 1 and 2) indicate the "Challenger" stations. - An excellent example of the existence of a submarine barrier being pointed out by a wide divergence in the temperature in contiguous areas of the ocean is met with in the Faroe Channel (see NORWEGIAN SEA, vol. xvii. p. 594, and NORTH SEA, p. 564, fig. 1).
Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/135
This page needs to be proofread.
