Page:EB1911 - Volume 19.djvu/1022

Rh fathoms is only 52° F. The whole ocean must thus form but a cold dwelling-place for the organisms of the deep sea. Sir John Murray calculates that at least 80% of the water in the ocean has a temperature always less than 40° F., and a recent calculation by Krümmel gave in fact a mean temperature of 39° F. for the whole ocean.



The normal vertical distribution of temperature is illustrated in curve A of fig. 1, which represents a sounding in the South Atlantic; and this arrangement of a rapid fall of temperature giving place gradually to an extremely slow but steady diminution as depth increases is termed anathermic (, back, and  , warm). Curve B shows the typical distribution of temperature in an enclosed sea, in this case the Sulu Basin of the Malay Sea, where from the level of the barrier to the bottom the temperature remains uniform or homothermic. Curve C shows a typical summer condition in the polar seas, where layers of sea-water at different temperatures are superimposed, the arrangement from the surface to 200 fathoms is termed dichathermic (, apart), from 1000 to 2000 fathoms it is termed katathermic (, down). In autumn the enclosed seas of high latitudes frequently present a thermal stratification in which a warm middle layer is sandwiched between a cold upper layer and a cold mass below, the arrangement being termed mesothermic (, middle). The nature of the change of temperature with depth below 2500 fathoms is entirely dependent on the position of the sub-oceanic elevations, for the rises and ridges act as true submarine watersheds. As the Arctic Basin is shut off from the North Atlantic by ridges rising to within 300 fathoms of the surface and from the Pacific by the shallow shelf of the Bering Sea, and as the ice-laden East Greenland and Labrador currents consist of fresh surface water which cannot appreciably influence the underlying mass, the Arctic region has no practical effect upon the bottom temperature of the three great oceans, which is entirely dominated by the influence of the Antarctic. The existence of deep-lying and extensive rises or ridges in high southern latitudes has been indicated by the deep-sea temperature observations of Antarctic expeditions. Temperatures so low as 31·5° to 31·3° F. do not occur much beyond 50° S. The “Belgica” even found a temperature of 33·1° F. in 61° S., 63° W., at a depth of 2018 fathoms. The conditions of temperature in the South Atlantic are characteristic. South of 55° S. in approximately 3000 fathoms the bottom temperature is 31·1° F.; in the Cape Trough it is 32·7°

in 45° S., and 33·8° to 34·3° in 35° S., while north of the Walfisch Ridge and east of the South Atlantic Rise bottom temperatures of 36° to 36·7° F. prevail right northwards across the equator into the Bay of Biscay, showing a steady rise, of bottom temperature as successive submarine elevations restrict communication with the Antarctic. On the other hand, in the more open Argentine Basin, which carries deep water far to the south, the bottom temperature in 40° S. is only from 32·2° to 32·7° F., and the same low temperature continues throughout the Brazil Basin to the equator; but in the North American Basin from the West Indies to the Telegraph Plateau no satisfactory bottom temperature lower than 35·6° F. has been reported. On the floor of the Indian Ocean temperatures of 33·3° to 33·6° occur south of 35° S. in depths of 2700 fathoms or more, but north of 35° S. the prevailing bottom temperatures are from 34·0° to 34·3°. In similar depths in the Pacific south of the equator temperatures of 33·8° to 34·5° are found, and north of the equator bottom temperatures at the same depth increase to 35·1° in the neighbourhood of the Aleutian Islands, again completely justifying the conclusion as to the Antarctic control of deep water temperature throughout the ocean.

The marginal rises and continental shelves prevent this cold bottom water from penetrating into the depths of the enclosed and fringing seas. Thus in the Central American Sea below 930 fathoms, the depth on the bar, no water is found at a temperature lower than that prevailing in the open ocean at that depth, viz. 39·6° F., not even at the bottom of the great Bartlett Deep in 3439 fathoms. Such homothermic masses of water are characteristic of all deep enclosed seas. Thus in the Malay Sea the various minor seas or basins are homothermic below the depth of the rim, at the temperature prevailing at that depth in the open ocean: in the China Sea below 875 fathoms with 36·5° F.; in the Sulu Sea (depth 2550 fathoms) below 400 fathoms with 50·5° F.; in the Celebes Sea below 820 fathoms with 38·6° F.; in the Banda Sea below 902 fathoms with 37·9° F. In other enclosed seas which are shut off from the ocean by a very shallow sill the rule holds good that the homothermic water below the level of the sill is at the lowest temperature reached by the surface water in the coldest season of the year, provided always that the stratification of salinity is such as to permit of convection being set up. To this group belongs the Arctic Sea; the Norwegian Sea is homothermic below 550 fathoms at 29·8° F., but this cold water does not penetrate into the Arctic Basin on account of the ridge between Spitsbergen and Greenland, and there the water below 1400 fathoms has a temperature of 30·6° to 30·7° F. because the surface layers of water are too light, on account of the low salinity due to ice-melting, to enable even the cold of a polar winter to set up a downward convection current. The Mediterranean Sea also belongs to this group; its various deep basins are homothermic (at the winter surface temperature) below the level of their respective sills—the Balearic Basin below 190 fathoms at 55° F.; the Eastern Basin below 270 fathoms at 55·9° F.; the Ionian Sea at 56·3° F.; and at 56·7° south of Cyprus. Similarly in the Red Sea the water below 380 fathoms is homothermic at 70·7° F.

An under-current flows out from the Red Sea through the Strait of Bab-el-Mandeb, and from the Mediterranean through the Strait of Gibraltar, raising the salinity as well as the temperature of the part of the ocean outside the gates of the respective seas. The action of the Red Sea water affects the whole of the Gulf of Aden and Arabian Sea, raising the temperature at the depth of 550 fathoms to 52° or 53° F. or 9 Fahrenheit degrees higher than the water of the Bay of Bengal at the same depth. The effect of the Mediterranean water in the North Atlantic does not require such large figures to express it, but is none the less extraordinarily far-reaching, as first indicated by the work of the “Challenger” and subsequently defined by H. N. Dickson’s discussion of the observations of Wolfenden in the little sailing yacht “Silver Belle.” The temperature at 550 fathoms is raised to 49° or 50° F. between Madeira and the Biscay Shelf, i.e. 5·4° F. above the temperature at the same depth off the Azores.

In shallow seas such as the North Sea and the British fringing