Page:EB1911 - Volume 21.djvu/994

 summer and winter, throughout the North Polar Basin, except near the surface, which in summer is covered by a layer of fresh water arising from the melting of the snow-covered surface of the floe-ice. This fresh-water layer may attain a thickness of 5 or 6 ft. between the floes. North of the Siberian coast the sea is, during summer, covered with a layer of warm water from the Siberian rivers, and the temperature of the surface may rise to several degrees above freezing-point.

In the Norwegian and Greenland Seas there are greater variations of temperature. Below a certain limit, which in the northern part (on the eastern side) is about 550 fathoms deep, and in the southern part between 300 and 400 fathoms deep, the whole basin of this sea is filled with water which has an unusually uniform salinity of about 34·92 per mille, and the temperature of which is below zero centigrade, gradually sinking downward from the above-mentioned limit, where it is 32° (0° C.), and down to 29·8° (−1·2° C.) or 29·6° (−1·3° C.) near the bottom in 1400 or 1600 fathoms. This cold underlying water of such a remarkably uniform and comparatively low salinity is formed chiefly in a small area between Jan Mayen and Spitsbergen, by the formation of ice and cooling down of the Atlantic surface water by radiation of heat during the winter. In this manner the surface water becomes heavier than the underlying water and gradually sinks to the bottom. This water seems to be distinctly different from the hitherto known water filling the deep of the North Polar Basin, as it has a lower salinity and lower temperature, the known bottom temperature of the North Polar Basin being between 30·7° (−0·7° C) and 30·4° (−0-9° C.), and the salinity about 35·1 per mille. This fact seems to indicate that there can be no direct communication between the deep depression of the North Polar Basin and the Norwegian-Greenland Sea, which are probably separated by a submarine ridge running from the north-west corner of Spitsbergen to Greenland.

The above-mentioned layer of uniform cold water of the Norwegian-Greenland Sea is, along its eastern side, covered by the warm and saline water of the Gulf Stream flowing northward along the west coast of Norway, Bear Island and Spitsbergen, and forming the upper strata of the sea about 300 to 500 fathoms deep. The maximum temperature of this water is on the surface about 46° (8° C) to 50° (10° C.) west of northern Norway, and about 37° (3° C.) to 39° (4° C.) west of Spitsbergen. The salinity is generally between 35 0 and 35-3 per mille.

Along the western side of this sea, towards the east coast of Greenland, the underlying cold water is covered by the less saline water of the polar current, which in the upper strata of the sea, from the surface down to about 100 fathoms, has very much the same temperature and salinity as in the upper cold and less saline strata of the North Polar Basin. Near the east coast of Greenland, a layer of comparatively warm and saline water, with a temperature of 32·7° (0·4° C.) and a salinity of 35·2 per mille, has been found (by the Ryder expedition in 1891) below the cold and lighter polar water in a depth of 70 to 90 fathoms. This warmer undercurrent is a continuation of the warm Spitsbergen current sending off a branch westward from Spitsbergen, and thus forming a great vortex movement in the Spitsbergen(Greenland Sea similar to the one mentioned farther south in the Norwegian Sea.

In Barents Sea the temperature and salinity are highest in the western part near Norway or between Norway and Bear Island, where the eastern branch of the Gulf Stream enters and where in summer the salinity generally is between 34·8 and 35 per mille from the surface down to the bottom, and the surface temperature generally is about 41° or 43° (5° C. or 6° C.), and the bottom temperature is above zero centigrade. The eastern part of Barents Sea is filled with water of a little lower salinity, the deeper strata of which are very cold, with temperature even as low as 28·9° (−1·7° C), but often with salinity above 35·0 per mille This cold and saline water is formed during the formation of ice on the sea-surface. The bottom temperature is everywhere in the eastern part below zero centigrade and generally below −1° C.

The Kara Sea is covered near the surface with a layer of cold water much diluted by the fresh water from the Siberian rivers, especially the Ob and the Yenisei. The salinity varies between 29 and 34 per mille, near the mouth of the rivers it is naturally much lower.

The vertical distribution of temperature and salinity in Baffin Bay seems to be very similar to that of the North Polar Basin, with a cold but less saline upper stratum of water-with a minimum temperature of about 28·9° (−1·7° C.)—and a warmer and more saline deeper stratum from 100 to 200 fathoms downwards, with a maximum temperature of 33·6° (0·9° C.) in about 200 fathoms, and the temperature slowly decreasing towards the bottom.

Arctic Ice.—As before mentioned, at least two-thirds of the Arctic Sea is constantly covered by drifting ice. This ice is mostly formed on the surface of the sea itself by freezing, the so-called floe-ice or sea-ice. A small part is also river-ice, formed on the rivers, especially those of Siberia, and carried into the sea during the spring or summer. Another comparatively small part of the ice originates from the glaciers of the Arctic lands. These pieces of glacier-ice or icebergs are, as a rule, easily distinguished from the floe-ice by their size and structure. They occur almost exclusively in the seas round Greenland, where they originate from the glaciers descending into the sea from the inland ice of Greenland. Some small icebergs are also formed in Franz Josef Land, Spitsbergen, Novaya Zemlya, Grinnell Land, &c., but they are comparatively insignificant, and are not as a rule carried far from the coasts. Sea-ice or floe-ice is formed during the autumn, winter and spring, especially in the North Polar Basin, but also in the Kara Sea, the greater part of Barents Sea, the northernmost part of the Norwegian Sea (near Bear Island and towards Jan Mayen), Greenland Sea and Baffin Bay. The floe-ice does not, as a rule, grow thicker than 7 or 8 ft in one year, but when it floats in the water for some years it may attain a thickness of 16 ft. or more directly by freezing. By the constant upheaval from pressure much greater thicknesses are attained in the piled-up hummocks and rubble which may be 20 to 30 ft. high above the water when floating. During the summer the floe-ice decreases again by melting partly on the surface owing to the direct radiation of heat from the sun, partly on the under side owing to the higher temperature of the water in which it floats. The first kind of melting is that which prevails in the North Polar Basin, which the second occurs in more southern latitudes. The floe-ice is constantly more or less in movement, carried by winds and currents. The changing wind, and also to a great extent the changing tidal current, causes diverging movements in the ice by breaking it into larger or smaller floes. When the floes separate, lanes and channels are formed, when they meet, ice-pressures arise, and the floes are piled up to form hummocks or ridges, and thus the uneven polar ice arises. In the North Polar Basin the floe-ice is slowly carried by the prevailing winds and the currents in an average direction from Bering Strait and the New Siberia Islands, north of Franz Josef Land and Spitsbergen, near the North Pole, towards the Greenland Sea and southward along the east coast of Greenland. Such a drift of an ice-floe from the sea north of Bering Strait to the east coast of Greenland probably takes, as a rule, four or five years, and the floes found in this part of the sea are not, therefore, generally older. What the drift of the ice is on the American side of the North Polar Basin is still little known. But there it is probably more or less blocked up in its southward movement by the islands of the American Arctic archipelago, and the ice-floes may thus grow very old and thick. Commander Peary found a strong easterly movement of the floes in the region north of Grant Land in 1907. The southward distribution of the drifting floe-ice (the pack ice) in Barents Sea, Norwegian-Greenland Sea and Davis Strait may differ much from one year to another, and these variations are evidently due to more or less periodical variations in the currents and also in the directions of the prevailing winds. In most places the ice has its most southerly distribution during the late winter and spring, while the late summer and autumn (end of August and September) is the most open season.