Fig. 8. Effect of diminution of storms on movement of water.
Now suppose that cyclonic storms should be greatly reduced in number so that in the zone of prevailing westerlies they were scarcely more numerous than tropical
hurricanes now are in the trade-wind belt. Then the more or less southwesterly winds in quadrant A´ in the right-hand part of Fig. 8 would not only become more frequent but would be stronger than at present. The total movement from that quarter might rise to sixty units, as indicated in the figure. In quadrants B´ and D´ the movement would fall to fifteen and in quadrant C´ to ten. B´ and D´ would balance one another as before. The movement in A´, however, would exceed that in C´ by fifty instead of ten. In other words, the current-making force would become five times as great as now. The actual effect would be increased still more, for the winds from the southwest would be stronger as well as steadier if there were no storms. A strong wind which causes whitecaps has much more power to drive the water forward than a weaker wind which does not cause whitecaps. In a wave without a whitecap the water returns to practically the original point after completing a circle beneath the surface. In a wave with a whitecap, however, the cap moves forward. Any increase in velocity beyond the rate at which whitecaps are formed has a great influence upon the amount of water which is blown forward. Several times as much water is drifted forward by a persistent wind of twenty miles an hour as by a ten-mile wind.[68]
In this connection a suggestion which is elaborated in Chapter XIII may be mentioned. At present the salinity of the oceans checks the general deep-sea circulation and thereby increases the contrasts from zone to zone. In the past, however, the ocean must have been fresher than now. Hence the circulation was presumably less impeded, and the transfer of heat from low latitudes to high was facilitated.
Consider now the magnitude of the probable effect of a diminution in storms. Today off the coast of Norway in latitude 65°N. and longitude 10°E., the mean temperature in January is 2°C. and in July 12°C. This represents a plus anomaly of about 22° in January and 2° in July; that is, the Norwegian coast is warmer than the normal for its latitude by these amounts. Suppose that in some past time the present distribution of lands and seas prevailed, but Norway was a lowland where extensive deposits could accumulate in great flood plains. Suppose, also, that the sun's atmosphere was so inactive that few cyclonic storms occurred, steady winds from the west-southwest prevailed, and strong, uninterrupted ocean currents brought from the Caribbean Sea and Gulf of Mexico much greater supplies of warm water than at present. The Norwegian winters would then be warmer than now not only because of the general increase in temperature which the earth regularly experiences at sunspot minima, but because the currents would accentuate this condition. In summer similar conditions would prevail except that the warming effect of the winds and currents would presumably be less than in winter, but this might be more than balanced by the increased heat of the sun during the long summer days, for storms and clouds would be rare.
If such conditions raised the winter temperature only 8°C. and the summer temperature 4°C., the climate would be as warm as that of the northern island of New Zealand (latitude 35°-43°S.). The flora of that part of New Zealand is subtropical and includes not only pines and beeches, but palms and tree ferns. A climate scarcely warmer than that of New Zealand would foster a flora like that which existed in far northern latitudes during some of the milder geological periods. If, however, the
general temperature of the earth's surface were raised 5° because of the scarcity of storms, if the currents were strong enough so that they increased the present anomaly by 50 per cent, and if more persistent sunshine in summer raised the temperature at that season about 4°C., the January temperature would be 18°C. and the July temperature 22°C. These figures perhaps make summer and winter more nearly alike than was ever really the case in such latitudes. Nevertheless, they show that a diminution of storms and a consequent strengthening and steadying of the southwesterlies might easily raise the temperature of the Norwegian coast so high that corals could flourish within the Arctic Circle.
Another factor would coöperate in producing mild temperatures in high latitudes during the winter, namely, the fogs which would presumably accumulate. It is well known that when saturated air from a warm ocean is blown over the lands in winter, as happens so often in the British Islands and around the North Sea, fog is formed. The effect of such a fog is indeed to shut out the sun's radiation, but in high latitudes during the winter when the sun is low, this is of little importance. Another effect is to retain the heat of the earth itself. When a constant supply of warm water is being brought from low latitudes this blanketing of the heat by the fog becomes of great importance. In the past, whenever cyclonic storms were weak and westerly winds were correspondingly strong, winter fogs in high latitudes must have been much more widespread and persistent than now.
The bearing of fogs on vegetation is another interesting point. If a region in high latitudes is constantly protected by fog in winter, it can support types of vegetation characteristic of fairly low latitudes, for plants are oftener killed by dry cold than by moist cold. Indeed,
excessive evaporation from the plant induced by dry cold when the evaporated water cannot be rapidly replaced by the movement of sap is a chief reason why large plants are winterkilled. The growing of transplanted palms on the coast of southwestern Ireland, in spite of its location in latitude 50°N., is possible only because of the great fogginess in winter due to the marine climate. The fogs prevent the escape of heat and ward off killing frosts. The tree ferns in latitude 46°S. in New Zealand, already referred to, are often similarly protected in winter. Therefore, the relative frequency of fogs in high latitudes when storms were at a minimum would apparently tend not merely to produce mild winters but to promote tropical vegetation.