We traced this distribution along four routes traversing the northern hemisphere from the pole to the equator in the most characteristic areas. °' The routes were as follows: 1) along the western coast of North America, through Central America, and along the western coast of South America to the equator; 2) along the eastern coast of North America through Central America, and along the eastern coast of South America to the equator; 3) through western Europe and the Mediterranean basin, and through western Africa down to the Equator; 4) along the eastern coast of Asia, including Japan, India and Indonesia, down to the equator.** Our data on atmospheric precipitation along the above routes were taken from the instructional climatic map of the world published by the Main Administration of Geodesy and Cartography of the Ministry of Internal Affairs of the USSR in 1955.
These data were used for plotting the four graphs in Figure 2. Each of these graphs reflects the variations in atmospheric precipitation from the equator toward the North Pole along one of the routes mentioned. According to all the graphs, the maximum atmospheric precipitation falls in the equatorial zone, decreasing gradually in a northward direction, but conforming to different patterns in each of the four cases.
Averaged data from the above four graphs were used for plotting a combined graph (Figure 3). The values of the ordinates of this graph were computed as the arithmetic means of the corresponding ordinates of the original four graphs. Consequently, curve B in Figure 3 does not portray the distribution of atmospheric precipitation along any single route, but reflects the general variation patterns of the amounts of atmospheric precipitation over the entire northern hemisphere.
Thus we have examined separately the two main factors in the formation of submarine canyons — fluctuations in ocean level and distribution of atmospheric precipitation. In nature, however, these factors act simul taneously, and therefore we must discover the possible results of their simultaneous action. In this case we shall also use the graphic method. Curve A in Figure 3 depicts amplitudinal variations of the fluctuations in ocean level from the equator to the pole. Curve B displays the variation in the quantity of atmospheric precipitation (also from the equator to the pole). The results of their simultaneous action may be determined by simply combining these two curves.
The following introductory explanation is necessary. The amplitude of fluctuations in ocean level and the amount of atmospheric precipitation are quite different phenomena, measured in different units, so that the combination of the respective measurements is a difficult task. However,
we consider it feasible to reduce both phenomena to a single scale, in their roles as prerequisites for the formation of submarine valleys. For this purpose we shall assume that the effect of both factors are approximately equal in the formation of submarine canyons. This statement is expressed graphically simply by assigning equal ordinate values to the maxima of each phenomenon.
This somewhat arbitrary use of the graphic method is justified in view of our use of the combined curve only as a qualitative characterization of the phenomenon under consideration rather than a quantitative indicator. If considerable portions of the two graphs being added are more or less in phase, both the above conditions for the formation of submarine valleys have been met, and each intensifies the other. In this case, the probability for the formation of submarine canyons is maximum, which is expressed by the maximum ordinate value of the resultant curve.
If large ordinates of one curve correspond to minimum ordinates of the other, their combined effect will reflect the case in which absence (or insignificance) of one of the conditions lowers the possibility of the formation of submarine canyons. In the case of coincidence of the minimum values of both constituent curves nature does not provide the conditions necessary for the formation of submarine valleys. In this case the probability of their formation is at its minimum and the combined graph will drop to a minimum.
Curve C in Figure 3 has been constructed by the method described above. It combines the graphs of the distribution of atmospheric precipitation and of the amplitude of the fluctuations of the oceanic spheroid. Ordinates of this curve for each of its abscissa values were obtained by addition of the ordinates of the first two curves (A and B). Since the absolute values of the graphs are unimportant in this case, the position of curve C was somewhat lowered for the sake of convenience.
Our resultant curve reflects the combined effect of both factors — fluctuations in ocean level and amount of atmospheric precipitation — upon the formation of submarine valleys. It indicates those cases in which conditions for the formation of submarine canyons are most favorable and those in which the probability of their formation is at a minimum. It is seen from the graph that curve C reaches maximum values in latitudes ranging from 32 to 50°, indicating conditions conducive to the formation of submarine valleys. This is, in fact, the zone in which the amplitude of fluctuations of ocean level is maximum and atmospheric precipitation is abundant, so that here the two factors exhibit the same characteristics, intensifying each other.