OCEANOGRAPHICAL CONDITIONS OF THE SEA IN THE PACIFIC OCEAN 789 



(c) Short periodic change. On the other hand, as mentioned above, 

 in the variation of the water temperature of each layer, a short periodic 

 variation is also recognized. This is especially the case for the layers 

 50 m., 100 m., 200 m. and 300 m. deep. 



Now, the maxima and minima of temperature of the 100 m, layer 

 as shown in Figure 2 were read off, and the time intervals between two 

 maxima or minima were measured and are shown in the following table. 



,, . r Period (in Months) 3 4 5 

 Time Interval of Temperature Maxmm s, ,. „ o 



I Frequency 5 12 



, ^ m ^ n/r- • f Period (in Months) 2 3 4 7 



Time Interval of Temperature Mmnna ■{ ^ 



1 Frequency 14 3 2 



(From 100 m. layer) 



As is seen in this table, the short periodic change in question has a 

 predominant period of 3 months. J. Fukuoka, one of the present 

 authors, and T. Yusa have studied the variation of water temperature 

 along the so-called "C-line" (Fig. 1) and it has been known that there 

 exists a change of water temperature of about 3-month period in the 

 sea adjacent to the Tohoku District (Northeastern Japan). 



3. Chlorinity 



Figure 4 shows the variations of chlorinity at different depths. From 

 this figure we can see that the annual variation of chlorinity in the 

 upper layers has a maximum in spring, which is due to the effect of 

 continued evaporation, and a minimum in autumn, which is caused 

 by heavy precipitation. Again, we can recognize that the chlorinity 

 in the layers 50-100 m. deep, tends to become high from early summer 

 to autumn. This could be due to the effect of the Kuroshio having 

 high chlorinity. On the other hand, the chlorinity in the layers 200- 

 500 m. deep is generally lower, which could be due to the effect of the 

 Oyashio creeping current. 



Further, if we look at the variation in chlorinity shown in Figure 4, 

 we can recognize that the long periodic change of chlorinity is similar 

 to that of water temperature, which is a fact worthy of being noted. 

 Thus it seems that the long periodic change of chlorinity is also due 

 to a long period change of water mass situation. 



4. Colour of the Sea, Transparency and Plankton Voluime 



At the point "Extra/' the observations of colour of the sea and 

 transparency have been performed since March 1948, and the collec- 

 tion of plankton since March 1950. In the following, we shall outline 

 the result. 



