GENERAL CHARACTERISTICS OF THE EASTERN SEAS 



689 



by that for 2°; at that season the isotherm 12° lies close to 40° N latitude, 

 where in summer the 20° isotherm passes. 



The southern limit of the cold layer is subject to substantial fluctuations 

 over many years (M. Uda, 1955), which have a pronounced effect on biological 



Fig. 328a. Diagram of continuous surface currents (summer) (Dobro- 



volsky, 1948). 



phenomena. In 1933 this limit passed close to the Kuril Islands; in subse- 

 quent years it moved farther and farther southeast, and in 1953 it had moved 

 away between 200 and 500 miles from its position of twenty years earlier. 



Fig. 328b. Diagram of continuous surface currents (winter) (Dobrovolsky). 



The Ivasi catastrophe may have been connected, either directly or indirectly, 

 with these fluctuations. 



A clear picture of the changes of temperature, salinity, oxygen, phosphorus 

 and silica content is given in Figs. 330, 331, 332 and 333. 



The amplitude of temperature fluctuations becomes less with depth. In the 

 Kuroshio region the amplitude is 13-5° (10-5° to 24°) on the surface; at a 

 depth of 200 m it is 2-5° (9° to 11-5°); at 500 m barely one degree; while at 



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