10. Western Pacific subtropical sea area (20° to 30° N. , 130° to 180° E.) 



o o o o 



The area between 130 and 180 E. longitude and 20 to 30 N. latitude will be 



discussed as the western Pacific subtropical sea area. This is an extremely broad area, but 



oceanographic conditions are comparatively uniform throughout the whole region. However, in 



the vicinity of the Volcano Islands, the Ogasawara Islands, and the northern Marianas where the 



topography is complex, the oceanographic conditions are also somewhat complicated. The same 



is true of the area around the Okinawan archipelago in the extreme west. 



The area is generally under the influence of the Kuroshio Countercurrent. From the 

 Volcano Islands to the Ogasawara Islands there is a rather strong northward flowing current 

 along the east side of the island chains. This is called the Ogasawara Current ajid is particularly 

 strong during the sunnmer. 



The southern part of the area is within the North Equatorial Current and here the 

 current flows from east to west throughout the year. Along the northern edge of the North 

 Equatorial Current there are here aind there breinch currents flowing northwest. These are 

 particularly conspicuous in the summer at which season they generally chajige the direction of 

 their flow from northwest to northward. In the northern part of the area there are branch cur- 

 rents of the Japam Current and the North Pacific Current flowing south or southeastward, and it 

 is said that here and there great clockwise gyrals are formed. The North Pacific Current is the 

 name given to that part of the Kuroshio (Japan Current) which leaves Japeinese waters and flows 

 eastward, and the southward or southeastward branches mentioned above are called the Kuroshio 

 Countercurrent. Where the North Equatorial Current and the Kuroshio Countercurrent come into 

 contact is the so-called subtropical convergence. This subtropical line of convergence runs 

 approximately east Jind west along the parallels of latitude. Accordingly, completely different 

 water systems are found to the north and south of the convergence. 



The position of the subtropical convergence differs greatly with the seasons. When 



the water temperatures are at their lowest around February and March, it runs east and west 



o o 



along a line at about 23 to 24 N. , and when water temperatures axe at their highest in 



September, it nnoves far to the north and runs east and west in the vicinity of 30 N, 



The distribution of water temperatures is controlled by the above mentioned currents 

 and the isotherms run roughly parallel to the parallels of latitude. Around February and March 



O o^JT r oo 



the 22 C. isotherm runs east and west along the line of 24 to 25 N. or roughly at the center of 

 this sea area. In the summer the 28 C, isotherm appears at approximately the same position. 

 Table 49, based on data from the waters adjacent to the Ogasawara Islands, shows by months the 

 water temperature, specific gravity, and transparency in the vicinity of 25 N, latitude. 



As the table shows, the water temperatures cire at their lowest in March with values 

 o o 



of about 19.5 C. from the surface to the 30-meter level. The maximum is in July with 27.9 C. 



at the surface. Table 49 represents records for one year from April 1931 to March 1932, From 



the 10-year averages given in the same report, it appears that the mininnum temperature at the 



surface is 20,6 C, in February and the maximum is 27 C, in August. 



In the winter strong northwesterly or westerly winds blow continuously, being 

 particularly violent in the northern part of the area. In the summer the sea is generally extreme- 

 ly calm except when typhoons are being generated. 



This sea area was opened as a fishing ground around the middle of the Tciisho Era 

 (around 1920) and all parts of it have been fished by large numbers of fishing boats. The fishing 

 grounds were expainded to the eastward and southward as the size of the fishing boats was in- 

 creased, and the albacore grounds in the eastern part of the area were developed very rapidly 

 after the esurly years of the Showa Era (from 1926 on). Thereafter, from 1936 on, the yellowfin 

 grounds of the equatorijil region attracted the attention of the fishing industry, and the fishing 

 boats jumped directly to that area, so that except for particular localities the significance and 



85 



