DISTRIBUTION OF SALINITY, TEMPERATURE, DENSITY 79 



ing winter, from the middle of November to the middle of February, an 

 inshore flow to the north, the Davidson Current, is present (p. 204). The 

 water of this inshore flow is characterized by relatively high and uniform 

 temperature, and appears in the annual variation of temperature as warm 

 water at subsurface depths. The upper homogeneous layer is relatively 

 thick, as is evident from the fact that the temperature is nearly the same 

 at 25 m as it is at the surface, and that at 50 m it is only slightly lower. 

 At the end of February the California Current again reaches to the coast, 

 and under the influence of the prevailing northwesterly winds an overturn 

 of the upper layers occurs which is generally described as upwelling 

 (p. 131). During the period of upwelling, vertical motion near the coast 

 brings water of relatively low temperature toward the surface. Conse- 

 quently, the temperatures at given depths decrease when the upwelling 

 begins. This decrease is brought out in fig. 17 A by the downward trend 

 of the temperature at 25, 50, and 100 m, at which depths the minimum 

 temperature is reached at the end of May. The much higher temperature 

 at the surface as compared to that at 25 m shows that a thin surface layer 

 is subject to heating by radiation, and from the variation of temperature 

 at 10 m, which is shown by a thin line, it is evident that the effect of 

 heating is limited to the upper 10 m. As the upwelling gradually ceases 

 toward the end of August, a sharp rise in temperature takes place both 

 at the surface and at subsurface depths, and the peaks shown by the 

 temperature curves in September are results of heating and conduction. 

 Thus, the annual march of temperature can be attributed to changes in 

 currents, to vertical motion associated with upwelling, to seasonal heating 

 and cooling, and to heat conduction. 



The annual variation of temperature in the Kuroshio, off the south 

 coast of Japan, as shown in fig. 17B, gives an entirely different picture. 

 The annual variation has the same character at all depths between the 

 surface and 100 m, with a minimum in late winter and a maximum in late 

 summer or early fall, but the range of the variation decreases with depth, 

 and the time of maximum temperature occurs later with increasing depth. 

 From the course of the curves, it may be concluded that the annual varia- 

 tion is due to heating and cooling near the surface and is transmitted to 

 greater depths by processes of conduction. This assumption appears to 

 be correct, but the heating and cooling also depend on excessive cooling 

 in winter by cold and dry winds blowing toward the sea, and are caused 

 only partly by variations in the net radiation. 



In order to be certain that observed temperature variations are 

 related to processes of heating and cooling only, it is necessary to deter- 

 mine whether the water in a given locality is of the same character 

 throughout the year. For this purpose, Helland-Hansen developed a 

 method that is applicable in areas in which it is possible to establish a 

 definite relation between temperature and salinity (p. 86). He assumed 



