74 DISTRIBUTION OF SALINITY, TEMPERATURE, DENSITY 



difference between evaporation and precipitation. From Bohnecke's 

 monthly charts of the surface salinity of the North Atlantic Ocean, mean 

 monthly values have been computed for an area extending between 

 latitudes 18° and 42° N, omitting the coastal areas in order to avoid 

 complications due to shifts of coastal currents. The results of this 

 computation show the highest average surface salinity, 36.70°/oo, in 

 March and the lowest, 36.59°/oo, in November. The variations from 

 one month to another are irregular, but on the whole the salinity is 

 somewhat higher in spring than it is in the fall. 

 Harmonic analvsis leads to the result 



(2^ I -80°), 



S = 36.641 + 0.021 cos (2t -^ - 80° ), (V, 3) 



and thus. 



as 

 dt 



0.021 ^ cos Utt ^ - 350°\ (V, 4) 



According to (V, 2), dS/dt is proportional to E — P, and it follows 

 therefore that the excess of evaporation over precipitation is at a mini- 

 mum at the end of June and at a maximum at the end of December. 

 This annual variation corresponds closely to the annual variation of 

 evaporation (p. 68), for which reason it appears that in the area under 

 consideration the annual variation of the surface salinity is mainly 

 controlled by the variation in evaporation during the year. For a more 

 exact examination, subsurface data are needed, but nothing is known as 

 to the annual variation of salinity at subsurface depths. 



From the open ocean, no data are available as to the diurnal variation 

 of the salinity of the surface waters, but it may be safely assumed that 

 such a variation is small, because neither the precipitation nor the evapo- 

 ration can be expected to show any considerable diurnal variation. 



Temperature of the Surface Layers 



Surface Temperature. The general distribution of surface temper- 

 ature cannot be treated in the same manner that Wiist employed for the 

 salinity, because the factors controlling the surface temperature are far 

 more complicated. The discussion must be confined to presentation of 

 empirical data and a few general remarks. 



Table 1 1 contains the average temperatures of the oceans in different 

 latitudes, according to Kriimmel, except in the case of the Atlantic 

 Ocean, for which new data have been compiled by Bohnecke. In all 

 oceans the highest values of the surface temperature are found somewhat 

 to the north of the Equator, and this feature is probably related to 

 the character of the atmospheric circulation in the two hemispheres. The 

 region of the highest temperature, the thermal Equator, shifts with the 

 season, but only in a few areas is it displaced to the Southern Hemisphere 



