DISTRIBUTION OF SALINITY, TEMPERATURE, DENSITY 83 



takes place, the vertical convection currents penetrate to greater and 

 greater depths until the density has attained a uniform value from the 

 surface to the bottom. When this state has been established, continued 

 increase of the density of the surface water leads to an accumulation 

 of the densest water near the bottom, and, if the process goes on in an 

 area that is in free communication with other areas, this bottom water of 

 great density spreads to other regions. If the increase of density at the 

 surface stops before the convection currents have reached the bottom, 

 the process may lead to the formation of water that spreads at an inter- 

 mediate level. 



In the open oceans the temperature" of the surface water in lower 

 and middle latitudes is so high that the density of the water remains low, 

 even in regions where high salinities occur through excess of evaporation. 

 In these latitudes, convection currents are limited to a relatively thin 

 layer near the surface and do not lead to formation of deep or bottom 

 water. Such formation takes place mainly in high latitudes, where, 

 however, the excess of precipitation in most regions prevents the develop- 

 ment of convection currents that reach to great depths. This excess of 

 precipitation is so great that deep and bottom water is formed only in two 

 cases: (1) if water of high salinity has been carried into high latitudes 

 by currents and is cooled, and (2) if water of relatively high salinity 

 freezes. 



The first conditions are encountered in the North Atlantic Ocean, 

 where water of the Gulf Stream system, the salinity of which has been 

 raised in lower latitudes by excessive evaporation, is carried into high 

 latitudes. In the Irminger Sea, between Iceland and Greenland, and in 

 the Labrador Sea, this water is partly mixed with cold water of low salinity 

 that flows out from the Polar Sea (p. 156). This mixed water has a rela- 

 tively high salinity, and, when it is cooled in winter, convection currents 

 that may reach from the surface to the bottom develop before any freezing 

 of ice begins. In this manner, deep and bottom water is formed that has 

 a high salinity and a temperature that lies several degrees above the freez- 

 ing point of the water (table 19, p. 170). A similar process takes place 

 in the Norwegian Sea, but there deep and bottom water is formed at a 

 temperature which deviates only slightly from the freezing temperature 

 (p. 157). 



In the Arctic the second process is of minor importance. There, in 

 the regions where freezing occurs, the salinity of the surface layers is very 

 low, mainly owing to the enormous masses of fresh water that are carried 

 into the sea by the Siberian rivers. Close to the Antarctic continent, 

 however, formation of bottom water by freezing is of the greatest impor- 

 tance. At some distance from the Antarctic continent the great excess 

 of precipitation maintains a low surface salinity, and in these areas winter 

 freezing is not great enough to increase the salinity sufficiently to form 



