DISTRIBUTION OF SALINITY, TEMPERATURE, DENSITY 89 



water types exist and that the T-S relations that are observed represent 

 the end results of mixing between the types. This concept presupposes 

 that the water types (often referred to as -water masses) are continually 

 renewed, because, if that were not the case, processes of mixing would 

 ultimately lead to the formation of homogeneous water. It is possible, 

 however, to account for the character of the T-S curves in the ocean by 

 considering other processes. 



In the first place, it should be observed that a water mass of uniform 

 temperature and saUnity is rarely formed in the open ocean. In high 

 latitudes, where convection currents in winter may reach to the bottom, 

 the deep and bottom water will mostly not be uniform, because in some 

 years the density of the surface water will be greater than in other years, 

 and the convection current will reach to different depths, depending 

 upon how much the density of the surface water has been increased. 

 As a consequence, even in these areas the density increases toward the 

 bottom; the bottom water is not homogeneous and shows, therefore, a 

 definite temperature-salinity relationship. In the second place, sinking 

 at convergences in middle latitudes may lead, as pointed out by Iselin, 

 to the formation of a water mass with a T-S curve that reflects the 

 horizontal distribution of temperature and salinity at the surface. 

 Fig. 21A illustrates this point. The figure represents a schematic cross 

 section in which are entered isotherms and isohalines that are all parallel 

 and that all cut the surface. The at curves have not been plotted, but 

 are parallel to the isolines. The indicated system will remain stationary 

 if sinking of surface water takes place between the lines a and h and if 

 the sinking water remains on the same ct surface. It will also remain 

 stationary if mixing takes place along or across at surfaces. These 

 processes will lead to the formation of a water mass which, between the 

 curves a and h, always shows the same temperature-salinity relation — 

 namely, the relation that is found along the sea surface. Iselin showed 

 that the horizontal T-S curve along the middle part of the North Atlantic 

 Ocean is very similar to the vertical T-S curve that is characteristic 

 between temperatures of 20° and 8° over large areas of the North Atlantic 

 Ocean, and he suggested that processes of sinking and lateral mixing are 

 mainly responsible for the formation of that water. Extensive use of 

 this concept will be made in the chapter dealing with the water masses 

 and currents of the oceans. 



However, a similar T-S relationship can be established by different 

 processes, as is illustrated in fig. 21 B. It is here assumed that two water 

 types, a and h, are formed at the surface and sink, following their charac- 

 teristic (Tt surfaces. It is furthermore assumed that, at subsurface 

 depths, mixing takes place between these two water types, whereas, 

 near the surface, external processes influence the distribution of tem- 

 perature and salinity in such a way that the different curves cross each 



