1 66 Salinity of the Ocean, its Variation in Oceanic Space and in Time 



decrease in temperature in these layers is thus associated with a strong decrease in the 

 salinity, This extends down to about 800 m where the salinity reaches a minimum of 

 34-3-34-9%o. There is then a second increase to about 34-8-34-9%o at about 1600- 

 2000 m and then a further slow decrease is generally observed down to the bottom. 

 The inversion in salinity at 800-1000 m becomes weaker and weaker towards higher 

 northern and southern latitudes, and from the polar fronts of both hemispheres 

 towards ^he poles it is entirely missing; the vertical differences are then small with 

 usually a slight increase in salinity if fresh water has not been added to the surface 

 layers by the melting of ice, but this becomes weaker and weaker towards the poles. 

 In contrast to this vertical distribution generally found, the North Atlantic shows a 

 pronounced peculiarity in middle latitudes which can be seen at some of the stations 

 in Fig. 69. The intermediate salinity minimum at about 800 m is missing here, and 

 from the core of upper layer of high salinity situated in middle latitudes the salinity 

 decreases almost uniformly down to the bottom. There is thus a marked asymmetry 

 between North and South Atlantic vertical distributions of salinity. 



ib) The Salinity of the Oceanic Troposphere 



The vertical distribution of salinity in the troposphere layers of the subtropics and 

 the tropics is worth a somewhat more detailed description. It has, of course, been 

 investigated more closely in the Atlantic (Defant, 1936). Almost all stations in the 

 tropics and subtropics show a nearly homo-haline top layer. Its thickness is not the 

 same as that of the thermal top layer but is usually somewhat smaller. In many cases 

 just below the quasi-isothermal top layer, however, still in the upper part of the ther- 

 mocline, there is a more or less well-developed salinity maximum. This maximum is 

 one of the most characteristic phenomena of the vertical salinity distribution of the 

 upper troposphere. Figure 70 shows an example of this. The "Meteor" 256 station 

 shows the maximum particularly well developed ; in a thin layer from about 50 m the 

 salinity rises from about 36-1 to 37-0%o and then falls again to the previous value. It is 

 worth noting that the salinity maximum appears there where the first drop in tempera- 

 ture occurs beneath the isothermal surface layer and not at about the maximum 

 temperature gradient of the thermocline (see Fig. 71). The sahnity maximum thus 

 extends just above the thermocline, but does not fully coincide with the density transi- 

 tion layer, the position of which is in turn fixed by the high salinity value. Careful 

 investigation of this sahnity maximum in the tropical and subtropical regions of the 

 Atlantic has shown that it is almost always present. Starting from the extensive sub- 

 tropical accumulation of very saline water (at about 25° S. and at about 30° N.), 

 where in a top layer down to the thermocline a homo-haline structure is found, a thin 

 layer of maximum salinity spreads out northward in the Southern Hemisphere and 

 southwards in the Northern immediately above or directly inside the thermocline. 

 This spreading occurs below the upper part of the top layer, in which salinity decreases 

 in both hemispheres towards the equator. 



From this it can be concluded that the layer of the salinity maximum is formed from 

 the lowermost parts of the subtropical high salinity water by currents flowing towards 

 the equator. It thus represents the intrusion of highly saline water under the surface 

 layers of lower salinity of the equatorial regions and forms a part of the upper tropo- 

 spheric circulation. 



