86 



BJ0RN HELLAND-HANSEN 



|REP. OF THE "MICHAEL SARS" NORTH 



Beloit' 1200 metres the vertical gradient of tempera- 

 ture decreases with depth towards the stratosphere and is 

 usually small except in those areas in the eastern part of 

 the ocean where water from the Mediterranean exerts a 



considerable influence. The difference is easily seen by 

 comparing, for instance, the curves for Stats. 17 and 25 B 

 (pp. 65* and 66*) with the curves for Stats. 63, 65 and 

 66 (pp. 69* and 70*). 



VII. THE SALINITIES IN THE NORTH ATLANTIC. 



38. Horizontal and Vertical Distribution of 



Salinity. 



The curves for the stations and the sections in Part 

 II clearly demonstrate a correspondence between tempe- 

 rature and salinity as regards their vertical and horizontal 

 distribution. 



With the scales used for the construction of the 

 station-curves on pp. 65*— 72*, the curves representing 

 the vertical distribution of temperature and salinity in the 

 open sea at a sufficient distance from the coasts, are of 

 nearly the same shape, at any rate when the upper part 

 is not considered. As most of the stations are worked 

 when the heating at the surface is well advanced, the 

 difference is considerable in the surface layers, and this 

 difference becomes more pronounced the later in the warm 

 season the observations are made. In the upper strata 

 the vertical variations in temperature in summer are great 

 as compared with the variations in salinity. From, say, 

 50 or 100 metres downwards for several hundred metres 

 the two curves usually run almost parallel to each other 

 in nearly all the regions investigated during the "Michael 

 Sars" Expedition. In the western North Atlantic the ver- 

 tical gradient of salinity becomes numerically smaller, in 

 relation to the vertical gradient of temperature, the deeper 

 one goes. In our arrangement of the curves in the graphs 

 this appears in their convergence (see, for instance, the 

 curves for Stats. 65 and 68). It corresponds to the 

 concave shape of the curve on p. 74* illustrating the 

 normal correlation of the two elements, in which a certain 

 variation of salinity corresponds to a greater variation of 

 temperature when the temperatures are low than when 

 they are high. 



In those regions of the eastern North Atlantic which 

 were investigated by the "Michael Sars" Expedition, water 

 from the Mediterranean exerted a very varying influence. 

 The appearance of this water creates an increase of sali- 

 nity and a rise of temperature at intermediate depths, or 

 makes the vertical gradients of salinity and temperature 

 smaller on the upper side of the layers in question (and 

 greater on the under side) than they would otherwise have 

 been. Where the Mediterranean water is especially prom- 



inent — in the region outside the Straits of Gibraltar — 

 the curves are of quite a different shape from those, say, 

 for the western part of the North Atlantic. The condi- 

 tions are cleaily demonstrated by the curves for Stats. 

 17, 23 and 25 B. 



Disregarding the irregularities caused by water from 

 the Mediterranean, we may state that the salinity decreases 

 from the surface (or a little below it) to the great depths 

 practically everywhere in the Atlantic N. of the tropics 

 except in areas influenced by coastal water or Arctic water. 



Water from the South Atlantic with relatively low 

 salinities advances northwards at intermediate depths as 

 far as the northern tropic, with its core at 800—1000 

 metres below the surface. This is pointed out more 

 especially by Merz and Wusi. The conditions are clearly 

 demonstrated by two sections constructed by Wust [1928] 

 and reproduced here in Figs. 32 and 33. The sections 

 follow a curved irregular course, mostly along the deepest 

 (central) parts of the western (Fig. 32) and the eastern 

 (Fig. 33) North Atlantic. The effect of water from the 

 Mediterranean is also distinctly seen in Fig. 33 (maximum 

 at about 1000 metres between 30° and 40° N.). 



In areas where the Atlantic water is covered by diluted 

 water near the coasts or by Arctic water with its compa- 

 ratively low salinities, the salinity naturally increases from 

 the surface downwards until a maximum is found in the 

 upper part of the Atlantic water. Where Atlantic and 

 Arctic currents meet the conditions may, however, be 

 rather complicated, with different kinds of water in suc- 

 cession vertically, as is demonstrated, for instance, by the 

 curves for our Stats. 66, 80 and 82 and to the right in 

 Fig. 32. 



Variations in the temperature of a definite mass of 

 water not mixed with other water, chiefly originate some- 

 where at the surface. Some heating is, however, also 

 caused by absorption of heat radiation penetrating through 

 the surface. Changes in salinity start e.xclusively at the 

 very surface (evaporation, addition of fresh water) and 

 the mean salinity cannot be altered except by phenomena 

 occurring there. Variations of salinity within the water 

 even close below the surface can only be induced by 

 mixing processes. The molecular diffusion, like the mole- 



