88 



BJ0RN HELLAND-HANSEN 



[rep. of the "MICHAEL SARS" NORTH 



cular conduction of lieat, is negligible. But the turbulence 

 has an effect upon the variations of salinity similar to 

 that which it has upon the variations of temperature, and 

 just as we speak of a virtual conductivity of temperature 

 we may speak of a virtual diffusion of salt, which is 

 generally very much greater than the molecular diffusion. 

 Such variations of salinity (S) with time (t) that are 

 solely due to the diffusivity in vertical direction may be 

 expressed by an equation analogous to that available for 

 temperature (p. 44): 



lS__^d^\ S_rSS_ 

 dt d Z^ dz dz 



where r is a "virtual coefficient of diffusion" which has 

 precisely the same value as the virtual coefficient of temp- 

 erature conductivity. 



The evaporation and dilution of the surface waters 

 are evidently subject to seasonal and annual variations 

 due to climatic changes. We shall not, however, try to 

 examine such variations here, but confine ourselves to a 

 short survey of the average conditions. 



Fig. 34 illustrates the average distribution of salinity 

 at the surface of the Atlantic, according to Professor G. 

 SciiOTT [1926). The chart is rather schematic and does 

 not show the many local differences to be found in places 

 where strong currents or eddies occur. 



In the region of the N.E. trade wind an excess 

 evaporation, takes place. The salinity increases all the 

 way in the surface current from Portugal past Madeira 

 and the Canary Islands, with the result that a maximum 

 of salinity appears in the southern part of the Azoric 

 high-pressure region. Within quite a large area W. and 

 SW. of the Canaries the salinity at the surface exceeds 

 37 "/oo, the absolute maximum being a little above 37-5 "/oo. 

 The salinities decrease S. of this area to a minimum with 

 salinities below 35 "/o". appearing in a belt across the 

 ocean in about 5° N. This belt almost corresponds to 

 the region of maximum temperature at the surface (average 

 for the year). Hence it follows that the positive corre- 

 lation between temperature and salinity which usually 

 appears in the ocean, does not exist in the case of the 

 surface waters, between about 25° and 5° N. where, in 

 fact, it is changed into a negative correlation. 



The isotherms representing the mean annual tempe- 

 rature at the surface of the North Atlantic lie compara- 

 tively close together on the coast of the United States 

 of America, where the isotherms for temperatures between 

 25° and 10° C. are crowded between 30° and 40° N. 

 They spread, fan-like, over the sea. At 20° W. we find 

 the isotherm for 25° C. at about 12° N., and that for 

 10° C. at about 58° N., a difference, in latitude of 46", 

 as against 10° on the American coast. The isotherms do 



not form closed curves as some of the isohalines do. A 

 comparison of the isotherms in question and the isohalines 

 shows an agreement in some regions, while in others the 

 two sets of lines run obliquely towards each other or even 

 straight across each other. 



In the northern regions except S. of Greenland the 

 agreement is quite good: 35-5°/oo corresponds to 13° C, 

 360/00 to I6V2", 36-5''/oo to about 20°. There is no agree- 

 ment between the 35-0-isohaline and the isotherms. The 

 curve on p. 74''' gives the following interdependence: 

 35-5° CO and 11° C, 367oo and 15-20°, 36-5«/oo and 18 1/4°, 

 or lower temperatures in relation to the salinities than 

 are given by the chart for the mean annual temperature, 

 the difference being about P/a — 2° for the regions in 

 question. This difference is easily explained by the fact 

 that our correlation curve (Fig. p. 74*) is based upon 

 observations from the deeper strata, which chiefly represent 

 a winter situation, so to say (cf. p. 52). In his ''Geo- 

 graphie des Atlantischen Ozeans" Schott has published 

 charts of the mean surface temperatures in February, May, 

 August and November. If we compare the isohalines 

 (Fig. 34) with the isotherms in Schott's chart for May 

 in the same regions as above we find a good agreement 

 with the data found from the correlation curve. The tem- 

 perature chart for February shows lower values in relation 

 to the salinities. Our curve of correlation (p. 74*) is based 

 upon observations from the western as well as the eastern 

 North Atlantic. It will be seen from the details shown 

 in the figure on p. 75* that the observations from the 

 western part tend to give a higher temperature at a cer- 

 tain salinity than the observations from the eastern part. 

 This may be explained by assuming that the waters of 

 the Great Atlantic Current are cooled on their way east- 

 wards, while the salinity is not changed to the same extent. 

 The observations made during the "Armauer Hansen" 

 expeditions in the eastern North Atlantic only, have been 

 used for the construction of a curve similar to that repro- 

 duced on p. 74* [Helland-Hansen and Nansen, 1926]. 

 This curve gives a lower temperature at a certain salinity 

 than the curve based upon the "Michael Sars" observations. 

 Using the curve for the "Armauer Hansen" observations 

 we find, that 35-0%o corresponds to 5°C., 35-5'Voo to 11 °, 

 360''/oo to 14.3°, and SS-So/oo to 16-8°. Scott's tempe- 

 rature chart for the surface in February compared with 

 the salinity chart (Fig. 34) shows that 35-5'''no corresponds 

 to about 10° C, but otherwise the correspondence with 

 the 'normal' conditions in the water-masses of the eastern 

 North Atlantic is as close as can be expected. There is 

 even a correspondence between the isohalines for 35-0%o 

 and the isotherm for 5" C. in the sea near Iceland. This 

 seems to agree well with our conception of the general 

 circulation between low and high latitudes: the surface 



