Chapter IV 



The Salinity of the Ocean, its Variation 

 in Oceanic Space and in Time 



1. Periodic and Aperiodic Variations of Salinity * 



If tidal effects are disregarded the most obvious periodic changes in salinity to be 

 taken into account are the diurnal and annual variations. There is little data on daily 

 variations. The diurnal variation of evaporation must give rise to a similar change in 

 the salinity but it can have only little signification. Apart from the small diurnal 

 variation in evaporation, the variations in salinity will be further diminished by the 

 vertical convection set up immediately in the homogeneous top layer by increased 

 salinity at the surface. The effect of an increase in salinity by a high evaporation rate 

 will thus spread very rapidly over a large water mass and will scarcely be detectable. 



The true salinity variation uninfluenced by other factors can only be shown by ob- 

 servations made at an oceanographic anchor station, and in this case also all stations 

 that showed any appreciable vertical salinity gradient should be left out of account. 

 At such stations the vertical displacements of water by the tides cause variations in 

 salinity with a tidal period which are usually several times greater than the normal 

 diurnal variations. A small diurnal variation can only be clearly shown in an almost 

 completely homo-haline top layer. Five "Meteor" anchor stations between 21° S. 

 and 4° N. gave the mean diurnal variation shown in Table 70. 



The second column of the table shows the diurnal salinity variation as hourly 

 values taken over three days at the "Altair" anchor station (44-5° N., 34° W.); see 

 Fig. 64. The range is very small and amounts to less than half of 1/100 part %o; there 

 is a broad flat minimum during night time until sunrise after which the salinity rises, 

 slowly at first and then rapidly, to a pronounced maximum in the late afternoon and 

 falls off just as rapidly to the night values. Physically the process can be regarded as 

 the effect of a positive transient source of salt at the surface, the surface amplitude 

 of the effect being somewhat modified by vertical exchange with the layers under- 

 neath. The variation proceeds so regularly that despite its small amplitude it deserves 

 more attention than it has hitherto received. Visser (1928) deduced a value for the 

 mean diurnal variation of the surface salinity by analysis of the observations of the 

 "William Snellius" Expedition; this is similar to that found in the Atlantic: minimum 

 at 04.00 h, maximum at about 17.00 h; but the amplitude was almost twice as large 

 probably due to climatic conditions in the area. 



Knowledge of the annual salinity variation is also rather meagre. Bohnecke 

 (1936) has prepared charts showing surface salinities for each month in the North 

 Atlantic and seasonal means of salinity for the total Atlantic which allow the annual 



154 



