Chapter V 



The Density of Water Masses in the 



Ocean ^ Vertical and Horizontal Density 



Distribution and its Stability 



1. Diurnal and Annual Variations at the Surface 



The diurnal and annual variations are uniquely determined by that of the tempera- 

 ture and salinity. Since the diurnal temperature variation is essentially parallel with 

 that of salinity, the effects of both factors on the density partly cancel each other out, 

 and apart from the fact that they are both small anyway, the diurnal surface-density 

 variation is thus a rather insignificant phenomenon. In general, the aperiodic changes 

 in density during the day are so large that they completely mask the regular diurnal 

 variation. At anchor stations the average diurnal variation in density, taken as the 

 average over several days, is of the order of 0-05-0-1 in a^ (Table 76), 



Table 76. Diurnal density (of) variation at the ocean surface (Atlantic Ocean) 



* Minimum; t Maximum 



The maximum occurs in the morning or in the forenoon; the density then falls, 

 probably due to the rising temperature — and in spite of the increasing sahnity — to a 

 minimum in the afternoon ; the amplitude is everywhere very small. 



The annual density variation is much larger and its amplitude usually is of the order 

 of 1 -00 and 2-00 in Of depending on whether the annual variation in the temperature is 

 parallel or inverse to the corresponding salinity variation. The annual density variation 

 can be conveniently presented by plotting the monthly values on a [rS'J-diagram. This 

 has the advantage of providing a visual impression of the variations in temperature and 

 salinity, and also in density. For annual variations in Tand S, following pure sine curves, 

 the annual variation in density will be shown on such a diagram as a straight line if the 

 annual variations of the two factors run either parallel or inverse. If the amphtudes are 

 normalized (choosing scales of equal length for T and S in the diagram) then the 

 straight line will be at an angle of 45° with the temperature axis, but for inverse 



185 



