PROCESSING PROCEDURES 



11 



(Wire angle = 35°) 



Figure 14. — Examples of calculating depths ol observations 

 by difference method, 



use of these plots is to scale off depths of desired 

 values of the variahlo. These are necessary for 

 construction of horizontal sections. The main 

 purpose of the plots, however, is to provide a 

 means for obtaining; interpolated vahips of 

 temperature, salinity, etc., at standard depths 

 for comparative purposes. Standard depths 

 according to the International Association of 

 Physical Oceanography arc: Surface, 10, 20, 30, 

 50, 75, 100, 150, 200, (250), 300, 400, 500, 600, 

 (700), 800, 1,000, 1,200, 1,500, 2,000, 2,500, 

 3,000, and 4,000 meters, and intervals of 1,000 

 meters thereafter to the greatest depth of 

 sampling. A data summary form is presented 

 in section D. 4 which shows a common practice 

 of presenting both observed and interpolated 

 data. 



In addition to vertical distribution curves, a 

 plot is made of the temperature-salinity (T-S) 

 relationship to detect error and to bring out 

 water mass characteristics of the data. In 

 figure 16 the corresponding observed values of 

 temperature and salinity are plotted for a single 

 station. After the T-S curve has been drawn 

 from observed data, corresponding interpolated 

 values of temperature and salinity read from 

 the vertical curves are also plotted. If these 

 do not fall on the T-S curve certain adjustments 

 must be made in the construction of the vertical 

 distribution curves. 



D. 3.— COMPUTATION OF DEPENDENT 

 QUANTITIES 



INTRODUCTION 



Wlien values of depth, temperature, and 

 salinity liave liecn determined in accor(hince 

 with the methods described in the preceding 

 section, certain calculations are necessary to 

 derive various dependent variables commonly 

 used to describe the field of mass in the sea. 

 These variables include: Specific volume, 

 anomaly of specific volinno from a standard 

 value, density in situ, and a,, which represents 

 density at surface pressure. In addition, cer- 

 tain calculations arc required to determine 

 relative currents pertaining to the distribution 

 of mass in the sea. Such relative currents are 

 deduced from a consideration of the balance of 

 forces in the sea. Tlie forces considered are 

 those which act along an isobaric surface. If 

 the isobaric surface is not level, a component of 

 the force of gravity acts downward along it, 

 and must be balanced by an equal and opposite 

 force if the slope of the surface is to be main- 

 tained. The balancing force is assumed to be 

 the deflecting force of the earth's rotation 

 (Coriolis force), which acts in the presence of 

 motion and indicates both the speed and direc- 

 tion of the cm-rent. 



By an isobaric layer is understood the layer 

 between two isobaric surfaces. The thickness 

 of an isobaric layer depends upon the average 

 specific volume of the layer. Therefore, the 

 slope of one isobaric surface relative to another, 

 which is assumed to be level, may be found. 

 Since djaiamic height is a measure of the work 

 performed agauist gravity in moving unit mass 

 from one level to another, the component of the 

 force of gravity acting down the sloping isobaric 

 surface between two stations is the difference 

 in dynamic height of the surface at the two 

 stations divided by the distance between sta- 

 tions. Equating this expression to the expres- 

 sion for the Coriolis force and solving for the 

 velocity, one obtains the component of current 

 normal to a line joining the two stations. The 

 current is that at the upper isobaric surface 

 relative to any current which may be present 

 at the lower reference surface. 



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