18 



PROCESSING OCEANOGRAPHIC DATA 



i= distance between stations in 

 meters, 



T=relative current velocity nor- 

 mal to a line joining the two 

 stations, in meters per 

 second, 



co= angular velocity of the earth, 

 equal to 0.729X10" ra- 

 dians per second, 



0=niean latitude between sta- 

 tions. 



Solve the equation for the velocity: 



IO(AD^-ADb) 



F=- 



L2 cosm<j) 



To simplify the computation of velocity in 



meters per second, the lactor -yt, '■ — 7 is giv- 



en in table XII for unit values of L and for each 

 degree of latitude. Conversion factors are also 

 given in table XII for various units of V and L. 

 A graphical aid in the calculation of current 

 velocity from dynamic height differences is the 

 nomogram illustrated in figure 20. If we use 

 the two scales on the left, the point where a 

 straight line through the appropriate dynamic 

 height dift'crence, ADa — ADb, and distance 

 between stations, L, crosses the center line is 

 marked on the diagram as P. On the right side 

 of the diagram, a straight line thi-ough the 

 established center line mark, P, and the 

 appropriate latitude crosses the velocity scale 



at the desired value of the velocity component 

 normal to the line joining the two stations. 

 As an example, given a dynamic height differ- 

 ence of 0.0525 dynamic meter and a distance 

 between stations of 27.8 kilometers at a mean 

 latitude of 29°56', a current velocity between 

 stations of 0.5 knot is found from the nomo- 

 gram. 



The usual graphical way not only of meas- 

 uring, but presenting, current data for analysis 

 is by the construction of charts showing the 

 dynamic topography of one or more isobaric 

 sm-faces relative to a reference surface, which is 

 assumed to be level. The anomaly of dj'namic 

 height, AD, between the reference surface and 

 the selected isobaric surface above is plotted on 

 a base chart at each oceanographic station, and 

 contours of equal values of dynamic height 

 anomaly are drawn. Such achart is illustrated in 

 figure 21, which shows the dynamic topography 

 of the 0-decibar surface (sea level) relative to 

 the 500-decibar surface. Each contour repre- 

 sents the line along which a level surface cuts 

 the selected isobaric surface (sea level in the 

 illustration), if we assume the lower reference 

 surface to be level. Since the force of gravity 

 acts downslope normal to the contours, and the 

 balancing Coriolis force is always normal to 

 the velocity, it follows that the current flows 

 along the contours of dynamic topography. 

 The surface slopes upward to the right of the 

 current in the Northern Hemisphere, to the left 

 of the current in the Southern Hemisphere. 



Fiaure 20. — Nomogram for determining cvurent velocity from dynamic height anomaly difference, distance 



stations, and latitude. 



