The Representation of Oceanic Movements and Kinematics 



365 



(c) 



(d) 



. ^^^^^^^^s:^ 



(b) 



(e) 'j 



(f) 



Fig. 155. Singularities in the current field: (a) neutral point, ib) one-sided neutral point, 

 (c), {d), (e) and (/) singularities in wave motions : (c) stream lines in a vertical cross-section, 

 (d) stream lines at the surface with a small translation parallel to the wave crests, (e) and (/) 

 the same with a somewhat stronger or a very strong translation oblique to the wave crests 



(according to V. Bjerknes). 



and the second to the wave troughs. If in addition to the wave motion there is also a 

 more or less strong translatory motion in the water mass, then the two current fields 

 will be superimposed on each other, and the resulting current field will consist of a 

 system of convergence and divergence lines moving parallel to each other with the 

 wave. Some fields of this type are illustrated in Fig. 155. 



The singularities are closely connected with the velocity field. Where stream lines 

 intersect the velocity must be zero ; the points of convergence and divergence and the 

 neutral points must therefore be points of zero velocity (places of no motion). The 

 isolines of velocity must be closed around these points. When approaching singular 

 lines there will always be more and more curvature in the lines of equal velocity. 

 This curvature becomes stronger the more the stream lines converge towards the 

 singular line. For weaker convergences this curvature is usually hardly noticeable 

 in the observations. 



Constructing stream lines usually offers little difficulty, especially if the position of 

 the singularities is fixed first. Usually some of the stream lines running out from the 

 singularities can be drawn in with some certainty and these fix the current field with 

 almost sufficient accuracy. Attention should also be paid, of course, to the velocity 

 field and to relationships with the dynamic phenomena expressed in the distribution 

 of other oceanographic factors (temperature, salinity, etc.). Sandstrom (1909) has 

 given a method for the accurate construction of stream lines. Auxiliary lines termed 

 isogons were drawn in first. An isogon is defined as a line along which the direction 

 of the current is constant, and for each direction there exists only one isogonal curve. 

 If the observed directions are expressed by numbers (usually 16 directions with the 

 numbers 2 to 32) then numbers can be entered on the chart in place of the arrows 

 indicating the direction of the current; the isolines of equal direction are then easily 

 constructed. These are covered with rather short dashes pointing in the direction of 



