362 



The Representation of Oceanic Movements and Kinematics 



from both sides the hnes of divergence and convergence (case (c) and {d)). Lines of 

 convergence and divergence in most cases represent the boundaries between different 

 water types moving relative to each other. They are generated when heavy water meets 

 lighter water or when lighter water spreads out over heavier water that is sinking. 

 Fig. 151 gives a vertical section showing current conditions on both sides of an inchned 

 gliding surface separating two different water masses. Similar vertical displacements 

 can also be expected for divergence and convergence lines from both sides. In all these 

 cases where there is a velocity component at right angles to the boundary surface the 

 inclined gliding boundary surface cannot be expected to remain stationary. 



The occurrence of divergence and convergence lines in oceanic current systems is a 

 general phenomenon closely connected with the oceanic circulation. They represent 

 the framework of the circulation and indicate the connecting places between the sur- 

 face currents and the three-dimensional vertical circulation. Some examples will be 

 given later. 



Rauschelbach ("1931) while making current measurements in the Ost-Friesband Gatje rtielow 

 Emden) took the opportunity to make measurements with a bifilar current meter at a convergence line 

 running through the observation point (an anchored vessel). The convergence line, which was visible 

 as a foam line, ran parallel to a dredging line; it moved the Ems upstream driving with the flood tide, 

 while at the same time it was displaced from the middle of the channel towards the east. It passed the 

 current meter at 1 7 h 3 min 30 sec. Figure 1 52 gives the velocity and direction of the current as measured 

 by the current meter before and after the passage of the convergence line; Fig. 153 shows the distribu- 

 tion of the surface current around it. The course of the boundary surface in the lower layers was not 

 that simple and according to current measurements at a depth of 1-2 m was disturbed by internal 

 waves. 



40 

 20 

 y I80°0 

 " 160 

 140 

 120 



100 



l7hQ'" |r 



3m ^r 



Time 



Fig. 1 52. Evaluation of a convergence line in the Ostfriesland Gatje (downstream of Emden) 

 according to Rauschelbach: (a) current velocities, (b) current directions (clockwise from 



0° to 360°). 



Convergence lines are frequently indicated at the sea siirface by more or less strong 

 agitation of the water and are then recorded in ships' logs as rips. Closer attention has 

 only been paid to them in more recent times. (Romer, 1935, 1936; Schumacher, 1935; 

 Thiel, 1937; Uda, 1936, 1938). It seems to be definitely established that rips in the 

 open ocean are formed at the boundaries between converging and diverging water 

 masses. Sometimes when lighter and heavier water are separated by either a converg- 

 ence or a divergence line, the wind forces the lighter one to move above the heavier, 

 as is often observed. Off the continental shelf and around island platforms there may 



