364 



The Representation of Oceanic Movements nad Kinematics 



(Q) / / (b) 



(d) 



Fig. 154. Singularities of the current field: (o) and (6) convergence and divergence point; 



(c) and id) superposition of singularities with convergence and divergence lines ; (c) cyclonic, 



id) anticyclonic vortex at the boundary between two water masses. 



The form d represents an anticyclonic vortex in the region between the two water 

 masses. Here the boundary surface sphts up into two divergence Hnes. In this case the 

 anticyclonic vortex causes a concentration of the hghter water in the central part of 

 the vortex. The dynamics of such cyclonic and anticyclonic vortexes will be discussed 

 later (see Chap. XIV, 4). 



Neutral points (Fig. 155fl, b) occur when currents flowing in opposite direction meet 

 each other and separate again without showing stronger vertical motion. Two asymp- 

 totes to the stream lines then intersect at the neutral point situated in the centre. 

 Singular points of higher order are also possible. The current field is then very com- 

 plicated, see, for instance, Fig. 155a. In place of the second water mass there may be a 

 solid boundary at a coast line where the current divides into two parts. The neutral 

 point then lies on the shore line. In the presence of a wave motion the stream lines take 

 on a special pattern. During the propagation of a wave the individual water elements 

 usually describe elliptical orbital motions in a vertical plane perpendicular to the wave 

 front. The longer axis of the ellipse is horizontal and the smaller is vertical. For such 

 a periodic wave motion it is of course only sensible to plot a stream-hne pattern for a 

 particular phase of the wave motion. Fig. 155c shows that for a propagation of a wave 

 to the right the water masses will converge in front of the wave and will diverge in its 

 rear. At the sea surface this gives rise to a convergence line in front of the wave and a 

 divergence line behind it. Both the singular lines move with the wave at right angles to 

 the wave front. Thus in a wave motion two types of strips occur, so that strips with a 

 movement from left to right alternate with strips moving from right to left. If the wave 

 is propagated to the right the first type of strips will correspond to the wave crests 



