The Representation of Oceanic Movements and Kinematics 



363 



also be disturbances of the water movement due to the bottom configuration; the 

 direction of the rips then usually corresponds with the main course of the shelf or of 

 the irregularity in the bottom. In many cases a connection has been shown with the 

 behaviour of the tidal currents in neighbouring oceanic regions. Particularly well 

 known to seamen are the rips in the Straits of Gibraltar and in the Straits of Messina, 



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Fig. 153. Current directions and stream lines during the passage through the convergence 

 line in the Ostfriesland Gatje (see Fig. 152). 



where they are definitely connected with tidal currents carrying different water types 

 and those off the eastern coast of North America in the area of the Gulf Stream, along 

 the west coast of Mexico and in West African waters where they are related to up- 

 welling phenomena. 



Points of convergence and divergence (Fig. \54a, b). These points represent the inter- 

 section of an infinite number of stream lines. For continuity reasons, movements 

 such as these must always be connected with movements perpendicular to the surface 

 of the sea ; thus a divergence point in a water layer near to the surface indicates up- 

 welling and a convergence point indicates a sinking movement. This need not, however, 

 be the case at greater distances from the surface. The divergence then merely indicates 

 that due to the vertical movement more flows in at one side than leaves from the other; 

 the reverse applies for convergence. The formation of curved stream lines near to the 

 centre point (cyclonic and anti-cyclonic vortices) as shown in Fig. 154 depends largely 

 on the effect of the Earth rotation. 



If there are different types of water masses in the near vicinity of the vortex they will be 

 drawn into it and combined singularities then occur. Cases of this type are shown in 

 Fig. 154c", d; c represents a cyclonic vortex in the region between a lighter and a heavier 

 water mass. Since the equilibrium state is upset at the boundary between the two water 

 masses the hghter water tends to spread out over the heavier while the heavier sinks 

 underneath the lighter. For such an inward spiraling motion a convergence line forms 

 at the boundary surface ; thereby one part of it will be an up-gliding surface where 

 the hghter water moves over the heavier and the other will be a down-gliding surface 

 where the heavier water sinks underneath the lighter. The lighter water will gradually 

 extend completely over the heavier and will finally give a cyclonic vortex (in the top 

 layer) with a simple convergence point of the form a. 



