Chapter XIV 



Water Bodies and Stationary Current 

 Conditions at Boundary Surfaces 



1. Water Bodies and the Boundary Surface Between Them 



The theory of ocean currents in a homogeneous sea gives results which allow in many 

 cases its application to actual conditions, although the sea itself is far from being homo- 

 geneous. In changing from a homogeneous to a stratified ocean it is necessary to 

 consider two homogeneous water masses (water bodies) situated side by side and 

 separated by a discontinuity surface (boundary surface). On passing through this, 

 changes in physical and chemical properties occur and also in the state of motion of 

 water masses. This is, of course, also only a schematic model, since in reality the indi- 

 vidual water bodies are not quite homogeneous and the transition from one to the 

 other is seldom abrupt. Usually in Nature there is a rapid "transition layer" between 

 the more or less homogeneous water bodies inside which a steady, rapid change of the 

 properties occurs, while passing through it. 



The genesis of boundary surfaces of this type is due to the circumstance that in 

 certain oceanic regions specific water types are continuously formed and carried away 

 by the ocean currents together with their characteristic properties. In this way two 

 different water bodies are brought into close contact at singularities in the current 

 field and a boundary surface between them is formed at convergence lines. The prin- 

 cipal changes in the horizontal distribution of a property (such as temperature or 

 salinity and others) occur always in connection with so-called deformation fields of 

 the motion (Bjerknes and co-workers, 1933). The most simple case of a horizontal 

 deformation field is the current field at a neutral point (see p. 365, Fig. 155 a) with 

 hyperbolic stream lines in each of the sectors formed by intersection of the two stream 

 lines in the neutral point (Fig. 198). These straight lines are the principal axes of defor- 

 mation of the field; one of them is an axis of dilatation and the other at right angles to 

 it is an axis of contraction. This deformation field when superimposed on the field of 

 one of the water properties will have a marked effect on the latter. The two full lines 

 in Fig. 198 represent two isolines of a property, such as for example, the temperature. 

 The current field will produce displacements in the position of these lines: all isolines 

 which initially are parallel to the axis of contraction will move away from it and 

 isolines parallel to the dilatation axis will move towards it. It can also be shown that 

 two isolines through the current will always tend towards a direction parallel to the 

 dilatation axis, so that they will first move away from each other to a maximum 

 distance, and then after reaching a certain angle to the dilatation axis will move to- 

 wards it again. In the case of a temperature distribution the effect of the deformation 



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