Part 2 

 DYNAMICAL OCEANOGRAPHY 



Introduction 



Dynamical oceanography is concerned with the movements of the water masses of 

 the oceans. In addition to the framework of the vertical and horizontal structure, in 

 the sea, of the oceanographic factors such as temperature, salinity and density dealt 

 with in detail in Vol, I, Pt. I, we have now to consider the forces present that cause 

 displacements of the water masses. These displacements are termed ocean currents; 

 they are phenom.ena that an observer will be directly aware of only occasionally, 

 near land or in narrow straits. In the open sea they are shown only by calculations 

 carried out for quite different purposes and only give a clear picture of the movement 

 of the water masses when taken together over a larger area of the sea. The system of 

 currents in the ocean, hke that in the atmosphere around the Earth, is among the most 

 striking phenomena in geophysics. The oceanic circulation involves the whole ocean 

 and the conditions are aptly described by Heraclitus' expression -navTa pTt (it all 

 moves). 



In principle, dynamical oceanography can be subdivided into two main parts. 

 One concerns ocean currents in the more restricted sense of the word as applied to the 

 steady continuous transport of water in definite direction. In these currents movements 

 in a horizontal plane predominate overwhelmingly, but there are also phenomena 

 where a vertical component becomes important. The second part of dynamical 

 oceanography concerns the phenomena associated with periodic water movements in 

 which the whole process is repeated after a certain time. These are the waves and 

 tides. Separate treatment of ocean currents (in the more restricted sense) on the one 

 hand and of the waves and tides on the other considerably simplifies their presentation, 

 although these phenomena are not separated in nature to the extent that might super- 

 ficially be easily assumed. 



Part II of this volume is therefore devoted mainly to ocean currents (in the narrower 

 sense) while Volume II deals with the dynamics of the periodic phenomena (Waves 

 and Tides). 



An explanation of the movements of water masses in the ocean requires in the first 

 place a study of the interplay between the oceanographic factors and of the effect of 

 external forces on the water masses. It is self evident that hydrodynamics must play a 

 major role in dealing with these questions, especially if the problems arising are treated 

 more from a geophysical standpoint. Thus, in addition to a more statistical-geographi- 

 cal description of observed oceanic phenomena, hydrodynamic considerations have 

 to be used and finally one attempts to explain them on a physical-mathematical basis. 



Ordinary classical hydrodynamics develops the theory of movement in a liquid on 

 the assumption that it is homogeneous and incompressible. As a first approximation 

 the results of pure hydrodynamics are applicable within wide limits to the water 



299 



