Water Bodies and Stationary Current Conditions at Boundary Surfaces 453 



on mixing processes in the transitional area between the North Sea and the BaUic. 

 These are real, progressing hydrographic fronts that are associated with the inter- 

 change of water between the two seas. 



The parts of the ocean where more or less stationary fronts are found are actually 

 closely connected with the occurrence of quasi-stationary deformation fields with an 

 axis of dilatation in the current system of the oceans deviating as little as possible 

 from the east-west direction. The position of these can be found directly from a chart 

 of ocean currents. In the Northern Hemisphere the most important are : 



(1) The North At/antic Polar Front which is present with its main part to the south 

 of Newfoundland and there forms the boundary between the Gulf Stream water and 

 the Arctic water of the Labrador Current; its continuation separates the cold low- 

 saline water of the East — and in part also of the West — Greenland Current from the 

 Atlantic water masses. Other parts lie south of Spitzbergen and in the Barents Sea. 



(2) The North Pacific Polar Front with its main part between the Kuroshio and the 

 Oyashio which can be traced to about the middle of the ocean. These fronts are a 

 consequence of quasi-stationary deformation fields in the current system in this part 

 of the ocean. 



(3) This is also the case, though less clearly, in the Southern Hemisphere Polar 

 Front which runs right around the Earth. It lies between the West Wind Drift and the 

 Antarctic Current. In the parts where it is particularly well developed (for instance, 

 south of South America and between the Falkland Islands and South Georgia) the 

 connection with the local deformation field is clearly shown. 



2. Stable Discontinuity Surfaces 



If two motionless water bodies are present together in the ocean for a stable equili- 

 brium, the heavier water type must lie underneath of the lighter and the discontinuity 

 surface between them must coincide with a level surface. Two water bodies at rest, 

 situated side by side, will never be in equilibrium, even if each water body by itself 

 has a stable vertical stratification. Since, due to their different densities, the pressure 

 in each water mass will increase with depth at diff"erent rates, pressure differences are 

 created; the resultant water movements will overturn the water bodies and they will 

 only cease when the water bodies are again situated one above the other, separated 

 by a horizontal boundary surface. However, two water bodies side by side can be in 

 stable equilibrium // they are in motion. The form and position of the resulting dis- 

 continuity surface was first given by Margules (1906) following up an investigation 

 by Helmholtz (1888); a more general representation was given later by J. Bjerknes 

 (1921) and later an application to the analogous conditions ia oceanic water bodies 

 has been given by Defant (1929 b). 



A stationary state of the boundary surface is possible only for a certain definite 

 state of motion in the two water bodies; thereby the boundary surfaces will lie at an 

 angle to the level surfaces, so that the denser water always spreads out in a wedge- 

 form underneath the lighter water. It will be a discontinuity surface for density 

 (temperature, salinity or both) but not for pressure, since otherwise movements 

 would immediately start directed towards the boundary surface. This, however, 

 would interfere with the condition of stationary state. On the contrary, the boundary 

 surface will be a discontinuity surface for the pressure gradient. According to the 



