Water Bodies and Stationary Current Conditions at Boundary Surfaces 471 



Hemisphere the arrow-directions indicating the velocities and the accelerations parallel 

 to the boundary surface have to be reversed. 



So far the discussion applies only for infinitely extended boundary surfaces. If they 

 intersect the sea surface (fronts) or the sea bottom the up- and down-gliding motions 

 will give rise to horizontal water currents in its vicinity and consequently to changes 

 in the position of the boundary surface. 



Cases of this type can be found at the oceanic polar fronts. Figure 2 14 shows the polar 

 front between the East Greenland Current and the Atlantic water to the south of the 

 Denmark Strait. The mass distribution requires larger velocities in the polar current 

 towards the south and smaller ones in the Atlantic water as is found by observation. 



Polar front 



(a) s. 



Fig. 214. Oceanic vertical stratification and currents at the East Greenland oceanic polar 

 front. Picture to the left: up-gliding of the polar water and down-gliding of the Atlantic 

 water for an accelerated East Greenland Current : boundary surface progresses towards east. 

 Picture to the right: down-gliding of the polar water and up-gliding of the Atlantic water for 

 an accelerated Atlantic current: boundary surface progresses towards west. 



In general, there exists a stable equilibrium in the current system between the mass 

 structure and the currents with a stable boundary surface position. If, however, an 

 easterly wind piles up polar water ("Anstau") along the east coast of Greenland, or if 

 other conditions in the North Polar Sea cause an increase in the strength of the East 

 Greenland Current, then the water masses of the current will be accelerated towards 

 the south and the boundary surface will become an up-gliding surface (Fig. 214 a). 

 This up-gliding along the boundary surface in the lighter polar water mass must come 

 to an end at the sea surface ; here it gives rise to a reduction in the inclination of the 

 boundary surface, that is, the extent of the East Greenland Current at the surface will 

 increase and will force the Atlantic water masses seaward. 



In the opposite case (Fig. 214 6) if the Atlantic water is accelerated towards the 

 north, the boundary surface becomes a down-gliding surface. It thus becomes steeper 

 and the extension of Atlantic water is increased. Pulsations in the basic currents will 

 be associated with variations in the mass distribution. The large-scale aperiodic 

 atmospheric disturbances of these regions must be accompanied by corresponding 

 large changes in the oceanic structure and the ideas outlined above are of major 

 importance in the coupling of these two phenomena. 



Similar conditions must apply for the much longer polar front in the Southern 

 Hemisphere. Here the temperature is the decisive factor for the mass structure and 

 the boundary surface between the West Wind Drift and the South Polar Current 

 slopes downward towards the north (towards the equator). In order to secure stationary 



