492 Ocean Currents in a Non-homogeneous Ocean 



cease and there will be a stable stratification with the temperature decreasing with 

 depth. A state of no motion is created since the circulations previously present will be 

 halted rapidly by friction. 



(b) Heat source at a lower level than cold source. This is the same experiment as in 

 (a) except that the position of the two sources is inverted. Convectional currents will 

 be set up in this case also, but soon there will form a steady circulation confined to the 

 layers between the levels of the two sources (Fig. 225, lower picture). Above there will 

 be a water movement from warm to cold and below from cold to warm; the most 

 heated water will be above the level of the heat source and the coldest below the cold 

 source. But these layers will not take part in the circulation which is solely confined to 

 the intermediate layers. 



Later on, Sandstrom modified the experiment in several ways, especially to show 

 more clearly its application to oceanographic conditions; basically, however, these 

 do not give any new results. Jeffreys (1925) has questioned the general validity of 

 Sandstrom's conclusions but Sandstrom's deductions from the circulation principle 

 are undoubtedly correct. The circulations produced by thermo-haline differences are 

 the more intense the greater the vertical distance between the level of the warm and that 

 of the cold source. However, conditions existing in nature in the ocean are not parti- 

 cularly favourable to the formation of any more intense circulations of this type, 

 since the principal heat supply in the ocean is primarily due to the combination of 

 solar radiation and back-radiation from the atmosphere and the loss of heat primarily 

 due to outgoing radiation. These processes operate to a very large extent at the 

 boundary between the ocean and the atmosphere (almost horizontal sea level and 

 evaporation and precipitation also act here. The vertical distance between the location 

 of the heat and cold sources is thus very small. Probably the heat source in equatorial 

 areas lies somewhat deeper than in higher latitudes, but nevertheless the thermo- 

 haline circulation must be limited to a very shallow top layer. Observations provide 

 complete confirmation of the consequences deduced from the circulation principle 

 (see p. 576). 



6. The "Reference -level" for the Conversion of the Relative Topography of the Press- 

 ure Surfaces into the Absolute One 



The relative topography of the isobaric surfaces (relative to the sea level) assumed 

 as plane) can be determined by the methods described on p. 309 and the following 

 pages. Using equation (XV. 12) this also gives the relative velocity differences from layer 

 to layer. In order to obtain a complete quantitative knowledge of the water move- 

 ments it is necessary to convert these relative topographies into absolute topographies. 

 This can be done if the relative topography can be referred to a known topography 

 of any isobaric surface. This determination of the absolute topography would be 

 easy if it were possible to determine from current measurements such a depth level at 

 which the velocity of the current is zero, since at this "depth of no motion" the isobaric 

 surface must coincide with a level surface ("Niveauflache"). 



In this way, for example, Wiist used the current measurements made by Pillsbury 

 in the Floriaa Strait in oraer to determine the current profile of the Gulf Stream from 

 the mass field. The number of current measurements available for the open ocean is, 

 however, insufficient to fix with some accuracy the position of such a "zero level" 



