576 



Basic Principles of the General Oceanic Circulation 



minor importance as is the case in the ocean. It was mentioned that in the ocean these 

 heat and cold sources are at approximately the same level and that therefore conditions 

 are not favourable for the development of powerful circulation systems. In any case 

 they can be only of small vertical extent and they will be entirely incapable of filling 

 the whole of the oceanic space from the poles to the equator. Conditions along a 

 meridian will be more or less the following: 



Since a salinity increase is equivalent to a heat loss and a salinity decrease to a 

 heat gain, the thermal and haline circulation will act in the same direction in the region 

 between the equator and in the Ross latitudes (0° until 30° N. and 30° S.). North and 

 south of the subtropical regions, however, they will counteract each other. A powerful 

 thermo-haline circulation can thus be expected only in the tropics and subtropics. 

 The water transport occurs towards the poles in the uppermost layer and toward the 

 equator underneath with an upward motion in the equatorial regions and a descending 

 one in the subtropics. This circulation can, however, develop only in a thin top layer 

 and the Lenz schematic circulation is restricted to this kind of shallow circulatory 

 water movement. The circulation of this tropical and subtropical top layer is dealt 

 with in Chapter XIX. 



5. Wind Effects and the Current System in a Hydographic Circular Vortex 



That the wind system of the atmosphere is also involved in the development of the 

 ocean circulation was not excluded by many investigators, but no agreement was 

 reached about the importance of its effects as long as the properties of wind drifts 

 were still unknown. The significance of atmospheric currents as a cause of the ocean 

 circulation was considerably clarified by Ekman's investigations. Probably the most 

 important result was to show that the wind affects directly only a top layer of not more 

 than 100-150 m thickness. The piling up of water at a coast by the wind will, however, 

 give rise to a slope in the physical sea level and to gradient currents reaching down- 

 wards to greater depths. In stratified water, mass compensation between upper and 

 lower levels (pp. 485 and 548) seems to prevent the development of deep-reaching 

 gradient currents. This remarkable compensation principle is readily illustrated by a 

 two-layered oceanic model. If in such a water mass (upper layer: pi, hi; lower layer: 

 p., and /72 — hi; Fig. 263) a current V is generated along AB in the upper layer, then 

 the physical sea level along AB will adjust itself to give a state of equilibrium between 

 the gradient and the Coriolis force. The deviation of the physical sea level from a level 

 surface ("Geoid") is denoted by Ci. Displacements of mass in the upper layer will also 

 disturb the equilibrium in the lower layer with a resultant mass transport in the 

 direction from D towards C, the internal boundary surface will decline {CD'), but 



