Water Bodies and Stationary Current Conditions at Boundary Surfaces 463 



layer carries the character of a real discontinuity surface and its position depends 

 principally on the currents in the top layer, since the cold water masses beneath are 

 almost motionless. Since the equatorial currents in both hemispheres flow from east 

 to west (North and South Equatorial Current) the dynamic equilibrium requires an 

 accumulation of the heavy water of the lower layer on the left side in the Northern 

 Hemisphere and on the right side in the Southern Hemisphere. The discontinuity 

 layer thus arches upwards in the equatorial regions and this must be associated with a 

 depression in the physical sea level at the equator. The vertical stratification of the 

 water bodies, the position of the isobaric surfaces and of the physical sea level is 

 presented schematically in Fig. 208a (Sverdrup, 1932, 1934a, Defant, 1936c, 



{a)S 



Equator 



Equator 



Fig. 208. Different positions of the thermocline and of the physical sea surface in the 



tropics and subtropics and the corresponding current systems (according to Sverdrup). 



IV, current towards west; E, current towards east. 



p. 315). When the currents are symmetrical about the equator, this stratification will 

 also be symmetrical. However, neither of these conditions actually occur in the 

 Atlantic nor in the Pacific and very probably also not in the Indian Ocean. The thermal 

 equator is at times found north of the geographical equator so that the equatorial 

 currents are not symmetrical about the equator. In the Indian Ocean the thermal 

 equator lies to the south of the equator during the southern summer. This complicates 

 the adjustments of the boundary surfaces, since the Coriolis force, the effect of which 

 is symmetrical about the equator, acts as a counter force to the non-symmetrical 

 pressure field. An accumulation of the subtropical water masses, asymmetric to the 

 equator, more or less as in case b in Fig. 208 with the position of the physical sea 

 level and of the isobaric surfaces indicated there, cannot be stable. This is because, for 

 stable stationary conditions, the topography of the sea level and of the isobaric surfaces 

 at the equator must always show either a maximum or a minimum. In case b there will 

 be a current from the west on the southern side of the equator and a current towards 

 the east on its northern side, and at the equator itself the velocities will be infinite 



