The Tropospheric Circulation 



595 



that intervenes between the homo-haline and weakly saline top layer and the deeper 

 lower salinity layers with an equally low salinity. Study of the position of these maxima 

 and their development showed that they intrude under the less saline top layer from 

 the extensive subtropical accumulations of highly saline water to the north and south. 

 These intrusions spread along preferred paths, the location of which throws some light 

 on movements within the middle and lower layers of the troposphere. This spreading 

 and its dynamics have been discussed in pt. I, Chapter IV, p. 166. There Fig. 72 

 (p. 168) shows that the salinity maxima are present everywhere except in two narrow 

 bands in both hemispheres where the density transition layer comes closest to the sea 

 surface. Evidently, the horizontal extension of the highly saline intermediate layer is 

 cut short in this region, and here the water masses must be deflected upward. The 

 region between the two bands without salinity maxima lies in the Equatorial Counter 

 Current. Here the supply of water that forms the salinity maxima comes from the 

 west, from regions which are not reached by the bands free from the salinity maxima 

 and are fed here from north and south. From these facts it is possible to derive a 

 three-dimensional system of currents in the oceanic troposphere of the tropics and 

 subtropics, that is illustrated schematically by the meridional section in Fig. 270. 



20° S 15 



20° N 25° 



200^ 



Fig. 270. Schematic representation of the zonal and meridional velocity components of the 



tropospheric circulation in the Atlantic Ocean (the topography of the thermocline is 



exaggerated in the vertical scale by about 1 :1 million; that of the physical sea surface even 



more); W, current towards west; E, current towards east. 



Where the stream lines are divergent in the top layer they are convergent in the dis- 

 continuity layer; the two bands with a low salinity are thus regions of upwelling water. 



The zonal components of motion do not appear in the meridional section and it 

 should not be forgotten that these are considerably more important. Compared with 

 these the transverse circulation is rather weak. This transverse circulation is primarily 

 a thermo-haline circulation and is the consequence of the internal forces of the mass 

 distribution (p. 575). It involves only the top layer down to the density transition layer 

 and in the strong zonal motions of the wind-driven equatorial currents it can hardly 

 be detected. It is, however, responsible for the pronounced vertical and horizontal 

 salinity distribution that is characteristic for the uppermost layers of the tropical 

 oceans. 



The water masses beneath the density transition layer (in the subtroposphere) are 

 very uniform and colourless and the water movements here must therefore be very 

 weak. Since they lie beneath the barrier, they can be only slightly aff'ected by turbulence 



