Thermohaline Features 169 



South Atlantic of sections 7 and 3 in the North Atlantic.^ Sections 3 and 16 

 are both situated in subtropical regions of high atmospheric pressure : the 

 regions of maximum curl of the mean wind stress between the mid-latitude 

 westerhes and the trades. Sections 3 and 16 show about the same equator- 

 ward transport; that across section 16 is, perhaps, a Uttle deeper. Each is 

 very much what would be expected from the wind-stress theory alone. It is 

 only when we compare the two western current sections 7 and 15 that we 

 see a striking difference between hemispheres. Although section 15 is very 

 complicated, the depth of the level of no motion may be ascertained fairly 

 weU from water-mass considerations. The choice shown in fig. 79 indicates 

 three such levels between different water masses. It is consistent with the 

 descriptions by previous investigators. The difference in appearance be- 

 tween section 7 and section 15 is so remarkable as to call for further 

 description and elaboration. In both sections the total vertically integrated 

 transport poleward must be equal to the total wind drift equatorward 

 across sections 3 and 16 respectively, from mass- conservation arguments. 

 The wind-driven component of section 7 and that of section 15 must be 

 essentially the same. The marked difference between the function for the 

 Gulf Stream and that of the Brazil Current must therefore be due to the 

 different way in which the thermohaHne western boundary currents enter. 

 In the Gulf Stream the thermohaline circulation reenforces the poleward 

 surface motion; in the Brazil Current the situation is quite the contrary. 

 The separation of the wind and thermohaline contributions is shown 

 schematically in fig. 81. 



In summary, we see, therefore, that the wind-driven component of the 

 circulation is much the same in the two hemispheres, but that in the western 

 boundary currents the thermohaline circulation plays quite different roles 

 in the Gulf Stream and in the Brazil Current, making the former appear 

 much more strongly on charts of the topography of surfaces, equal pressure, 

 or density. 



For purposes of comparison, the function T{z, d) is drawn also for the 

 transport through a north-south Discovery section across the Antarctic 

 Circumpolar Current, section 19, 4500 m. being taken as the reference level. 

 The trifling Benguela Current is shown in section 17. 



Transport curves for a rough two-layer division of the circulation of 

 the Atlantic Ocean are indicated in fig. 82, a and h. Each fine represents 

 about 10 X 10^ m.^/sec. 



* Since this chapter was written, a much more precise analysis of deep water 

 transports in the South Atlantic Ocean has been published by Georg Wiist : in Papers 

 in Marine Biology and Oceanography, Supplement to Vol. 3 of Deep-Sea Research, 

 1955, entitled ' Stromgeschwindigkeiten im Tiefen- und Bodenwasser des Atlantischen 

 Ozeans auf Grund dynamischer Berechnung der ilfefeor-Profile der Deutschen 

 Atlantischen Expedition 1925/27', pp. 373-397. 



