The Tropospheric Circulation 



593 



It is usually difficult to fix a definite boundary between the troposphere and the 

 stratosphere. In the vertical density profile it appears as a slight intensification of the 

 vertical gradients; but often it is quite indistinct because of the very great distance 

 between observation levels at these depths. It should probably be referred to only as a 

 boundary layer. An approximate boundary can be obtained using the oxygen content 

 as a criterion (Wust, 1936Z?, see pt. I, p. 66 et seq.); it is then defined by the inter- 

 mediate oxygen minima. The method is based on the assumption that these minima 

 indicate layers where the air supply is least, that is, those localities where the renewal 

 of the water masses is particularly slow and where horizontal movement of the water 

 is entirely missing. It has frequently been pointed out (p. 494) that in the uppermost 

 layers the position of the oxygen minima is affected by biological processes. However, 

 oxygen minima can be used at greater depths to specify approximatively the different 

 circulations. In the Atlantic the oxygen minimum extends across the 1 10 degrees of 

 latitude (from 45° S. to 55° N.) between the oceanic polar fronts of both hemispheres; 

 its mean depth along a meridional section is given in Table 151. 



From the Southern Hemisphere polar front the lower limit of the oceanic tropo- 

 sphere sinks rapidly down to 600 m in the southern convergence region (between 35° 

 and 25° S.), and rises again to about 300 m in the tropics. Just north of the equator, it 

 is at first somewhat irregular and then sinks gradually down to about 950 m in the 

 northern convergence region (30° to 40° N.). 



Reasonably accurate data are available for the tropospheric circulation which 

 extends throughout the space between the sea surface and the lower boundary of the 

 troposphere. Defant's (1936c) representation of conditions in the Atlantic also 

 includes subsurface data over the whole area. For the other oceans the series observa- 

 tions are sufficient for interpretation only along single meridional or zonal sections. 

 No major differences between the oceans in the principal features of circulation are to 

 be expected. 



Table 151. Lower limit of the troposphere in the Atlantic Ocean 

 (Determined from the position of the oxygen minimum. Depth in metres.) 



N. Northern Hemisphere; S. Southern Hemisphere 



* Minimum values; f Maximum values. 

 Q 



