The Tropospheric Circulation 



597 



pressure surfaces are of the same form as the sea surface but the pressure gradient 

 decreases rapidly with depth (Fig. 272). The lower limit of the tropical and subtropical 

 circulation must lie at the 500 decibar surface where the pressure gradient is almost 

 zero; already at 200-300 m depth the velocity of the currents is very slight and the 

 Equatorial Counter current does not reach nearly as deep as this (approx. down to 

 1 50 m). A comparison of the topography of the physical sea level and the gradient 



ra° W 



Fig. 272. Absolute topography of the 100-decibar (upper picture) and 500-decibar surface 



(lower picture) of the subtropical and tropical region of the Atlantic Ocean (dynamic isobaths 



are drawn from 2-5 to 2-5 dyn cm). 



