650 



The Tropospheric Circulation 



Fig. 305. Position of the physical sea surface and of the internal thermohaline boundary 

 surface and the corresponding circulation cells of the upper layer during the cruises 1 and 2. 

 In the first case: "Anstau" at the coast (piling up of water); in the second case: upwelling 

 off the coast. The inclinations of both boundary surfaces are strongly exaggerated, that of 

 the physical sea surface by far more than that of the thermocline. 



Stationary equilibrium) in a sea composed of two layers, the displacement of the physical 

 sea surface is always inverse to that of the internal discontinuity surface. However, 

 the fluctuations of the internal discontinuity surface is many times greater (inversely 

 proportional to the difference in density of the two water masses). Figure 306 shows a 

 schematic cross-section for cruises 1 and 2. The effect of the wind on the sea surface 

 gradually builds up to such a stage where the wind effect is exactly in balance with the 

 developing pressure gradients. While approaching this stage circulations have developed 

 mainly in the mixed layer, and must take the form shown in Fig. 306. On cruise 1 the 

 water accumulation at the coast causes a downward circulation here and a sinking of 

 the density transition layer. Upwelling occurs in the trough forming outside this 

 region of accumulation. 



In contrast to these conditions, during cruise 2 the water is driven away from the 

 coast, where upweUing thus takes place and the water masses sink down in the accumu- 

 lation region away from the coast. These primary circulations at the coast are followed 

 further out by successive secondary circulations of diminishing intensity. 



