in fig. 3b between 30° and 0°), a layer of no motion may develop 

 (with steady state conditions) in a certain depth a bove the bottom 

 by "mass-compensation" of the hydrostatic pressure differences re- 

 sulting from sea surface slopes. In this case, the necessary balance 

 for the mass transport in the layer of frictional influence has to 

 be provided for completely by the wind induced gradient current. 



If the hydrostatic equation holds in vertical direction, the 

 pressure p at a depth, h, is given by 



z=0 

 V = ~ g f pdz+gp Q £ 

 y z=-h 



where b is the elevation of the sea surface above the level sur- 

 face z = 0, and p is the density of the surface. 



Assume two oceanographic stations, A and B, fairly close to- 

 gether, for which the vertical density stratification is known, and 

 suppose A b is the elevation of the sea surface at station B re- 

 lative to station A where b A =0. Then, the depth of the level 

 of no motion for the current perpendicular to the line AB, between 

 these stations is given at an average (constant) depth, z = -D, where 



( *z } A = (p z } B ' or 



P 



p A dz = / p B dz " p o A ^ > 

 -D -D 



and 







X = - r / (p a - p B ) dz • <i7) 



u 



/ 



A ( 



-D 



Let the coordinate system be oriented with the x-axls in the (zonal) 

 wind direction. Then, in the layer of frictional influence 



24 



