WIND CURRENTS AND WIND WAVES 



129 



formed, the stability will be great at the lower boundary of the layer, 

 where the eddy viscosity will be small, and a further increase of the thick- 

 ness of the homogeneous top layer will be effectively impeded, although 

 the thickness may be much less than that of the layer within which 

 normally a wind current should be developed. The further increase must 

 be very slow, but no estimate can be given of the time required for the 

 wind current to penetrate to the depths that it would have reached in 

 homogeneous water. The gradual increase of the homogeneous top layer 

 is shown schematically in fig. 32A. 



If the wind dies off, heating at the surface may again decrease the 

 density near the surface, but, as soon as the wind again starts to blow. 



DENSITY 



DENSITY 



Fig. 32. Effect of wind in producing a homogeneous surface 

 layer demonstrated by showing progressive stages of mixing. 



a new homogeneous layer is formed near the surface, and, consequently, 

 two sharp bends in the density curves may be present (fig. 32B). 



If conclusions are to be drawn from the thickness of the upper homo- 

 geneous layer as to the depth to which wind currents penetrate, cases 

 must be examined in which the wind has blown for a long time from the 

 same direction and with nearly uniform velocity. In middle and high 

 latitudes the wind current will reach its final state more rapidly in winter, 

 when cooling takes place at the surface, than in summer, when heating 

 takes place. 



Secondary Effect of Wind in Producing Ocean Currents 



In the open ocean the total transport by wind is equal to Xa/X and is 

 directed normal to the mnd regardless of the depth to which the wind 

 current reaches and regardless of the variation with depth of the eddy 

 viscosity. This fact is of the greatest importance, because the transport 

 of surface layers by wind plays a prominent part in the generation and 



