The prevailing winds are east winds between the equator and 30° , 

 west winds between 30° and 60°, and weaker east winds between 60° 

 and the pole. The primary effect of the winds acting at the sea 

 surface is to produce drift currents with velocities which decrease 

 with depth and which are essentially limited to the upper 50 m or 

 100 m of depth (layer of frictional influence). In our model, fig- 

 ure 3b, which is considered to represent the Northern Hemisphere, 

 the total mass transport of water in the layer of frictional in- 

 fluence (d) is directed toward the north between the equator and 

 30°N, and between 60°N and the pole, whereas the total mass trans- 

 port is directed towards the south between 60° and 30°. The con- 

 sequences are a divergent flow in equatorial regions and around 60° 

 and a convergent flow in the latitudes around 30° and near the pole. 

 The divergence (or convergence) of this mass transport is propor- 

 tional to the curl of the wind stress, and it can easily be computed. 



The unequal divergence at different localities, which strongly 

 depends on details of the wind field, sets up a slope of the sea 

 surface, in our simplified model, which drops from 30° toward the 

 equator and towards 60° and rises again from 60° toward the pole as 

 indicated schematically in figure 3b. From this, hydrostatic pres- 

 sure gradients result in the water, and, as an indirect consequence 

 of the wind effect, gradient currents will develop in the deeper 

 layers (in addition to the "density gradient currents", simply called 

 "density currents", due to the original horizontal temperature dif- 

 ferences). The depth of the wind induced gradient currents depends 

 on the vertical stratification of the ocean water. 



In what follows, we may neglect the "density currents" as they 

 would result from the original temperature distribution in the model. 



21 



