WIND CURRENTS AND WIND WAVES 123 



The stress against the bottom, r^, has been neglected, because in the open 

 ocean the velocities at great depths are negligible. In the ocean, slopes 

 produced by the wind can be recognized, because the wind leads to a 

 piling up of the lighter surface water. Thus, Montgomery and Palm^n 

 have shown that the trade winds of the North Atlantic and the North 

 Pacific lead to a piling up of the warmer and lighter water masses against 

 the western boundaries of the oceans. In the Atlantic Ocean the effect 

 reaches to a depth of 150 m, and in the Pacific to a depth of 300 m. 

 Below these depths the isobaric surfaces are horizontal, but above they 

 are inclined, sloping from west to east, so that the trade winds actually 

 blow uphill, but the slopes are small and decrease rapidly with depth. 

 In the Atlantic the slope of the sea surface is 3.8 X 10~^, and in the Pacific 

 it is 4.5 X 10~^. It appears that only part of the wind stresses are 

 balanced against the slope, so that the complete equation (VII, 12) must 

 be considered if the total effect of the wind is to be discussed. 



In the calm belt between the trade-wind regions, no stress is exerted 

 on the surface. Here the water must flow downhill as a countercurrent 

 flowing toward the east between the west-flowing currents of the trade- 

 wind regions. These equatorial countercurrents are completely reflected 

 in the distribution of mass. Where they are located in the Northern 

 Hemisphere the lighter water lies on the right-hand side and the denser 

 water on the left-hand side of the current. This is a striking example 

 of a case in which the distribution of mass is maintained by the current, 

 and not vice versa. 



Wind Currents in Homogeneous Water 



In homogeneous water where no piling up of water takes place, equa- 

 tion (VII, 12) is reduced to 



M:, = - Ta,y and My = - - Ta,x, (VII, 15) 



provided that no stress is exerted against the bottom. Here M^ and My 

 represent the components of the mass transport between the surface and 

 the bottom (p. 114), and the equations state that, regardless of the char- 

 acter of the viscosity, the total transport due to the stress of the wind is 

 directed at right angles to the stress, in the Northern Hemisphere to the 

 right, in the Southern Hemisphere to the left. 



In order to examine the character of the wind currents, it is necessary 

 to return to the equations of motion, which, on the above assumption, 

 are reduced to 



d) d\ ^^^^'^^^ 



