I£ a steady uniform wind blows from south to north across a rectangular 

 lake with a bottom that slopes downward uniformly from east to west, the 

 energy transmitted from the air to the water vrill be nearly proportional 

 to the surface area of the lake. This energy must be distributed over a 

 much thicker water column at the western side of the lake. Thus, water 

 velocities generated on the east side will be higher than those generated 

 on the west side. As a result, water will be accumulated at the north- 

 eastern comer of the basin more rapidly than elsewhere. A return flow 

 will be generated in an east to west direction at the northern boundary 

 of the lake, from north to south at the western boundary and from west 

 to east at the southern boundary. It is possible that a shallow current 

 from south to north would be generated throughout the lake. However, on 

 the western side of the lake the mean flow will be from north to south 

 since the return flow required by continuity can be more readily developed 

 in deeper water where the velocity of generated currents are much lower 

 in magnitude. It is because of the physical boundaries and Coriolis 

 force (discussed below) that a wind may generate currents in all possible 

 directions in different parts of a basin at the same time. 



3. Modification of Currents by the Rotation of the Earth . 



Newton's laws of motion pertain to a reference frame fixed in space. 

 A reference frame that is fixed with respect to a point on the earth's 

 surface rotates about the earth's axis with a velocity equal to the 

 velocity of the earth's surface. In the Northern Hemisphere, an apparent 

 force (an apparent force since it arises only because a rotating reference 

 frame is used) deflects moving bodies to the right with respect to a 

 rotating reference frame that is fixed on the earth's surface. In the 

 Southern Hemisphere, the deflection is to the left. This apparent force 

 is zero at the equator and is a maximum at the poles. Since Gaspard 

 Gustave De Coriolis (1835) first explained this phenomenon, the force is 

 termed the Corn-otis force. It must be considered to explain observed 

 circulation patterns in large lakes and oceans. This explains the large 

 number of clockwise rotating currents in the oceans of the Northern 

 Hemisphere and the corresponding counterclockwise rotating currents of 

 the Southern Hemisphere. 



In ignoring Coriolis effect and the interaction between fluid layers, 

 one might be led to believe that in the middle of the ocean, where lateral 

 boundaries and the bottom do not have any significant effect on the flow, 

 wind-generated currents would be parallel with the wind. However, this 

 is not the case. Wind- generated currents are almost always deflected to 

 the right of the wind direction in the Northern Hemisphere and to the 

 left of the wind direction in the Southern Hemisphere. Ekman (1905), 

 observed that friction between two fluid layers could be described as a 

 diffusion of motion from one fluid layer to another. A finite time is 

 required for this diffusion process to take place. By considering the 

 diffusion of momentum through the fluid column along with tlie Coriolis 

 force, Ekman found that the angle between the mean wind direction and the 



