76 



to the horizontal motion of the water, and is directly pro- 

 portional to the horizontal velocity. 



The current velocity is inversely proportional to Coriolis 

 force, but in qualitatively comparing the velocities associated 

 with varying slopes of isothermal surfaces within a restricted 

 range in latitude (Santa Monica Bay), Coriolis force can be 

 considered as contant and the relative velocities are therefore 

 directly proportional to the slopes. 



Centrifugal and deflective forces always act perpendicular 

 to the instantaneous direction of a wind in the atmosphere and 

 the same is true of water motion in the sea. It can be shown, 

 mathematically, that when the three forces (gradient, centri- 

 fugal, and deflective), are in balance, a steady state is 

 reached. The resultant wind (or current, in the case of water) 

 is called a gradient wind. Such a gradient flow occurs after 

 the gradient has initiated the movement, although the gradient 

 itself may be formed by a current driven by some external force 

 (the wind, for example). If a gradient current is present which 

 flows along isolines that are parallel and straight, it is called 

 a geostrophic current . Currents of this nature are know to 

 exist in the open sea, and undoubtedly they also occur in shelf 

 waters. However, in the shelf area, where many conditions tend 

 to disrupt water motion, it is unlikely that a "steady state** 

 of more than a few hours duration ever occurs. 

 Some Conditions Causing Density Currents 



Wind . In the open ocean the stress that the wind exerts 

 on the sea surface leads directly to the development of a 

 shallow wind drift, and the transport of water by the wind 



