46 



WINDS 



The discussion of the most important of all forces classified as 

 external and provoking currents in the sea — the winds — has been 

 reserved until the last. Winds, as they are treated in this connection, 

 are divided into two groups: (a) Those winds the effects from which 

 impel the surface layers — propagate frictionally downward — and 

 produce a drift current only; and (h) those winds which by virtue 

 of (a) drive water particles against boundary surfaces in the sea and 

 give rise to gradient currents. 



Let us first examine {a) the current caused by the wind and the 

 earth's rotation alone. Nansen, on board the Fram while held fast 

 in the Arctic pack, observed that the drift of his ship was 20° to 40° 

 to the right of the direction of the wind. More recently Ekman has 

 made some very interesting theoretical investigations regarding a 

 wind blowing tangentially over a water surface, taking the rotation 

 of the earth into account. (See Ekman 's ''Earth Rotation and 

 Ocean Currents," Arkiv for Matematik, Astronomi, och Fysik, Band 

 II, No. 11, Uppsala, 1905.) To begin with, Ekman has made the 

 following assumptions : 



1. The ocean be unlimited in a horizontal direction. 



2. The depth to the bottom be great. 



3. The water mass be homogeneous. 

 The following deductions are then made: 



ia) The surface water particles are driven 45° cum sole (to the 

 right in the Northern Hemisphere) of the direction of the wind. 



(&) This effect is propagated frictionally downward. The direc- 

 tion of the current will alter cum sole in direct ratio to the increase 

 in depth, and the velocities will decrease in geometrical progression. 



(c) At a certain variable depth, called the frictional depth (or 

 depth of wind current), the water will flow in a direction exactly 

 opposite to that of the surface current, but its velocity will be only 

 about one-twenty-third of that on the surface. 



In order to obtain a clearer conception of the penetration of wind 

 currents in the upper levels of an ocean, relative values of c (the 

 velocity of the drift current throughout the vertical range of fric- 

 tional depth) and D (the frictional depth) may easily be calculated 

 by means of Ekman 's formulae and placed in table form : 



Table VIII 



