184 



WATER MASSES AND CURRENTS OF THE OCEANS 



currents, and estimate the coefficient of horizontal eddy viscosity to be 

 7 X 10^ g/cm/sec. This concept leads to the important conclusion that 

 on both sides of the countercurrent the equatorial currents are subjected 

 to great lateral frictional stresses that are directed opposite to the hori- 

 zontal stresses exerted by the trade winds on the surface. Similar lateral 

 stresses must be directed opposite to the general flow along the conti- 

 nental boundaries of the two large counterclockwise gyrals of the two 

 hemispheres, and the torque exerted by these stresses perhaps balances 

 the torque exerted by the stress of the wind on the sea surface. The 



25' S 20° 



20' N 25** 



200 



Fig. 50. Schematic representation of the vertical circulation within the equatorial 

 region of the Atlantic. The main direction of the currents is indicated by the letters 

 W and E. The water below the discontinuity surface, which is supposed to be at 

 rest, is shaded. 



lateral stresses between the equatorial currents and the countercurrent 

 probably contribute very materially to the total torque, and the counter- 

 current represents, therefore, a dynamically important link in the entire 

 system of ocean currents. 



Within the countercurrent and the adjacent equatorial currents the 

 frictional forces must lead to a transport of water across the isolines — that 

 is, to a transverse circulation. This question was examined theoretically 

 by Defant, who considered friction due to vertical mixing, but the con- 

 clusions are probably also applicable if the friction is due to lateral mixing. 

 Defant's results are shown schematically in fig. 50, according to which 

 four "cells" are present, representing gyrals with horizontal axes, 

 neighboring gyrals rotating in opposite directions. Within the southern 

 cell the water sinks in the region of the Tropical Convergence and rises 

 at the Equator, and within the next cell, located between the Equator and 

 the southern boundary of the countercurrent, the water rises at the 

 Equator and sinks at the boundary of the countercurrent. Within the 

 countercurrent the water rises at the northern boundary and sinks at 

 the southern, and within the northern cell sinking motion takes place at the 



