Basic Principles of the General Oceanic Circulation 565 



current which keeps the water masses constantly in motion around the earth from west 

 to east. It is of much greater strength and constancy than the corresponding West 

 Wind Drift in the North Atlantic. South of 35° S. and east of 20° W. it flows mainly 

 in a north-easterly direction. There are widely differing opinions about the position of 

 its northern boundary in the area of the subtropical convergence; the southern 

 boundary is found at about 63° S. but is not sharply defined either. At the core of the 

 West Wind Drift lies the boundary between two quite different water types, the 

 subantarctic water of middle latitudes and the Antarctic polar water. In the Atlantic 

 this latter water type has its origin almost entirely in the Weddell Sea. A small part 

 only comes from the Pacific through the Drake passage. The boundary between the two 

 water bodies is denoted the South Polar Front {Antarctic Convergence) on both sides 

 of which the currents flow between east and east-north-east but the velocity is greater 

 on the northern side. For the dynamics of this front see p. 549. 



The Polar Current in the Southern Hemisphere flows in the coastal regions of the 

 Antarctic carrying cold polar water westward until the Weddell Sea where it turns in a 

 great arc around a central almost motionless region and flows towards north or north- 

 east to become the southern part of the West Wind Drift. East of 10° W. the course of 

 this Antarctic polar current coincides almost entirely with the mean pack-ice limit of 

 the southern summer. 



The framework of the circulation system of the sea surface formed by singular 

 lines and regions inside the current field is shown in Fig. 258. In the tropical and 

 subtropical circulation the divergence lines stand out clearly in the eastern parts of the 

 North and South Equatorial Currents. In almost all months there is a narrow area of 

 divergence off the West African coast in particular between the Canaries and the Cape 

 Verde Islands that extends towards the south-west beyond 35° W. as a two-sided 

 divergence line and forms the southern boundary of the North Equatorial Current. 

 This is connected with the upwelling of cold water off the West African coast. Its 

 counterpart in the Southern Hemisphere is the extended divergence line in the area 

 of the Benquela Current off the coast of South West Africa; the upwelling of cold 

 water also occurs here (Defant, 1936a). Reference has already been made to the 

 divergence line along the equator between the northern and southern branches of the 

 Equatorial Current (p. 559) and also to the convergence line in the Equatorial Counter 

 Current. The Cape Verde divergence line, the equatorial divergence line and the con- 

 vergence line that lies between them are all part of the tropospheric circulation system 

 and are associated with contrary singularities in the lower layers of the troposphere 

 (p. 595). 



The oceanic regions between the Equatorial Currents and the West Wind Drifts 

 in both hemispheres contain weak and variable currents. Stream lines deflected to the 

 right from the Atlantic Current and from the North Equatorial Current together form 

 the region of subtropic convergence. This extends across the Atlantic from 75° to 20° W. 

 but is not a continuous uniform convergence line. Vortex formations are the charac- 

 teristic type of motion with the existing slight density differences. In these vortices 

 warm water sinks to become part of the warm-water mass of the troposphere in this 

 region. This convergence is always indistinct and shows everywhere large seasonal 

 variations (Felber, 1934) and is therefore more appropriately called a subtropical 

 convergence region than a convergence line. In this convergence region the interaction 



