558 Basic Principles of the General Oceanic Circulation 



highly deficient and must be supported by theoretical deductions. For details in parti- 

 cular areas of the ocean, reference must be made to special charts; the literature 

 sources will be indicated below. 



As is apparent from the current charts in Plate 8, the more schematic distribution of 

 oceanic currents known from earlier work is really present to a large extent in all 

 oceans. Northern and southern equatorial currents characterize everywhere the 

 tropical surface circulation and are usually separated by an equatorial counter current 

 flowing in the opposite direction, while the surface circulation of higher latitudes is 

 composed principally by the West Wind Drift and the Polar Current. Separation of 

 these current regions gives convergence and divergence lines which are specially 

 indicated in the current chart. They are rarely clear-cut lines; instead they are usually 

 rather wide areas intruding between individual currents. It is often difficult to deter- 

 mine their position accurately since they move backward and forward periodically 

 in time. The connection of this surface current system with the currents of the deeper 

 layers lies in these singularity areas, and they are thus of great importance. 



In the following sections a brief description will be given of the surface-current 

 conditions in the individual oceans and of their seasonal variations. The dynamics of 

 single currents will be dealt with later. 



{b) The Surface Currents of the Atlantic Ocean 



The backbone of the system of currents present in the Atlantic is formed by the two 

 equatorial currents; that in the Southern Hemisphere is the stronger one and is more 

 constant and of greater extent. During the whole of the year this current crosses 

 the equator from west of the island of St Thome until the South American coast. 

 The meridional distribution of the current intensity shows a double current core for 

 nearly all months; one of the two just north of the equator at about 1° to 2° N. and the 

 other one at about 4° to 5° S. (especially between 20° to 30° W.). Between them along 

 the equator is the equatorial region of divergence which belongs to the tropospheric 

 deep sea circulation (p. 595). This divergence coincides with the tongues or island of 

 cooler water that are shown in temperature charts, particularly in the period from 

 June to August and indicate the upwelling of deep water accompanying the diver- 

 gence. In the central part (8° to 40° S.) the South Equatorial Current is most intense 

 from June to July and hardly drops below 20 nautical miles in 24 h. The southern 

 current core divides into two parts at Cape San Roque — one turning south and be- 

 coming the Brazil Current, and the other joining the northern current core in the 

 latitude of the Amazon estuary to form the strong Guiana Current flowing along the 

 South American coast. 



The Northern Equatorial Current is less constant in extent and strength. Its northern 

 boundaries fluctuate, but from about 20° N. its itensity decreases and it passes into 

 an extensive region of weak and variable currents with frequent motionless areas. 

 South.of 20° N. its average intensity is about 15-17 nautical miles in 24 h. Schumacher's 

 monthly charts (1940) which give greater detail show the eff"ect of the bottom topo- 

 graphy on the current system where it passes over the mid-Atlantic Ridge (see p. 435). 



During the winter months when the equatorial counter current is very weak the 

 North and South Equatorial Currents flow together along a convergence line from 

 about 20° W., 4° N. to approximately 50° W., 11° N. but during the summer months 



