154 A TEXTBOOK OF OCEANOGRAPHY 



area more south-south-west and south-west. At its northern 

 limits its strength falls much below 4 of the Beaufort scale, and 

 calms are frequent. 



In the northern summer the whole system shifts in a 

 northerly direction, and the zone of calms is broader, especially 

 in the sea off Cape Verde. The strength of the north-east 

 trades is usually the same as in winter in two areas only 

 namely, north from Cape Verde in a north-easterly direction 

 and by the Lesser Antilles. Within the zone of calms a so- 

 called south-west monsoon develops. The south-east trade 

 blows in summer with greater force, over 5 on the Beaufort 

 scale, in the westerly region between Fernando Noronha and 

 Ascension. 



These trade winds must produce surface currents. In the 

 northern summer, when the south-east trade blows strongest, 

 the velocity of the Southern Equatorial Current reaches its 

 maximum. The Northern Equatorial current also waxes and 

 wanes with the north-east trade winds. Moreover, in the open 

 ocean currents purely due to the wind have been determined 

 in numerous localities. For example, in the area between 10 

 and 16 N. L. and 35 and 40 W. L., in the jnonth of 

 February, the average direction of the wind is stem? 51 east 

 (from 256 observations). The corresponding currents in this 

 area give a direction of north 81 west, a difference which is 

 due to the deviation of the current to the right from the wind's 

 direction. The deviation (279 to 231) amounts to 48, and is 

 due to the earth's rotation. The Southern Equatorial Current 

 is driven by the south-east trade wind to Cape San Roque, 

 when it divides into two branches, one running north-west, 

 the other south. Part of the north-west arm (the Guayana 

 Current) and the south arm (the Brazil Current) are bent back 

 on themselves into the South Equatorial Current. 



The GuineaCurrent is not to be attributed solely to the wind, 

 especially in summer, when the south-west monsoon blows. 



In a typical region (from 15 to 25 W. Long, and 5 to 

 10 N. Lat.) the average direction of the wind is south 29 



