SUBTROPICAL WATER: MOVEMENTS 73 



suggests that the water has a southward component above the 10-5° C. isotherm and the 

 34'9 7oo isohaline, both of which are found where the discontinuity between the two 

 layers is sharpest. 



THE MOVEMENTS OF SUBTROPICAL WATER 



North of the region of prevailing west winds in the Southern Ocean the winds blow 

 from the south-east or east. They are most regular on the northern side of the sub- 

 tropical regions of high pressure, which are centred in about 30 S, but near to the land, 

 and especially in summer, they extend farther south. South of Africa the winds alternate, 

 blowing chiefly from the west in winter and from the east in summer and in a narrow 

 coastal region south of Australia there are similar changes. New Zealand lies wholly 

 within the region of west winds, but they are weaker and less constant towards the 

 northern end of North Island, especially in the summer months when the subtropical 

 anticyclones take a more southerly path. 



The subtropical convergence — the southern boundary of the subtropical water — has, 

 however, been found to lie some distance to the south of the boundary of the west and 

 east winds, and in the southern part of the zone the wind will cause the surface water to 

 flow towards the east and north. The current charts give ample confirmation of the 

 eastward movement ; but the formation of the sharp convergence with the sub-Antarctic 

 water shows that the subtropical water must either flow southwards, or northwards 

 with a smaller velocity than the sub-Antarctic current. It is not unreasonable to suppose 

 that the southward movement of the subtropical water is continued beyond the boundary 

 of the east and west winds ; the movement may be partly due to another factor, the 

 thermohaline differences between the Antarctic and subtropical regions ; the southward 

 movements set up by the strong east winds which prevail as far north as the equatorial 

 regions must also be strong enough to overrun the northern boundary of the west 

 winds. 



In the western part of the Atlantic Ocean subtropical water is carried southwards by 

 the Brazil current. This current has its origin in the trade wind drift and the south 

 equatorial current, which as it approaches the Brazilian coast turns southwards and 

 flows along, or obliquely towards, the land. As far as the River Plate, especially in 

 summer, the southward movement is strengthened by the wind, which has a major 

 component from the north. 



Farther south, where the north winds cease, the effect of the earth's rotation and the 

 prevailing west winds tend to destroy the southward movement, and to drive the current 

 away from the land towards the east. In spite of the retarding influences, however, the 

 current continues to force its way 10-15 farther towards the south, and in the tempera- 

 ture and salinity charts of the region it appears as a tongue of warm highly saline water 

 between the cold and poorly saline waters of the Falkland current on the west and the 

 main drift of sub-Antarctic water towards the north-east on the east. 



An investigation made by Klaehn (191 1, p. 650), based on a very large number of 

 surface temperature observations, shows that the current — still known as the Brazil 



