SUBTROPICAL WATER: MOVEMENTS 75 



There is not sufficient evidence to show that the subtropical water in the eastern part 

 of the Atlantic Ocean actually has a component of movement towards the south, 

 but the existence of a sharp convergence shows at least that its northward component 

 is less than that of the sub-Antarctic water. 



In the western part of the Indian Ocean the subtropical water is carried strongly 

 towards the south in the Agulhas current, which flows towards the south-west skirting 

 the African coast. It is caused by the great Trade Wind Drift across the Indian Ocean ; 

 as this drift advances towards the African coast it meets the island of Madagascar and 

 divides, one stream flowing towards the northern end of the island and the other towards 

 the south. The first stream, joined by more water from the east, flows round the northern 

 end of the island towards the mainland, where it again divides, part of it turning towards 

 the north and the remainder flowing southwards through the Mozambique Channel to- 

 wards Natal. The second stream after flowing southward of Madagascar and Mauritius 

 also flows towards Natal, and joining with the southward current from the Mozambique 

 Channel, gives rise to the Agulhas current (Africa Pilot, III, 1929, p. 35). The current 

 is clearly recognizable from 3 to 120 miles off the land, and it generally flows fastest near 

 the edge of the bank. Between Durban and 23° E it attains a velocity of 3-4-2 knots, but 

 in its subsequent progress towards the south-west it becomes weaker, and on reaching 

 the Agulhas bank tends to follow the edge of the bank and to branch off to the south. 

 A small portion of the current passes round and over the bank and turns towards the 

 north round the Cape of Good Hope. 



Where the main body of the current meets the sub-Antarctic current from the south- 

 west it gives rise to a marked salient of warm highly saline water towards the west in 

 about 40-43 S (see Figs. 4, 8, 1 1, pp. 19, 29, 45) in which the current curves towards the 

 south and back to the east. The boundary region between the warm and cold currents 

 is a region of irregular movements and confused seas, and there are sudden and irregular 

 changes of temperature and salinity. These changes have already been described briefly 

 on pp. 59, 60, and graphic accounts have been given by Krummel (191 1, 11, pp. 673-5), 

 Brennecke (1909, p. 128) and Schott (1902, pp. 130-2). The greater part of the sub- 

 tropical water appears to turn back near the edge of the Agulhas bank in about 22 E, 

 but sometimes the current has been found to penetrate westwards of io° E. The surface 

 water at St. 1165 (section 7, Plates X-XII) in 9 34' E was undoubtedly subtropical. 



The large volume of subtropical water which is turned back towards the east from the 

 Agulhas current is generally known as the Agulhas return current. The temperature and 

 salinity distribution in the Indian Ocean suggest that in addition to its eastward move- 

 ment it has a small southward component, and the isotherms and isohalines in the 

 neighbourhood of the subtropical convergence show on the whole a small advance to- 

 wards the south. The presence of so much highly saline water in the surface and sub- 

 surface strata as far south as Sts. 869 and 870 in section 9 (Plate XVII) also indicates 

 that the subtropical water tends to force its way southwards. 



Most of the older current charts of the Indian Ocean indicate that the West Wind 

 Drift across the ocean divides off Cape Leeuwin, the south-western extremity of the 



