FISHERY BULLETIN: VOL. 72, NO. 4 



of Bengal the general circulation is counter- 

 clockwise. Unlike in the Pacific and Atlantic 

 oceans, there is little evidence of an eastern 

 boundary current in the Indian Ocean along the 

 west coast of Australia. According to Wyrtki 

 (1973), water movements in this region are weak 

 and variable and most of the northward flow is 

 situated farther offshore between long. 95°E and 

 105°E. In the Indian Ocean the Equatorial Under- 

 current is also seasonal, being more pronounced 

 during the NE monsoon than in the opposite sea- 

 son (Taft and Knauss, 1967; Taft, 1971). 



At the onset of the SW monsoon the surface 

 currents in the Indian Ocean change dramati- 

 cally. The westward flowing North Equatorial 

 Current is replaced by an eastward flowing 

 Southwest Monsoon Current, thereby reversing 

 the surface circulation in that region. Together 

 with the Somali Current it is the outstanding cur- 

 rent in these latitudes. The surface current north 

 of lat. 5°S is then directed eastward and the 

 Equatorial Countercurrent is not distinguishable 

 as a separate flow. This is contrary to the condi- 

 tions prevailing in other oceans where the inten- 

 sity of the Countercurrent reaches a maximum 

 during this period. The Somali Current, now 

 flowing northeast as an intense western boundary 

 current, extends to about lat. 8°N. The current 

 transports southern forms into the Arabian Sea 

 thereby serving to increase the faunal complexity 

 of the region. In addition, enrichment of surface 

 water north of the current produces zooplankton 

 blooms. North of lat. 8°N the data suggest that this 

 current leaves the coast and turns eastward (Taft, 

 1971). The South Equatorial Current becomes 

 intensified in the western Indian Ocean; near the 

 African coast it contributes to part of the northern 

 flow. This brings species of the southern latitudes 

 close to the Equator in the western Indian Ocean 

 and even occasionally farther north to about lat. 

 10°N along the western boundary. During the SW 

 monsoon the flow off the Arabian coast is toward 



the east and northeast but is much less intense 

 than the Somali Current. This flow starts in April 

 and is reported to persist until August. The south- 

 eastward flow off the southwest coast of India re- 

 mains the same as in the NE monsoon. In this 

 region the current flows northward only for 2 mo 

 and it is essentially opposite to the prevailing 

 wind stress. Taft ( 1971 ) pointed out that the circu- 

 lation in the eastern Arabian Sea is therefore 

 influenced by thermohaline processes. 



During the SW monsoon three areas are promi- 

 nent in the Arabian Sea for upwelling: off the 

 Somali, Arabian, and southwest Indian coasts. 

 There is evidence that the upwelling off" the Ara- 

 bian coast is wind driven; that off" the Somali coast 

 is partly wind driven and partly due to dynamic 

 constraint on the current (Taft, 1971 ); and that off 

 the coast of southwest India is due to the dynamic 

 response which involves tilting of the thermocline 

 up toward the coast (Darbyshire, 1967; Banse, 

 1968). The cool surface waters off the coasts of 

 Arabia and Somalia during the SW monsoon are 

 clear indication of intense upwelling (See Wyrtki, 

 1971:45). Along the western Indian Ocean Diiing 

 (1971) commented on the presence of many 

 offshore anticyclonic and cyclonic vortices which 

 are indicated by alternating highs and lows of 

 dynamic height persisting through both monsoon 

 phases. Taft (1971) singled out the importance of 

 these vortices in the mixing processes of the near- 

 surface waters which would increase the rate of 

 nutrient enrichment of the surface layer. In the 

 western Arabian Sea such enrichments are max- 

 imum during the June-August period (Table 1). 

 High values of NO3-N and SiOa-Si persist for a 

 longer period along the coast of Somalia than off 

 the Arabian coast. The plankton atlas of the IIOE 

 (Indian Ocean Biological Centre, 1968) shows that 

 the maximum abundance of zooplankton is also 

 associated with these upwelling areas. 



The distribution of surface temperature during 

 the NE monsoon indicates no cool surface water in 



Table 1. — Surface nutrients at four coastal regions of the Arabian Sea. A = off the coast of Arabia, lat. 20°N; B = off the coast of India, 

 lat. 20°N; C = off the coast of Somalia, lat. 10°N; D = off the coast of southwest India, lat. 10°N. All values are in fjg-at/\. (Data from 

 Wooster et al., 1967.) 



1044 



