only gradual modification from the southern to the northern limits 

 of the ocean. Of the waters below the surface layer the subsurface 

 water masses are the most limited in spatial extent and remain 

 within well defined areas of the ocean. With this in mind^ the 

 areal limits of the principal subusrface water masses of the Indian 

 Ocean are designated on the accompanying chart (Fig. 20 ). As an 

 additional aid in locating the extent of these water masses the 

 chart bears Marsden Square numbers, which are numbers assigned to 

 each 10° square of- latitude and longitude. It can be seen that the 

 Laccadive Island Ridge affords a sharp division between Equatorial 

 Water on the west and Indo-Australian Subsurface Water to the east. 

 The Indo-Australian Water is apparently carried west by the South 

 Equatorial Current until it meets the barrier of the ridge and is 

 then deflected southward. The Equatorial Water also follows the 

 ridge south for some distance. Farther to the north the Equatorial 

 Water finds its way across the ridge and proceeds into the Bay of 

 Bengal and part way along the coast of Sumatra. 



The division between Equatorial Water and Arabian Sea Water is 

 not so well defined, as these two water masses have many properties 

 In common. The Equatorial Water in one sense can be considered the 

 end product of the modification of Arabian Sea Water with distance 

 from the Red Sea and Persian Gulf. It is defined as a separate water 

 mass here because it is more homogeneous and does not display the 

 rapid change in salinity with distance which is so characteristic of 

 the Arabian Sea. • • 



In the transition zone south of the Equatorial Vfater Mass both 



Equatorial and Indian Central Water are found. Even farther south 



in the area of the Central Water Mass the influence of Equatorial 



70 



