this part of the section is an indication of shear along the southern boundary 

 of the Great Whirl. Between about 2°N and 2°S, 260 and 40 nmi from the end of 

 the section, isotherm spreading between the 25°C and 22^0 isotherms was ob- 

 served to increase from 40 to 120 m. This feature, which is characteristic of 

 both the Atlantic and Pacific along the equator, indicates the presence of a 

 shallow, easterly setting equatorial undercurrent (Neumann and Pierson, 1966). 

 Although it is generally believed that the Equatorial Undercurrent in the 

 western Indian Ocean is absent during the southwest monsoon (Neumann and 

 Pierson, 1966; and Wyrtki , 1973), indications of its presence along section 10 

 may be explained by the relative weakness of the 1979 southwest monsoon and 

 the lateness of the date of the observations, which were taken during the 

 first week of September. 



IV. WATER MASS ANALYSIS 



Figures 16 and 17 show the sea surface bucket temperatures and salinities, 

 respectively, for late June and early July 1979. Surface waters are in direct 

 contact with the atmosphere, and temperature and salinity are not always 

 sufficiently conservative at the surface to directly tag water masses. However, 

 the currents in this area are sufficiently rapid to allow "this. Salinity at 

 the surface in this area is a better indicator than temperature of the nature 

 and origin of the water. 



From figures 16, 17, 18, and 19, it is possible to distinguish two dis- 

 tinctly different kinds of surface water in the Somali Basin. The fresher 

 water with salinities less than 35.3 °/oo is advected into the Somali Basin by 

 the South Equatorial Current from the eastern side of the Indian Ocean (Warren, 

 et^ al_. , 1966). Such fresh water is the result of the excess of precipitation 

 and river runoff over evaporation in the Bay of Bengal and the region of the 

 Indonesian archipelago. Another much more localized source of relatively 

 fresh water is the coastal upwelling induced by the prevailing southwesterly 

 monsoons paralleling the Somali Coast. Such water can be readily identified 

 by low temperatures (less than 22°C) accompanied by low salinities. 



The second type of surface water, which is characteristic of the Socotra 

 Eddy, found in this region is warm (>26°C) and saline (>35.6 °/oo). Its high 

 temperature and salinity identify it as originating in the Gulf of Aden and 

 the Arabian Sea, where evaporation exceeds precipitation and river runoff, and 

 incoming solar radiation shows higher values than at any other location on the 

 earth's surface (Sellers, 1965). 



The complex intermingling of these two surface water masses can be seen 

 in figures 16 and 17. The warm (>270C), low-salinity (<35.3 °/oo) water 

 straddling the equator off the Somali-Kenya coast originates in the South 

 Equatorial Current and forms a clockwise rotating gyre called the Southern 

 Eddy. This warm and relatively fresh water is separated from warm yet more 

 saline (>35.5 °/oo) water by a ribbon of cold (<24°C), fresh (<35.1 °/oo) 

 upwelled water located between 3°N and 5°H. The presence of two warm gyres 

 south of Ras Hafun (10°N) is indicated in the XBT cross-section taken by the 

 tankers AL DURIYAH and ESSO HONOLULU between 220N and 20S from 28 June to 

 3 July and 14-18 July 1979 (figures 20 and 21). Along these sections, two 

 strong frontal zones centered at approximately 6°H and 10°N are apparent. 



