I. INTRODUCTION - GENERAL CHARACTERISTICS OF THE AREA 



An oceanographic survey of the area between the Horn of Africa and the 

 meridian of 60°E was conducted from USNS WILKES (T-AGS 33) during August and 

 September of 1979. This is the region of the Somali Current, Somali Front, or 

 East African Current, which are names given to the large ocean circulation 

 system influenced by the southwest monsoon. The southwest monsoon, caused by 

 differential solar heating of the vast Afro-Asian land mass and the Indian 

 Ocean on its southern and eastern boundaries, is a seasonal wind that blows 

 along the coast of East Africa and bends to the northeast as it crosses the 

 Horn of Africa beginning in May and ending in September. Although oceanogra- 

 phers and meteorologists are in general agreement concerning the existence of 

 a causal link between the southwest monsoon and the shallow but rapid transport 

 of water northeastward that is called the Somali Current, the exact nature of 

 the processes involved is not clearly understood. In particular, the effects 

 of a strong and rapidly varying wind stress on the sea surface topography in 

 low latitudes are poorly understood (Warren et^ al_. , 1966). 



One of the difficulties in understanding the dynamics of the Somali 

 Current is posed by the necessity of separating the barotropic from the 

 baroclinic field of mass. Hurlburt and Thompson (1976) characterize the 

 Somali Current as a time-dependent, baroclinic, inertial boundary current. The 

 large anticyclonic gyre south of Socotra Island, accompanied by supergeostro- 

 phic flow, is characterized by anticyclonic inflow in the upper layer and 

 cyclonic outflow in the lower layer (Hurlburt and Thompson, 1976). Lighthill 

 (1969) in ascribing the formation of the Somali Current to mass flux deposited 

 by baroclinic and barotropic waves in the western boundary region neglects 

 wind stress acting within 500 km of the coast. However, Leetma (1972; 1973) 

 found that local winds are crucial to the onset of the Somali Current. 



Although theoretical models have undoubtedly contributed a great deal to 

 an improved understanding of the Somali Current, they are incomplete. They 

 must be supplemented by future ship and aircraft surveys in the area owing to 

 the relative paucity of oceanographic surveys dedicated to a thorough under- 

 standing of the Somali Current. Standard sections allowing time-series 

 analyses, such as have been completed from EXXON tankers transiting the area, 

 would be of great value in achieving an improved understanding of the temporal 

 evolution of the Somali Current (Bruce, 1979). Figure 1 presents the location 

 of the sea lane transited by the EXXON tankers during the taking of the XBT 

 cross-sections. 



Although the amount of historical data available from this area is 

 considerable, it is of limited value in understanding the fine structure of 

 the strong and highly variable Somali Current. Colborn (1975) performed an 

 excellent climatological analysis of the entire Indian Ocean from a total of 

 28,669 bathythermograph and hydrocast observations distributed among 274 

 subareas. Poor distribution of the observations in both space and time, 

 together with aliasing and averaging, makes it impractical to glean from them 

 the fine structure of the complex Somali Current System. 



Although the Somali Current is found on the western boundary of the 

 Indian Ocean off the Horn of Africa and thus has been called a "western 

 boundary current", it differs in several important respects from the Gulf 



