stream and Kuroshio. Driven principally by the monsoon wind system characteris- 

 tic of this part of the world it is the most seasonably variable of the major 

 ocean currents. Although the Gulf Stream and Kuroshio show significant varia- 

 tions in the form of meanders and eddies, such variations are small compared 

 with the mean circulation of these two mid-latitude western boundary currents. 

 The Somali Current, on the other hand, is known from reports of ship set and 

 drift to disappear and even reverse in surface and near-surface layers during 

 the months of the northeast monsoon. 



The Somali Current has been found to be a comparatively shallow current. 

 Duing (1970), in a statistical study of dynamic height anomalies from all 

 months in the Indian Ocean north of 20°S, found the dynamic height anomalies 

 of the 1000 and 2000 dbar surfaces relative to the 3000 dbar surface to be 

 almost randomly distributed in a Gaussian distribution indicating the absence 

 of significant horizontal baroclinic pressure gradients at these levels. 

 Values of 600 dbar relative to 3000 dbar were found to depart from the random 

 distribution indicating the presence of geostrophic motion at 600 dbar relative 

 to the 3000 dbar surface. Studies of surface dynamic height values with 

 respect to 1000 dbar from July, August, and September only showed a very broad 

 non-Gaussian distribution which is indicative of appreciable horizontal pres- 

 sure gradients and strong geostrophic currents at the surface relative to 1000 

 dbar. This shallowness of circulation of the western Indian Ocean seems 

 reasonable in light of the extreme variability of the monsoon winds in this 

 area. In the deeper layers, below 1000 dbar, the field of mass probably does 

 not have time to adjust to rapid changes in wind stress at the surface. In 

 the western Atlantic and Pacific Oceans where isopycnals show an appreciable 

 slope below 2000 m, the surface wind stress clearly shows less annual variation 

 than in the northwestern Indian Ocean where the prevailing winds shift from 

 northeasterly in January and February to southwesterly in July and August. In 

 the great western boundary currents of the North Atlantic and Pacific Oceans, 

 there is time for the field of mass to adjust to a much greater depth owing to 

 the more persistent surface wind stress. Although there has been considerable 

 controversy among classical oceanographers concerning the placement of a 

 reference level or level of no motion in both the Gulf Stream and Kuroshio 

 (Stommel , 1966), such a level occurs at a much shallower depth in the Somali 

 Current system. 



The rapid response of the Somali Current needs to be understood in terms 

 of the modal structure of baroclinic waves. It is not sufficient to ascribe 

 the Somali Current to a mere local response of the ocean to the local winds 

 (Lighthill, 1969). The Somali Current is highly baroclinic and is spun up 

 within a month of the onset of the southwesterly monsoon winds. Although the 

 barotropic and the baroclinic spin up times are comparable at the low latitudes 

 of the Somali Current system, the baroclinic mode is the dominant mode (Duing 

 and Szekelda, 1971). 



II 

 From his studies of the historical ocean data in the Indian Ocean, Duing 

 (1970) surmised the presence of alternative cyclonic and anticyclonic gyres in 

 this area. These Indian Ocean gyres, inferred from dynamic topographies 

 calculated from a total of 4,390 oceanographic stations from the entire Indian 

 Ocean, were found to be about 300 to 500 nmi in extent. This is considerably 

 larger than the meanders and eddies associated with the Gulf Stream and 

 Kuroshio which are on the order of 50 to 100 nmi in diameter. Although the 



