The depth analysis of the 20°C isotherm (figure 30) is an especially 

 effective presentation. The 20°C isotherm, which is located near the top of 

 the upper thermocline, is an excellent tracer of the vertical displacement of 

 this thermocline. Because fronts are regions of strongly sloping isotherms, 

 the vertical location of the thermocline is a reliable indicator of the posi- 

 tions of fronts and eddies. Two depth maxima of the 20°C isotherm are shown 

 in figure 30. The 20°C isotherm breaks the surface off the Somali coast 

 between 9°N and lO^N and is found at a depth of less than 100 m between the 

 Socotra Eddy and the Great Whirl. The minimum in the depth of the 20^0 

 isotherm persists in an elongated pattern between the Socotra Eddy and Great 

 Whirl and a large anticyclonic eddy immediately to the east of the survey 

 area. 



The analysis of the depth of the IS^C isotherm (figure 31) shows the 

 anticyclonic circulation of the two eddies together with the strong zone of 

 shear between them. The zone of shear to the east of the two eddies is still 

 evident, but not nearly as pronounced as it is at the level of the 20^0 

 isotherm. The lack of a strong discernible pattern in the depth of the 15°C 

 isotherm near the eastern edge of the survey area is further evidence of the 

 shallowness (<200 m) of the strong currents observed there. 



V. DYNAMIC HEIGHT ANOMALIES 



The anomalies of the dynamic topography in dynamic centimeters of the 

 surface relative to 1500 dbar are shown in figure 32. This figure, which is 

 based on data taken from 27 STD stations during the last two weeks of August 

 and the first week of September 1979, shows the double-cellular nature of the 

 circulation in the region. The two anticyclonic cells centered at approxi- 

 mately 5°N, 53°E and 11°N, 56°E are indications of the presence and strength 

 of the Great Whirl and Socotra Eddy, respectively. Cold upwelled coastal 

 water, which is denser than the warmer offshore oceanic water, is the cause of 

 the low in the dynamic topography between 9°N and 11°N just off the Somali 

 coast. Cold upwelled water advected offshore is entrained between the Great 

 Whirl and the Socotra Eddy, causing strong shear between these two anticyclonic 

 gyres. The small, weak cyclonic cell south of the Socotra Eddy, indicated by 

 the broken line, is a result of offshore entrainment of cold upwelled water. 



The dynamic topography anomalies in dynamic centimeters of the surface 

 with respect to 1000 dbar and 500 dbar shown in figures 33 and 34, respectively, 

 both show the double-cellular pattern evident in figure 32. Their similarity 

 in dynamic topography configuration shows that there is little relative geo- 

 strophic motion at either the 500 dbar or 1000 dbar surface with respect to 

 1500 dbar. The analysis of the dynamic height anomalies of the 500 dbar 

 surface with respect to 1500 dbar (figure 35) shows a weak reversal from the 

 surface circulation. The two anticyclonic gyres become cyclonic with depth, 

 and the circulation adjacent to the coast of Somalia centered near 10°N 

 becomes anticyclonic. This weak reversal is a further indication of the 

 location of a reference level or level of no motion at a depth somewhat 

 shallower than 500 dbar. To further determine a shallow level of no motion, 

 the method of Defant (1951) of determining the difference in dynamic heights 

 between pairs of adjacent stations was used. This method indicated a refer- 

 ence level situated between 300 dbar and 500 dbar for most of the station 



13 



