VII. FRONTAL CROSSINGS 



During the survey, numerous pronounced sea surface temperature changes, 

 as measured by the hull-mounted shipboard thermometer, were observed. Such 

 sea surface temperature changes are not readily apparent on the sea surface 

 temperature analysis chart (figure 18) for two reasons. The temperature 

 analysis is based on discrete hourly bucket temperatures rather than a con- 

 tinuous analog trace. Such discrete observations alias out the effects of 

 strong horizontal temperature gradients, thus leaving unknown the exact loca- 

 tion or nature of thermal discontinuities. The second difficulty with the 

 sea surface temperature analysis arises from its small scale: it is simply 

 impossible to present a feature only a few nautical miles in extent on such a 

 small-scale chart. In order to present sea surface temperature changes in a 

 meaningful way, the locations of each significant surface thermal discontinuity 

 are plotted and labeled in figure 48. The analog traces are reproduced and 

 presented in figures 48a, 48b, and 48c. 



The first group of temperature traces (A, B, and C) is associated with 

 the remnants of the Southern Eddy before its merger with the Great Whirl. 

 Trace A is a result of the strong thermal boundary between cold upwelled 

 coastal water and warm oceanic equatorial water at the southern edge of the 

 Southern Eddy. Trace B is an indication of cold upwelled coastal water at the 

 shoreward boundary of the Southern Eddy. The eastern boundary of the Southern 

 Eddy is evident in trace C, which was made on 21 August 1979 just as the 

 Southern Eddy and the Great Whirl were merging. This front is also seen in 

 the strong salinity gradient depicted in figure 19. Traces F, Q, and R are 

 associated with the Great Whirl. The northern edge of the Great Whirl is 

 evident in traces F and Q, while R is an indication of strong shear and tur- 

 bulence near the center of the Great Whirl. The extemely strong upwelling in 

 the area north of the Great Whirl adjacent to the coast south of Ras Hafun is 

 indicated in traces G, H, I, and J. The boundaries of the Socotra Eddy are 

 reflected in traces K, L, M, and P. The southern boundary of the Socotra Eddy 

 is indicated by traces K and L. For ease of presentation, the orientation of 

 trace L has been reversed. That is to say, the cold water on the right hand 

 side of the trace is located south of the warm water on the left hand side. 

 Traces M and P are crossings of the northern and eastern edges, respectively, 

 of the Socotra Eddy. The thermal boundaries apparent in traces D, E, N, and 

 indicate the presence of the shear zone between the Socotra Eddy and Great 

 Whirl and a large anticyclonic gyre to the east of the survey area. The use 

 of continuous analog sea surface temperature traces makes it possible to 

 determine immediately when a thermal discontinuity is crossed. For this 

 reason, the shipboard hull -mounted thermometer is of great value in pinpointing 

 the exact locations of frontal boundaries. 



VIII. XBT FAILURES 



Approximately 300 nmi from the Somali Coast along section 1 many XBT 

 malfunctions occurred. Since there are numerous causes of XBT failures 

 (Blumenthal and Kroner, 1977), T-4 and T-7 probes from several batches were 

 launched to investigate the possibility of malfunctions being caused by a 

 defective lot of probes. Examination of the analog traces produced from 

 several batches of XBT probes revealed wire breakage between the bottom of 



