III. TEMPERATURE SECTIONS 



Figure 6 is a temperature section (section 1) based on XBT data taken 

 between the Seychelles plateau and the Somali Coast from 16 to 19 August 1979. 

 Approaching the equator from the south, a gradual deepening and packing of 

 isotherms in the upper 150 m is observed. This is indicative of a shallow 

 (<150 m) easterly-flowing countercurrent between the Seychelles plateau and 

 the equator. The sharp downslope of isotherms between 560 and 580 nmi is 

 indicative of strong horizontal shear. Between 300 and 400 nmi from the 

 coast, packing of isotherms between 20°C and 15°C straddling the equator is 

 especially noticeable. The observation is at variance with equatorial obser- 

 vat":ons in the Atlantic and Pacific, which generally reveal spreading of 

 isotherms on sections crossing the equator. In these two oceans, southeast 

 tradewinds crossing the equator are the cause of mass divergence along the 

 equator, giving rise to an upward displacement of the thermocline (Neumann and 

 Pierson, 1966). Such is not the case in the western Indian Ocean during the 

 southwest monsoon, when the winds crossing the equator have a westerly instead 

 of an easterly component. The deepening and packing of isotherms observed on 

 this section in the immediate vicinity of the equator are the result of mass 

 convergence at the equator caused by southerly monsoon winds blowing across 

 the equator with a slight westerly component. The current shear appears to be 

 quite weak in the region of the equator as inferred from the relatively gentle 

 slope of the isotherms. The abrupt peaking of isotherms between 22^0 and 25°C 

 at approximately 260 nmi is an indication of strong shear in the upper 100 m. 

 This peaking of isotherms is coincident with the strong gradient in sea surface 

 salinity (figure 19) just north of the equator and is an indication of a zone 

 of transition between relatively fresh water (S<35.1 °/oo) in the near-coastal 

 circulation and the more saline oceanic water to the east. The sinking of 

 isotherms from the coast out to 200 nmi indicates northeastward-setting geo- 

 strophic flow. 



Section 2, depicted in figure 7, is a temperature cross-section based on 

 XBT data taken from 19 to 22 August across the center of the Great Whirl. The 

 sharp slope of isotherms near the coast is especially pronounced out to about 

 250 nmi. As the colder isotherms (T<20°C) level off, two major breaks in ^he 

 data are encountered. These breaks are the result of repeated failures to 

 achieve successful XBT drops below about 200 m. Between 290 and 310 nmi a 

 weak front in the upper 100 m is evident in the sharp upturning and breaking 

 of the surface of the 24°C and 25°C isotherms. Particularly sharp upsloping 

 of isotherms is noticeable between 500 nmi and 580 nmi. The 20°C isotherm 

 rises 100 m in 60 nmi between 520 and 580 nmi from the Somali coast. The 

 sharp deepening of isotherms between 580 nmi from the coast to the end of the 

 section is also noticeable. The 20°C isotherm deepens from 80 m at 580 nmi to 

 about 150 m at the end of the section. Of particular interest is the pro- 

 nounced reversal in slope of the isotherms between 500 nmi and 640 nmi from 

 the coast. 



Section 3, shown in figure 8, is about 90 miles long and contains nine 

 observations. This section parallels the front which forms the eastern edge 

 of the Great Whirl and was a traverse made to avoid crossing the Great Whirl 

 at the same place. The thermocline rises slightly between 50 and 90 nmi 

 indicating an easterly-setting geostrophic current. However, the major com- 

 ponent of the current is most likely meridional since this section parallels 

 rather than crosses the front. 



