difference is not the result of a recognizable artifact of the XBT profile. The largest positive 

 difference was 0.45°C associated with XBT 166. This profile is shown in figure 19. On 

 the XBT grid, the towed thermistor temperature was 6.1 1°C. Again, this large positive 

 difference is not the result of an obvious artifact. 



As previously indicated, the calibration lines for seven of the XBT profiles exceeded 

 16.7°C by more than ±0.34°C. All seven profiles were made after XBT 1 66. Six of the 

 seven were consecutive XBT profiles. An inspection of this series of six profiles, made 

 during propagation loss experiment A3, showed that the calibration mark shifted abruptly 

 between XBT 175 and 176. The calibration line for XBT 175 was at 16.54°C, 0.1 6°C lower 

 than the 16.7°C standard and well within the permissible interval of ±0.34°C. The calibra- 

 tion line for XBT 176 shifted to 16. 15°C, a variation of 0.55°C from the 1 6. 7° C standard. 

 The line varied from 16.1 1°C to 16.21°C for the next six XBT profiles when it again 

 abruptly shifted to 16.76°C. The temperature differences between the XBT surface temper- 

 atures and the towed thermistor for these six XBT profiles were as follows: 



XBT 176 



0.60°C 



XBT 179 



0.69°C 



XBT 177 



0.17°C 



XBT 180 



1.06°C 



XBT 178 



0.76°C 



XBT 181 



0.23°C. 



The reason for this abrupt shift in the calibration line is not known. 



Finally, since all seven profiles that had an erroneous calibration line were made 

 after XBT 165, it appears that the presence of erroneous calibration lines is related to a 

 system malfunction. 



To check whether the differences for XBT 165 to 334 are linear shifts in temperature, 

 the 400-m temperatures for all XBT profiles obtained in water mass 2 were plotted. The 

 data set was limited to the water mass 2 differences since the variation of the 400-m temp- 

 erature in this water mass is small. Figure 20, similar in format to figure 15, is a plot of 

 these temperatures. Pertinent statistics are tabulated in table 21. An inspection of figure 20 

 suggests that the 400-m temperatures exhibit characteristics similar to, but not the same as, 

 those exhibited by the surface temperature differences. As shown in table 21 , the average 

 400-m temperature as measured by 54 STD/SV and SVTP profiles taken during the same 

 time interval as the XBT profiles were taken is 3.59°C, with a standard deviation of 0.03°C. 

 The standard deviation of 0.03°C confirms the stability of the 400-m temperature in water 

 mass 2. The average temperature for XBT 1 to 165 was 3.65°C, with a standard deviation 

 of 0.12°C;forXBT 166 to 334 it was 3. 77°C, 0.12% higher than for XBT 1 to 165, and 

 had a standard deviation of 0.31 °C, which is almost three times greater than the standard 

 deviation for XBT 1 to 165. A comparison of figure 15 with figure 20 shows little corre- 

 lation of the surface temperature differences with the 400-m temperatures for individual 

 XBT profiles. For example, XBT 197, shown in figure 21 , measured a surface temperature 

 0.22°C less than the towed thermistor and a 400-m temperature 0.9 1°C higher than the 

 average STD/SV and SVTP 400-m temperature. Such errors would grossly distort an 

 application requiring temperature as an input. A visual examination of the XBT 197 profile 

 does not suggest any reason to suspect the validity of the measurement. 



48 



