XBT measurements with the thermistor chain measurements at the 1 7 sensor depths in the 

 near-surface layer where vertical temperature gradients were small showed that the 6 1 XBT 

 profiles measured temperatures slightly higher than the thermistor chain. The average of 

 1037 comparisons was 0.04°C, with a standard deviation of 0.13°C; 62.8% of the differ- 

 ences were positive, 35.3% negative, and 1 .9% zero. These results are consistent with those 

 obtained from the hydrocast and STD/SV, as well as the quasisimultaneous STD/SV 

 comparisons. In addition, a serial tabulation of the average differences, standard deviations, 

 and the number of positive and negative differences for each of the 61 XBT profiles, showed 

 temperature measurement biases both with respect to measurements made by individual 

 XBT probes and with respect to "runs" of consecutive probes. 



DISCUSSION OF PROFILES WITH LARGE DIFFERENCES 



The analyses presented in the previous section were concerned primarily with 

 average temperature differences. The results suggested that, on the average, the XBT system 

 measured temperatures in the surface-to-400-m depth interval that were sHghtly higher 

 and vertical temperature gradients that were slightly larger than those measured by 

 hydrocast, STD/SV, and thermistor chain systems. In many applications, however, the 

 concern is not with average values but rather with the use and interpretation of individual, 

 or a short series of individual, profiles. This section discusses these biases with respect to 

 individual profiles. 



For the set of 559 profiles containing no profiles made with malfunctioning systems 

 (profile set I) and reaching a minimum depth of 200 m, the 200-, 3 00-, and 400-m temper- 

 atures for 54 (9.7%) of the profiles exceeded the average hydrocast and STD/SV temper- 

 atures by more than or equal to ±0.50°C at one or more of the three depths.* These XBT 

 profiles are listed in appendix G. Also Hsted are the 200-, 300-, and 400-m differences in 

 °C and the 200-300-m and 300-400-m hnear temperature gradient biases in °C/100 m. 

 An examination of the data presented in appendix G shows that of the eight 460-m XBT 

 systems used to make the 559 profiles, seven measured 200-, 300-, or 400-m temperatures 

 that exceeded the average hydrocast and STD/SV temperatures by more than or equal to 

 ±0.50°C at one or more of the three depths. Of the 5 1 profiles extending to 400 m, the 

 differences for 41 (80.4%) profiles were positive at all three depths; for only five (9.8%) 

 profiles they were all negative; and for the remaining five (9.8%) profiles they were mixed 

 in sign. The XBT profiles measured, over the 200- to 400-m depth interval, a positive 

 gradient bias for 15 profiles and a negative gradient bias for 18 profiles. 



At 200 m, 46 (85.2%) of the differences were positive and eight (14.8%) were 

 negative. The comparable numbers at 300 m are 47 (88.7%) and six (1 1.3%); at 400 m, 

 the comparable numbers are 44 (86.3%) and seven (13.7%). In addition, more of the larger 

 differences are positive. For example, at 400 m, three differences are greater than 1 .00°C, 

 with the largest difference being 3.02°C, while the largest negative difference was -0.37°C. 

 For the 200-300-m depth interval, 21 (39.6%) of the temperature gradient biases were 

 positive, 30 (56.6%) negative, and two were zero with more of the larger gradient biases 

 being positive. For the 300-400-m depth interval, 26 (5 1 .0%) of the temperature gradient 



•■The use of a difference of ±0.50°C is arbitrary. The intent was to select a subset of profiles that 

 measured temperatures obviously different from the average hydrocast and STD/SV temperatures. 



93 



