volume. The number of XBT profiles plotted on each figure is indicated in the upper left of 

 the right-hand figure.. The symbol (x) indicates the average value at each depth. Also 

 shown in each plot are the wind conditions present during the run. An inspection of these 

 figures shows that the distribution of the differences for station 4, runs 3 and 4, is quite 

 different from the distribution of the differences for the other data sets. For station 4, run 

 3, the differences are nearly all positive, with all average differences being positive. In 

 addition at all depths the largest positive differences, shown by the circled "dots," are 

 associated with XBT 266L. The station 4, run 4, data set also contains one anomalous 

 profile. The largest positive differences for depths greater than 40 m are all associated 

 with XBT 269L. The differences for this XBT are also shown by the circled "dots." XBT 

 266L and 269L are shown in figures 34 and 35. An examination of these profiles provides 

 little reason to consider them to be inaccurate. However, comparison with the thermistor 

 chain data clearly shows them to be in error. The two profiles will not be included in the 

 following analysis. Thus, of the original 64 visually acceptable profiles, 61 remain for 

 further consideration. 



Figure 36 is a composite plot of the differences for the remaining 61 XBT profiles. 

 The "dots" are the individual differences, and the average difference for each depth is 

 shown by the symbol (x). An inspection of the average thermistor chain plots in appendix 

 E shows that the depth of the top of the thermocline, or the bottom of the near-surface 

 mixed layer, is about 90 m. Note that all the above-thermocline average differences, except 

 the surface difference, are positive, while all below-thermocline average differences are 

 negative. Recall that the thermistor chain depth sensor was inoperative during these 

 measurements, so that no independent thermistor chain sensor depths were measured. 

 From indirect evidence based on average thermistor chain and hydrocast, and on STD/SV 

 temperature measurements, it was previously concluded that the chain was vertical in the 

 water during the 3-knot propagation loss runs, with the deepest temperature sensor at 

 242 m. Perhaps the bias in the below-thermochne differences may be attributed to the lack 

 of validity of this conclusion. Since the vertical temperature gradient below the surface 

 layer is not zero, the thermistor chain sensor would record a slightly higher temperature 

 at a slightly shallower depth. Thus there remains a possibility that below-thermocline 

 differences are not real but rather related to the assumption made regarding the depth of 

 the sensors. Only the above thermocline differences are considered in the remainder of 

 this discussion. 



Table 38 lists for each depth in the near-surface layer the number of differences, the 

 average difference, the standard deviation, and the number of positive, zero, and negative 

 differences. Also shown are the statistics for all 1037 comparisons. The average of all 

 differences was 0.04°C, standard deviation 0.1 3°C; 62.8% of the differences were positive, 

 35.3% negative, and 1 .9% zero. Since the vertical temperature gradient associated with this 

 data set is small, the reasoning used to explain the bias in connection with the thermocHne 

 differences is not apphcable to these comparisons. Thus it is concluded that in this compar- 

 ison of 61 underway XBT profiles with the thermistor chain, the XBT measures temper- 

 atures that are, on the average, slightly higher than the thermistor chain. 



Statistics'associated with the individual XBT measurements made in the surface to 

 90-m near-surface layer are tabulated in appendix F. Tabulated for each XBT are the 

 average differences, the standard deviations, and the number of positive and negative differ- 

 ences. Note that some XBT numbers are missing. These profiles were made at times 

 when the thermistor chain was not making measurements. Also shown are the station and 



86 



