using tlie same limits as in the first test to deter- 

 mine if tlie point was significant. If botli of 

 these tests were negative, we again checked the 

 departure of levels from points immediately 

 above and below. If the absolute departure was 

 greater than 0.09 for both temperature and salin- 

 ity, the level was significant. If the limits were 

 not exceeded in any of the three tests, the level 

 was not significant. After running the same 

 checks for salinity, the top level of the five level 

 group was dropped and the next new level was 

 added onto the bottom end, and the testing was 

 begun again. A punched card output from the 

 STSP program was then processed for sigma-t 

 and dynamic heights. The final number of levels 

 was usually reduced to 50-100 levels. 



Typical results of the processing procedures 

 are shown for Ice Patrol station 10969 (fig. 3). 

 The DDL recorded 2250 levels of data from the 

 STD. After processing, 120 levels were selected 

 as significant or falling closest to specified stand- 

 ard depths. Shown are the upper 300 meters of 

 the raw DDL temperature and salinity distribu- 

 tion curves, over which have been plotted the 

 processed DDL values. 



Temperature and salinity curves from the 

 analog trace, as originally recorded from the 

 STD, were added to the processed DDL curve 

 (fig. 4). The computed dynamic heights from 

 the processed DDL data and the STD analog 

 data differed by 83 dynamic millimeters. In this 

 case, the STD trace could not be read accurately 

 in the upper 250 meters due to rapid temperature 

 variations with depth. In 40 percent of the sta- 

 tions taken during the May multiship survey, 

 the difference between the dynamic height com- 

 puted from the processed DDL data and the 

 value from the STD analog trace was greater 

 than 10 dynamic millimeters. In a majority of 

 the cases, the difference could be attributed di- 

 rectly to one or more major problems in reading 

 the analog trace in the surface layer. 



STD Analog Trace versus Digital Data 



During the 1970 Ice Patrol surveys, significant 

 differences were noted between STD analog trace 

 data and digital data logger records at the same 

 level (Ettle and Wolford, 1973). When the ini- 

 tial soaking depth of the underwater unit was 

 accounted for prior to reading the STD trace, 

 the differences were reduced but remained sig- 

 nificant. Analysis of the 1971 data showed that 



when the quality control values (Xiskin tempera- 

 ture and salinity values) were applied to the raw 

 DDL values, the resulting quality control correc- 

 tions for temperature and salinity were generally 

 the same as the quality control corrections cal- 

 culated using the analog trace. In general, com- 

 parisons of analog and DDL values at corre- 

 sponding depths between 250 and 1000 meters 

 were remarkably similar. Large variations were 

 found, however, in the surface layers. For ex- 

 ample, at Ice Patrol station 11003 dynamic 

 heights were calculated from analog trace tem- 

 perature and salinity values and DDL values, 

 and were found to agree within 1 dynamic milli- 

 meter. However, in the top 100 meters the tem- 

 perature differences between the two methods 

 averaged 0.42° C. and the salinity difference was 

 0.11°/o„. The differences can be traced directly 

 to the inability of the technician to read the 

 analog trace accurately because of extremely 

 rapid variations in temperature and salinity with 

 depth in the surface layer. The ability of the 

 DDL to reduce the chance of human eri'or and 

 the increased data accuracy are major advantages 

 of the DDL- STD system. 



STD Quality Control Procedures 



STD data were quality controlled by compar- 

 ing STD analog trace and DDL values with 

 temperature and salinity values obtained from 

 Niskin bottles attached just above the underwater 

 sensor unit. Normally, quality control correc- 

 tions were computed based on the averaged cor- 

 rections for each station. The Niskin bottle was 

 equipped with two protected and two unprotected 

 deep sea reversing thermometers. When the STD 

 cast reached its deepest depth, the thermometers 

 were allowed to soak for ten minutes to reach 

 equilibrium before the Niskin bottle was tripped. 

 The conductivity ratios of the quality control 

 samples were determined using an inductive lab- 

 oratory salinometer and were converted to salin- 

 ities utilizing the method established in the 

 International Oceanographic Tables published 

 jointly by UNESCO and the National Institute 

 of Oceanography of Great Britain (1966). 



Data Processing 



The reduced size of the field party during the 

 April Ice Patrol cruise necessitated the use of 

 modified data processing procedures. All quality 

 control values were computed aboard EVER- 



