50 

 20-24 September 1984 



Figure 3-g 



Hydrographic Survey 



A two part oceanographic cruise 

 was conducted from USCGC 

 HORNBEAM (WLB-394). The 

 first part was a hydrograpiiic 

 survey conducted between 1 

 and 1 1 July 1984 (Figure B-4). 

 One of ttie objectives of this 

 year's cruise was to compare TOD 

 drift with geostrophic current. 

 The International Ice Patrol has 

 assumed TOD drift tracks can be 

 used to calculate geostrophic 

 currents since first using TODs in 

 1976. The Ice Patrol uses a 

 computer program to remove 

 wind driven current based on 

 Ekman dynamics from TOD drifts 

 and computes a "quasi- 

 geostrophic" current. This 

 current information is used to 

 modify the time-invariant historical 

 geostrophic current field used to 

 predict iceberg motion. Another 

 objective of the cruise was to 

 determine how accurately Fleet 

 Numerical Oceanography Center 

 (FNOC) environmental products 



compared with actual conditions. 

 We were interested in the 

 magnitude of the errors in the 

 products we received from 

 FNOC. 



Seventy-five of a planned 1 00 

 hydrographic stations were 

 occupied during the first phase 

 (Figure B-5). Water samples were 

 collected using Nansen bottles at 

 all standard depths to 500m 

 (Table B-2). Nansen bottles were 

 used because the Coast Guard's 

 Ocean Sampling System ( a 

 Conductivity, Temperature, 

 Depth instrument) was 

 inoperative. A minimum of two 

 protected deep sea reversing 

 thermometers were attached to 

 each Nansen bottle sampling 

 above 200m. An unprotected 

 deep sea reversing thermometer 

 was also used on bottles 

 sampling 200m and deeper. The 

 conductivity ratios and 

 corresponding salinities of all 

 water samples were determined 

 at sea, while corrected 



temperature values were 

 determined after returning to 

 Groton. The dynamic height 

 contours relative to the 500 

 decibar pressure surface for the 

 survey area are shown in Figure B- 

 6. In previous surveys, dynamic 

 heights were computed from CTD 

 or STD data relative to the 1 000 

 decibar surface and then 

 contoured at 0.02 dynamic meter 

 intervals (Kollmeyer, 1966; 

 Scobie and Schults, 1976). Due 

 to the inherent errors associated 

 with Nansen casts (as compared 

 to CTD data) and the spacing of 

 the station lines, the data from 

 this cruise were contoured at 

 0.05 dynamic meter intervals. All 

 of the water samples from the 

 500m bottles had nearly the same 

 temperature and salinity values, 

 indicating the water at 500m was 

 nearly homogeneous for the 

 entire survey area. Historical IIP 

 data shows the geopotential 

 surfaces below 500 decibars are 

 mainly isosteric and contribute 

 only a small fraction to surface 

 dynamic height variations, making 

 relative measurements to 500 

 decibars valid for examining 

 circulation in this area. 



TOD #2633 was deployed 

 immediately after occupying 

 station #23 (48°20'N48°30'W) 

 on 6 July (188). The anticipated 

 drift was through the survey area. 

 For the first three to four days, 

 TOD #2633, for the most part, 

 followed the observed 

 geostrophic flow (Figure B-6). 

 The geostrophic current in this 

 section of the hydrographic 

 survey was well-defined. Forthe 



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