OCEANOGRAPHIC CONDITIONS, 1976 



Two oceanographic cruises were conducted to 

 the Grand Banks of Newfoundland from 25 

 March to 25 April and 18 May to 30 June dur- 

 ing the 1976 Ice Patrol Season to provide real- 

 time sea current data from dynamic topography 

 surveys. 



Additional objectives of these cruises aboard 

 the USCGC EVERGREEN (WAGO 295) were 

 research investigations of iceberg drift and de- 

 terioration. The research program included the 

 use of satellite-tracked drogued drifting bouys 

 (RTTs). in situ moored current meter arrays, 

 and iceberg and drogue tracking experiments 

 designed to aid in drift modelling. 



In June 1976, a full dynamic topography sur- 

 vey encompassing section A4 to AIR (Figure 22) 

 was performed in conjunction with an extensive 

 survey conducted simultaneously in the waters to 

 the east of the Grand Ranks Ice Patrol standard 

 sections by the USCGC SHERMAN (WHEC 

 720). The purpose of the USCGC SHERMAN 

 cruise was to better understand the detailed 

 dynamic characteristics of the North Atlantic 

 Current after it leaves the Grand Ranks and to 

 determine the water properties of the associated 

 water masses. The USCGC SHERMAN cruise 

 data is the subject of a separate report. 



The dynamic topography surveys were con- 

 ducted by field parties from the Coast Guard 

 Oceanographic Unit and the crew of the USCGC 

 EVERGREEN using the Plessey Environmental 

 System, Inc. Salinity/Temperature/Depth 

 (S/T/D) or Conductivity/Temperature/Depth 

 (C/T/D) Model 9040, Environmental Profiling 

 Systems. The measurements were made to 1000 

 decibars (or to near bottom if shallower) and 

 were recorded on magnetic tape (Kennedy Co., 

 Model 1600R tape recorder) after formatting by 

 a digital data logger (Sonicraft, Inc. DDL). For 

 data processing details see Mountain (1978). The 

 method of calculating dynamic height in water 

 depth less than the reference level (1000 decibars 

 for the Ice Patrol) is described in Kollmeyer 

 (1967). 



During the first cruise three current meter 

 moorings were established in a triangular array 

 centered at 42-47N, 47-47AV. Deployment was 

 accomplished by launching from the fantail of 

 the CGC EVERGREEN with the anchor last 

 technique while the ship steamed slowly forward. 

 The moorings (Figure 23) each had two Vector 

 Averaging Current Meters (VACM) and an 

 acoustic release. Flotation was supplied by glass 

 ball floats above each instrument and by two 31" 

 fiberglass covered syntactic foam floats at the 

 top of each mooring. Mooring materials con- 

 sisted of 3/16" wire from the top floats to the 

 release and 5/8" nylon line from the release to 

 the anchor. 



Attempts to recover the current meter arrays 

 during the second cruise were unsuccessful. The 

 acoustic releases were interrogated and com- 

 manded to release. All releases responded as if 

 disengagement had occurred. However, no signal 

 from the submersible radio transmitters and no 

 sighting was made of the current meter mooring 

 on the surface even after an extensive search 

 was made. The acoustic releases were heard ping- 

 ing continuously in place for the life ^ uie 

 battery (about 5 hours). Dragging attempts in 

 both 1976 and 1977 also failed to recover the 

 moorings. The cause of failure of the moorings 

 to surface remains unknown. 



The results of the iceberg tracking study for 

 drift and deterioration appears in a separate sec- 

 tion in this bulletin. 



The contoured field of dynamic topography on 

 the first cruise (Figure 24 and 25) reveals a pat- 

 tern of flow similar to the average conditions 

 (Figure 26). The Labrador Current flows south- 

 ward through Flemish Pass following the eastern 

 edge of the Grand Banks. Surveys from the 

 second cruise (Figure 27) provide a full coverage 

 of the area bounded by the standard Ice Patrol 

 sections. The dynamic height field at this time 

 exhibits good agreement with the normal topo- 

 graphy in the northern sections of the survey, 



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