APPENDIX A 

 OCEANOGRAPHIC CONDITIONS ON THE GRAND BANKS DURING THE 1981 



INTERNATIONAL ICE PATROL SEASON 

 JOSEPH L. SHUHY 



The validation of the currents contained in the 

 model used to drift icebergs on the Grand Banks 

 was accomplished using TIROS Oceanographic 

 Drifters (TOD) during the 1981 International Ice 

 Patrol (IIP) Season. Five drifters were deployed 

 (Table A-1): drifters 2593, and 2635 were deployed 

 in the Labrador Current; drifter 2594 to the west 

 of the Labrador Current, and drifter 2596 was 

 deployed to the east of the Avalon Peninsula. The 

 drifters were airdeployed from a Coast Guard 

 C-130. Drifter 2593 stopped transmitting after 7 

 days, but all the other drifters performed without 

 casualty throughout the season. The drifters were 

 equipped with 10 meter long window shade 

 drogues tethered 10 meters below the surface. The 

 drifters measured and transmitted sea surface 

 temperature and drogue tension (which verified 

 the presence of the drogue). Drifter position was 

 determined from the doppler shift of the transmit- 

 ting frequency. 



Drifter trajectories are shown in Figure A-1. 

 The drift trajectories were prepared using stand- 

 ard computer programs developed at the Coast 

 Guard Oceanographic Unit. 



A computer program was developed to modify 

 the IIP current file based on drifter trajectories. 

 This model will be evaluated using the 1981 drift 

 data. Basically, a time dependent Ekman wind 

 current will be calculated and subtracted from the 

 surface current as obtained from the drifter 

 resulting in a value for the local sea current. This 

 local surface current will then be smoothed into the 

 IIP current file and relaxed to the historical value 

 within two weeks. The evaluation of the program is 

 planned for completion prior to the start of the 

 1982 IIP season. 



Two drifters (2631 and 2635) were air-deployed 

 on 1 April 1981. Drifter 2635 was deployed at posi- 

 tion 48°59.7'N, 49°57.8'W which was in the 

 Labrador Current. The sea surface temperature as 

 recorded by the drifter was 2.7°C. The drifter in- 



itially moved in a southeasterly direction remain- 

 ing between the 500 and 1000 meter contour, but 

 then started to go north of Flemish Cap generally 

 parallel to the 1000 meter contour. The average 

 velocity of the drifter during this time was about 

 24 cm /sec. The drifter then traveled south on the 

 western side of Flemish Cap continuing parallel to 

 the 1000 meter contour to about 46°20'N where it 

 was caught by the anticyclonic circulation around 

 Flemish Cap traveling at about 20 cm/ sec. By the 

 end of July 1982, the drifter traveled around 

 Flemish Cap and was located on the southern side 

 of the Cap. Shortly thereafter, (not shown in 

 Figure A-1) it was caught in the North Atlantic 

 Current and traveled in a general northeasterly 

 direction. 



Drifter 2631 was also deployed on 1 April 1981 

 about 30 nautical miles to the east of drifter 2635. 

 The trajectory of 2631 took it north of Flemish Cap 

 traveling at about 28 cm/ sec. The sea surface 

 temperatures during this time as recorded by the 

 drifter were about 3 to 3.5°C. 



The drifter then moved to the northwest and the 

 sea surface temperature as recorded by the drifter 

 quickly rose to about 9°C indicating that the 

 drifter was no longer in the Labrador Current but 

 had entered the North Atlantic Current. The 

 drifter continued moving in a generally easterly 

 direction after leaving the Ice Patrol region. 



Drifter 2596 was deployed just to the east of 

 Avalon Peninsula, and circled in a cyclonic direc- 

 tion before starting a slow eastward movement. 

 This drift was contrary to the expected southerly 

 drift along the coastal branch of the Labrador Cur- 

 rent. The sea surface temperature as measured by 

 the drifter during this time ranged from about 

 0.9°C to 1.6°C. The sea surface temperature chart 

 published by the Canadian Forces METOC Center 

 for the period 4 May to 10 May 1981, which is 

 derived from satellite imagery, shows a cyclonic 



A-1 



