(Appendix D 



Observations of a Warm-Core Eddy Near the 



Grand Banks of Newfoundland 



INTRODUCTION 



In April - May 1987, the Interna- 

 tional Ice Patrol conducted a 

 surface hydrographic and remote 

 sensing study of a warm core 

 eddy near the eastern edge of the 

 Grand Banks of Newfoundland 

 (Figure D-1). The surface vessel 

 was USCGC BITTERSWEET 

 (WLB 389). The primary objective 

 of the study was to improve Ice 

 Patrol's ability to interpret images 

 of the ocean's surface made with 

 its side-looking airborne radar 

 (SLAR). 



Imaging radars map the sea 

 surface roughness primarily 

 through Bragg scattering (Robin- 

 son, 1985), which for the 3 cm 

 wavelength and incidence angles 

 (45° to 87°) of the Ice Patrol 

 SLAR, results in a sensitivity to 

 wavelengths of 2 cm. These 

 waves are in the capillary-gravity 

 part of the spectrum, thus they are 

 influenced by molecular viscosity, 

 which is a function of sea surface 

 temperature and salinity. The 

 physics of radar returns from the 

 sea surface is receiving increased 

 research attention (see for ex- 

 ample, Phillips, 1988 and Donelan 

 and Pierson, 1987) mostly be- 

 cause radar is used to measure 

 oceanic wind distributions. Ice 

 Patrol is interested in using radar 

 images of the sea surface to map 

 the major water-mass boundaries 

 within its operations area. 



A previous study (fwlurphy et al, 

 1986 and Thayer and Murphy, 

 1987) showed that the SLAR 

 mapped the location of sharp 



surface thermal gradients that 

 marked the boundary between a 

 warm-core eddy and the Labrador 

 Current. They found that the 

 warm surface-water within the 

 eddy was always marked by a 

 stronger radar return than the 

 surrounding cooler water. How- 

 ever, they were unable to map the 

 entire boundary around the eddy. 

 A likely explanation of this obser- 

 vation is that their flight patterns 

 permitted only two look angles 

 (with respect to the wind) at the 

 eddy boundary and these were 

 reciprocals of each other. This 

 means that along some portions of 

 the eddy's boundary, the radar 

 was looking along the Bragg wave 

 field rather than into it. This 

 reduces the intensity of the radar 

 return from within the eddy and 

 makes the location of the thermal 

 front difficult to determine. 



One of the goals of the 1987 

 experiment was to improve on the 

 previous experimental design by 

 including four look angles at the 

 eddy in hopes of defining the 

 entire boundary of the feature. In 

 addition, the 1987 experiment 

 provided an opportunity to conduct 

 the surveys under different 

 environmental conditions than 

 those encountered in 1986. 



The intent of this report is to 

 describe the experiment that was 

 conducted in 1987 and to present 

 some of the preliminary results. 

 None of the SLAR data are 

 available for presentation at this 

 time, but a portion of the surface- 

 truth data is. This presentation, 

 made before a thorough analysis 



Donald L. Murphy 



is completed, is nonetheless 

 worthwhile because it helps 

 understand the oceanographic 

 conditions in the Ice Patrol opera- 

 tions area during 1987. The eddy 

 studied during IIP-87-1 dominated 

 the circulation near the southeast- 

 ern edge of the Grand Banks early 

 in the season 



OBSERVATIONAL PROGRAM 



The study site was chosen prior to 

 BITTERSWEET's departure from 

 port based on a satellite infrared 

 image obtained from the National 

 Marine Fisheries Service (NMFS) 

 laboratory at Narragansett, Rhode 

 Island. It showed a warm-core 

 eddy near the eastern slope of the 

 Grand Bank at about 44°N. In 

 addition, data from Ice Patrol 

 operational drifting buoys showed 

 apparent eastward movement in 

 the Labrador Current north of the 

 eddy. Hence, the region had 

 waters of the cold and relatively 

 fresh (< 2°C and < 34.3 ppt) 

 Labrador Current and the warm 

 and more saline (> 12°C and > 

 35.5 ppt) North Atlantic Current in 

 close proximity. The substantial 

 surface temperature gradients 

 presented a good location to test 

 the SLAR. 



Hydrographic Survey 



The hydrographic survey was 

 divided into two phases, the first 

 during 5-10 May and the second 

 from 16-20 May. The objective 

 was to survey the eddy and its 

 surroundings twice, in an attempt 

 to describe the evolution of the 

 feature over the entire three-week 



99 



