the drift track is indicated by the heavily thickened track line in 

 Figure 5« The seasonal appearances of the scattering layer are all 

 seen to be on the northern and eastern sides of the gyre. In 1966 

 and 1967 when the layer failed to appear, T-5 was drifting on the 

 southern and western sides of the gyre. By the summer of 1968 the 

 ice island was back in the same location it had been in 1963 when 

 the layer was first discovered and, indeed, the layer reappeared 

 in 1968. This suggests that the siammer appearance of the layer 

 has a definite geographic pattern which does not change from year 

 to year. As T-5 drifts around the gyre the geographic pattern is 

 superimposed on the seasonal pattern to give the observed be- 

 havior in time. 



Observations With a 100 KHz Sounder 



A high-freq.uency, narrow-beam echo sounder was installed at T-3 

 in March 1967 in an attempt to resolve individtial scatterers within 

 the layer. The instrument is a Ross model 200 A Fish Finder oper- 

 ating at 100 KHz. The sounding rate is 120 per minute with 250 

 watts of peak pulse power delivered to the transducer. The eight- 

 element transducer has a beam angle of 5° x lO^. This soionder of- 

 ten resolves individual targets within the scattering layers. The 

 short pulse length, rapid chart speed, high frequency and narrow 

 beam all contribute to the increased resolution. 



The most interesting finding with this sounder was a thin con- 

 tinuous scattering or reflecting layer at 50 m depths. This layer 

 normally forms a thin straight line across the chart with a thick- 

 ness of one or two meters. At times the line may thicken or thin 

 slightly. At other times it may be deformed into undulations which 

 probably represent internal wave motions (Fig. ?)• There is gen- 

 ereilly a sharp increase in individual scatterers below this layer. 

 This thin layer is never observed with the 12 KHz sounder. The 

 records are not as continuous as they are for the 12 KHz sounder, 

 but there does seem to be a tendency for the thin layer to disap- 

 pear in late summer. 



Water Structure and the Arctic Scattering Layers 



The scattering layers are associated with characteristic fea- 

 tures of the arctic water column. The distributions of tempera- 

 ture, salinity and sigma-t with depth which are shown in Figure 6 

 are typical of the entire area of the Arctic Ocean under discus- 

 sion. 



Four water masses are generally recognized: Arctic surface 

 water. Pacific water, Atlantic water, and Arctic deep water. The 

 svirface water, with a temperature of about -1.7°C and salinity be- 

 tween 29 and 31°/oo, extends to a depth of 50 m. The profile in 

 Figure 6 was taken in late winter when the surface water was well 



440 



