Both scattered and reflected arrivals appear on echo-sounder 

 records made from a floating ice station in the Arctic Ocean (Hunk- 

 ins, 1965). Scattering layers often appear at depths between 50 

 and 200 m in this ocean and often persist for several months. The 

 arctic scattering layers have a diffuse, cloud-like appearance on 

 echograms made with a 12 KHz soixader and sometimes occur as two or 

 more layers (Fig. l). One of the striking characteristics of the 

 arctic scattering layer is its seasonal behavior. The layer gener- 

 ally appears only during the summer months, disappearing during the 

 winter. This behavior is probably related to the special light con- 

 ditions in the Arctic Ocean wheri: the sun is above the horizon 

 throughout the summer and below the horizon throughout the winter. 

 The scattering layer, presumably biological in origin, responds to 

 the predominantly seasonal light cycle. 



Discrete reflected echoes from shallow depths are also noted on 

 the echograms from T-3. These discrete echoes are frequently ob- 

 served at shallow depths, even in the winter when no scattering 

 layer is present. The discrete echoes often take a hyperbolic shape 

 indicating relative movement between the ice station and the object. 



The paper by Hunkins (1965) reported on the first two years of 

 scattering layer observations at Fletcher's Ice Island, T-3. The 

 same KHz Precision Echo Sounder has continued in operation almost 

 without interruption at T-3 since it was first installed in June 

 1963. The system consists of a Gifft ESRTR-3 Sonar Transceiver, 

 a UQN transducer, and a spark-type drum recorder. The repetition 

 rate is once per second and the rate of chart advance is one cm 

 per hour. The present paper describes the results of nearly six 

 years of continuous monitoring from T-3 as it drifted under the in- 

 fluence of winds, currents, and pack ice pressxn-e in the western 

 Arctic Ocean. Since the data were taken from a single moving plat- 

 form it is not possible to completely separate the temporal from 

 the spatial variations of the layer. However, there is enough 

 information to determine many of the layer's characteristics. 



Variability in Time 



The seasonal appearance and disappearance of the layer occurs 

 suddenly and is easily determined from the records. The behavior 

 of the arctic scattering layer as a function of time is shown in 

 Figure 2 where its presence is indicated by the solid black bar. 

 The layer was already present on June 19, 1963, when the echo- 

 soionder began operation. It persisted, except for a short break, 

 until August 2k of that year. The next year, 1964, it appeared in 

 late June and persisted until early November, except for a short 

 break in September. In 1965 it made only a short appearance in 

 September and then disappeared for the following two years. Ko 

 evidence of the arctic scattering layer was seen in 1966 and 1967, 

 although the same soimder continued to operate in nearly the same 



438 



