A PRELIMINARY STUDY OF THE DIRECTIONAL SPECTRUM 

 OF SHORT -PERIOD INTERNAL WAVES 



by 



H, Charnock 

 UK National Institute of Oceanography, Wonnley, Surrey 



INTRODUCTION 



The material on which this work is based was collected on Cruise 21 of 

 R.V. 'Chain' (Woods Hole Oceanographic Institution) in September 1961. The 

 Chief Scientist was Professor J. B, Hersey and the observations were made 

 under the supervision of Mr. R. Frassetto. 



The object was to study the short-period internal waves on the seasonal 

 thermocline in deep water between Madeira and Gibraltar and the main tool 

 was the Woods Hole thermistor chain which was used to record temperattire 

 fluctuations at intervals down to a depth of about 100 m. 



To gain some information about the directional properties of the 

 internal waves, the ship followed a track in the form of a regular octogram. 

 Each leg was about 4.0 km long and took about 2 hours. 



ANALYSIS AND RESULTS 



The thermistor chain provides an analogue record of the depth of 

 selected isotherms as a function of time. These were first read off and 

 prepared in digital fonn for analysis on a digital computer. 



The mean temperat\ire distribution had an almost isothermal layer about 

 50 m thick, overlying a thermocline in which the temperature fell by about 

 5 C in 30 m. This basic temperature structure stayed fairly constant over 

 the 16 hours of recording but the individual isotherms fluctuated in depth. 

 The typical range was 5 to 10 m with extremes beyond 20 m. 



Further analysis has so far been limited to the study of fluctuations 

 in the depth of the 20°C isotherm, which is thought to be typical of the 

 motion about the mid -depth of the thermocline. The mean square fluctuation 

 in the depth of this isotherm varied, but its variation was not systematic 

 with either course or time. Most of the variation was associated with the 

 longer-period components of the spectrum which were poorly sampled. 



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