7 



pulsations in current. Sagar (1960) found a diurnal cycle of 

 growth and decay of insolation-produced microstructure in the 

 surface layer in the summer months and showed a direct relation- 

 ship between the existence of microstructure and acoustic 

 intensity fluctuations. Helle (1964) measured thermal micro- 

 structure from a submarine using fast-response thermistors. 9 

 Murphy (1965) used fast-response thermistors mounted on an 

 unmanned torpedo- like research vehicle and recorded tempera- 

 ture deviations of 0. 3°C at 50 meters and 0.02°C at 1500 

 meters. Horizontal gradients measured near the surface were 

 of the order of 0. l°C/nautical mile and 0. OOlC/nautical mile at 

 2000 meters. 



From the above discussion it can be seen micro-horizontal 

 thermal gradients describe temperature changes over distances 

 from a few inches to a few yards, while average horizontal 

 gradients describe temperature changes over distances of several 

 miles or even several hundred miles. Here we shall deal with 

 distances between these extremes, being concerned with tem- 

 perature changes over horizontal distances of about 1 mile. 



Horizontal-temperature gradients in this intermediate 

 range (1 nautical mile) were first investigated in July 1964 when 

 the NEL Thermistor Chain was used in collaboration with the 

 Marine Physical Laboratory who were making convergence-zone 

 bearing-accuracy measurements. The thermistor chain recorded 

 continuous temperature cross sections parallel and perpendicular 

 to the sound path between the two MPL participating ships . The 

 temperature profiles were resolved into vertical- and horizontal- 

 temperature-gradient fields , which revealed regions of high- 

 intensity gradient. The horizontal-temperature gradient alter- 

 nated in sign at nearly equal intervals and was consistently two 

 orders of magnitude smaller than the corresponding vertical 

 gradient (Smith, 1965). n Fisher et al (1965) reported on bearing 

 fluctuations observed. 12 These results, particularly the period- 

 icity of the horizontal-temperature-gradient field, generated 

 interest in gradient fields in other areas already surveyed with 

 the thermistor chain. The speculation followed that, if other 

 areas displayed a similar periodicity in internal temperature 

 structure and if the two-orders-of-magnitude ratio between the 

 vertical and horizontal gradients held, a predictive acoustic model 

 might be constructed. 



