A MOBILE INSTRUMENT FOR STUDY OF OCEAN TEMPERATURE 

 IN THE THERMOCLINE REGION 



A. A. HUDIMAC, J. R. OLSON and D. F. BRUMLEY 



U. S. Navy Electronics Laboratory 



San Diego, California 



INTRODUCTION 



A heavy fluid with a free surface, having a 

 density variation only in the vertical direction 

 (for the undisturbed medium), is subject to two 

 basically different types of gravity wave excita- 

 tion: (l) surface waves and (2) internal waves. 

 Internal waves are characterized by having, for 

 every mode, the maximum vertical particle dis- 

 placement below the free surface. 



For many analytical purposes most of the ocean 

 for most of the year can be taken to consist of 

 three strata. The first stratum is the surface 

 layer consisting of well mixed, nearly isothermal 

 water. The second part is the lower, more dense 

 water in which the temperature and density vary 

 slowly. Separating these two parts of the ocean 

 is the (seasonal) thermocline region in which the 

 vertical temperature gradient is relatively 

 large. Since the horizontal salinity gradient 

 is very slight in this region, isothermal sur- 

 faces nearly coincide with isopycnal surfaces. 

 Thus, an obvious way to observe internal waves 

 is to measure the vertical temperature structure 

 in the vicinity of the thermocline. 



A vertical string of thermistor temperature 

 sensors for detection of the passage of internal 

 waves was first used by Ufford;-'-'^ such arrays 

 are commonly used today. This technique is 

 widely applicable in shallow water and can even 

 be used in deep water when attached to deep-sea 

 mooring devices . When it is necessary to survey 

 an area, an instrument on a moving platform is 

 desirable. One such scheme is a towed chain 

 designed for measurement of thermal microstruc- 

 ture in the upper 500 to 1,000 feet. 3 To mea- 

 sure the vertical displacement of isotherms in 

 the thermocline where steep temperature gradients 

 are encountered, fairly close spacing of sensor 

 elements, precisely located, is required. Close, 

 precise spacing can be achieved with the moving 

 strut internal wave recorder described herein. 



APPARATUS 



The vertical sensing array consisted of 

 normalized thermistors spaced equally in the 

 leading edge of a vertical strut mounted on the 

 bow of a submarine. The first strut (Fig. l) 



Fig. 1. Low drag strut array mounted on USS BAYA. 



was 18 feet long and made of wood with a stream- 

 lined cross -section. Its base was mounted in a 

 steel boot and the main member was held in place 

 by a number of smaller struts. The temperature 

 sensors were placed 2.5 feet apart. The shoulder 

 of the sleeve of the sensor element was flush 

 with the leading edge and the thermistor bead 

 extended about 1/8 inch into the fluid stream. 



The strut current in use (Fig. 2) is made of 

 three 10-foot sections of a commercial antenna 

 tower. Each section is triangular in cross- 

 section. The tower is guyed laterally as well as 

 fore and aft. Operation is satisfactory up to 



Superior numbers refer to similarly numbered references at the end of this paper. 



^9 



