LECTURE 12 
THERMAL MICROSTRUCTURE IN THE SEA AND ITS 
CONTRIBUTION TO SOUND LEVEL FLUCTUATIONS 
E.J. Skudrzyk 
Ordnance Research Laboratory 
The Pennsylvania State University 
University Park, Pennsylvania 
U.S.A. 
12.1. THE TEMPERATURE STRUCTURE OF THE SEA 
Bodies of water such as the sea are acoustically inhomogeneous because of 
the dissolved substances, suspended particles, turbulent motion and water cur- 
rents, temperature gradients, and inherent microscopic temperature fluctuations. 
Temperature, which has the greatest effect on sound propagation, varies with 
daily and seasonal changes in the radiation from the sun. Variations in solar 
radiation cause temperature changes that take place over long distances and 
vary slowly with time. The turbulent motion of the water, on the other hand, 
generates microscopic temperature fluctuations from one point in the medium 
to another. 
The sea surface is coldest at midnight and warmest at noon, and these sur- 
face temperature variations penetrate deeply into the medium as Fig. 12.1 
illustrates. The curves of Fig. 12.1 are interesting because they have been 
computed on the basis of some very reasonable assumptions (concerning the 
radiation from the sun, evaporation at the sea surface, and turbulent heat con- 
duction of the water). The unknown constants have been selected so that the 
curves coincide with a set of experimental curves measured at Key West, Florida. 
The only discrepancy occurs when the slopes become negative. These parts of 
the curves are not realized experimentally: the upper surface layers are heavier 
than the lower layers, and turbulent motion sets in. This surface-temperature 
instability generates strong turbulent motion in the sea, causing equalization 
of the temperature and generation of the so-called isothermal layer. The tem- 
perature gradients and long-scale temperature fluctuations lead to bending of 
the sound rays; they have been thoroughly investigated in recent years. 
Figure 12.2 shows the microscopic variations of the temperature for various 
depths, as recorded by Urick and Searfoss [1] with a resistance thermometer 
(the temperature fluctuates by a few thousandths up to a tenth or over a full 
degree). These readings exhibit a surprising space periodicity, as if the tem- 
perature distribution had been generated by a resonance phenomenon. In the. 
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