Additional statistics are computed to provide information on the strength of the 

 observed gradients. For positive profiles the gradient is computed and tabulated from the 

 absolute sound speed maximum to the shallower sound speed minimum. For non-positive 

 profiles the gradient is computed from the near surface maximum to the deeper absolute 

 minimum. If the profile has a deeper positive gradient leg below the absolute minimum, this 

 value is also computed. Examples of possible positive and non-positive gradient profile types 

 are shown in Figure 23 of the main body of this report. With this statistical summary, it is 

 possible for the investigator to determine how many profile types are necessary to 

 represent adequately the particular shallow water site and season. With the aid of the 

 gradient strength and depth information, suitable representative profiles may be selected 

 from the individual data subsets for acoustic model inputs. 



Discussion of Selected Shallow Water Environments 



This initial investigation for four selected shallow water sites confirms the suspicion 

 that shallow water environments tend to be complex, highly variable and generally cannot 

 be summarized by the same analysis techniques applicable to deep open ocean waters. 

 Although each of the four selected shallow water regions displays unique environmental 

 characteristics, we can draw some tentative general conclusions concerning the distribution 

 of vertical sound speed structures. 



Vertical sound speed characteristics for the winter season in mid and high latitudes 

 can be summarized reasonably well in terms of basic structure. Winter mixing and surface 

 heat losses normally produce a near-isothermal water column and result in a slightly posi- 

 tive sound speed gradient from the surface to the bottom. The single equatorial site sampled 

 East of Singapore also indicated a tendency for this environment to produce this positive 

 gradient profile in winter and to some extent in summer. The absolute sound speeds, 

 of course, vary significantly with location (and latitude). 



The summer season indicates some consistency in overall sound speed gradient for 

 the different locations. With the exception of a potential shallow mixed layer (surface 

 duct), the summer profiles usually have negative gradients to the bottom. The actual 

 shapes of the profiles, however, may vary considerably at a given site. The North Sea 

 provides an example of two distinctly different vertical structures observed in the summer. 



The spring and fall are transition seasons where both positive and negative profile 

 gradients are observed. Based on the sites evaluated, it appears that the high latitude and 

 mid latitude transition months show a preponderance of negative gradients. The fall 

 appears to have deeper positive gradient surface ducts than the spring. The low latitudes 

 (based on the single equatorial example) tend to display relatively more positive gradients 

 throughout the year. 



These tentative conclusions are based primarily on reasonable and general effects 

 expected from seasonal surface heat exchange and vertical mixing. The presence of nearby 

 land masses and the influence of the bottom itself make most shallow water environments 

 subject to strong advecture perturbations. This movement of water masses can modify or 

 dominate the more classical seasonal effects on the vertical structure of temperature and 

 salinity. Results from this limited shallow water study should be applied very cautiously 

 to other apparently similar locations. 



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