STRAIT OF JUAN DE FUCA 



This shallow water location was selected because it represents a narrow continental 

 shelf environment and an adequate number of observations are available for analysis. The 

 location is mid latitude and is under direct influence of the open ocean environment off- 

 shore. The one-degree square site selected for analysis is located on the La Perouse Bank 

 outside of the strait. Data were screened to select only deep profiles covering 80 percent 

 or more of the water column and were processed for four three-month seasons: winter 

 (January-March), spring (April-June), summer (July-September) and fall (October- 

 December). Each season will be briefly described. Figures containing statistical summaries, 

 gradient statistics, composite plots and T-S diagrams are ordered by season at the end of 

 the discussion. 



Winter — The predominant sound speed profile gradient is positive from the surface 

 to the bottom of the profile. Almost 98 percent of all profiles are classed as positive in the 

 gradient summary table. In a few instances the positive gradient is modified by a negative 

 gradient tail near the bottom. This is indicated in the increased percentage occurrence of 

 negative gradients from the surface to depths below 100 m in the sound speed statistical 

 summary. A narrow bottom channel exceeding 1 80 m depth crosses the shelf in the 

 southeast part of the one-degree square. Several of the deep negative gradient tails appear 

 in or near the channel. Because of the high percentage of positive gradient profiles during 

 the winter, it is possible to select a typical positive gradient profile to represent this season 

 for acoustic modeling. The T-S diagram indicates a moderately large salinity range created 

 by dilution of the surface layer by rain and runoff. A single near-shore profile with a very 

 low surface salinity is evident on the diagram. 



Spring — A large percentage of non-positive gradient profiles is produced by the 

 spring warming. Most of the positive gradient profiles were observed during early to mid 

 April before the advent of strong surface heating. The T-S diagram for spring indicates that 

 the high variability in surface sound speed is primarily temperature controlled with some 

 salinity contribution in a few instances where surface dilution is evident. The variability 

 realistically precludes the selection of a single typical profile to represent the entire transi- 

 tion season. Increased stability of the water column produced by the large temperature 

 and salinity ranges is indicated by the sigma-T (density) range on the T-S diagram. About 

 34 percent of the profiles are classified as positive gradient with the remainder indicating 

 a large variabihty in the strength of the negative gradients. Because spring is a season of 

 transition, it is likely there would be better resolution of profile types in the monthly 

 analysis. 



Summer — The summer season exhibits 100 percent non-positive sound speed 

 gradients. High surface sound speeds and very strong upper layer gradients characterize the 

 profile set. The few profiles with shallow surface ducts are products of local wind mixing or 

 eddies. A weak minimum is observed in the 50 m to 100 m depth range in many of the 

 profiles. The T-S diagram indicates the overall variability is strongly temperature dependent. 

 The profiles with surface temperatures above 12°C have the highest surface sound speeds. 

 Also indicated is an example of salinity dilution produced near shore by local river runoff. 

 A single non-positive profile with a strong thermocline can be used to represent the summer 

 season at this location for modeling purposes. 



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