NORTH SEA 



This location provides a good example of an isolated adjacent sea that has limited 

 communication with the open ocean as well as high latitude seasonal variations. Data were 

 processed and analyzed by the standard three-month seasons of winter (January-March), 

 spring (April-June), summer (July-September) and fall (October-December). Figures 

 containing statistical summaries, gradient statistics, composite plots and T-S plots are 

 ordered by season at the end of the discussion. 



Winter — Essentially all profiles have positive sound speed gradients from the sur- 

 face to the bottom of the profile. This is the result of total vertical mixing and isothermal 

 conditions surface to bottom. The only profile differences are year to-year and January- 

 to-March shifts in the absolute values. The highest sound speeds appear in early January 

 and the lowest, in late March. These comments are restricted to the 1° x 1-1^2° site 

 analyzed ; however, we presume these variations are reasonably consistent in the central 

 North Sea. The selection of a single typical profile for acoustic purposes is reasonable 

 because the overall positive gradients were observed to be consistent. The extent of homo- 

 geneity and weak stability created by winter mixing is evident by the low individual 

 profile temperature and salinity ranges exhibited on the winter T-S diagram. 



Spring — Three basic profile types are evident during spring. The positive gradient 

 profiles are observed primarily in early April during some years and are remnants of the 

 winter profile type, prior to the onset of spring warming. About 33 percent of the pro- 

 files are classified as positive gradient. Two other basic profile types have been observed: 



• Type I has sharp negative gradients indicated to 75 m, some with a shallow 

 mixed layer. Below 75 m a weak positive gradient extends to the profile 

 bottom with a slope similar to the deep winter profiles. More than 50 per- 

 cent of the profiles have a sound speed minimum at 50 m to 75 m. 



• Type II has weak near-surface gradients that may be positive or negative with 

 somewhat negative gradients below 30-50 m. Bottom sound speeds are near 

 winter values. 



Type I profiles appear to represent the expected effect of increasing surface heat exchange 

 during spring warming. The dominance of the strong temperature gradient on the resultant 

 sound speed structure and increased stability of the water column is clearly shown on the 

 T-S diagram. Type II profiles seem to be influenced by positive heat advection in addition 

 to the surface warming. A more extensive study of North Sea oceanography would have to 

 be made to establish explanations for the two separate profile types. It would be possible 

 to select three typical profile types to represent the spring. However, it is not possible 

 from this analysis to predict the occurrence of Type I or Type II. 



Summer — These data indicate the presence of the same basic Type I and Type II 

 profiles observed during spring. The remnant winter positive profiles are gone. However, 

 24 percent of the profiles are still classified as positive because of the depth of the surface 

 layer. The difference between Type I and Type II profiles seems more pronounced. Most 

 of the Type I profiles have a surface layer to 20-30 m and a strong negative gradient to 

 75 m. Some have a sound speed minimum at 75-100 m and a weak positive gradient to 

 the bottom. The Type II profiles usually have negative sound speed gradients from 

 surface to bottom with Uttle or no surface layer. The slope is nearly hnear or increases 

 slightly with depth. The T-S diagram indicates that the structure is primarily temperature 

 controlled with a few instances of surface layer salinity dilution. Water column stability is 



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