LANDS END 



This location represents a higli latitude continental shelf exposed to the influence of 

 open ocean circulation of the North Atlantic current extension of the Gulf Stream. This 

 can be contrasted to the relatively isolated North Sea location. For consistency, data were 

 screened to select only deep profiles covering 80 percent or more of the water column and 

 were processed for the same four three-month seasons used for the Strait of Juan de Fuca 

 and the North Sea. Statistical summaries, composite profile plots and T-S diagrams are 

 provided following the discussion. 



Winter — The predominant winter sound speed profile shape is positive from the 

 surface to the bottom, resembling the structure observed for winter in the other shallow 

 water locations. The profile gradients are quite consistent with a mean of 0.017 (m/s)/m to 

 0.020 (m/s)/m. The vertical temperature distribution tends to be isothermal with a very 

 weak positive salinity gradient indicated on the winter T-S diagram. Spatial variation, 

 with higher sound speeds observed at lower latitudes, and year to year variations both 

 contribute to the observed spread in absolute sound speeds within the profile set. All pro- 

 files are classified as positive gradient and a choice of a representative profile for acoustic 

 modeling is not difficult. 



Spring — Seasonal warming in the upper layers produces negative sound speed 

 gradients and 80 percent of the profiles are classified as non-positive. This surface warming 

 increases vertical stability (see the T-S diagram) and inhibits overturn and mixing. The 

 deep sound speed structure tends to maintain the positive gradient of the winter, producing 

 a sound speed minimum at 50 m to 75 m for many of the deeper profiles. Surface warming 

 proceeds through this transition season and by June essentially all profiles are classed as 

 non-positive gradients. The choice of a single non-positive gradient profile to represent 

 this data set can be made, although the range of observed gradients is relatively high. 



Summer — Upper layer sound speeds are higher than the spring, but several profiles 

 have a greater surface layer depth resulting in a slight increase in the percentage of positive 

 gradient profiles over the spring. This is opposite to the trend expected as a result of surface 

 heating where the percentage of non-positive profiles would increase during the summer as 

 observed in the North Sea. This may indicate the influence of strong circulation in the 

 Lands End situation in contrast to the more isolated North Sea. A high gradient layer is 

 observed below the surface layer and a sound speed minimum is produced in the 50 m to 

 75 m depth range. The high stability of the water column above the sound speed minimum 

 is produced by the large temperature gradient indicated on the T-S diagram. Because the 

 presence of a relatively deep surface duct results in a positive classification for 27 percent 

 of the observed profiles, this must be considered when selecting a single profile to represent 

 the summer season. 



Fall — Seasonal cooling and overturn of the surface layer causes the fall surface duct 

 to deepen considerably. This results in a positive classification for over 95 percent of the 

 observations. Most of the deeper profiles, however, still have an overall negative sound 

 speed gradient from surface to depths of 75 m and below. By mid-November much of the 

 thermochne is destroyed resulting in deep surface layers and only a weak negative gradient 

 layer above the sound speed minimum near 75 m. Two profile types result from the pro- 

 gressive cooHng of the upper layer as seen on the T-S diagram and the composite plot. The 

 basic difference is in the depth of the surface duct and strength of the below-layer gradient. 



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