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J. A Whitney.

Evaluation of Surface Ducts in Shallow Water online

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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.



97



Fall — Surface layer cooling reduces the strength of the negative gradient in the
upper layer during the fall. About 75 percent of the observations indicate a positive grad-
ient in the upper layer. A majority of the profiles classified as non-positive were observed
during October or early November. Most of these profiles have a surface duct indicating the
advent of seasonal mixing and layer deepening which characterizes this transition season. A
weak sound speed minimum near 50-75 m is observed sometimes for profiles at locations
where the shelf depth exceeds 100 m. The T-S diagram indicates the complexity and
weakening of vertical stability in some of the profiles, in comparison to the summer profiles.
Both gradient statistical tables indicate that the large percentage of positive profiles is
produced by positive gradients in the upper 50 m. Overall gradients from the surface to
depths below 50 m are primarily negative. It would be difficult to represent the entire fall
season with a single profile.



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119



EAST OF SINGAPORE

This location was selected as an example of an equatorial shallow water environment.
It also is isolated from open ocean influence. Six one-degree squares were processed to
provide a reasonable number of observations. This area is very shallow with bottom depths
ranging from about 25 m to 75 m. There were very few Nansen cast data in the set so
shallow profiles were retained and the sample was supplemented with a large percentage
ofXBT data.

Strong seasonal variations would not be expected in the equatorial zone. However,
some seasonal pattern was evident from atlas sea surface temperature charts, so two six-
month seasons were selected based on this information. The northern hemisphere spring
and summer months (April-September) were combined for summer and the standard fall
and winter months (October-March) were processed as winter. Each season is briefly
described and associated figures containing statistical tables, composite plots and T-S
diagrams are located at the end of this section.

Winter — The shape of the winter profiles resembles winter data observed at much
higher latitudes. The profiles are isothermal with weak positive sound speed gradients from
the surface to the bottom of the profiles. About 82 percent of the observations are classified
as positive gradient profiles. The one profile extending to 75 m has a strong negative
gradient near the bottom, indicating a local cool bottom layer. The T-S diagram shows
that a positive salinity gradient is relatively important in maintaining vertical stability at
this location. The selection of a single positive gradient sound speed profile to represent
the upper 50 m in this shallow water site for winter would seem to be supported. How-
ever, in water deeper than 50 m the presence of cooler deep water is suggested by the
single deep observation, which, if present would create a gradient change near the bottom.

Summer — The strong seasonal signal in the upper layer temperatures and sound
speeds observed in the other examples is not apparent for this equatorial location. The
sound speed gradient in the upper 30 m primarily is positive, resulting in 60 percent of
the profiles being classified as having a positive gradient. A thermocline below this
depth is indicated on most of the profiles. Negative gradients with deep sound speed
values less than winter sound speeds are observed. A few profiles extending below 30 m
again indicate the presence of a cool bottom water while others do not. The data sample
is inadequate to support firm conclusions about the causes of the observed sound speed
structures. Both seasons indicate weak positive sound speed gradients in the upper 30 m.
Below this depth, the winter is still primarily positive, but the summer is less certain.



120




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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


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Online LibraryJ. A WhitneyEvaluation of Surface Ducts in Shallow Water → online text (page 6 of 7)