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ESSA TR ERL 167-AOML 2

A UNITED STATES
DEPARTMENT OF
COMMERCE
PUBLICATION

/y\

ESSA Technical Report ERL 167-AOML 2

U.S. DEPARTMENT OF COMMERCE
Environmental Science Services Administration
Research Laboratories

An Oceanographic Investigation

Adjacent to Cay Sal Bank, Bahama Islands

ROBERT B. STARR

MIAMI, FLA.
JUNE 1970

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U. S. DEPARTMENT OF COMMERCE
Maurice H. Stans, Secretary

ENVIRONMENTAL SCIENCE SERVICES ADMINISTRATION
Robert M. White, Administrator
RESEARCH LABORATORIES
Wilmot N. Hess, Director

ESSA TECHNICAL REPORT ERL 167-AOML 2

An Oceanographic Investigation

Adjacent to Cay Sal Bank, Bahama Islands

ROBERT B. STARR

o
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ATLANTIC OCEANOGRAPHIC AND METEOROLOGICAL LABORATORIES
PHYSICAL OCEANOGRAPHY LABORATORY
MIAMI, FLORIDA
June 1970

For sale by the Superintendent of Documents, U. S. Government Printing Office, Washington, D. C. 20402

Price 55 cents.

TABLE OF CONTENTS

ABSTRACT 1

1. INTRODUCTION 1

2. PROCESSING AND DISPOSITION OF THE DATA 4

3 . SETT ING 8
If. OCEANOGRAPHIC BACKGROUND 9

5. TIDES AND TIDAL CURRENTS 12

6. OCEANOGRAPHY 17

7. THE SURFACE CIRCULATION 30

8. THE SUBSURFACE CIRCULATION 32

9. SUMMARY AND CONCLUSIONS 44

10. REFERENCES 47

APPENDIX A DRIFT BOTTLE RECOVERY RECORD 49

APPENDIX B SUMMARY OF SHIP'S DRIFT, WIND DATA,

AND WIRE ANGLES ON OCEANOGRAPHIC STATIONS 51

AN OCEANOGRAPHIC INVESTIGATION ADJACENT
TO CAY SAL BANK, BAHAMA ISLANDS

Robert B. Starr

Forty1seven oceanographic stations
were occupied in the Cay Sal Bank region
of the Straits of Florida to investigate
the water structure in the straits here
and in the entrances to Nicholas and
Santaren Channels. The water exchange
through Nicholas Channel appears to be
negligible; Santaren Channel contributes
water to the northern Florida Straits
below 350-m depth. Evidence of a possible
south-flowing countercurrent in the Straits
of Florida is also presented.

1, INTRODUCTION

An investigation of the physical and geological oceanogra-
phy of the area of the Straits of Florida adjacent to Cay Sal
Bank was conducted from June 20 through 26, 1962. This study
consisted of V7 oceanographic stations and bathythermograph
observations. There were 22 bottom sediment cores taken and
a release of 10 drift bottles at each station. The locations
of the stations are shown in figure 1 and were planned to
investigate the water structure across Nicholas and Santaren
channels, as well as the Straits of Florida to determine the
effect of Cay Sal Bank on this structure and to learn the
nature of the bottom in the area.

The layout and numbering of sections (see figs. 7"ll) are
shown in figure 1.

81°

80°

OCEANOGRAPHIC STATIONS

O NANSEN CAST
— BOTTOM SAMPLE

NAUTICAL MILES
^™ 10 20

KILOMETERS

Figure 1. Chart of station locations, section locations, and
general bathymetry (in meters).

The observations were made from the USC&GSS HYDROGRAPHER
by the writer and Dr. Robert E. Burns, assisted by the
officers and crew. The HYDROGRAPHER was commanded by
Captain Raymond E. Stone.

Ship's navigation was by Loran A, radar, and visual fixes.
The depths of 100 fathoms along the Florida Keys and 200
fathoms elsewhere were used as a secondary control when it
was desired to position stations relative to the banks
bounding the area. Fixes were taken every 15 min while the
ship was on station so that when Loran reception was good or
when the ship was close to land relative positions for deter-
mining the ship's drift were good to one-quarter mile. The
plotted locations of the stations are accurate to 1 n mi,
except when Loran reception was poor well away from land
(see fig. l). The oceanographic stations consisted normally
of 2-8 bottle Nansen casts with modifications for shallow
depths. These were taken starting from the surface to as
near bottom as possible. A 180 lb steel ball was used as
the Nansen cast weight to keep wire angles at a minimum. A
bathythermograph observation to 900 ft or the bottom was
taken at the time of each shallow cast.

All Nansen bottles were equipped with two deep-sea
reversing thermometers, except one had three. Five unpro-
tected thermometers were used on each cast for thermometric
depth determinations. The thermometers had been recently

calibrated at the Naval Oceanographic Instrumentation Center
and were periodically exchanged among the bottles to reveal
malfunctions or erratic operation.

The water samples for salinity analysis were bottled in
aged citrate of magnesia bottles and shipped to Washington,
D. C, where their salinities were determined by dual
analyses on a South African conductive salinometer. Check
analyses on a selected batch of samples were run on a
HYTECH inductive salinometer.

2. PROCESSING AND DISPOSITION OF THE DATA
Processing of the serial oceanograph:' c data included
plotting station profiles of temperature and salinity
against depth, the plotting of individual Temperature-
Salinity (T-S) curves, and the comparison of these against
a composite T-S curve. This composite curve is shown in
figure 2. The verified station data were then transmitted
to the National Oceanographic Data Center, where standard
depth interpolations for the stations were made and the
dependent parameters computed. A machine listing of the
station data was then reviewed, density as sigma-t (cr-jO
was plotted as a check, and hand interpolations of tempera-
ture and/or salinity were made where the machine ones were
unacceptable. The final listings are available from NODC.

35.0

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36.6

37.0

y>^^

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Figure 2. Composite Temperature-Salinity (T-S) Diagram Deoths

NlcWlna:rsrS;h A^ Atlanti° °«r*l »*« shor^iX ked
NACW, and South Atlantic Central Water shown by line marked SACW.

The NODC reference number is 3101+2. Except where questioned,
the depths of the individual observations are considered to
be accurate within 5 in, the temperatures to -0.02°C, and the
salinities to -0.01#c with relative accuracy to 0.003/fc> Where
two protected thermometers were paired, the average was used
in most cases, but where the thermometers differed by more
than 0.05°C, the more reasonable value was used. This some-
times happeded with observations in the steep thermocline,
probably because of different thermal responses of the
thermometers.

The bathythermograph observations taken on the oceano-
graphic stations are available from NODC as cruise number
5296. The individual traces at the station locations are re-
produced in figure 3*

The 28 returns from the ^60 drift bottles released are
listed in appendix A. These amount to a 6.1$ recovery, all of
which came from only 11 of the *+6 locations where bottles were
released. The nine bottles recovered from station 38 account
for 32.1$ of all returns.

Twenty- two bottom sediment cores were taken with 60- and 80-
pound Phleger Corers with 3"foot barrels. Immediately after
recovery, 'the cores were preserved with 5ml of alcohol and
sealed. Their visual physical characteristics were logged,
and they were stored in the ship's refrigerator. These cores
were transferred to Florida State University and used by

24°

NAUTICAL MILES

10 20

KILOMETERS

20 40

Figure 3. Bathythermograms taken at the

oceanographic stations.

Donald Milligan as part of a Master's thesis (Milligan, 1962).

3. SETTING

Cay Sal Bank may be considered an outlier of the Florida-
Bahama Province. This province is not a continuous platform
now but consists of extensive shallow water areas of general-
ly less than 20 m depth transected by narrow, relatively
deep channels of which the Straits of Florida and Santaren
and Nicholas channels are examples. These are shown in
figure 1 with their general bathymetry.

The Cay Sal Bank area is located where the trend of the
Straits of Florida changes from generally east-west to north-
south. This change in conjunction with the bank and its
associated Santaren and Nicholas channels significantly
influneces the Florida Current. Since the current probably
reaches to the bottom in this part of the straits, a
knowledge of the general bathymetry (fig. l) and the con-
trolling sill depths is necessary for understanding the
structure of the current in this area.

Recent Coast and Geodetic Survey soundings in the northren
Straits of Florida have established that depths increase
gradually from a sill of 730 m at Latitude 27°20°N., Longi-
tude 79°3Lt-'W. This southward gradient increases appreciably
west of Cay Sal Bank. The straits are also considerably
wider from this point to the west (fig. l). The sill of the

Yucatan Channel between Cuba and Mexico at 2,100 m (C&GS
Chart 1007) is so deep that it does not restrict the water
properties of the straits.

The relatively wide and shallow Santaren Channel joins
the Straits of Florida northeast of Cay Sal Bank, while the
deeper but narrower Nicholas Channel connects with the
straits southwest of the bank (fig. l). At the southeast end
of Cay Sal Bank, Santaren and Nicholas channels merge into
the Old Bahama Channel that separates the Great Bahama Bank
from Cuba. The sparse sounding data from these channels
indicate that their controlling sill depth occurs in the Old
Bahama Channel and is roughly *+10 m. This channel also is
considerably narrower than either Santaren or Nicholas
Channel (C&GS Chart 1002).

h. OCEANOGRAPHIC BACKGROUND
The water masses occurring in the Cay Sal Bank area are
defined best by referring to the composite Temperature-
Salinity (T-S) curve of the oceanographic stations (fig. 2).
This reveals the admixture of several water masses of diverse
origin. The scatter in the plot to about 75~m depth reflects
the influence of locally generated modifications, particular-
ly a secondary salinity maximum at 50 m, which is apparently
caused by the sinking of relatively dense bank water intro-
duced into the Bahamian and Florida Keys margins by tidal

currents.

Below this surface layer the main salinity maximum of
36.60 to 36. 77 %oa"t an average sampled depth of 150 m com-
prises a relatively thin stratum of water that Wust (196*+)
calls the Subtropical Underwater. This is water that has
passed through the Yucatan Channel from the Caribbean Sea.
It appears likely that the maximum salinity of this stratum
lies closer to 125~m depth, but this level was not sampled
frequently enough for this to be established. The 100-m
salinity of 36.99%>in this layer at station 10 appears
anomalously high, but it has been retained because there
appeared to be no evaporation from the sample bottle and
because there was a 0.07°C. temperature inversion at this
depth established with paired reversing thermometers and the
BT observation. Furthermore, the density determined from
these data did not imply instability. The lower salinities
evident at 150 m occurred in the stations taken in the
center and left-hand side of the Florida Current. These
correspond to the Continental Edge Water of Wennekens (1959),
which he interprets as being derived from the surface waters
of the northern and eastern Gulf of Mexico and having sunk
to their equilibrium level after winter cooling.

Below 300 m, local and seasonal effects disappear from
the composite T-S plot so that from 3°0 to about 700 m the
curve reflects, for the most part, the result of a mixture

of North Atlantic Central Water with some South Atlantic
Central Water. The influence of the South Atlantic Central
Water appears to be most prominent at about 550 m (27*0 a+-
level) where it comprises up to 30% of the water type
(fig. 2), but most of the water in this range is of North
Atlantic origin. In particular, stations 2, 9, and h-2 (see
fig. l) appear to have relatively high percentages of North
Atlantic Water at some levels.

The minimum salinity evident in the T-S curve at roughly
800 m indicates the influence of Antarctic Intermediate
Water. This water, which is also known as Subantarctic
Intermediate Water, is considered to be formed at the
Antarctic Convergence by mixing and sinking of Antarctic
and Subantarctic surface waters. As the resulting water mass
moves north it gradually mixes with adjacent waters so that
by the time it reaches the North Atlantic at about 27° N.
Latitude, the salinity minimum used to trace this water is
gone. Its presence in the Straits of Florida with a value
of 3I+«87 Vindicates an appreciable quantity of water of South
Atlantic origin at about 800 m.

The observations below the salinity minimum show a posi-
tive gradient in the salinity to the greatest depths sampled.
This reflects the presence of Upper North Atlantic Deep Water
with possibly the traces of an admixture of Mediterranean
Water. These depths were attained at only the stations west

of Cay Sal Bank, and were well below the sills to the north
and east. While for any level above the minimum, salinity
increases to the right in the Florida Current, below the
minimum it increases to the left. Temperature apparently
always decreases to the left.

5. TIDES AND TIDAL CURRENTS
Because the channels of the Cay Sal Bank area are rela-
tively restricted, there is an appreciable bathymetric
influence on the currents. Since tides and tidal currents
become amplified in restricted waters, their effect probably
influences the oceanographic station observations signifi-
cantly; consequently, some of the irregularities evident in
the charts and sections of properties may reflect tidal
modification of the water column depending on the time of
the individual station relative to the tidal cycle.

The predicted and observed tides at Key West, the nearest
reference station, were compared for the period of the
oceanographic stations and were in good agreement. - These,
in turn, were compared with the tide predictions nearest the
oceanographic stations at Elbow Cay on Cay Sal Bank and
Tennessee Reef on the Florida Keys side of the straits.
While Key West has a mixed tide, the tide at Elbow Cay and
Tennessee Reef is predominantly semidiurnal. According to
Dietrich (1963) the tide wave progresses upstream against

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the flow of the Florida Current.

Tidal current data in the area are available as pre-
dictions only, and these are restricted to the Florida Keys
side of the straits. In figure h, the predicted tide at
Tennessee Reef is presented with the closest tidal current
predictions at a point east of Long Key Drawbridge and at
Lcng Key Viaduct. The direction of flood current at these
sites is north and of the ebb current, south. Also included
in this figure are the messenger times of the shallow casts
of the oceanographic stations taken adjacent to the margins
of the channels and of those that show the shallow, secondary
salinity maximum.

This secondary maximum has its greatest areal development
along the Florida Keys but is more saline by approximately
0-3%0al°ng "the Bahama Banks. Along the Florida Keys the
salinity gradient indicates a source from the west, but the
gradient along the Bahama Banks indicates a warm saline
tongue with a probable northern source. This is corroborated
by the ship's drift at station 7 (fig* 5) • Station 3 along
the Bahama Banks that would be expected to show the shallow
maximum but did not was outside of this tongue. This
salinity distribution is evident on the 50-m depth chart
(fig. 6). None of the southern stations along Cay Sal Bank
and the Cuban coast show the secondary maximum. This is due
possibly to the phase and speed of the tidal currents during

80°

- if

CUBA

ftitt

rfS^S

CURMNT SPHO SCAIE
0 12 3 KNOTS

10 20
KILOMETERS

81'

30'

80°

30'

Figure 5. Distribution of temperature, salinity, and density at
the sea surface. Arrows are vectors of ship's drift. Zero
indicates no discernible drift.

0 12 3 KNOTS

TEMPERATURE
SALINITY
DENSITY (Sigma-t)

50 METER DEPTH

50 meters=27.3 fathoms
NAUTICAL MILES

Figure 6. Distribution of temperature, salinity, and density at
50 m. Arrows are vectors of ship's drift. Zero indicates no
discernible drift.

the time these stations were occupied (fig. h) , but it is
more likely due to the lack of a shallow area of adequate
size for evaporation to produce high salinities. The higher
average velocity and longer duration of flow of the ebb
current compared to the flood current at Long Key indicate
that the net transport of water is from the Gulf of Mexico
side of the Keys into the Straits of Florida. The water
introduced into the western side of the straits by this net
transport is probably the source of the secondary salinity
maximum found here.

6. OCEANOGRAPHY
The results of the HYDROGRAPHER oceanographic stations are
presented in the accompanying charts and sections of the
distribution of temperature, salinity, and derived
sigma-t (cr+-)*. Isopleths of sigma~t are a convenient ex-
pression for the density of water at surface pressure for a
given temperature and salinity. They approximate the
distribution of potential density very closely and may be
considered as defining quasi-isentropic surfaces. Since
these isopleths indicate the variation of density at any
given level, they are useful for determining the approximate

* The layout and numbering of sections (see figs. 7~ll) are
shown in figure 1.

STATION

46 45 44 43

TEMPERATURE

NUMBER

11 10 5

II I I

J L

Figure 7a. Distribution of temperature (in degrees Celsius) along section 1.

SALINITY
STATION

46 45 44 43

I 1 I

£ 400
ill

i—
ul

z 500

X
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£ 600

J I I L

Figure 7b. Distribution of salinity (in parts per thousand) along section 1.

STATION

46 45 44 43
_L„ I I L

SIGMA - T

NUMBER

11 10 5 4 3

I I I I L

ex 400 -
ui

III

Z 500 -

900 -

Figure 7c. Distribution of density (as Sigma-T) along section 1.

TEMPERATURE

J L

Figure 8a. Distribution of temperature (in degrees Celsius) along section 2.
STATION SALINITY

30 29 24 23

J I I I

z 600

J L

Figure 8b. Distribution o£ salinity (in parts per thousand) along section 2.

£ 500 -
III

■z 600

ui 700
Q

1200

J L

Figure 8c. Distribution of density (as Sigma-T) along section 2.

TEMPERATURE

Figure 9a. Distribution of temperature (in degrees Celsius) along section 3.

SALINITY

20 IS 17

J I L

J I I L

Figure 9b. Distribution of salinitv (in parts p;r thousand) along section 3.

STATION

SIGMA

Figure 9c. Distribution of density (as Sigma-T) along section 3.

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relative velocity of currents that are related to the distri-
bution of mass by applying the rule that, except for near-
surface anomalies, in the northern hemisphere the lighter
water lies on the right side of a current. The relationship
of the water properties and the deduced currents in the
vicinity of Cay Sal Bank to those near the adjacent banks is
seen best on the charts (see figs. 5> 6, 12, 13 & 1^), while
the sections (see figs. 7_ll) illustrate the distribution of
properties and the characteristics of the water masses in
the Florida Straits and adjoining channels. The layout and
numbering of the sections are indicated in fig. 1. These
are drawn at a vertical exaggeration of approximately 120
to 1.

The charts present the distribution of properties and
sigma-t at the surface, 50, 150, 300? and 600 m. Levels
below 600 m are not shown, because greater depths exclude
over h0% of the stations including all of those in Santaren
Channel. The surface and 50-m depth charts include current
velocities derived from the ship's drift on station when the
positioning was considered reliable and wind speeds, for the
most part, were 12 knots or less. These wind speeds were
read from the ship's anemometer. Wind velocities up to 20
knots were accepted on stations where the ship drifted into
the wind or where the ship was in the high velocity core of
the Florida Current. The current and wind velocity data for

CUBA

i*&

Jffr*%

KILOMETERS

81'

30'

80°

30'

Figure 12. Distribution of temperature, salinity, and density

at 150 m.

25°-

TEMPERATURE
SALINITY
DENSITY ISigmo-t)

300 METER DEPTH

300 meters = 164 fathoms
NAUTICAL MILES

Figure 13. Distribution of temperature, salinity, and density

at 300 m.

Figure 14. Distribution of temperature, salinity, and density

at 600 m.

the acceptable stations are summarized in table form in
appendix B with the wire angles and directions of the oceano-
graphic casts.

7. THE SURFACE CIRCULATION

In the central portion of the Straits of Florida the high
drift velocities found correspond to water temperatures
above 28.0° C, except for station 20 (see fig. l) which
appears to be influenced by upwelling (fig. 5)- The distri-
bution of surface salinity does not correlate as well with
the deduced current, except perhaps for a low salinity tongue
associated with a southeastward drift in the entrance of
Santaren Channel. Here low salinities at stations 6, 7 and
10 (see fig. 1 and 5) are possibly analogous to those associ-
ated with the high velocitycore of the stream, but at these
stations they seem to coincide with a zone of transition
between Straits of Florida waters and those found in Santaren
Channel. This zone of transition is defined by the 36.2%c
salinity band on the surface chart (fig. 5) and is apparent
in the near-surface layering evident in the BT traces at
these stations (fig. 3)-

In Santaren Channel a thin film of slightly warmer surface
water is apparent at the southern stations along the Bahama
Banks side, and there is an appreciable salinity increase
toward the banks as well (see fig. 7 and fig. 5) • A much

more varied temperature and salinity distribution is evident
here than in Nicholas Channel. Along the western side of
the Florida Straits, the cause of a relatively low salinity
at station hO (fig. 5) is unknown but it is not from rain or
contamination from ship overboard discharge.

A decrease in velocity near Cay Sal Bank in the Straits of
Florida is apparent in the reduced drift at stations 19 and
22 and by the absence of a detectable current at station 16,
while a northwesterly drift was found at station 15 (see fig.
1 and 5) • These stations were all within visual bearing and
radar range of the cays on Cay Sal Bank. The high surface
density at station 19 and configuration of the adjacent
isopycnics suggest upwelling, which appears on section h
(see fig. 10) to be from a depth of about only *+0 meters.
This probably reflects a damming effect on the subsurface
high velocity core of the Florida Current by Cay Sal Bank.

The ship's drift, surface temperature and salinity, and
density structure evident in the western entrance to Nicholas
Channel agree, in that, except for a possible southward drift
at the three eastern stations north of Cuba (fig. 5)> little
appreciable surface current appears to exist here. This
southward drift may be the eastern side of a weak anti-
cyclonic gyre. Santaren Channel, however, as noted previous-
ly, does not seem to be quite so passive. Here the velocity
decrease indicated by the reversal of slope of the near

surface isopycnics along the Bahama Banks extends to a depth
of only about 20 m, as is evident in sections 1, 2, and 3
(see figs. 7, 8, and y) . The associated surface salinity
gradient is the result of high salinities generated on the
bank in the lee west of Andros Island (Cloud, 1962). The
low salinity tongue with corresponding relatively low
temperatures and resulting 23 •*+ isopycnic extending north-
east from Cay Sal Bank (fig. 5) not only separates the
northerly flow of Santaren Channel from the Florida Straits
water but appears to represent a zone of mixing and a path
of southerly flow toward Cay Sal Bank. Unfortunately most
of the ship's drift observations in this area were unaccepta-
ble, but those at stations 1, 2, and 3 (see figs, 1 and 5)
support the interpretation of a slow northerly flow decreas-
ing in velocity toward the banks, while observations at
stations 7> 15> l*+> and 13 indicate that this zone may be
the eastern side of a gyre between the Florida Current and
the Santaren Channel outflow (fig. 5) • This inference is
also upheld by the sigma-t structure in sections 1, 2, and
3 (see figs. 7> 8, and 9) which indicates that this circu-
lation is more than 100 m deep.

8. THE SUBSURFACE CIRCULATION
The 50-m depth was chosen as one of the levels to show
the distribution of properties because its proximity to the

surface permits correlation with the drift measurements and
it reflects the oceanographic conditions associated with the
highest current velocities, but it is deep enough to be out
of the influence of short term meteorological factors. In
addition, this is the level of most of the secondary salinity-
maxima and is near the top of the high gradient of the
pycnocline. It can be seen on the composite T-S plot (fig. 2)
that his level posseses considerable variability in the T-S
relationships.

The Straits of Florida at the 50~m level exhibit a pattern
considerably different from the surface chart in detail but
similar in general features. The core of the current at this
depth appears to be delineated by a sigma-t of 23*8, which
hugs the Cay Sal Bank side of the channel in the vicinity of
Nicholas Channel but trends toward the western side of the
straits north of Cay Sal Bank (see fig. 6). The distribution
of the temperature, salinity, and sigma-t isopleths along the
Florida Keys side of the straits reflects the effect of the
influx of water from the Gulf of Mexico with its associated
secondary salinity maximum.

The sources and characteristics of these maxima have
already been discussed. The absence of this feature at
station 3 along the Bahama Banks reflects the dominance of
the northward-moving Santaren Channel flow at this point over
the Florida Straits counterflow, which carries the high

salinities generated on the Bahama Banks in the lee of Andros
Island, as previously indicated. This Santaren Channel
water, which is colder at 50 m than the water to the north,
appears to have its core at station 5 (see fig. 6). As it
travels north it mixes with the warmer, more saline south-
ward-flowing Bahama Banks water and becomes part of the anti-
cyclonic eddy suggested in the discussion of the surface
circulation. This pattern is defined by the 25° c« and
2lf.6 sigma-t isopleths in sections 1, 2, and 3 (see figs. 7,
8, and y) . The high salinity found at the 100-m level of
station 10 may be a remnant of the secondary salinity
maximum caused by tidal currents, a salinity maximum that
has descended as it moved southwestward in the eastern arm
of the gyre.

In the western entrance to Nicholas Channel at the 50-m
level a slight penetration of the Florida Current appears as
the anticyclonic eddy mentioned previously. This extends to
about 100 m depth (see fig. 8). It is well defined by the
2'?. 0° C. temperature and 23*8 sigma-t isopleths and shows up
also as a core of minimum salinity (fig. 6). The available
reliable drift determinations at stations hk, h-5 , ^6 and 30>
shown in figures 1 and 6, are in agreement with this. As
far as the water budget in the area is concerned, this gyre
does not appear to be particularly significant, and ap-
parently very little net water transport is occurring in

Nicholas Channel above 100 m.

The average depth of the main salinity maximum, and
therefore the core of the Subtropical Underwater, is close
to 150 m. For this reason and because 150 m is in the lower-
part of the high gradient of the main pycnocline, this level
was chosen to display the distribution of properties. Al-
though this is the depth of the salinity maximum, there is
still a considerable spread evident in the composite T-S
curve (fig. 2). The lower salinities at 150 m evident in
this plot result from an admixture of Gulf of Mexico water
described previously.

In the Straits of Florida the 150 m level is the first to
show a sinuous pattern in temperature, salinity, and sigma-t
(see fig. 12) that continues to deeper levels. The vertical
displacement of the 20° isotherm equivalent to the meander
evident on this level is approximately 1+5 m, and that of
sigma-t, 30 m, neither of which are excessive amplitudes for
internal waves or tides. However, the fact that the
disturbance is uniform across the channel suggests that it
is not caused by short-period fluctuations. This pattern
does seem to be associated with a marked change in trend of
the depth contours along Pourtales Terrace, as shown by
Jordan et al. (196*+) and as evidenced by the 600- and 800-
m isobaths of figure 1. Sections k and 5 (see figs. 1, 10,
and 11) cross the straits immediately downstream from where

it changes in direction from east-west to northeast-south-
west. These sections also follow the maximum constriction
in width of the straits, at the surface by Cay Sal Bank to
roughly 65%, and at 600 m by Pourtales Terrace and Cay Sal
Bank to approximately 30% of what it was west of the bank.
It is evident from the trend of the plotted variables at
150 m and other depths that the Florida Current is already
flowing toward the northeast before it reaches sections h
and 5 (see fig. 12). This is probably due to the hydraulic
block caused by Cay Sal Bank and the relatively inert water
of Nicholas Channel. Because these changes in the configu-
ration of the straits are upstream from the two lines of
stations, the undulation evident in the properties is most
probably an expression of the accommodation of the Florida
Current to the change in direction and to the widening of
the channel downstream from the constriction.

The extreme gradient evident in the properties at 150 and
300 m along the Florida Keys (see fig. 12 and 13 ) is not
apparent in the 50-m or 600-m charts. The depth interval
between 150 and 300 m includes most of the area of Pourtales
Terrace (see figs. 1, and 11). The gradient in the water
properties may be due in part to their accommodating them-
selves to the transverse component of the sinuous wave as it
interacts with the shoaling surface of Pourtales Terrace,
but there appears to be another factor operating as well:

Below 150 m, the water at oceanographic stations 39 and ^0
was isothermal, isohaline, and of indifferent stability.
The bathythermograms obtained at these stations (fig. 3)
illustrate the isothermal structure very well. The depth
range of 150 to 300 m is the depth at which the principal
artesian aquifer (Floridan) of Florida is expected to out-
crop in this area (Stringfield and LeGrand, 1966 ; Kohout,
1967). It is therefore suggested that the strong gradients
evident in the charts and the absence of isopleths near the
bottom at stations 39 and *+0 in sections h and 5 (fig* 10
and 11) respectively reflect the introduction of artesian
water into the Straits of Florida.

This possibility has been suggested by Stringfield and
LeGrand (1966). From Kohout ' s (1967 ) interpretation of the
hydrology and thermology involved, it is impossible to judge
whether relatively fresh artesian water is expected to be
relatively warm or cold. Jordan et al.(l961+) present bathy-
thermograph sections obtained by the HYDROGRAPHER in the
area in 1953* These give a more detailed thermal picture of
this phenomenon. The unusual layer at station 39 is colder,
fresher, and less stable than that at station ^0. This
suggests that the source of the anomalous water is closer to
s cation 39 and is being absorbed as it moves northeast with
the Florida Current. The bathythermogram obtained at
station 38 on Pourtales Terrace also shows this structure

from 530 feet (160 m) to the bottom. Unfortunately the
Nansen cast observations at depth differ from the BT temper-
atures because biological fouling of the oceanographic wire
required the bottom bottles to be relowered; meanwhile the
ship drifted out of the immediate area of the BT cast. It
should be noted that this is the only BT observation (see
fig. 3) that shows appreciable hysteresis in the thermocline
between the times of lowering and raising the bathythermo-
graph. The particular area of Pourtales Terrace over which
station 38 was begun is one of Karst topography (Jordan et aL
196^-), so that the anomalous water found there could possibly
have originated from a sink hole.

At 150 m both Nicholas and Santaren channels appear to
lack any appreciable circulation (fig. 12). In Nicholas
Channel the isopleths of the physical properties and sigma-t
suggest a possible slight inflow along the Cuban coast. At
150 m in Santaren Channel the anticyclonic eddy described
previously apparently dominates the entire entrance to the
channel and probably blocks any effective circulation
between the channel and the Straits of Florida at this level.

The next depth chosen to show the distribution of proper-
ties is at '300 m. This is below the steep gradient of the
pycnocline and is approximately the level at which the
curves of the composite T-S plot (see fig. 2) converge,
indicating that external influences are minimal and that the

water masses here and below have uniform properties except
for several special cases. The 300-m chart (see fig. 13)
is the first to show an appreciable reduction in the width
of the Straits of Florida other than that caused by Cay
Sal Bank itself. This contraction is due principally to
the presence of Pourtales Terrace along the Florida Keys.

At 300-m, the structure of the physical properties of
the Straits of Florida is very similar to that at 150 m.
The isopleths of properties are still quite closely packed
and continue to trend northeast-southwest, while the
sinuous pattern they display persists in the same region
and at about the same magnitude that it did at 150 m.
Cay Sal Bank station 19 shows an anomalously low salinity
at this level on the composite T-S plot, (fig. 2). This
low salinity possibly results from upwelling caused by
blockage of the Florida Current by the northern flank of
Cay Sal Bank.

Nicholas Channel, at 300 m, exhibits slight evidence in
section 1 (see fig. 7) of a possible weak anticyclonic eddy
that appears to exist from this depth to the bottom. Since
the sill depth at the eastern end of Nicholas Channel is
approximately ^10 m, this gyre probably has no connection
there but is driven by a segment of the Florida Current that
is deflected into Nicholas Channel by the steep western end
of Cay Sal Bank. There is an indication of this deflection

in the trend of the isopleths of temperature and salinity in
the northern half of the entrance to Nicholas Channel on the
300-m chart.

In Santaren Channel, the 300-m depth appears as part of a
transition between the distribution of properties above and
below. According to these (fig. 13) the core of the anti-
cyclonic eddy that has dominated the upper layers of the
entrance to the channel seems to be located at station 6.
The last trace of this gyre apparently dies out at about
350 m at this station (sec. 2, fig. 8) and at approximately
this same depth in sections 1 and 3 (fig* 7 and 9). At 300
m the isopleths of properties and sigma-t indicate a
probable northwesterly flow into the Straits of Florida
between the eddy and the northeastern end of Cay Sal Bank.

Between 350 and 550 m, the northwesterly flow appears to
become more northerly and to occupy the entire width of
Santaren Channel. This is evident in sections 1, 2, and 3
where the reversal in gradient along the eastern half of
the channel and steepening of the isopleths, particularly
that of sigma-t, indicate a zone of high shear that suggests
a probable considerable outflow reaching close to the bottom.
Apparently- the major contribution of water from Santaren
Channel to the Florida Current occurs in this interval.
This outflow of water is defined best by its range of
properties rather than an average depth range. These are

temperatures between 17*5 and 10.5° C., salinity between

3 5» 3 and 36. h%>, and sigma-t between 26.5 and 27*1. A

comparison of these values with those found in the Straits

of Florida between the average levels of 350 to 550 m is

shown in table 1.

Table 1. Comparison of Physical Properties
of Florida Straits and Santaren Channel Waters

Temperature Salinity Sigma-t
(° C.) (%)

Santaren Channel 17-5 - 10.5 36 A - 35-3 26.5 - 27-1
Straits of Florida l!+.5 - 09.0 35-8 - 35-1 26.7 - 27-2

It is clear that at these depths the water in Santaren
Channel is about 2.0° C. warmer, 0.3%c more saline, and 0.2
sigma-t units less dense than in the Straits of Florida.
This flow extends beyond section 3 (fig* l) and obviously
contributes to the flow along the eastern side of the
Florida Current along the Bahama Banks. The markedly
increasing temperatures and salinities to the north and east
at any given level apparently result from the Santaren
Channel water accommodating itself to the increasing depth
as it moves northward into the Straits of Florida.

In the core of this flow, station 9 deviates from the
composite T-S curve (see fig. 2) in most of the observations
below ^-00 m because of a consistently high salinity anomaly

of about 0.1%cor low temperature anomaly of about 0.5° C.
At this station, the values at h of the 5 depths sampled
from ^-00 m to the bottom of the cast produced this deviation
so that there can be little question as to the validity of
the observations. The resulting T-S curve is very close to
that of North Atlantic Central Water (Sverdrup et al., 19*+2),
except for the observation at 558 m that agrees with the
composite T-S curve.

The deepest level chosen to be charted is that at 600 m.
Below this, at 700 m, the bottom excludes almost 50% of the
stations, including all of those in the entrance to Santaren
Channel. The reduction in area of the straits between Cay
Sal Bank and the Florida Keys relative to the upper levels,
and to the straits to the west and north is very evident on
the 600-m chart (fig. ih) . This level is well down in the
intermediate gradient of the pycnocline and is about 200 m
above the salinity minimum.

In the Straits of Florida, the northeast trend paralleling
the isobaths and the curvature of the isopleths is as evident
here as at the upper levels (fig. ih) . In sections h and 5
(see fig. 10 and 11) it can be seen that the cross-stream
gradients of the properties and sigma-t from about 500 m
to close to the bottom intensify markedly. The increased
shear that this intensified gradient implies suggests an
increasing velocity to compensate for the narrowing and

shoaling of the straits.

It has been reported that there is a south flowing
counter-current along the "bottom of the Straits of Florida
(Hurley and Fink, 1963). Because of the extreme difficulty
of sampling the water immediately adjacent to the bottom in
the high velocity core of the Florida Current, it is diffi-
cult to assess the oceanographic regime in this very
critical part of the water column. However, in section 5
(see fig. 11) where the sampling was deepest, the 27*6
sigma-t isopleth does indicate a decrease in cross-stream
gradient. Possibly of more significance, however, is that
from 600 m to the bottom of the cast, the water at station
k-2 (fig. 1 and 1*+) is significantly colder at a given level
than at any other station and that at 500 m, which is just
above the maximum depth sampled by station 37? "the cold
water at station \2 relative to station 37 reverses the
sigma-t gradient. On the T-S plot (fig. 2) station h-2 is
the line to the right of the main group of curves between
600 and 800 m and is very similar to North Atlantic Central
Water. Whether this cold water and reversed gradient are
indicative of a southerly flow along the continental slope
here is conjectural, but a southerly flow along the slope
to the north has been reported (Stewart, 1962). In addition,
from 700 m to the bottom of the casts, the salinity, which
above this normally decreases to the left of the Florida

Current at any given level, consistently increases from the
middle of the channel to the Florida Keys side of the
straits. There is not, however, a cross stream reversal of
the density gradient, as described for stations 37 and h2.

In the entrance to Nicholas Channel, the pattern of the
isopleths on the 600-m chart (fig. l^f) clearly suggests an
anticyclonic gyre. This appears to extend from the 300-m
level to the bottom, which, at the entrance to the channel
is slightly over 100 m.

In Santaren Channel, the 600-m level is very close to the
bottom. Consequently the isopleths of the properties and
sigma-t reflect the influence of the bathymetry, particu-
larly a midchannel ridge. A relatively warm, saline tongue
of water appears to be moving north and sinking along the
Bahama Banks side of the channel, as it does at shoaler
depths (fig. ih) . Most of it enters the Straits of Florida,
but some appears to recurve southward to the west side of
the ridge. The final destination of this south flowing
water is unknown, but it has properties intermediate between
the north flowing tongue and the Straits of Florida water
with which it is mixed.

9. SUMMARY AND CONCLUSIONS
Forty-seven oceanographic stations taken in the vicinity
of Cay Sal Bank across the Straits of Florida, Nicholas, and

Santaren channels provide an insight into the effects of
these features and the change in direction of the straits on
the Florida Current. The deep, relatively narrow Nicholas
Channel appears to have very little net circulation. An
anticyclonic eddy, driven by the Florida Current, occupied
its entrance. The Florida Current changes from east flowing
to northeast flowing before it reaches Nicholas Channel.

The structure of the Florida Current indicates that along
the Cay Sal Bank side between 200 and 500 m, the velocities
probably are much less than above and below. Along the
Florida Keys side, overlying the Pourtales Terrace, the
structure of the water column near the bottom indicates the
possibility of artesian water seeping into the straits from
sink holes in the terrace. East of the terrace there is
some indication of a possible south 'flowing undercurrent
along the bottom on the Florida Keys side of the straits.

Santaren Channel, from the surface to 300 m, appears to
be blocked by an anticyclonic eddy driven by the Florida
Current. There appears to be a considerable net transport
into the straits below this to the bottom.

10. REFERENCES

Cloud, P. E., Jr. (1962), Environment of calcium carbonate
deposition west of Andros Island, Bahamas, U.S. Geo-
logical Survey Professional Paper 350? 138 p.

Dietrich, G. (1963), General Oceanography, ( Interscience
Publishers, John Wiley and Sons, New York, 588 p.).

Hurley, R. J., and L. K. Fink (1963), Ripple marks show
that countercurrent exists in Florida Straits,
Science, 139, 603-605.

Jordan, G. F., R. J. Malloy and J. W. Kofoed (I96*f),

Bathymetry and geology of Pourtales Terrace, Florida,
Marine Geology, 1, 259-287-

Kohout, F. A. (1967), Ground-water flow and the geothermal
regime of the Floridian plateau, Trans, of the Gulf
Coast Assoc, of Geol. Soc. 1£, 339~351+.

Milligan, D. B. (1962), Marine geology of the Florida

Straits, M.S. Thesis, Florida State University, 120 p,

Stewart, H. B., Jr. (1962), Oceanographic cruise report

USC&GS Ship EXPLORER-1960, (U.S. Government Printing
Office, Washington, D. C), 162.

Stringfield, V. T. and H. E. LeGrand (1966), Hydrology of
limestone terranes in the coastal plain of the south-
eastern United States, Geol. Soc. Amer. Special Paper
93, ^6 p.

Sverdrup, H. U., M. W. Johnson and R. H. Fleming (19^2),
The Oceans, (Prentice-Hall, New York, 1087 p.).

U.S. Coast and Geodetic Survey (1967), Hydrographic Chart:
Straits of Florida and Approaches, C&GS 1002.

U.S. Coast and Geodetic Survey (1968), Hydrographic Chart:
Gulf of Mexico, C&GS 1007 .

Wennekens, M. P. (1959), Water mass properties of the

straits of Florida and related waters, Bull. Marine
Science of the Gulf and Caribbean, 9, 1-52.

Wust, G. (196*+), Stratification and Circulation in the

Antillean-Caribbean Basins (Columbia University Press,
New York, 201 p.) .

48.

APPENDIX A
DRIFT BOTTLE RECOVERY RECORD

Release

Recovery-

Sta.

No.

Latitude

Longitude

Date

Latitude

Longitude

Date

Days
Before
Found

J-

une 1962

2i+°38'N

80°^-5'W

25°00'N

80°31'W

2^°30'N

80ol+0IW

26°l1flN

80°05'W

23°55'N

80°52'W

26°52'N

80°03'W

23°55'N

80°52'W

26°^9'N

80°02'W

23°55'N

80°52'W

26°52'N

80°03'W

2If°30'N

80OIfO'W

26°21»N

80°0^'W

2If°30'N

80°^0'W

26°03'N

80°07'W

2I+°30'N

80°^fO'W

26°1I+,N

80°05'w

2I+°30'N

80°^0'W

26°09'N

80°06'W

2I+°30'N

80°^0'W

26°20'N

80°0I+'W

30
July

21+°38'N

80°^5'W

25°00'N

80°31'W

21+°30'N

80OI+0'W

260¥+'N

80°02'W

2Lf°38'N

80oif5,w

21+°56'N

80°37'W

2M-°30'N

80o[f0'W

27°15'N

80°13'W

2I+°30'N

80oLfO'W

27°09'N

80°09'W

2I+°38'N

8ooI+5,w

25°00'N

80°31'W

23
Oct.

±5

23°2Lf'N

80°31+,W

29°55'N

8l°17'W

110

21+°21'N

79°38'W

2lf°lf3»N

77°i+7'W

129

APPENDIX A (cont.)

Release

Recovery

Sta. Latitude Longitude
No.

Date

Latitude Longitude

Date

Days
Before
Found

1+5 23°21+,N 80°31+?W 2h 22°15'N 77°lf9»W

6 21+°25'N 79°28'W 21 23°0^f'N 71+051^

IfO 2I+°IfO,N 80°38'W 22 25°25'N 80°19'W

28 23ol*VN 80°50»W 26 32°20'N 6^°1+1,W

13 21+°29'N 79°^f8'W 21 25°11+»N 78°08'W

33 23oLfl'N 8l°05'W 25 39°05'N 28°03'W

1+0 21+°I+0'N 80°38'W 22 25°16'N 80°l8»W

2 2^f°28'N 79°19'W 20 59°20'N 6°00'E

1+0 2^oI+0'N 80°38'W 22 25°29'N 80°20"W

1+0 2^*0^ 80°38'W 22 25°29'N 80°20'W

Nov.
6

1963
Jan.

1
Mar.
20
Apr.

2
Sept.

8
Oct.

if
Dec.

1
V)6k
Oct.

19
Nov.

Dec.
6

135

19^

271

280

¥*3

^66

556

851

918

925

APPENDIX B
SUMMARY OF SHIP'S DRIFT, WIND DATA, AND
WIRE ANGLES ON OCEANOGRAPHIC STATIONS

Sta. Current Wind Wire Wire

No. Speed Direction Speed Direction Angle Direction

(knots) (deg. T) (knots) (deg. T) (degrees) (deg. T)

—

180

220

0.7

007

200

205

3, 17

1.5

12>k

180

1.2

033

160

180

li+

1.3

009

135

155

1.9

302

150

—

1^0

—

1.0

0^1

135

185

1.2

070

120

—

3.5

0^9

100

125

2.5

028

090

110

1.5

039

090

085

070

3-8

066

090

080

3-2

060

090

205

303

090

100

0.5

286

090

095

APPENDIX B (cont.)

Sta. Current Wind Wire Wire

No. Speed Direction Speed Direction Angle Direction
(knots) (deg. T) (knots) (deg. T) (degrees) (deg. T)

2.7

okh

110

—

1.8

038

110

—

0.8

093

077

0^0

0.7

101

050

oh 5

1.0

057

090

—

2.6

0^-8

090

0.25

—

2.2

028

120

—

032

115

230

3.5

051

135

185

2.3

0^+2

120

180

120

110

1.0

215

120

110

0.6

197

090

030

0.5

210

075

060

52 GpO 859-256

PENN STATE UNIVERSITY LIBRARIES

ADQ0Q72D2D03

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