Characteristic features of the Florida Current.

NPS ARCHIVE >\
1967 j{
CLAUSNER, E. |

liiiiiiiiiiiii

v: ;1 'ERISTIC FEATURES OF
THE FLORIDA CURRENT

K.DVVAUD CLAUSNER, JR.

LISKAi\J

NAVAL POoioKADUATE SCHOOL
MONTEREY. CALIF. 93940

THE UNIVERSITY OF MIAMI

CHARACTERISTIC FEATURES OF
THE FLORIDA CURRENT

BY
Edward Clausner , Jr.

A THESIS

Submitted to the Faculty

of the University of Miami

in partial fulfillment of the requirements for

the degree of Master of Science

Coral Gables , Florida
May, 1967

GLAVSN£fc,g,.

LIBRARY

NAVAL POSTGRADUATE SCHOOL

MONTEREY, CALIF. 93940

THE UNIVERSITY OF MIAMI

A thesis submitted in partial fulfillment of

the requirements for the degree of

Master of Science

Subject

Characteristic Features of
the Florida Current

Edward Clausner, Jr.

ABSTRACT

Characteristic features of the velocity and temperature fields
in the Florida Current are isolated and discussed on the basis of
time-averaged free instrument data obtained during approximately 40
transects across the current at four separate sections during 1965-
1966. The sections, from Marathon - Cay Sal Bank to Ft. Pierce -
Matanilla Shoal, encompass a 225 km downstream distance within the
Florida Straits.

The dominant feature isolated is a downstream acceleration of the
subsurface current, associated with the convergent downstream cross-
sectional area of the Straits. A uniaxial surface current and the
cross-stream component of the subsurface current are also intimately
linked with the convergence and divergence of the cross-channel width.
The mass field adjusts to changes in current velocity as would be
anticipated from Bernoulli's Equation. Comparison of the results
of the free instrument technique with previous time- averaged data
taken in this area shows close agreement.

ACKNOWLEDGEMENTS

This is the first report of the results of data obtained by the
free instrument technique encompassing two years of experiments in the
Florida Straits. However, it is due to the kindness and interest of
Dr. William S. Richardson, Professor of Oceanography, and his asso-
ciates that I was permitted to utilize the total data and to assist in
the planning and execution of a small portion of this work. I am
particularly indebted to Dr. William J. Schmitz, Jr., for showing me
the fine structure of scientific research and for changing my fathoms
to meters, my knots to cm/sec, and my spin to vorticity.

I also want to thank the rest of the Southeastern Massachusetts
Technological Society; Fred White, who has forgotten more about boats
than I ever knew, and Angelo Cangiamila, who made the blueprints come
to life. In addition, I would like to express my appreciation to
Fred Koch for assisting in the computer programming, and to the young
ladies who punched innumerable IBM cards and typed the many drafts of
this thesis; Ann Calvert, Elaine Hallett, Ann Dolney, and Car la
Cangiamila.

Many thanks are also in order for the members of my thesis
committee for their constructive guidance in the preparation of this
thesis: Dr. William S. Richardson, Dr. William J. Schmitz, Jr.,
Dr. Leonard J. Greenfield, Dr. Eugene F. Corcoran, Dr. Russel L. Synder
Dr. Edwin S. Iverson, and Dr. Saul Broida.

Finally, I wish to convey my gratitude and respect to my

iv

running-mate, LT Edward J. O'Brien, III, USN, whose moral, mental, and
physical support during the last two years was invaluable.

Support for this work was provided by the Office of Naval Research

CDR Edward Clausner, Jr., USN

Coral Gables , Florida
May, 1967

TABLE OF CONTENTS

Page

LIST OF TABLES vii

LIST OF FIGURES viii

I. INTRODUCTION 1

II. THE EXPERIMENT 5

A. Method 5

B . Sampling Requirements 5

C. Field Program 11

D. Data Analysis 11

E. Errors 14

III. RESULTS 16

A. Discussion 16

B. Comparison with Other Methods 40

IV. SUMMARY 52

LITERATURE CITED 54

vi

LIST OF TABLES

TABLE Page

I . The Field Program 12

II . Positioning Cons tants 13

III. Basic Data for Section 1 17

IV. Basic Data for Section II 18

V. Basic Data for Section III 19

VI . Basic Data for Section IV 20

vii

LIST OF FIGURES

FIGURE Page

1. Section Locations 3

2. Station Location and Drop Spacing

for Sections I and II 7

3. Station Location and Drop Spacing

for Sections III and IV 9

4. Downstream Surface Current (Vs) vs Cross-Stream

Distance for Sections I and II 21

5. Downstream Surface Current (Vs) vs Cross-Stream
Distance for Sections III and IV 23

6. Smoothed Downstream (V) Isotachs for

Sections I and II 25

7. Smoothed Downstream (V) Isotachs for

Sections III and IV 27

8. Smoothed Cross-Stream (U) Isotachs for

Sections I and II 29

9. Smoothed Cross-Stream (U) Isotachs for

Sections III and IV 31

10. Isotherm Depths for Sections I and II 33

11. Isotherm Depths for Sections III and IV 35

12. Comparison of Free Instrument Surface and
Subsurface Current Data at Section II

with Current Meter Data 41

13. Comparison of Free Instrument Surface Current

Data at Sections I and II with GEK Data 44

14. Comparison of Free Instrument Mass Field Data

at Sections I and IV with Hydrographic Data 48

15. Comparison of Free Instrument Mass Field Data

at Section II with Time-Averaged Hydrographic Data 50

I. INTRODUCTION

The purpose of this thesis is to isolate and discuss certain
characteristic features of the Florida Current on the basis of data
obtained by direct measurement using the free instrument method
(Richardson and Schmitz, 1965). From six to twelve transects were
made along each of four sections across the current encompassing a
225 km downstream distance within the Florida Straits (Figure 1) „
Within this segment of the Florida Current, the surface and sub-
surface structure and isotherm distribution will be presented and
discussed with particular emphasis on their adjustment to the
changing downstream geometry of the Florida Straits. The data has
been time-averaged to keep tidal influences to a minimum in approxi-
mating steady state conditions, and is the first detailed description
of the results of free instrument measurements.

Although there is extensive literature on the Florida Current,
this paper represents a considerable addition to the previous work
done in this area. Undoubtedly, the most thorough investigation of
the Florida Current by any method was made by Pillsbury (1890).
However, the majority of information obtained in the past has been
made by indirect methods, such as the GEK (geomagnetic electro-
kinetograph) and geostrophic interpretations based on hydrographic
data. In addition, due to the nature of the current fluctuations,
previous attempts to approximate steady state conditions have been
biased in inverse proportion to the time and number of samples taken.

A comparison will be made between the results of the free instrument
method and results obtained by current meter measurement (Pillsbury,
1890), the GEK (Webster, 1961), (Murray, 1952) and (Chew and Wagner,
1956) , and hydrographic data (Chew and Wagner, 1956) , (Worthington,
1966) and (Broida, 1962a, 1962b, 1963 and 1964),

The technique used to obtain the data in this thesis will be
referred to as the "free instrument method", and yields a direct
measurement of the vertically averaged current over a water column,
Preliminary evaluation of the free instrument technique and its
applicability to the Florida Current was conducted in 1964
(Richardson and Schmitz, 1965). This pilot data clearly indicated
the existence of well defined features of the mass and velocity
fields of the Florida Current. Based on this pilot data, a series
of experiments were devised and conducted in 1965-1966 in order to
observe both the cross-stream distribution and downstream changes
of mass and current field structure over the greater portion of the
Florida Current.

Figure 1. Section Locations

mataniula^---^

.., SHOAL Lr'-->^

II. THE EXPERIMENT

A. Method

The free instrument technique yields the magnitude and direction
of the vertically averaged current and the transport per unit width
of a water column from measurements of run time, depth, and hori-
zontal deflection of a freely falling instrument „ A thorough
description of this method has been presented by Richardson and
Schmitz (1965).

Briefly, an instrument falls freely to a preselected depth,
where ballast weights are released, and then returns to the surface
under its own buoyancy. If the horizontal displacement (X) of the
instrument from drop to surfacing is known, together with the
elapsed run time (t) and the depth (h) to which the instrument
travels, then the vertically averaged velocity over the depth of
flight is — and the transport per unit width to h is — . In
addition, a 16mm camera housed within the instrument takes time-
lapse photographs of a thermometer and pressure gauge at about every
20 meters of depth, yielding a vertical temperature distribution
for each drop.
Bo Sampling Requirements

Downstream section spacing between Sections I and IV was
dictated by a desire to observe the Florida Current over varying
topography and cross-sectional area on the maximum downstream scale
possible within the confines of the Florida Straits and the

limitations imposed by the political situation to the South, The
location of Sections II and III was chosen in order to study a
convergent sector of the channel in isolation.

Cross-stream station spacing was determined by the results of
pilot data and previous work in the Florida Current, Station spacing
of 10 km in the anticyclonic zone was considered adequate and a
5 km spacing was used in the cyclonic zone since cross-stream varia-
tions in this region are about twice those in the anticyclonic zone.
Abrupt shear zones of 3-5 km were expected on both edges of the
channel .

Vertical sampling consisted of dropping one to six instruments
at each station in each cross-section, depending upon depth. One
bottom drop was made at each station. In Sections II and III,
instruments were dropped to the expected depth of the layers of
vertically averaged sigma-t of 25 and 26, In Sections I and IV,
instruments were more closely spaced and dropped to standard depths
in order to obtain a more detailed picture of the velocity structure
of the current. Station and drop spacing for each Section is shown
in Figures 2 and 3.

Time sampling was determined by the speed of the research
vessel, the limitation of being able to operate only in the daytime,
and the requirements for random sampling over periods of significant
fluctuations of the mean current. Based on pilot data accumulated
over a nine-month period in 1964-65, the dominant time variations
appeared to be of tidal character. Therefore, an attempt was made
to minimize rectification from tidal time scales. The previous method
of sampling each station on every transect was abandoned in Sections I

Figure 2. Station Location and
Drop Spacing - Sections I and II

SECTION I SAMPLING DEPTHS

KM CROSS-STREAM ►

10 20 30 40 50 60 70 80 90 100 110 120

SECTION H SAMPLING DEPTHS

KM CROSS -STREAM ►

10 20 30 40 50 60 70 80 90 100 110 120

Figure 3. Station Location and
Drop Spacing - Sections III and IV.

SECTION HI SAMPLING DEPTHS

KM CROSS- STREAM ►

6 10 20 30 40 50 60 70 80 90 100 1 10 120

0 -i^— ®-®-<g>-«>-

SECTI0NTJ7 SAMPLING DEPTHS

KM CROSS-STREAM ►

0 10 20 30 40 50 60 70 80 90 100 110 120

0

100

200

300

w400
a:

LU

^500
jE 600

Q.
Q

700
800
900
1000
1100

ll

and IV due to the speed and handling limitations of R/V AUSTAUSCH.
C. Field Program

Observations were made along two sections completed in 1965^
Miami, Florida to Bimini, B.W.I, (Section III), and from just south
of Fowey Rocks to Cat Cay, B.W.I, (Section II), and two sections
completed during the period June-August 1966; Marathon (Vaca Key) ,
Florida to Cay Sal Bank, B.W,I, (Section I), and from Ft„ Pierce,
Florida to Matanilla Shoal on the N.VL corner of Little Bahama Bank
(Section IV), see Figure 1. It was planned to sample each section
over a period of a month. However, Section I was interrupted for a
week due to bad weather and Section IV was terminated 13 days early
due to structural failure on the R/V AUSTAUSCH, (see Tables I and
ID.
D„ Data Analysis

Depth as a function of time for each run is plotted and extra-
polated slightly at the drop and surfacing points as well as the
bottom of the run in order to obtain both run time and run depth.
Surface current is computed from three fixes taken on a surface
buoy. The surfacing position of each instrument is then extrapolated
back from its recovery position,, The average velocity and transport
per unit width are calculated for each run, and are then plotted
against depth on a station basis and time-averaged by curve fitting*
The distribution of transport per unit width is differentiated
for the velocity distribution, which is then plotted on a station
basis. The same basic sequence is followed for temperature-depth
for each run, i.e., plotted on a station basis and time-averaged by
curve fitting, A section plot is then made of six selected isotherms

12

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and smoothed . The velocity components at the smoothed isotherm depths
are read off the station plots and smoothed. The smoothing was
used in order to obtain the most reasonable possible representation
of the mean mass and current fields These data are then listec for
each of the six selected isotherms at up to fifteen cross-stream
points (at each station and at the boundaries) for each section (see
Tables III-VI) , This format was chosen for ease in handling and
presentation and because it is convenient for the testing of three-
dimensional models of inertial currents in x,y,T (cross-stream,
downstream, temperature) coordinates (Robinson, 1965) „
Eo Errors

Errors may be introduced from three sources: (1) inter-
pretative errors associated with the assumption of steady state
conditions over the time and space scales of a run, (2) instrument
and computational errors, and (3) errors due to rectification in
time-averaging .

Based on previous experience in the Florida Straits, interpre-
tive errors due to fluctuations over the time and space scales of
an instrument run are very small (1-3%) and measurements are to be
considered as desirable averages (Richardson and Schmitz, 1965).

System errors in a single measurement of vertically averaged
current (or transport per unit width) and isotherm depths are in
the 3-10% range (Schmitz, 1966). Station locations are known to
about 100 m or better, Subsurface current profiles obtained by
differentiation of the transport profile are smoothed on both a
station and section basis in order to minimize random system errors .
Errors associated with obtaining a representation of the mean stream

15

using this technique are thought to be bounded by 5-10% „ Individual
errors are assumed to combine in a random manner yielding character-
istic errors in time-averaged currents and isotherm depths of 3-5%,
Rectification errors in time-averaged downstream current speed
and isotherm depth are estimated to about 3%. However, errors in
cross-stream speeds due to this source may reach 25% (Schmitz and
Richardson, 1966).

16

III. RESULTS

The basic data from this experiment are presented in Tables III-
VI, and in Figures 4-11. These tables list cross-stream distance,
time-averaged values of isotherm depth, downstream current speeds, and
cross-stream speeds at six selected isotherms at up to fifteen points
along each cross-section. Figures 4 and 5 give the downstream surface
current component as a function of cross-stream distance with an
envelope of maximum observed variations from time-averaged values.
The remaining graphs are (for each section) downstream current compo-
nent contours (Figures 6 and 7), cross-stream current component
contours (Figures 8 and 9), and isotherm depths (Figures 10 and 11).
A. Discussion

(1) Topography. The topography of the Florida Straits is
discussed in this thesis as it is cogent to downstream changes of
velocity and mass field structure of the Florida Current. Further-
more, the navigational and depth accuracy of the instruments used
make this description a meaningful addition to the topographical
knowledge of this area.

In general, the cross-sectional area of the Straits decreases
downstream. Cross-sectional areas are approximately 60 km2 at
Section I, 55 km2 at Section II, 48 km2 at Section III, and 36 km2
at Section IV. There is a reduction in channel width at the surface
as the cross-sectional area decreases downstream with the exception
of Section IV, where the width at the surface diverges slightly

17

TABLE Grid // ■+
Isotherms

TABLE III - BASIC DATA FOR SECTION I

26

*

10

15

20

30

AO

50

60

70

80

90

100

103

105

22

6

10

15

20

30

AO

50

60

70

80

90

100

103

104

18

**

13

15

20

30

AO

50

60

70

80

90

100

103

104

1A

**

**

18

20

30

AO

50

60

70

80

90

100

103

104

10

**

**

**

**

ft*

**

51

60

70

80

90

100

102

**

8

**

**

**

**

**

**

57

60

70

80

90

100

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

26

*

0

17

26

Al

53

69

8A

91

99

110

112

112

112

22

20

39

59

72

87

100

111

128

1A3

155

166

170

170

170

18

**

100

108

119

138

157

177

193

217

233

2A3

257

260

261

1A

**

**

170

175

189

213

260

293

320

348

380

A08

A18

420

10

**

**

**

**

**

**

393

AA2

A95

5A5

589

623

630

**

8

**

**

**

**

**

**

550

569

628

680

729

770

**

**

47 65 99 116 113 105

48 53 77 97 99 93

0

25

37

AO

53

72

81

79

75

72

70

52

0

:*

0

17

23

27

AA

62

6A

57

A9

A7

37

0

t*

**

**

**

**

0

38

A7

38

23

9

0

**

r*

**

**

**

**

0

30

35

26

1A

0

**

**

c*

**

**

**

**

**

**

**

**

**

**

**

**

0

6

10

17

26

30

25

21

18

10

2

-3

0

5

17

27

28

33

3A

25

18

16

11

3

-2

0

0

16

26

23

25

28

23

17

13

8

1

-2

0

t*

0

2

9

16

20

18

13

9

5

-2

-5

0

r*

**

**

**

**

0

8

5

A

1

-3

0

**

c*

**

**

**

**

0

3

-1

-1

1

0

**

ft*

* Denotes Isotherm rises to the surface to the East of this station
** Denotes solid boundary

18

TABLE Grid # -
Isotherms

TABLE IV - BASIC DATA FOR SECTION II

26

5

10

15

20

25

30

35

45

55

65

75

80

83

87

90

22

6

10

15

20

25

30

35

45

55

65

75

80

83

87

89

18

8

10

15

20

25

30

35

45

55

65

75

80

83

87

88

14

9

10

15

20

25

30

35

45

55

65

75

80

83

87

**

10

**

10

15

20

25

30

35

45

55

65

75

80

83

84

**

8

**

12

15

20

25

30

35

45

55

65

75

80

81

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

26

0

22

40

51

62

70

78

86

90

90

90

90

90

90

90

22

44

61

80

94

105

115

123

138

150

160

166

168

169

170

170

18

80

90

108

124

141

156

170

198

230

265

289

300

304

308

308

14

103

110

140

167

191

218

240

290

337

384

421

446

458

470

**

10

**

146

192

232

273

316

357

421

490

549

599

618

628

629

**

** 250 281 333 376 412 447 510 580 661 728 752 756

26

0

80

116

131

139

142

140

134

124

105

84

69

57

37

0

22

0

62

94

114

126

129

129

122

111

94

77

66

57

37

0

18

0

49

78

99

111

113

113

107

94

78

63

53

47

32

0

14

0

39

61

78

88

90

91

86

74

60

49

39

27

0

**

10

**

0

26

43

49

52

57

58

49

38

30

24

17

0

**

8

**

0

0

4

13

23

35

40

34

23

15

8

0

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

26

0

6

10

12

16

20

19

14

10

5

0

-7

-13

-13

0

22

0

9

11

12

15

20

18

14

9

3

-4

-9

-14

-14

0

18

0

11

12

11

14

17

16

12

6

-1

-7

-10

-15

-16

0

14

0

11

12

10

12

15

14

11

5

-3

-8

-10

-8

0

**

10

**

0

5

9

10

9

9

8

4

-1

-4

-6

-6

0

**

8

**

0

3

6

6

3

4

5

3

3

3

0

0

**

**

* Denotes isotherm rises to the surface to the East of this section.
** Denotes solid boundary

19

TABLE Grid # -
Isotherms

BASIC DATA FOR SECTION III

10 15 20 25 30 35 45 55 65

22

8

10

15

20

25

30

35

45

55

65

70

75

80

83

85

18

9

10

15

20

25

30

35

45

55

65

70

75

80

83

84

14

**

10

15

20

25

30

35

45

55

65

70

75

80

83

**

10

**

10

15

20

25

30

35

45

55

65

70

75

79

**

**

8

**

13

15

20

25

30

35

45

55

65

70

75

77

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

26

0

12

38

49

52

55

60

65

70

74

75

76

77

78

78

22

46

60

81

90

92

94

96

106

120

135

140

142

146

147

147

18

80

83

100

109

120

133

151

180

210

239

251

262

271

278

279

14

**

105

121

143

168

192

220

284

328

362

381

397

416

421

**

10

**

135

171

203

234

284

323

39 7

^61

527

555

586

610

**

**

8

**

252

270

315

355

400

438

508

561

621

650

677

687

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

26

0

70

105

127

148

157

160

159

144

123

111

102

94

68

0

22

0

49

85

112

130

138

137

131

124

115

108

101

94

68

0

18

0

45

75

98

114

119

117

112

105

98

91

85

81

60

0

14

**

0

55

83

90

87

81

79

79

78

71

63

44

0

**

10

**

0

39

53

47

41

43

50

55

52

43

27

0

**

**

8

**

0

9

12

12

12

17

29

39

38

29

17

0

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

**

26

0

2

3

5

8

11

12

11

11

10

8

7

8

7

0

22

0

2

3

5

8

10

11

11

10

9

8

8

8

5

0

18

0

2

3

4

5

8

10

9

6

3

5

7

6

2

0

14

**

0

2

4

5

7

8

6

3

0

3

5

2

0

**

10

**

0

2

3

3

4

5

3

1

0

2

3

0

**

**

8

**

0

1

2

3

3

3

1

0

0

1

3

0

**

**

Denotes isotherm rises to the surface to the East of this station.
Denotes solid boundary

20

TABLE Grid // ■*
Isotherms

4-

TABLE VI - BASIC DATA FOR SECTION IV

34

35

A0

45

50

55

65

75

85

95

105

115

30

35

40

45

50

55

65

75

85

95

105

112

30

35

40

45

50

55

65

75

85

95

105

109

**

36

40

45

50

55

65

75

85

95

105

**

**

**

42

45

50

55

65

75

85

95

101

**

50 55 65 75 85

* Denotes isotherm rises to the surface to the East of this station
** Denotes solid boundary

** **

26

*

0

11

50

80

99

107

120

125

127

130

131

134

**

ft*

22

*

0

47

85

117

139

151

161

166

172

185

200

220

**

ft*

18

0

18

68

112

150

176

192

212

223

240

265

295

310

ft*

ft*

14

**

**

110

140

173

212

243

284

323

355

380

405

**

*ft

ft*

10

**

**

**

210

230

270

304

354

390

428

466

484

**

**

**

8

**

**

**

**

310

325

359

430

497

567

615

**

**

**

ftft

**

**

**

A*

**

**

**

**

**

**

**

**

**

**

ft*

26

0

0

61

93

109

128

147

146

136

123

98

59

0

**

ft*

22

0

0

48

67

77

101

127

139

138

122

93

56

0

**

ft*

18

0

17

33

42

52

75

102

122

127

111

85

53

0

ft*

ft*

14

**

**

0

18

33

54

78

102

109

93

59

0

**

**

ftft

10

**

**

**

0

9

27

51

81

92

81

54

0

**

ft*

ft*

8

**

**

**

**

0

11

28

57

64

37

0

**

ft*

**

**

**

**

**

*ft

**

ft*

**

**

**

**

**

**

**

**

**

26

0

0

-3

-1

3

2

0

3

3

0

2

4

0

**

ft*

22

0

0

-1

-1

2

0

-2

2

2

-2

1

4

0

**

ft*

18

0

3

1

-1

1

-1

-3

1

0

-4

0

4

0

ft*

ft*

14

**

**

0

-3

-1

-2

-3

1

-1

-5

-3

0

**

ft*

**

10

**

**

**

0

2

-3

-1

3

1

-2

-1

0

**

**

**

8

**

**

**

**

0

-5

2

4

2

1

0

**

**

**

ft*

ft* **

21

Figure 4. Downstream Surface Current (V )

vs Cross-Stream Distance with Envelope

of Observed Variations - Sections I and II,

SECTION I

200 -I

t 160-

4> 120-

E
o

"51 80-

cfr^A

40-

/ i

0-

-40-

1 /
1

i

/

-80-1

0 10 20 30 40 50 60 70 80 90 100

110

120

KM CROSS -STREAM *

SECTION I

200-

/ ^

*■* — .

t 160-

/ /

^\\

\ 120-

Y r-

-^^s

o

A J

\ ^O^N

>" 80-

r

X\!

40-

0-

-40-

1 1 1 1 1 1 1

!

(

,

C

) 10 20

30 40 50 60 70 80 90
KM CROSS-STREAM >

100

no

120

23

Figure 5. Downstream Surface Current (V )

vs Cross-Stream Distance with Envelope

of Observed Variations - Sections III and IV.

SECTION m

-40-1

0 10 20 30 40 50 60 70 80 90 100 NO 120

KM CROSS -STREAM »

SECTION is:

200-

r^^=r — \

t 160-

| 120-
<->

/ ^\\

>w 80-

il

40-

/ \\

0-

C

) 10

20

30 40 50 60 70 80 90 100 110 120
KM CROSS-STREAM — >

25

Figure 6. Smoothed Downstream (V)
Isotachs for Sections I and II.

SECTION I ISOTACHS (v-cm/s)

KM CROSS-STREAM >

0 10 20 30 40 50 60 70 80 90 100 110 120

SECTION II ISOTACHS (v-cm/s)

KM CROSS -STREAM >

0 10 20 30 40 50 60 70 80 90 100 110 120

100-

\^v^ J \

200-

\\\^^-^ 100 ^/ J

cc

^400-
UJ

S 500-
2

\\ \-_ 60 ^y \\

\\ //

x 600-
h-

£i 700-
Q

I \20^/ /

800-

900-

1000-

1100-

27

Figure 7. Smoothed Downstream (V)
Isotachs for Sections III and IV.

SECTION IE ISOTACHS (v-cm/s)

KM CROSS -STREAM »

0 10 20 30 40 50 60 70 80 90 100 110 120

SECTION 12 ISOTACHS (v-cm/s)

KM CROSS -STREAM ►

0 10 20 30 40 50 60 70 80 90 100 110 120

29

Figure 8. Smoothed Cross-Stream (U)
Isotachs for Sections I and II.

SECTION I

ISOTACHS (u-cm/s)

KM CROSS- STREAM >

c

0-
100-

) 10 20

30 40 50 60 70 80 90 100

no 120

<CjoD ) \ \

200-

"^^^-20^^^ /

co 300-

QC

LU

h- 400-
UJ

500-

V^"*^--— JQ^/ /

X 600-

1-

Q-

UJ 700-

Q

\^--_o^/ /

800-

\ u<0 /

900^

1000-

1100-

SECTION I

ISOTACHS (u-cm/s)

KM CROSS -STREAM *

0 10 20 30 40

50 60 70 80 90 100 110 120

' l ' ' ) 7

100-

\\ ^°

200-

co300"

^400-

UJ

2 500-

\ "v_3

0

-10

2

x600-
h-

UJ 700-
Q

800-
900-

1000-

1100-

31

Figure 9. Smoothed Cross-Stream (U)
Isotachs for Sections III and IV.

SECTION m ISOTACHS (u-cm/s)

KM CROSS- STREAM »

0 10 20 30 40 50 60 70 80 90 100 110 120

SECTION W

ISOTACHS (u-cm/s)

KM CROSS -STREAM »

0 10 20 30 40

50 60 70 80 90 100 NO 120

100-

^Wo\

u>0

200-

f) /\ ,

IN METERS

o o o
o o o

V vv

X 600-

1-

Q.

W 700-

Q

800-

900-

1000-

1100-

33

Figure 10. Isotherm Depths
for Sections I and II.

SECTION I

ISOTHERM DEPTHS
KM CROSS- STREAM ►

0 10 20 30 40 50 60 70 80 90 100 110 120

*~ ^-^26°

ICCH

•—- — Zll^?--* • • _^

200-

»-— - ~-^-__^l8° "~ •" •"■

300-

\^*^-04° *~"

£400

\\io° ^^~*~«

h-

5 500-

■z.

V 8° ^»^^

fE 600-

Q.

Q

700

800

900

1000

1100

SECTIONH

ISOTHERM DEPTHS
KM CROSS- STREAM ►

C

10 20 30 40 50 60 70 80 90 100 110 120

100

vT^*^^~~*--*- 26°

~~*~——~J^__ |

200-

^-^ |

300-

^4» 1

w400-

\ ^j

^500

\ NIO" 1

H

(E 600

Q.

\8° ^^W

700-

800

900

1000

1100-

35

Figure 11. Isotherm Depths
for Sections III and IV.

SECTION m

ISOTHERM DEPTHS
KM CROSS- STREAM ►

c

100-

10 20 30 40 50 60 70 80 90 100 110 120

26° J

~~*~— — *— §S°

200-

^*^-~~~U3°

300-

^K^I4° 1

£400-

u

Ll)

2 500-

\_ \l°° /

jE 600-

\8° ^VJ

LU
Q

700-

800-

900-

1000

1100-

SECTION M ISOTHERM DEPTHS

KM CROSS-STREAM ►

0 10 20 30 40 50 60 70 80 90 100 110 120

37

prior to opening onto the Blake Plateau. The eastern profile of
the Straits maintains its characteristic steep slope in Sections I,
II, and III with a reduced steepness in Section IV. The western
slope of the Straits is more gradual, with a shelf at 200 m in
Section I, deepening to about 300 m in Sections II and III, and
losing its identity in Section IV. The average cross-sectional area
of this 225 km downstream sector is about 50 km2, the average depth
about 600 m, and the width of the current is 80-100 km.

(2) Surface Current. With reference to Figures 4 and 5, the
surface current is uniaxial. The surface current is fairly
symmetric at Section I and clearly asymmetric to the west of center
at Sections II, III, and IV. The surface current is characterized
by a cyclonic zone to the west of the axis and an anticyclonic zone
to the east, with thin layers of high shear at the channel boundaries.
A typical value of the shear in the thin layer at the western boundary
is 1 m/sec in 3-5 km. The shear in the cyclonic zone is about one -
half of that value, decreasing to one-quarter of that value (and
negative) in the anticyclonic zone. A typical value of the shear
in the thin layer at the eastern boundary is 0.75 m/sec in 3-5 km.

Figures 4 and 5 show envelopes of fluctuation bounds (dotted
lines) in the downstream surface current component (V ) for each
section, along with the .mean profiles. The amplitude of the bounds
are characteristically in the 20-30% range except at the current
edges, particularly for Sections I and III and at the western edge
of the current. The large bounds at the current edges are associated
with a meandering motion. At Section I, the position of the edge
of the current was observed to vary by 30 km, from X=5 to 35 km.

38

The mean position of the current edge was chosen at X=20 km and the
:'mean" profile brought to zero at that point. This large fluctuation
in surface current on the western side of the channel is apparent
in all four sections, although most pronounced in Section I and
least pronounced in Section IV.

The symmetry of the surface current at Section I might be
expected from the curvature effect as the east-bound current in
the southern Straits -begins its swing to the northeast. The current
axis is found to the west of center as the channel converges at
Section II. Between Sections II and III the curvature effect is
small, but the channel further converges as the cross-stream distance
at the surface is reduced by 4 km at the eastern boundary. The
surface current remains asymmetric to the west of the channel center
at Section IV.

The most important feature of the surface current is its down-
stream acceleration. The surface current accelerates downstream as
the cross-channel width decreases between Section I and III and then
decelerates slightly as the surface width diverges at Section IV.

(3) Subsurface Current. With reference to Figures 6 and 7, the
subsurface current contours, like the surface current, show abrupt
shear zones at both edges of the channel with characteristic regions
of cyclonic and anticyclonic shear to the west and east of the axis,
respectively. The cyclonic shear becomes more pronounced downstream.

The dominant feature of the current is a downstream acceleration.
The higher velocity contours widen, deepen, and are displaced more
to the west downstream, while the lower velocity contours tend to
conform to the bottom topography of the area. As would be expected

3C>

from the continuity equation, the decrease in the cross-sectional
area of the channel leads to the acceleration of the current. There
is, however, a slight deceleration of the high speed layers between
Sections III and IV as the cross-channel width increases at the
surface.

As can be seen in Figures 8 and 9, the absolute value of cross-
stream speed decreases downstream. There is a well-defined
positive core at Section I, decreasing to Section III, and dis-
appearing at Section IV. The change in cross-stream speeds
between Sections I and II is associated with the curvature effect.
The zones of negative cross-stream speeds at Sections I and II are
associated with convergence on the east side of the channel, and
the zone of positive cross-stream speed at the eastern side of the
channel at Sections III and IV is associated with the diverging
channel.

(4) Isotherm Distribution. The distribution of isotherms shows
the cross-stream slope from west to east characteristic of Gulf
Stream Regions, see Figures 10 and 11. On the average, the 8° and
10° isotherms always rise downstream. The upper isotherms also
rise downstream except for the 22°, 18° and 14° isotherms, which
lower between Sections I and II, and the 26°, 22° and 18° isotherms,
which lower between Sections III and IV. The pattern of downstream
changes in isotherm depths described above is associated with the
downstream changes in the current field, and are qualitatively those
anticipated from the Bernoulli Equation. Downstream changes along
streamlines will not be discussed in this thesis.

40

B. Comparison with Other Methods

Free instrument data will be compared with (1) current meter
measurements (2) GEK surface current measurements, and (3) hydro-
graphic data. The comparison with these methods is shown in
Figures 12-15. In all figures, the dotted lines show free instrument
results and solid lines the compared data.

(1) Pillsbury (1890) made over 1,100 hours of direct current
measurements during 1885-1886. Using an anchored current meter of
his own design, current measurements were made at depths of 6, 27,
55, 119, and 238 m at each of six anchor stations between Fowey
Rocks and Gun Cay, B.W.I. Current measurements to a depth of 370 m
were made at the easternmost station. Pillsbury assumed a linear
decrease in velocity with depth below the depth of his measurements.
Therefore, his description of the deep current structure was based
on this extrapolation from his measured values, and resulted in a
contour of zero velocity above the bottom. Below this zero velocity
contour he concluded that the current was either at rest or setting
southward. He also reported a uniaxial surface current with a
displacement of the axis to the west of channel center.

A comparison of free instrument data at Section II with that
of Pillsbury 's is shown in Figure 12. It should be noted that
Pillsbury 's data was taken 8.4 km downstream of this section.
However, the correlation in surface current is good, with the only
meaningful deviation being near the western boundary. This is
probably due to his linear extrapolation from the 14 km distance of
his westernmost station to the point of zero velocity. Pillsbury's
subsurface speed contours, with the exception of his zero velocity

41

Figure 12. Comparison of Free Instrument Surface
and Subsurface Current Data at Section II with
Current Meter Data.

0 10 20 30 40 50 60 70 80 90 100 110 120
KM CROSS -STREAM »

SECTION n ISOTACHS (v-cm/s)

KM CROSS- STREAM >

0 10 20 30 40 50 60 70 80 90 100 110 120

43

contour and those contours constructed from extrapolated values,
compare favorably with our results. However, there are measurable
differences in the depths of the high speed contours. Considering
the fact that there were over twice as many free instrument stations
as current meter stations (in particular note the differences on
the western edge of the current), the contours are quite similar
in shape.

(2) Webster (1961) computed time-averaged surface currents on
the basis of 632 GEK measurements made during 42 cruises between
Fowey Rocks and Gun Cay, B.W.I. , during the period 1952-1958. These
cruises were made by the Institute of Marine Science of the University
of Miami, for example see Anon, 1954. Webster divided the current
into eleven zones of 5-8 km each, and averaged the surface velocity
components in each zone. A comparison of the average surface current
resulting from these measurements with the free instrument surface
current profile at Section II is shown in Figure 13a. Current velocity
is brought to zero at boundaries coincident with those at Section II
for ease of comparison. In general, the comparison shows a remark-
able similarity, the only exception being an increased speed at the
current axis.

In contrast to time-averaged data, Figure 13b shows a comparison
of free instrument data at Section III with a single uncorrected GEK
transect (Murray, 1952) taken on May 26, 1951. The multiaxial current
structure shown from this GEK data is seldom observed on a single
transect with the free instrument technique, and is certainly not a
feature of the steady state Florida Current. A multiaxial current
structure is often reported from data obtained along a single transect

44

Figure 13. Comparison of Free Instrument
Surface Current Data at Sections I and II
with GEK Data. (a) Webster (1961)

(b) Murray (1952)

(c) Chew and Wagner (1956)

200-

| 160-
| 120-

"~~^v

\

3

\X

>" 80-

N. \

40-

\\

o-

-40-

(

) ib

20

30

40

50 60 70 80 90

100

iio

120

KM

CROSS -STREAM — ►

200 -I

t 160-

/ /

"""-^

| 120-
> 80-

/

"^v^

40-

-40-

(

) ib

20

30

40
KM

50 60 70 80
CROSS-STREAM —

90

100

iio

120

200 1

| 160-

,''' ""'■>..

"^ 120-

^-^"~ — ^^X

_

/ y n.

*!

>• 80-

/ / \

40-

/

-40-

C

) ib

20

30 40 50 60 70 80 90
KM CROSS -STREAM —

100

iio

120

46

across the current, where the time scale of crossing is greater than
six hours, suggesting tidal aliasing.

A comparison of free instrument surface current data at Section I
is made with GEK measurements taken in this region during a single
transect in July 1953, see Figure 13c. The corrected measurements
were smoothed by Chew and Wagner (1956) as their interpretation of
time-averaged data. Although there is agreement in absolute values
and symmetry of the curves, the current axis is displaced to the east
of the free instrument results.

(3) The same cruise report (Chew and Wagner, 1956) listed
isotherm depths from a series of hydrographic stations completed
on August 25, 1955. Figure 14a shows these isotherm depths as they
compare with free instrument data at Section I. There is agreement
in the general symmetry of the isotherms, although the hydrographic
results show the characteristic undulations attributable to tidally
biased data.

Worthington (1966) has kindly made available data taken from
hydrographic stations during an Atlantis cruise in the Ft. Pierce-
Matanilla Shoal region on June 25, 1955. Figure 14b shows a
comparison of this data with the isotherm depths from free instrument
measurements at Section IV. Although there is a favorable correlation
in shape and slope of the isotherms, there are variations in their
vertical separation, again possibly showing a tidal influence on data
from a single transect.

In marked contrast to non-synoptic data, the averaged temperature
data from four summer hydrographic cruises across the Florida Current
near Section II (Broida, 1962a, 1962b, 1963, 1964) is compared with

47

results of the free instrument method. Figure 15a shows a comparison
of this time-averaged data and is in good agreement with the results
of our experiment. Figure 15b shows the fluctuations in the 26°, 18°,
and 8° isotherms over the four cruises. These same isotherms from
free instrument data fall within the envelope of the variations.

48

Figure 14. Comparison of Free Instrument
Mass Field Data at Sections I and IV with
Hydrographic Data. (a) Chew and Wagner (1956)
(b) Worthington (1966)

SECTION I

ISOTHERM DEPTHS

KM CROSS -STREAM ►

0 10 20 30 40 50 60 70 80 90 100

no 120

100-

200-

26°

\ --- —

----___ —

\ ^Xr-

\ — ~~""~--r~~ ~^i~~-~^ZS'

X--- 18°

IN METERS

8 8 8

\^^_j4°

V<<JO° —
V^8° ^V

DEPTH

-si CD

8 8

\ ^X /

800-

900-

1000-

1100-

SECTION IE-

ISOTHERM DEPTHS

KM CROSS - STREAM »

0 10 20 30

40 50 60 70 80 90 100 110 120

100-
200-

VVv:rrrr^ ~^=^^__J26°

\v\s- 722°

v^-X^_^^^ ~~/22°

en300"
<r

\^ 400-
UJ

5 500-

X 600-

1-

CL

UJ 700-

Q

VaX;. ^^:::^-/l8°
\\ V-^\~^-~ /l8°

V^^nL ^ — ~\ """714°

\\ X^\ ~~~oVi4°

\\ "x^^\ /l0°

\ \ "N j/o°

V^8°

800-

900-

1000-

1100-

50

Figure 15. Comparison of Free Instrument
Mass Field Data at Section II with Time-
Averaged Hydrographic Data.

(a) Comparison of Data

(b) Variations in Hydrographic Data

SECTION I ISOTHERM DEPTHS

KM CROSS- STREAM ►

0 10 20 30 40 50 60 70 80 90 100 110 120

v^- 1

100-
200-

V^l~"~-^^^_ 26° I

^^^r^^^ "1

^^S^T^^W i

DEPTH IN METERS

o o o o o
o o o o o

Ti|

800-

\ y

900-

1000-

1100-

SECTION I FLUCTUATIONS IN ISOTHERM DEPTHS

KM CROSS -STREAM »

0 10 20 30 40 50 60 70 80 90 100 110 120

DEPTH IN METERS

— o to co >i oi w * w n -
oooooooooooc

^fe=^=^^_26° __— -—

\CT^--~_ ---^___

^

52

IV. SUMMARY

Characteristic features of the velocity and temperature fields
in the Florida Current have been isolated and discussed on the basis
of free instrument data obtained during approximately 40 transects
across the current at four separate sections. The data was obtained
during the summers of 1965-1966 and encompasses a 225 km downstream
distance within the Florida Straits. Each transect consisted of
about 10 stations, and each station was sampled several times in a
serious effort to average over fluctuations in order to obtain a
coherent representation of the mean or steady state mass and velocity
fields. The experiment was designed to provide data for testing
models of the current dynamics. The results have been presented in
x,y,T (cross-stream, downstream, temperature) coordinates at six
temperature levels since data in this form are convenient to the
testing of three-dimensional inertial current models (Robinson, 1965).
As a first step, this thesis presents a description of the dominant
features of the mass (temperature) and velocity fields. Also, the
data obtained has been compared with previous observations in the
area of the experiment.

The most important original result obtained is the determination
of the nature of downstream responses of the mass and velocity fields
to downstream changes in channel geometry. The most significant
result of the comparison with previous time-averaged data is the
general agreement obtained.

53

The major conclusions are:

(1) The surface current is uniaxial, symmetrical about the
current center at Section I due to the curvature effect and asymmetric
to the west of current center as the channel converges at Sections
II-IV. The surface current accelerates between Sections I and III
and decelerates slightly at Section IV, clearly associated with
changes in channel width.

(2) The downstream subsurface current retains the shape of
the surface current. Between Sections I and II there is a strong
(30%) acceleration in the upper layers, which penetrates in depth
between Sections II-IV. As would be expected from the continuity
equation, this phenomenon is associated with changes in cross-
sectional area of the channel.

(3) Cross-stream current fields near the channel boundaries are
primarily associated with the converging and diverging topography,
while absolute values of cross-stream speed decrease downstream as
the curvature effect decreases.

(4) Isotherms rise downstream where the current is accelerated
and lower where the current is decelerated, in agreement with
Bernoulli's Equation.

(5) The comparison of the free instrument method with Pillsbury's
current meter data and GEK and hydrographic data shows close agreement
where time-averaged data is the basis for comparison.

54

LITERATURE CITED

55

Anon, 1954. Some results of the Florida Current Study, 1953. Inst.
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VITA

CDR Edward Clausner, Jr., USN, was born in Buffalo, New
York, on October 27, 1929. His parents are Maurice Edward
Clausner and Lucille Emily Clausner. He received his elementary
education at South Mountain School, Millburn, New Jersey, and his
secondary education at Millburn High School, Millburn, New Jersey.

In August 1947 he entered the U.S. Naval Academy, Annapolis,
Maryland. Upon graduating in June 1951 with a B.S., he was commis-
sioned an Ensign in the U.S. Navy. Subsequent naval service
included one and one half years of service in amphibious force
ships of the Pacific Fleet, followed by twelve years of submarine
service in the Atlantic Fleet. He attended the U.S. Naval War
College, Newport, Rhode Island during 1962-1963 prior to two years
of service as Commanding Officer of USS TIRANTE (SS420) .

He was admitted to the Graduate School of the University of
Miami in September 1965. He was granted the degree of Master of
Science in June 1967.

Permanent address: 1444 S.E. 15th Court

Deerfield Beach, Florida

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