INDICATED GEOSTROPHIC VELOCITIES AND VOLUME
TRANSPORTS, CENTRAL AND EASTERN GULF OF
MEXICO, WARMEST AND COLDEST MONTHS

by

William Louis Wunderly, Jr,

! United States

Naval Postgraduate School

THBSI

-

INDICATED GEOSTROPHIC
TRANSPORTS, CENTRAL ANE
WARMEST AND

VELOCITIES AND
I EASTERN GULF OF
COLDEST MONTHS

VOLUME
MEXICO,

■

by

William

Louis Wunderly,

Jr.

September 1970

TMs 'document Kas tfeen approved "for pubTTc"
release and sale; its distribution is unlimited,

Indicated Gcos trophic Velocities and Volume Transports,
Central and Eastern Gulf of Mexico, Warmest and Coldest Months

by

William Louis Wunderly, Jr.
Lieutenant Commander, United States Navy
B.S., United States Naval Academy, 1962

Submitted in partial fulfillment of the
requirements for the degree of

MASTER OF SCIENCE IN OCEANOGRAPHY

from the

NAVAL POSTGRADUATE SCHOOL
September 1970

IARY

iL POSTGRADUATE SCHOOB

'EEEY, CALIF. 9394Q ^

ABSTRACT

■ To make comparisons to seven similar cruises, the geostrophic
method of volume transport and velocity analysis was applied to
ALAMINOS cruises 67-A-6 of 4 to 22 August 1967 and 68-A-2 of 13
February to 6 March 1968. An average velocity of 83 cm/sec and a
volume transport of 27.5 Sverdrups was found in the Yucatan Channel
in August and an average velocity of 79 cm/sec and a volume transport
of 26.6 Sverdrups was found in the channel for February to March. A
subsurface westward flow occurred in August along the southern coast
of Cuba providing input into the Loop Current north of the Yucatan
Channel. The Loop Current never crossed 25 N latitude. A cold ridge
extended from the Florida shelf to the Campeche Bank.

An analysis of East-West volume transport in the central Gulf
indicated a merging of east and west Gulf waters between 87 50 'W and
89°30'W longitude for the MABEL TAYLOR cruise of 1932 and the ATLANTIS
cruise of 1935. The GERONLMO cruise of February-March 1967 and cruise
68-A-2 indicated a merging of east and west Gulf waters between 89 30'W
and 91°00'W longitude.

TABLE OF CONTENTS

I. ! INTRODUCTION 9

II. PROCEDURES 11

A. GEOSTROPHIC VELOCITY AND VOLUME TRANSPORT COMPUTATIONS. 1]

B. USE OF THERMAL STRUCTURE AN) THE 22°C ISOTHERM TO

LOCATE THE LOOP CURRENT AND DETERMINE ITS EXTREMITIES . 14

C. SHALLOW STATION ANALYSIS 16

III. CRUISE 68-A-2 19

A. GENERAL 19

B. VELOCITIES 21

C. VOLUME TRANSPORT 22

D. EAST-WEST TRANSPORT IN THE CENTRAL GULF 25

IV. CRUISE 67-A-6 35

A. GENERAL 35

B. VELOCITIES 37

C. VOLUME TRANSPORT 39

V. COMPARISON OF CRUISES 67 -A- 6 AND 68-A-2 44

VI. COMPARISON OF CRUISES 65-A-ll, 65-A-13, 66-A-15, AND

67-A-6 46

VII. SUMMARY OF LOOP CURRENT AND EDDY VELOCITIES AND VOLUME
TRANSPORTS FOR NINE SUMMER AND WINTER CRUISES IN THE

GULF OF MEXICO FROM 1965-1968 55

VIII. CONCLUSIONS 58

APPENDIX A - Equations Utilized to Compute Geostrophic Volume

Transport and Velocity 60

COMPUTER PROGRAM 73

BIBLIOGRAPHY 76

INITIAL DISTRIBUTION LIST ....... "77

FORM DD 1473 79

3

4

LIST OF TABLES

Table _ Page

I Volume Transport, Relative to 1000 Meters, for
Assumed Velocities at 1000 Meters (Stations 17-18,
Cruise 68-A-2) 13

II Sea Surface Geostrophic Velocities of the Loop

Current Relative to 1000 Meters (Cruise 68-A-2) ... 21

III Loop Current Geostrophic Volume Transports Relative

to 1000 Meters (Cruise 68-A-2) 22

IV Geostrophic Volume Transport, Relative to 1000

Meters, Between Station: 25 and 38 (Cruise 68-A-2) . . 23

V Central Gulf East-West Geostrophic Volume Transport,
Velocity, and Direction Relative to 1000 Meters-
Leg I (Cruise 68-A-2) 26

VI Central Gulf East-West Geostrophic Volume Transport,
Velocity, and Direction Relative to 1000 Meters-
Leg II (Cruise C -A-2) 27

VII Central Gulf East-West Geostrophic Volume Transport,
Velocity, and Direction Relative to 1000 Meters -

Leg III (Cruise 68-A-2) 28

VIII Net East-West Deep Water Geostrophic Volume
Transport and Direction, Relative to 1000 Meters,

Across Legs I, II, and III (Cruise 68-A-2) 29

IX Net Geostrophic Volume Transport Inputs and
Outputs to Areas X and Y, Relative to 1000 Meters
(Cruise 68-A-2) 30

X Net East-West Deep Water Geostrophic Volume
Transport and Direction, Relative to 1000 Meters,
Across Legs I, II, and III for Selected Winter

Cruises 32

XI Net Geostrophic Volume Transport Inputs and
Outputs to Areas X and Y, Relative to 1000 Meters,

- for Selected Winter Cruises ~ 33

XII Loop Current Sea Surface Velocities Relative to

1000 Meters (Cruise 67-A-6) 38

XIII Loop Current Geostrophic Volume Transport

Relative to 1000 Meters (Cruise 67-A-6) 40

Table
XIV

XV

XVI

XVII

XVIII

XIX

Axial Geostrophic Volume Transport in Selected
Layers Relative to 1000 Meters and to the Bottom
of the Respective Layers (Cruise 65-A-ll)

Axial Geostrophic Volume Transport in Selected
Layers Relative to 1000 Meters and to the Bottom
of the Respective Layers (Cruise 65-A-13)

Axial Geostrophic Volume Transport in Selected
Layers Relative to 1000 Meters and to the Bottom
of the Respective Layers (Cruise 66-A-15)

Loop Current Geostrophic Volume Transport in
Selected Layers Relative to 1000 Meters and to
the Bottom of the Respective Layers
(Cruise 66-A-15)

Summary of Loop Current Sea Surface Velocities and
Volume Transports, Relative to the Chosen Reference
Level, at the Yucatan Channel for Selected Cruises

Summary of Observed Anti-cyclonic Eddy Sea Surface
Velocities and Volume Transports, Relative to the
Chosen Reference Level, for Selected Cruises . . .

Page

47

48

50

51

56

57

LIST OF FIGURES

I
j

Figure Page

1 Temperature Cross-section of Loop Current 61

2 Depth of 22 C Isotherm versus Dynamic Height
Anomaly of the Sea Surface Relative to 1000

Meters 62

3 Station Locations, Cruise 68-A-2 63

4 Dynamic Topography of the Sea Surface Relative

to 1000 Meters (Cruise 68-A-2) 64

5 Dynamic Topography of the 200 Meter Surface

Relative to 1000 Meters (Cruise 68-A-2) 65

6 ■ Dynamic Topography of the 500 Meter Surface

Relative to 1000 Meters (Cruise 68-A-2) 66

7 Location of Loop Current (Cruise 68-A-2) 67

8 Station Locations, Cruise 67-A-6 68

9 Dynamic Topography of the Sea Surface Relative

to 1000 Meters (Cruise 67-A-6) 69

10 Dynamic Topography of the 200 Meter Surface

Relative to 1000 Meters (Cruise 67-A-6) 70

11 Dynamic Topography of the 500 Meter Surface

Relative to 1000 Meters (Cruise 67-A-6) 71

12 - Location of Loop Current (Cruise 67-A-6) 72

ACKNOWLEDGEMENTS

I would like to express my sincere appreciation to Dr. D. F.
Leipper for the assistance he provided in the preparation of this
thesis. His patience, personal int. rest, and encouragement made the
work much easier.

I would also like to thank Professor J. J. von Schwind for his
constructive criticism and suggestions in the format of this thesis.

A special thank you to my wife, who spent many hours typing and
retyping.

I. INTRODUCTION

!

Emphasis on the study of the circulation in the Gulf of Mexico
began about 1925. This interest was motivated by the apparent miscon-
ception that the circulation in the Gulf consisted of the Yucatan Cur-
rent which entered the Gulf at the Yucatan Channel and flowed clockwise
around the Gulf to exit at the Florida Straits as the Florida Current.
Parr [1935] made a cruise in the Gulf on the MABEL TAYLOR in 1932, and
found at that time (February to April) that a surface current entered
the Gulf at the Yucatan Channel and flowed, without deviation, toward
the Florida Straits. He also found that a subsurface flow in the cur-
rent intruded into the eastern Gulf.

Since that time the Gulf has been studied extensively by numerous
people. The water masses of the Gulf have been identified by their
temperature and salinity relationship. Observations of the Loop Cur-
rent have shown various patterns of flow. Detached eddies have been
observed which were apparently once a part of the Loop Current. The
Loop Current itself has intruded into the eastern and central Gulf as
far north as 28 N latitude.

The definition of the Loop Current varies with authors. Nqwlin
and McLellan [1967] referred to the Loop Current as only that portion
of the current in the Gulf, excluding the Yucatan Current and the
Florida Current. However, since the Yucatan and Florida Currents are
really part of the Loop Current, they have been included, in any ref-
erence in this paper, in the Loop Current.

The determination of the current patterns is not the only reason
the Gulf has been studied so extensively. Pilot charts issued by the

Naval Oceanographic Offices have indicated that there is flow into the
west Gulf with no apparent return. The east and west Gulfs are con-
sidered by some people to be isolated from each other. However, it has
been shown that powerful eddies, which were once part of the Loop Cur-
rent, have moved into the Gulf and dissipated, altering the water
characteristics. This East-West exchange is important to the fishing
industry because the change in water characteristics effects the en-
vironment in which fish live.

The extremities of the Loop Current can be generally located by a
T-S diagram. Above 17 C, water on the right (looking downstream of the
current) side of the current has a different T-S curve than the water
on the left side of the current [Leipper 1970]. The water in between
contains the Loop Current. Locations (extremities) can also be deter-
mined from the slope of the 22 C isotherm in the upper 200-300 meters
of water.

The objective of this paper is to analyze the Loop Current, as
observed by ALAMINOS cruises 67-A-6 of 4-22 August 1967, 68-A-2 of
15 February to 6 March 1968, 65-A-ll of 10-24 August 1965, 65-A-13 of
12-24 September 1965, and 66-A-15 of 27 October to 13 November 1966,
and to analyze East-West volume transport in the Central Gulf. The
results are presented so that they may assist future studies of the
Loop Current c.:d general current pattern in the Gulf of Mexico.

10

II. PROCEDURES

I
A. I GEOSTROPHIC VELOCITY AND VOLUME TRANSPORT COMPUTATIONS

To compute the relative velocities and volume transports of geo-
strophic currents, the assumption is made that the pressure gradient
acceleration and the coriolis acceleration are the only accelerations
present and that they are equal in magnitude and opposite in direction.
Friction is neglected. Under these assumptions it can be shown that
the currents are normal to the slopes of isobaric surfaces which means
the currents flow parallel to contour lines of dynamic height anomaly.
The term relative is used since the currents are determined with res-
pect to a reference level where some residual motion may exist.

The choice of a reference level is arbitrary but is usually taken
where minimum motion seems to exist. In the eastern Gulf of Mexico,
Hubertz [1967] chose 1350 meters as a reference level. Nowlin and
McLellan [1967] state that a depth of 1000 meters may be chosen with-
out introducing errors of much more than 10 cm/sec in current compu-
tations. In this study, partly because of this small error and partly
because data were not regularly available at greater depth, a depth of
1000 meters was chosen as the reference level. All currents and volume
transports were computed relative to it.

Appendix A has a numbered list of equations used for various com-
putations of velocity and volume transport. Future reference to an
equation by number will indicate an equation listed in this appendix.

Equation (1) provides a method of calculating volume transports
between two stations, in a layer between the sea surface and a selected
depth, assuming the selected depth to be a level of no motion. The

11

equation was used by Hewitt [1970] to determine volume transport in
specific layers (e.g. 0-200 meters relative to the 200 meter surface).

Schneider [1969] used the equation to determine the total volume trans-

I

port between the sea surface and 1000 meters.

If the notation of Equation (1) is modified, it can be used to
calculate volume transport in a given layer referred to a reference
level, below that layer, at which no motion may be assumed to exist
[Hubertz 1967]. The dynamie height anomaly (AD) is the summation of
the dynamic height anomalies from the sea surface to the depth of the
chosen reference level. The volume transport: function (Q) is the dif-
ference between the summation of the transport functions at each depth
from the sea surface to the top and the summation from the sea surface
to the bottom of the layer of interest. The term (Z ) is the differ-
ence in depth between the top and bottom of this layer. It is there-
fore possible to use data which was computed assuming the sea surface
as the reference level to compute geostrophic volume transport and
velocity relative to another reference level.

It was found that the use of Equation (1), with a chosen reference
level, produced ambiguities in interpretation of the resulting signs
(+ or -) of velocity and transport. Computed sea surface velocities
indicated at times a flow in one direction, while computed volume
transports indicated total volume transport in the opposite direction.
To eliminate the ambiguity, Equations (2) and (3) were used to compute
the increment of volume transport in the layers of water between the
standard depths, relative to 1000 meters. This method permits complete
analysis, by layers, of the entire column of water between two stations
from the chosen reference lovel to the sea surface. It is of particu-
lar interest when the direction of volume transport is different at

12

different depths in the column due to a change in the direction of the
slope of the isobaric surfaces. Cruises 68-A-2 and 67-A-6, as well as
the; remaining cruises conducted in the Gulf of Mexico series, were
analyzed this way.

To determine the effect of possible motion (velocity) at the depth
selected as the reference level for geostrophic volume transport, the
column of water between stations 17 and 18 of cruise 68-A-2 was analyzed
assuming velocities of 0, 5, and 10 cm/sec respectively at 1000 meters.
These values were added to the velocities at each depth of calculation
from 1000 meters to the sea surface, and the volume transports, in
Sverdrups (Sv), were calculated for the various layers, 0-200 meters,
0-500 meters, and 0-1000 meters. The results are shown in Table I.

TABLE I

VOLUME TRANSPORT, RELATIVE TO 1000 METERS, FOR ASSUMED VELOCITIES AT
1000 METERS (Stations 17-18, Cruise 68-A-2)

Assumed Velocity
(cm/sec)

0

5

10

LAYERS (m)

VOLUME TRANSPORT
(Sv)

VOLUME TRANSPORT
(Sv)

VOLUME TRANSPORT
(Sv)

0-200
0-500
0-1000

11.20
17.97
20.82

12.01
20.02
24.91

12.55
21.43
27.78

Substitution of Equation (2) into Equation (3), Appendix A, indi-
cated that the total geostrophic volume transport from the sea surface
to the chosen reference level between station pairs was independent of

13

the distance between the station pairs, but was dependent only on the
difference in dynamic heights of the sea surface relative to the chosen
reference level. Station pairs with the same differences in dynamic
heights of the sea surface, relative to the chosen reference level, will
have the same total volume transport between the sea surface and the
chosen reference level and the same volume transport error for a given
velocity at the chosen reference level. The distribution of the volume
transports by layers may differ for the different station pairs.

If, as Nowlin and McLellan state, the maximum error in velocity is
about 10 cm/sec at the 1000 meter reference level, the maximum error in
volume transport through 1000 meters for stations 17 and 18 is approxi-
mately 6 to 7 Sverdrups. This error is approximately 30 to 35 percent
of the volume transport found when the assumed velocity is zero. The

dynamic height difference of the sea surface, relative to 1000 meters,

2 2

for station pair 17-18 of cruise 68-A-2 was 0.5 m /sec .

B. USE OF THERMAL STRUCTURE AND THE 22°C ISOTHERM TO LOCATE THE LOOP
CURRENT AND DETERMINE ITS EXTREMITIES

Dynamic topography at various depths is used to locate and study,
in detail, geostrophic currents at those depths. Unfortunately the
location of currents can vary from day to day, making it very diffi-
cult to plan a cruise so that the positions of the hydrographic stations
will cover satisfactorily any strong and variable (in position) current
which is of interest. Other methods may be combined with dynamic topo-
graphy analysis in order to more exactly locate a current.

Leipper [1970] utilized a method in which the thermal structure in
the upper 300 meters of water locates a current. Leipper's analysis
indicates that the 22 C isotherm is representative throughout the year

14

of the field of isotherms In the Loop Current in the Gulf of Mexico.
Bathythermograph data taken across a strong current, such as the Loop

Current, can present a good indication of its existence, location, and

I

direction of flow. Hewitt [1970] adapted this method to determine the
extremities of the Loop Current and major eddies in the Gulf of Mexico
for cruises 65-A-ll, 65-A-13, and 66-A-15 conducted by Leipper.

Figure 1 presents a section of bathythermograph data taken across
the Loop Current for cruise 68-A-2. The slope of the 22 C isotherm
between BT stations 70 and 80 indicates a current out of the paper as
indicated by the symbol (l) . A reversed slope would indicate flow into
the paper indicated by the symbol Q9 . The point of maximum slope of
the 22 C isotherm is a good indication of the location of the maximum
velocity of the current, as indicated by Leipper.

Sections of bathythermograph data which crossed the strong currents
were used to indicate the boundaries (extremities) of the current. The
extremity of the current is assumed to have been reached when the gen-
eral trend of the 22 C isotherm is reversed. Figure 1 is a good example
of how this method of analysis is applied. It indicates a general trend
of the slope of the 22 C isotherm between BT stations 70 and 80 to be
up toward the sea surface. The trend of the slope is reversed for
stations 68 to 70; therefore, the extremities of the current on. the
right of the dashed line are considered to be at stations 70 and 80.

Two problem areas arise when using the 22 C isotherm. First, this
method of analysis is only applicable when strong currents exist. An
attempt was made to correlate weaker currents and thermal structure,
but the definite features of the thermal structure in Figure 1 were
not indicated across the weaker current. Secondly, slight reversals
in the slope of the 22 C isotherm were indicated even though the

15

obvious trend of the slope was in one specific direction. The locati

on

of the current was determined by the general trend of the slope of the

22 C isotherm.

C. SHALLOW STATION ANALYSIS

A major problem was encountered computing volume transport and
velocity for the portion of the current which passed over an area
where the depth was less than the selected reference level (1000 meters).
Geostrophic volume transport between a water surface and the chosen ref-
erence level and. the geostrophic velocity of that water surface, relative
to the chosen reference level, can only be calculated when the slope of
that water surface, relative to the chosen reference level, is known.
In order to compute the volume transport in an upper layer relative to
a reference level below that layer, the dynamic height anomaly of the
top and bottom of the layer, relative to the reference level, must
therefore be known at two locations.

Fomin [1964] derived a method which is formulated by Equation (4),
Appendix A. This equation enables the computation of a theoretical
addition (A) to the dynamic height anomaly of the shallow station so
that the shallow station can be compared to deep water stations in
order to determine the geostrophic velocity of the sea surface rela-
tive to the chosen reference level, and the geostrophic volume trans-
port between the sea surface and the chosen reference level.

Hewitt [1970] utilized another method of obtaining a useable
dynamic height of the sea surface for a shallow water station. This
method is based upon an apparent correlation of dynamic height of the
sea surface, relative to 1000 m, versus the depth of the 22 C isotherm.
From the observed isotherm depth at a shallow station, the correlation

16

indicates the dynamic height the sea surface might have had if the
depth of the station had been 1000 meters. Figure 2 indicated Hewitt's
correlation curve and the correlation curve used for cruises 67-A-6 and
68-A-2. These curves represent a least-squares best fit to the observed
data. The depth of the 18 C isotherm was used for cruise 68-A-2 because
the 22 C isotherm was not present everywhere. This curve did not coin-
cide with the curves for the depth of the 22 C isotherm^ The slope at
various points on the curves were calculated and compared, since the
slopes should be nearly the same if the correlation is to be a good
one. The slope of the curves for cruises 67-A-6 and 68-A-2 were very
similar at all locations. Hewitt's curve and the curve for cruise
67-A-6 had similar slopes at shallow depths of the 22 C isotherm, but
for a small portion of the curves at the deepest depths, the slopes

differed. This difference in slope could cause a maximum difference

2 2
of 1.25 m /sec in the selected dynamic height at a depth of 200 meters.

This is a large difference and indicates that the value of such a curve
as a means of determining the dynamic height of the sea surface depends
on how much the observed data varies from the curve.

Fomin's method was used when the shallow station was a hydro-
graphic station, but it did not seem to be applicable when the depth
of the shallow station was very shallow compared to the deep station.
For cruise 68-A-2 the depth of the station at the western extremity of
the Loop Current in the Yucatan Strait was 30 meters, and the depth at
the station at the eastern extremity was 1000 meters. Fomin's method
provided current velocities and volume transports which were considered
too large to.be reasonable when compared to the velocities and volume
transports farther north in the same current. The 22 C isotherm

17

correlation method was used both in this case and for the case of the
shallow BT station.

After computing volume transport involving a shallow station,
another problem presented itself. The computed volume transport was
analyzed for two stations as though their depths had been 1000 meters.
However, the actual depth between the stations was, in places, less
than 1000 meters, so the computed volume transport was too large. To
determine what fraction of the computed volume transport was actual
volume transport, a plot of depth versus distance between stations was
made. This plot "provided an estimate of the areal extent of the water
in the cross-section. The estimated area divided by the entire rectangu-
lar area between the two stations was multiplied by the computed volume
transport to obtain an approximation of the actual volume transport.

These methods are not the only ways of analyzing a shallow water
station. Fomin's method is only one of the methods he presented.
Nowlin and McLellan [1967] used a method of extrapolation of the
dynamic height anomaly for the shallow station from the dynamic
height anomalies for the two closest deep stations. The method used
depended on the data available and the depth of the station. Hydro-
graphic data were used as much as possible, but such data were not
always available. -

18

Ill, CRUISE 68-A-2

A. GENERAL

Cruise 68-A-2 was planned and conducted by Leipper [1968] from 13
February to 6 March 1968. This cruise was one of a series of eight
conducted from 1965-1968 to study the temperature-depth structure of
the Gulf of Mexico in the summer and winter seasons. This particular
one was also planned to study the East-West volume transport across
the central Gulf. Figure 3 indicates three full North-South legs of
the cruise across the deep Gulf which were made to accomplish this
purpose .

Figures 4, 5, and 6 present the contours of dynamic topography for
the surface, 200 meters, and 500 meters, relative to 1000 meters, the
selected reference level. Using STD and BT data and the dynamic height
contour charts, the location^ geostrophic volume transport and velocity
of the Loop Current were established. An analysis of the geostrophic
volume transport in the Gulf across Legs I, II, and III was also made.

The Loop Current entered the Gulf through the western side of the
Yucatan Channel between stations 24 and 30. It intruded into the Gulf
369 km (from the western tip of Cuba to the outer extremity of the cur-
rent) before it turned eastward. This northward intrusion is approxi-
mately 131 km less than that found by Nowlin and McLellan [1967] in
their analysis of cruise 62-H-3 of 12 February to 31 March 1962. How-
ever, the extent of the intrusion of the current -was observed by Nowlin
and McLellan on the second or third of March, while the extent of the
intrusion found for cruise 68-A-2 was observed on 19 February. Since
the northward extent of the intrusion may be increasing at this time

19

of year, the approximately 10 days difference in observation times may
explain, in part, the difference in the extent of the intrusion, as
observed on the two cruises. Leipper [1970] found indication that the
intrusion increases about 150 km per month, during mid-February to late
March. At this rate, the 10 days difference in time would allow the
intrusion for cruise 68-A-2 to advance approximately 48 km farther into
the Gulf. It is also possible that the full extent of the intrusion
for cruise 68-A-2 was not observed because the northern end of the Loop
Current was not adequately covered by hydrographic stations.

Figure 7 indicates the location of the extremities (dashed lines)
and the location of the maximum current velocity (solid lines) across
the two sections A and B. These two sections represent the only two
crossings of the Loop Current made during cruise 68-A-2 .

The upper waters were so cold during cruise 68-A-2 that the 22 C
isotherm was not present. A study of BT data indicated that the 18 C
isotherm was a good substitute for the 22 C isotherm as an indicator
of current location, thus a plot of the depth of the 18 C isotherm
versus the dynamic height anomaly of the sea surface relative to 1000
meters was used to infer dynamic topography for analysis of shallow
water BT stations. The observed data for the cruise varied very little

from the correlation curve. There was a lack of stations with.-sea sur-

2 2
face dynamic heights (relative to 1000 meters) of 1.3-1.6 m /sec be-
cause these dynamic heights were representative of the Loop Current,
and for this cruise there were very few stations in the narrow Loop
Current. However, the correlation of the depth of the 18 C isotherm
and dynamic height of the sea surface was considered good for this
cruise.

20

The Loop Current was 137 km wide, measured between extremities, as
it entered the Gulf at section A, Figure 7. The maximum sea surface
velocity was located over the 300 meter isobath, as was also found by
Nowlin and McLellan [1967]. Methods described previously were used to
determine the current's location, velocity, and transport involving the
stations of limited depth on the western side of the current. The width
of the current increased to the north, becoming 172 km, from extremity
to extremity, at section B where it turned eastward.

B. VELOCITIES

Table II_shows sea surface velocities computed for the Loop Current
at sections A and B.

TABLE II

SEA SURFACE GEOSTROPHIC VELOCITIES OF THE LOOP CURRENT RELATIVE TO
1000 METERS (Cruise 68-A-2)

CROSS -SECTIONS

SECTION *
EXTREMITIES

AVERAGE VELOCITY
(cm/sec)

MAXIMUM VELOCITY
(cm/sec)

A

" B

24-80 (BT, S)
17-20

79.2
68.3

91.3
119.1

*Note: BT-Bathythermograph station
S-Shallow station

The average sea surface velocities were computed from the velocities
of the sea surface (relative to 1000 meters) between pairs of stations
along the cross-section of the Loop Current. The sea surface velocities
for each station pair were added together and then averaged. The maximum

21

surface velocity (referred to 1000 meters) increased as the current
proceeded northward; however, the average velocity of the current de-
creased, probably because of the broadening of the current.

Figure 4 indicates an apparent anti-cyclonic eddy in the western
Gulf centered at 24°30'N, 93°42*W. The southern half of the eddy was
not observed during the cruise. The sea surface velocity of the
observed portion of the eddy reached a maximum of 21 cm/sec between
stations 89 and 90.

C. VOLUME TRANSPORT

Volume transport across sections A and B for cruise 68-A-2 were
computed for layers of, 0-200 meters, 0-500 meters, and 0-1000 meters
(all referred to 1000 meters) when the hydrographic stations were in
deep water (1000 meters or greater) .

Table III indicates the volume transports calculated for both
cross-sections .

TABLE III

LOOP CURRENT GEOSTROPHIC VOLUME TRANSPORTS RELATIVE TO 1000 METERS

(Cruise 68-A-2)

CROSS -SECT ION

STATION PAIRS *

0-200 m
(Sv)

0-500 m
(Sv)

0-1000 m
(Sv)

A

24-25
25-80(S,BT)

6.4

10.7

13.2
13.4

B

17-18
18-19
19-20

11.2
6.7
2.3

18.0

12.1

5.0

20.8

14.4

7.1

22

*Note : S-Shallow water station

BT-Bathythermograph station

The volume transport across section A (0-1000 meters) is 26.6 Sv.
Since the majority of the depths are less than 300 meters, the greatest
part of the volume transport between stations 25 and 80 (BT) was in
shallow water. Therefore, the largest portion of the volume transport
through section A was probably in the upper 200 meters of water. Section
B had a volume transport of 42.3 Sv in the 0-1000 meter layer. Table
III indicates that 20.2 Sv occurred in the layer of 0-200 meters, while
14.9 Sv occurred in the layer of 200-500 meters. Only 7.2 Sv occurred
in the layer of 500-1000 meters. Therefore, the 0-200 meter layer had
more volume transport than either of the layers below it.

The increase in volume transport of 15.7 Sv at section B over
section A seems to have come from the west across the Campeche Bank
and from the area north of Cuba. The analysis of volume transport
between stations 25 and 38 is indicated in Table IV. All of the trans-
ports are to the east and the calculations assume a depth of greater
than 1000 meters .

TABLE IV

GEOSTROPHIC VOLUME TRANSPORT, RELATIVE TO 1000 METERS, BETWEEN
STATIONS 25 AND 38 ' (Cruise 68-A-2)

LAYER

TRANSPORT (Sv)

0-200 m
0-500 m
0-1000 m

10.2
16.6
18.8

23

The bottom topography of the cross-section between station 25 and 38
decreases to a minimum depth of approximately 500 meters, and all the
water approaching the cross-section between the stations, from the
west and south, comes across the Campeche Bank or through the Yucatan
Channel. Figure 4, the dynamic topography of the sea surface relative
to 1000 meters, indicates that the water movement in the southern part
of the Gulf, between latitudes 22 N to 24 N and longitudes 88 W to
91 W, is generally toward the east. Also^the analysis of the North-
South legs of the cruise indicated eastward volume transport in this
area .

Although the water is shallow (less than 1000 meters) between
stations 25 and 38, the computed volume transport in the upper 500
meters of water should be fairly indicative of total transport. The
volume transport (16.6 Sv) in this layer accounts for approximately
88 percent of the total volume transport between the two stations.

A portion of the volume transport between stations 25 and 38,
included that of the Loop Current, so only a portion of the volume
transport could be considered as an input into the current. The dif-
ference of 15.7 Sv probably consisted partially of an input from the
western Gulf caused by eastward flow over the Campeche Bank. An
analysis of stations 20 to 24, which lie to the east of the Loop
Current and north of section A (see Figure 3), indicated a net
(0-1000 meters) volume transport of 8.3 Sv to the west. This indi-
cated flow may have been another input into the Yucatan leg of the
Loop Current caused by flow to the west along the northern coast of
Cuba and flow toward the northwest along the southern coast of Cuba.

The fact that the largest portion of the volume transport generally

occurred in the upper 200 meters of water was also supported by the

24

velocity analysis (at different depths) with respect to 1000 meters.
At all times the velocities at; 0m (surface), 10m, 20m, 30m, 40m, 50m,
etc. decreased with depth. The higher velocities occurred in the upper
100 meters and water velocity decreased rapidly between 100 and 1000
meters .

D. EAST- WEST VOLUME TRANSPORT IN THE CENTRAL GULF

There have been differing thoughts concerning the possible con-
nection between the east and west Gulf waters. In the winter these
waters seem to be connected either by direct flow or by large detached
eddies which- have originated from the strong Loop Current. Leipper
made three North-South legs during cruise 68-A-2 along longitudes
87°50'W, 89°30'W, and 91°00*W respectively. The succeeding pages
contain an analysis of the East-West volume transports and the velo-
cities of selected water surfaces between the deep water stations only,
although data were also taken at shallow water stations.

Figure 3 indicates the three North-South legs in their entirety.
Leg I includes the segments between deep stations 38-43 and 47-49.
Leg II includes the segment between deep stations 61-68. Leg III
includes the segments between deep stations 74-79 and 82-83. Eastward
transport is indicated by a double line and westward transport (for
deep stations) by a single line. The eastward flow across the indicated
southern portions of each leg seemed to correlate well with the cyclonic
eddy indicated on Figure 4 centered in the eastern Gulf and extending
across the central Gulf. _

Tables V, VI, and VII indicate net geostrophic volume transport
and its direction in the indicated layers for pairs of hydrographic
stations for cruise 68-A-2.

25

TABLE V

CENTRAL GULF EAST-WEST GEOSTROPHIC VOLUME TRANSPORT, VELOCITY, AND
DIRECTION RELATIVE TO 1000 METERS - LEG I (Cruise 68-A-2)

STATION
PAIRS

LAYERS (m)

TRANSPORT (Sv)

VELOCITY (cm/sec)
*

DIRECTION
*

38-39

0-200
0,-500
0-1000

1.0

1.8
2.3

4.4

3.8

14.6

E
E
E

39-40

0-200
0-500
0-1000

2.4
5.2
6.2

16.7

7.0

15.9

E
E
E

40-41

0-200
0-500
0-1000

1.7
4.5
5.7

26.0
14.4
13.6

W
W
W

41-42

0-200
0-500
0-1000

J. 4
5.7
6.8

17.0

7.5

27.7

W
W
W

42-43

0-200
0-500
0-1000

0.4
0.7
0.9

1.0
1.5
2.8

E
E
E

43-48

0-200
0-500
0-1000

1.0
1.4
1.4

2.8

0.1
5.5

E
E

_ E

48-49

0-200
0-500
0-1000

0.5
0.8
1.1

3.1
1.4
4.7

W
W

w

*Note

E-East
W-West

The velocities for the 0-200 and 0-500 meter layers are those
of the bottom of the layers. The velocity for the 0-1000 meter
layer is that of the sea surface.

26

TABLE VI

CENTRAL GULF EAST-WEST GEOSTROPHIC VOLUME TRANSPORT, VELOCITY, AND
DIRECTION RELATIVE TO 1000 METERS - LEG II (Cruise 68-A-2)

STATION
PALRS

LAYERS (m)

TRANSPORT (Sv)

VELOCITY (cm/sec)
*

DIRECTION
*

61-63

0-200
0-500
0-1000

3.8
7.1
8.7

20.8
10.0
28.6

E
E
E

63-64

0-200
0-500
0-1000

2.8

5.4
6.6

14.5

7.2

19.5

W
W

w

64-65

0-200
0-500
0-1000

2.0
3.5
4.2

11.0

4.6

17.6

w
w
w

65-66

0-200
0-500
0-1000

~0.8
1.8
2.2

6.0
3.5
3.0

w
w
w

66-67

0-200
0-500
0-1000

0.7
1.2
1.5

3.3
1.8
3.1

w
w
w

67-68

0-200
0-500
0-1000

1.1
2.3

3.0

6.9
4.4
6.7

E
E

_ E

*Note

E-East
W-Wost

The velocities for the 0-200 and 0-500 meter layers are those
of the bottom of the layer. The velocity for the 0-1000 meter
layer is that of the sea surface. —

27

TABLE VII

CENTRAL GULF EAST-WEST GEOSTROPHIC VOLUME TRANSPORT, VELOCITY, AND

DIRECTION RELATIVE TC

l 1000 METERS - LEG III (Cruise 68-A-2)

STATION

LAYER (m)

TRANSPORT (Sv)

VELOCITY (cm/sec)

DIRECTION

PAIRS

*

*

0-200-

1.3

1.6

E

74-75

0-500

1.3

-0.2**

E

0-1000

1.2

15.3

E

0-200

0.7

4.9

E

75-76

0-500

1.4

2.0

E

0-1000

1.6

5.8

E

0-200

0.5

4.7

W

76-77

0-500

1.2

2.5

W

0-1000

1.4

1.2

w

0-200

0.-2

1.0

w

77-78

0-500

0.5

1.2

w

0-1000

0.7

5.2

w

0-200

0.4

1.2

w

78-79

0-500

0.5

0.2

w

0-1000

0.5

4.5

w

_

0-200

1.9

5.6

E

79-83

0-500

2.8

2.7

E

0-1000

3.2

20.9

E

0-200

0.3

0.5

w

82-83

0-500

0.5

1.1

w

0-1000

0.7

3.0

w

*Note :

E-East
W-West

The velocities for the 0-200 and 0-500 meter layers are those
of the bottom of the layers. The velocity for the 0-1000 meter
layer "is that of the sea surface.

**The negative sign for the velocity of the 500 meter surface for
station pair 74-75 represents a reversal in flow.

28

Station pairs 74-75 and 78-79 of Leg III were the only station
pairs where there was a reversal in the direction of flow between the
surface and 1000 meters. These reversals of flow account for the fact
that there was greater flow in the 0-200 and 0-500 meter layers than in
the 0-1000 meter layer for station pair 74-75 and that station pair
78-79 had no net flow from 500-1000 meters. In both cases, flow in the
deeper water was in the opposite direction of the surface waters, but
the magnitude of the flow was less than that of the surface layers.

To analyze East-West transport, the net volume transport and
direction in the layer from 0-1000 meters was used. Table VIII indi-
cates the total volume transport in the east and west direction and the
net volume transport and direction across each leg.

TABLE VIII

NET EAST-WEST DEEP WATER GEOSTROPHIC VOLUME TRANSPORT AND DIRECTION,
RELATIVE TO 1000 METERS, ACROSS LEGS I, II, AND III (Cruise 68-A-2)

LEG

WEST TRANSPORT
(Sv)

EAST TRANSPORT
(Sv)

NET TRANSPORT (Sv)
AND DIRECTION

I

13.6

10.8

2.8 West

II

14.5

11.7

2.8 West -

III

3.3

6.0

2.7 East

To analyze for volume continuity, the area enclosed by Legs I and
II and lines drawn from stations 49 to 61 and 38 to 68 will be called
area X, and the area enclosed by Legs II and III and lines drawn from
stations 68 to 74 and 61 to 82, area Y (see Figure 3). Table IX

29

indicates the total volume transport inputs and outputs across the
boundaries of these areas. Since the northern and southern boundaries
represent the beginning of shallow water (less than 1000 meters), in-
puts across these boundaries represent flow from shallow to deep water
and output represents flow from deep to shallow water.

TABLE IX

NET GEOSTROPHIC VOLUME TRANSPORT INPUTS AND OUTPUTS TO AREAS X AND Y,
RELATIVE TO 1000 METERS (Cruise 68-A-2)

AREA

BOUNDARY

TRANSPORT (Sv)

X

Leg I
Leg II

STATIONS 49-61
STATIONS 38-68

2.8 Input

2.8 Output
2.0 Input

2.9 Output

Y

Leg II
Leg III

STATIONS 68-74
STATIONS 61-82

2.8 Input
2.7 Input
0.6 Output
4.3 Output

These calculations indicate a deficit of 0.9 Sv in area X and a 0.6 Sv
excess in area Y. These apparent imbalances may have been caused by
motion of the 1000 meter surface.

Area Y was the apparent meeting place for eas-fe and west Gulf
waters. The majority of the water in area Y apparently flowed north
between stations 61 and 82.

30

In an attempt to compare the East-West volume transport of this
cruise with that of previous winter cruises, the positions of the hydro-
graphic stations of Legs I, II, and III were replotted on available
analyses of the depth of the 22 C isotherm for the following cruises:
the MABEL TAYLOR cruise of 8 February-27 April 1932; the ATLANTIS
cruise of 15 February- 13 April 1935; and the GERONIMO cruise of 21
February-31 March 1967. Using the available 22 C isotherm topography
provided a simple method of obtaining volume transport. Also, since
the MABEL TAYLOR cruise had no unprotected thermometers for accurate
depth determination, this method provided a means of calculating volume
transport for the cruise.

Comparison of the selected cruises was made by selecting the depth
of the 22 C isotherm for the replotted hydrographic stations. The
dynamic height anomaly for each station was determined by using Hewitt's
curve (Figure 2) for the correlation of the depth of the 22 C isotherm
versus the dynamic height anomaly of the sea surface, relative to 1000
meters. As discussed previously, this method of determining the dynamic
height anomaly permitted the calculation of volume transport, between
stations, only for the column of water from the water surface to 1000
meters, relative to 1000 meters.

The 22 C isotherm was not present at all stations for the cruises
because of the cold water. Stations 47 to 49 of Leg I, located on the
northern part of the leg, could not be used for any cruise because of
the absence of the 22 C isotherm.

Tables X and XI indicate the results of this analysis. The stations
analyzed for each leg are indicated in the tables.

31

TABLE X

NET EAST-WEST DEEP WATER GEOSTROPHIC VOLUME TRANSPORT AND DIRECTION,
RELATIVE TO 1000 METERS, ACROSS LEGS I, II, AND III FOR SELECTED
WINTER CRUISES

LEG

MABEL TAYLOR
(1932)*

ATLANTIS
(1935)*

GERONIMO
(1967)*

I

13.2 Sv (E)
(Stations 38-41)

15.8 Sv (E)
(Stations 38-43)

1.0 Sv (W)
(Stations 38-43)

II

13.8 Sv (W)
(Stations 61-67)

18.6 Sv (W)
(Stations 61-68)

1.7 Sv (W)
(Stations 61-68)

III

4.4 Sv (W)
(Stations 74-82)

7.8 Sv (W)
(Stations 74-82)

16.8 Sv (E)
(Stations 74-82)

*Note : E-East
W-West

An analysis of volume transport continuity for areas X and Y (see
Figure 3) was also made using boundaries which include only those
stations where the 22 C isotherm was present. Table XI indicates the
results .

The apparent imbalances varied from 0.1 Sv to 2.9 Sv . This error
may again have been caused by motion of the 1000 meter surface,, use of
a correlation curve which was not obtained from data for each specific
cruise, or by misinterpretation of the value chosen for the depth of
the 22 C isotherm as compared to its actual depth. The latter reason
was probably the cause of the large imbalance of 2.9 Sv. However, the
method of analysis, use of the depth of the 22 C isotherm, seemed to
provide a good indication of the volume transport for the. selected,
cruises .

32

TABLE XI

NET GEOSTROPHIC VOLUME TRANSPORT INPUTS AND OUTPUTS TO AREAS X AND
Y, RELATIVE TO 1000 METERS, FOR SELECTED WINTER CRUISES

AREA

CRUISE

BOUNDARY

TRANSPORT (Sv)

X

MABEL

TAYLOR

(1932)

Leg I (Sta. 38-41)
Leg II (Sta. 61-67)
STATIONS 41-61
STATIONS 38-67

13.2 Output
13.8 Output
12.8 Input
13.2 Input

. ATLANTIS
(1935)

Leg I (Sta. 38-43)
Leg II (Sta. 61-68)
STATIONS 43-61
STATIONS 38-68

15.8 Output
18.6 Output
21.5 Input
13.4 Input

GERONIMO
(1967)

Leg I (Sta. 38-43)
Leg II (Sta. 61-68)
STATIONS 43-61
STATIONS 38-68

1.0 Input
1.7 Output
4.4 Output
5.0 Input

Y

MABEL

TAYLOR

(1932)

Leg II (Sta. 61-67)
Leg III (Sta. 74-82)
STATIONS 67-74
STATIONS 61-82

13.8 Input
4.4 Output
0.0
8.2 Output

ATLANTIS
(1935)

Leg II (Sta. 61-68)
Leg III (Sta. 74-82)
STATIONS 68-74
STATIONS 61-82

18.6 Input
7 .8 Output
3.5 Output
4.4 Output

GERONIMO
(1967)

Leg II (Sta. 61-68)
Leg III (Sta. 74-82)
STATIONS 68-74
STATIONS 61-82

1.7 Input
16.8 Input
18.2 Output

0.7 Output

33

Use of the 22 C isotherm topography indicated that the Loop Current
and a large anti-cyclonic eddy in the central Gulf provided the large
inputs into the northern and southern boundaries of area X for the
MABEL TAYLOR cruise. The Loop Current apparently split in this area.
Part of it flowed west and was the driving force for the anti-cyclonic
eddy, and part of it turned eastward eventually turning toward the
Florida Straits. A similar circulation pattern existed for the GERONIMO
cruise, but the eddy did not provide input into the northern boundary of
area X.

An approximation of the volume transport input into the Gulf through
the Yucatan Channel was made for all three cruises. The volume trans-
ports for the MABEL TAYLOR and GERONIMO cruises were 24.6 Sv and 26.5
Sv respectively. These values correspond closely to the 26.6 Sv calcu-
lated for cruise 68-A-2. The volume transport for the ATLANTIS cruise
was 39.5 Sv.

Area X was the apparent meeting place of the water from the east
and west Gulf for the MABEL TAYLOR and ATLANTIS cruises. The GERONIMO
cruise and cruise 68-A-2 indicated area Y as the meeting place.

34

IV. CRUISE 67-A-6

A. GENERAL

Cruise 67-A-6 was conducted from 4 August to 22 August 1967 by
Leipper [1968], The cruise lasted 18 days, permitting only limited
coverage of the Loop Current. Figure 8 shows the cruise stations.
Five legs of the cruise crossed the Loop Current providing a good
indication of its location. Figures 9, 10, and 11 present the dynamic
topography (relative to 1000 meters) of the sea surface, 200 meter and
500 meter surfaces respectively. The locations of the extremities of
the current, as determined by bathythermograph data and dynamic topo-
graphy analysis, are indicated on Figure 12. The extremities of the
current are indicated by dashed lines and the location of the maximum
current is indicated by the solid line.

The dynamic topography for the surface (Figure 9), relative to
1000 meters, indicates a well defined anti-cyclonic eddy at 24 24'N,
88 55'W. The southern portion of the eddy had apparently moved on to
the Campeche Bank. Figure 9 also indicated an anti-cyclonic eddy
associated within the Loop Current whose flow apparently provided some
water which flowed west along the northern coast of Cuba and returned
to the Yucatan Channel as a southern current. Cruise station 60 was
not useable for this analysis because of obvious errors in recorded
salinities and temperatures at the standard depths from 500-1000
meters. Station 62 did not have any data for the_water surface, so
it was assumed that the dynamic height anomaly at the surface was the
same as that for 10 meters.

35

Analysis of the water between stations 61 and 62 indicated that the
surface layers of water (0-100 meters) and the water from 400-1000
meters did flow south while the water layers from 100-400 meters flowed
north. The net transport (0-1000 meters), referred to 1000 meters, was
only 0.2 Sv to the south. However, the maximum current (5.6 cm/sec) in
the entire column of water was found at 200 meters flowing to the north.
A section between stations 59 and 61 also indicated flow to the south-
east in the water layers from the surface to 250 meters with a maximum
velocity of 35 cm/sec at 100 meters (referred to 1000 meters). The
layers below 250 meters down to 800 meters indicated weak flow to the
northwest. A subsurface current to the west along the southern coast
of Cuba was also found by Gordon [1967]. Therefore, there was probably
some contribution to the Loop Current north of section A (Figure 12)
from subsurface flow to the northwest near the western tip of Cuba.

The Loop Current crossings were labeled A through E, as indicated
by Figure 12. The current entered the Gulf flowing to the north, it
turned northeast toward Florida, and as it reached the 3000 meter
depth contour it turned sharply to the southeast toward the Florida
Straits. Its outer extremity intruded into the Gulf 342 km from the
western tip of Cuba.

Analysis of section A indicated a broad Loop Current (from -extremity
to extremity). The sections between hydrographic stations 62, 63, and
64 only had an average sea surface velocity of 44 cm/sec and a maximum
sea surface velocity of 47 cm/sec located between stations 63 and 64.
The BT cross-section indicated a maximum velocity close to station 63.
However, Fomin's analysis of a shallow water hydrographic station, used
for station 65 on the western extremity of the Loop Current, indicated

36

a maximum sea surface velocity of 161 cm/sec between stations 64 and
65 flowing over the shallow bottom near the eastern coast of the
Yucatan Peninsula. This maximum was probably influenced by the fun-
neling of water between the Yucatan Peninsula and Cozumel Island.
Cochrane [1962] indicated that the maximum current velocity was within
an interval of 72 to 108 km north of Cozumel Island which is approxi-
mately the location of section A.

The largest portion of the total volume transport, relative to
1000 meters, in the Loop Current was in the upper 200 meters of water.
With the exception of the shallow water stations where the volume
transport in the upper layers could not be calculated, all but one
station pair indicated a greater transport in the 0-200 meter layer
than in the 200-500 meter or the 500-1000 meter layers (see Table
XIII).

The analysis of section B was difficult because the depth of the
22 C isotherm at that section varied so much in such short distances
that use of BT stations and the plot of the depth of the 22 C isotherm
versus the dynamic height anomaly for the surface relative to 1000
meters was not practical. A small error in station location would have
produced significant errors in the value of the dynamic height anomaly.
Calculations were made utilizing only hydrographic station data".

B. VELOCITIES

Table XII indicates the maximum sea surface current velocity and
the average sea surface velocity found at sections A through E.

The current broadened as it cleared the Campeche Bank and turned
to the northeast between sections B and C. This broadening probably
accounts for some of the decrease in the average velocity as the

37

TABLE XII

LOOP CURRENT SEA SURFACE VELOCITIES RELATIVE

(Cruise 67-A-6)

TO 1000 METERS

CROSS -SECTION

SECTION
EXTREMITIES *

AVERAGE VELOCITY
(cm/sec)

MAXIMUM VELOCITY
(cm/sec)

A

62-65 (S)

83.1

161.0

B

78-159 (BT)

101.1

107.0

C

52-55

74.4

122.5

D

91-95

58.9

98.9

E

89-183 (BT)

50.1

60.8

*Note : S-Shallow water station

BT-Bathythermograph station

current moved toward the Florida Straits. At section E the current
narrowed again, but the average velocity still decreased. The indi-
cated decrease is probably due to the fact that the northern portion
of the cruise crossing at section E crossed the current at an oblique
angle, and the geostrophic velocity components, being perpendicular to
a line between stations, are not representative of the actual Loop Cur-
rent velocity. The "Volume Transport" section of this cruise indicates
a large increase in geostrophic volume transport at section E as com-
pared to sections C and D. The cross-sectional area at section E
decreased as compared to sections C and D. To permit an increase in
volume transport and a decrease in cross-sectional area, the velocity
must have increased. Assuming that the northern portion of cross-
section E had crossed the Loop Current at a right angle, it was

38

calculated that the average velocity would have been 83 cm/sec and the
maximum velocity would have increased to 105 cm/sec. These values are
considered more indicative of the velocities of the Loop Current at
section E than the values indicated in Table XII.

The section of the eddy indicated on Figure 12 at 24 24'N,
88 55 !W was located over deep water (greater than 1000 meters). The
maximum velocity of this section was 76 cm/sec and the average velocity
was 54 cm/sec. Figure 9 indicates that water from the western Gulf
flows eastward around the eddy over deep water and then returns to the
western Gulf over the Campeche Bank. It is probable that some of this
water continued into the eastern Gulf and became part of the Loop Cur-
rent, providing an exchange of water from the western to the eastern
Gulf.

C. VOLUME TRANSPORT

Table XIII indicates the volume transports calculated for layers
of 0-200, 0-500, and 0-1000 meters with respect to 1000 meters.
Shallow water analysis was used for sections A, B, and E.

To analyze volume transport continuity, the entire water column
from 0-1000 meters was used. This presented an overall view of the
volume transport in the Loop Current as the current proceeded from
the Yucatan Channel to the area approximately 198 km west of the
Florida Straits (Section E) .

A net volume transport of 27.5 Sv crossed section A (Figure 12)
with the majority of the water flowing across the— section in the upper
200 meters of water. At section B, 34.9 Sv crossed the section.
Station pair 77-75 of this section (Table XIII) indicates volume
transports in the layers 0-200 and 0-500 meters, relative to 200 meters

39

TABLE XIII

LOOP CURRENT GEOSTROPHIC VOLUME TRANSPORT RELATIVE TO 1000 METERS

(Cruise 67-A-6)

CROSS SECTION

STATION PAIRS
*

0-200 m (Sv)

0-500 m (Sv)

0-1000 m (Sv)

A

62-63
63-64
64-65 (S)

4.0
4.0

6.5
7.4

8.2

10.3

9.0

B

77-78
77-75

7.0
(2 .7)***

12.5
(10.0)***

15.9
19.0

C

52-53
53-54
54-55

10.0
3.2
0.3

16.2
6.0
1.2

19.3
7.4
1.3

D

91-92
92-94
94-95

4.4
1.5
9.6

7.1

0.2

17.1

7.9
-0.3**
20.6

E

88-89

88-183 (S,BT)

5.8

(4.6)***

10.4
(16.5)***

12.7
17.0

*Note : S-Shallow water station

BT-Bathythermograph station

**Note : The minus sign indicates the volume transport in the opposite
direction.

***Note : The numbers in parentheses are the volume transports in the
layer relative to the bottom of the layer.

40

and 500 meters respectively. These volume transports were calculated
relative to the bottom of the layer because the stations were in
shallow water and because the values indicate which layer had the
greatest volume transport. Volume transport for the 0-1000 meter
layer for station pair 77-75 was calculated by computing the volume
transport from hydrographic data for the water surface to 800 meters
(referred to 800 meters) between stations 77 and 76 and then, assuming
that a similar volume transport from 800-1000 meters as computed be-
tween stations 76 and 78 also flowed between stations 76 and 77, an
additional 0.5 Sv was added. To this value was added 0.8 Sv , esti-
mated from shallow water calculations between stations 75 and 76. The
increase of volume transport from section A to B is probably due to
the westwardly subsurface flow along the southern coast of Cuba and by
a possible input from the anti-cyclonic eddy north of Cuba.

At section C there was a decrease in volume transport of 6.9 Sv as
compared to section B. It was found that approximately 5 Sv passed
between stations 51 and 52 (see Figure 8). These stations are located
adjacent to the northwest extremity of the Loop Current at section C.
This water was probably lost by the current as the current turned
eastward. Nowlin and McLellan [1970] indicated that there was a loss
from the Loop Current in the region of the northern Campeche Bank due
to an apparent branching off to the west of part of the current.
Stations 98 and 99 indicated a flow to the northwest which may have
been caused by the branching off of the Loop Current. Therefore, the
difference in volume transport of 6.9 may be accounted for, at least in
part, by the losses due to the turning and possible branching off of
the current. -

41

Section D indicates an increase of 0.5 Sv in volume transport as
compared to section C. Figure 9 indicates a cyclonic eddy to the north
of section D. The increase in volume transport of the Loop Current at
section D was probably due to the entrainment of water circulation from
around this eddy. The negative sign for the volume transport in the
0-1000 meter layer for stations 92 and 94 of this section was due to a
volume transport to the west in the layer from 200 to 700 meters below
the surface. This westward volume transport of 1.8 Sv , between these
two depths, provided a net westward volume transport of 0.3 Sv in the
layer from the surface to 1000 meters, relative to 1000 meters. BT
data for section D did not indicate a reversal in current direction
because the reversal occurred below 200 meters. The magnitude of the
velocity component at the sea surface, relative to 1000 meters, was
18 cm/sec between stations 92 and 94, which is considered low for the
Loop Current. However, the magnitudes of the velocities components
between stations 91 and 92 and stations 94 and 95 were 59 cm/sec and
99 cm/sec respectively. This indicates that stations 92 and 94 were
probably in the Loop Current in spite of the low velocity.

As the current approached the Florida Straits, the volume trans-
port increased to 29.7 Sv at section E. The increase was probably
caused by entrainment of water from the Florida shelf. However, there
was no data available to verify southward flow over the shelf.

The apparent difference of 2.2 Sv in volume transport into the
Gulf by the Loop Current and that approaching the Florida Straits was
probably due to the southwestward flow around the anti-cyclonic eddy
just north of Cuba.

42

The volume transport across a section of the anti-cyclonic eddy
north of the Yucatan Peninsula was 21.3 Sv between stations 105 and
231 (BT) . This is of significant magnitude and indicates that the
eddy contained a strong current and may have become detached from the
Loop Current .

43

V. COMPARISON OF CRUISES 67-A-6 AND 68-A-2

Cruise 67-A-6 was conducted during the late summer of 1967 and
cruise 68-A-2 was conducted late during the following winter. A com-
parison of these cruises (Figures 3-11) gave a good indication of
what changes in water circulation occurred primarily to the Loop
Current, during the fall and early winter.

The Loop Current entered the Gulf in the late summer with an
average sea surface velocity of 83 cm/sec and a questionable maximum
sea surface velocity of 161 cm/sec (calculated from a shallow station),
By late winter the average sea surface velocity and the maximum velo-
city had decreased. The locations of the maximum sea surface velocity
and the extremities of the current were farther east of the Yucatan
Peninsula in the summer than in the winter. Volume transport into the
Gulf was about the same for both cruises.

Both cruises passed along the same line on their southwest transit
toward the Yucatan Channel. Stations 45 to 55 for cruise 67-A-6
(Figure 8) and stations 14 to 22 for cruise 68-A-2 (Figure 3) define
this path. Section C of cruise 67-A-6 and section B of cruise 68-A-2
were contained between the respective stations. A comparison of
sections B and C indicated that the location of the Loop Current had
apparently moved to the northwest and had broadened from the summer
to the winter. The location of the maximum sea surface current had
moved 115 km, but the magnitude had only decreased 3 cm/sec. With
the exception of the indicated 161 cm/sec maximum sea surface velocity
as the current entered the Gulf on cruise 67-A-6, the value of the
maximum sea surface velocity for both cruises was largest at these

44

two sections. Further, the magnitude of the average velocity had only
decreased by 6 cm/scc. The volume transport between the two sections
increased from the summer to the winter from 28 to 42.3 Sv. This
increase may have been partly caused by the apparent eastward flow
across the Campeche Bank during the winter.

The eastern portion of the Loop Current was not observed. No
comparison therefore could be made with the current as it approached
the Florida Straits.

The apparent detached anti-cyclonic eddy indicated by the summer
cruise possibly moved off the Campeche Bank during the winter resulting
in the observed anti-cyclonic eddy in the western Gulf for the winter
cruise. If so, the intensity of the eddy apparently decreased over
this time.

The analyzed charts of the depth of the 22 C isotherm, for both
cruises, indicated a cold ridge crossing the Gulf from the shelf off
the western coast of Florida to the Campeche Bank. The charts of
dynamic topography for the water surface, relative to 1000 meters,
(Figures 4 and 9) indicated the presence of this cold ridge by areas
of low dynamic topography north of the Loop Current. This cold ridge
apparently persisted between the cruises, so the Loop Current did
not move any farther north than 25 N latitude.

45

VI. COMPARISON OF CRUISES 65-A-ll, 65-A-13, 66-A-15, AND 67-A-6

Cruise 65-A-ll was conducted from 10-24 August 1965. Hurricane
BETSY occurred immediately after this cruise and an opportunity was
provided to make observations in areas just surveyed by cruise 65-A-ll
This was cruise 65-A-13 of 12-24 September 1965. Cruise 66-A-15 was
conducted from 27 October-13 November 1966. These three cruises were
analyzed by Hewitt [1970] using Equation (1), Appendix A, to compute
volume transport .in the layers 0-200m, 0-500m, and 0-1000m, relative
to the bottom of the respective layers. The Loop Current and eddy
extremities were also determined by Hewitt using the BT method previ-
ously discussed in this paper.

A comparison of the cruises was made, since the three cruises were
conducted during approximately the same season (late summer to fall)
as cruise 67-A-6. A comparison of Hewitt's volume transports in the
selected layers relative to the bottom of the layers, to those rela-
tive to 1000 meters, was made using the same station pairs. The
inferred dynamic heights from the depth of the 22 C isotherm for BT
or shallow stations were not changed. Tables XIV, XV, XVI, and XVII
indicate the BT and shallow stations by an (I).

Previous analyses in this paper were made of volume transport
around the Loop Current. Hewitt also analyzed volume transport around
observed eddies, referring to this transport as axial volume transport
Axial volume transport was computed using the station at the apparent
center of an eddy together with a station at the outer extremity of
the eddy. Tables XIV, XV, and XVI indicate the axial volume trans-
port in the 0-200, 0-500, and 0-1000 meter layers, relative to 1000

46

meters and to the bottom of the respective layers. The volume trans-
port in the 0-1000 meter layer was, of course, the same when using
1000 meters or the bottom of the layer. However, there were signifi-
cant differences in volume transports in the 0-200 meter and 0-500
meter layers when the deeper reference level was used.

Cruise 65-A-ll observed one well defined anti-cyclonic eddy cen-
tered at 25 15'N, 87 25'W which apparently became detached from the
Loop Current. The eddy extended from 23°20'N to 28°N and from 85°W
to 89 W. This eddy was crossed five times, providing a good volume
transport continuity analysis. The Loop Current was crossed only once

TABLE XIV

AXIAL GEOSTROPHIC VOLUME TRANSPORT IN SELECTED LAYERS RELATIVE TO 1000
METERS AND TO THE BOTTOM OF THE RESPECTIVE LAYERS (CRUISE 65-A-ll)

STATION
PAIRS *

LAYERS
(m)

VOLUME TRANSPORT
RELATIVE" TO 1000 m (Sv)

VOLUME TRANSPORT RELATIVE
TO BOTTOM OF LAYER (Sv)

22(I)-26
(E)

0-1000

34.9

34.9

18-26
(E)

0-200
0-500
0-1000

21.0
35.5
41.9

6.0

20.4
41.9

26-27
(E)

0-200
0-500
0-1000

20.0
34.5
41.4

6.0
18.5
41.4

24(I)-26
(E)

0-1000

43.8

43.8

25-26
(E)

0-200
0-500
0-1000

18.5
30.5
36.3

6.3

17.7
36.3

25(I)-25(J)
(Y)

0-1000

20.3

20.3

*Note : I or J-station used an inferred dynamic height
E-Eddy
Y-Yucatan current

n

One large well defined anti-cyclonic eddy was observed by cruise
65-A-13. This eddy consisted of two smaller eddies referred to as the
"upper eddy" and the "lower eddy". The "lower eddy" was centered at
24°19'N, 87°25'W. The "upper eddy" was centered at 26°20'N, 86°55*W.
The combination of eddies extended from 23 10'N to 26 30'N and from
85 10'W to 88 30'W. A small anti-cyclonic eddy was observed to the
north of the "upper eddy". The Loop Current was not observed on this
cruise .

TABLE XV

AXIAL GEOSTROPHIC VOLUME TRANSPORT IN SELECTED LAYERS RELATIVE TO 1000
METERS AND TO THE BOTTOM OF THE RESPECTIVE LAYERS (Cruise 65-A-13)

STATION
PAIRS *

LAYERS
(m)

VOLUME TRANSPORT
RELATIVE TO 1000 m (Sv)

VOLUME TRANSPORT RELA-
TIVE TO BOTTOM OF LAYER
(Sv)

34-16
(U)

0-200
0-500
0-1000

11.8
17.2
19.2

5.3
12.2
19.2

13-16
(U)

0-200
0-500
0-1000

- 10.8
16.8
19.0

4.1
11.2

19.0

16-28(1)
(U)

0-1000

19.0

19.0

16-19
(U)

0-200
0-500
0-1000

10.5
16.0
18.2

4.3
10.6
18.2

22-23
(L)

0-200
0-500
0-1000

8.4
13.3
15.5

3.2

8.4

15.5

22-20

(10

0-200
0-500
0-1000

9.1
13.9
16.5

4.0 -
8.6
16.5

22-21(1)
(L)

0-1000

13.4

13.4

12-13
(N)

0-200
0-500
0-1000

3.1
5.3

6.4

1.2
2.9

6.4

10(I)-12
(N)

0-1000

6.7

6.7

*Note : I-station used an inferred dynamic height
U-Upper eddy
L-Lower eddy
N-Northern eddy

48

The Loop Current was observed nine times on cruise 66-A-15. It
entered the Gulf at the Yucatan Channel and flowed north to the northern
tip of the Campeche Bank and then turned to the west into the central
Gulf. The farthest extension to the west was 91 30'W. The current
then turned northeast until it turned at 28°30'W, 88°30'W toward the
Florida Straits. The current consisted of a northern and southern eddy.
Hewitt [1970] analyzed the axial volume transport around the eddies
(Table XVI) and the Loop Current volume transport (Table XVII). The
stations which are underlined in these tables represent a second
station with the same number as a previous station on this same cruise.

The variation in volume transport in the 0-200 meter and 0-500
meter layers, using the layer bottoms on one hand and the 1000 meter
surface of the other, was caused by variation in the geostrophic
velocities of the 200 and 500 meter water surfaces. Plots of water
velocity, relative to 1000 meters, versus depth for cruises 65-A-ll,
65-A-13, and 66-A-15 indicated that the majority of the velocities
(relative to 1000 meters) of the 200 meter surface were at least 40
percent of the velocity (relative to 1000 meters) of the sea surface.
The 500 meter water surface had an average velocity (relative to 1000
meters) of 10 cm/sec for most cruises. Therefore, any assumption that
these water surfaces were levels of no motion would produce differ-
ences in the volume transport indicated in the preceeding tables.

There was a small range of volume transport differences for cruise
65-A-ll of 12-15 Sv (Table XIV) at each layer for all station pairs.
An analysis of the velocities of the 200 and 500 meter water surfaces
and the difference in dynamic heights of the sea surface, both rela-
tive to 1000 meters, at each station pair indicated the following ;" a

49

TABLE XVI

AXIAL GEOSTROPHIC VOLUME TRANSPORT IN SELECTED LAYERS RELATIVE TO 1000
METERS AND TO THE BOTTOM OF THE RESPECTIVE LAYERS (Cruise 66-A-15)

STATION
PAIRS *

LAYERS
(m)

VOLUME TRANSPORT
RELATIVE TO 1000 m (Sv)

VOLUME TRANSPORT RELATIVE
TO BOTTOM OF LAYER (Sv)

12-T(I) (S)

0-1000

30.3

30.3

11-5(1) (S)

0-1000

49.2

49.2

12-6(1) (S)

0-1000

48.6

48.6

12-16(I)(S)

0-1000

46.1

46.1

12-21

(S)

0-200
0-500
0-1000

19.5
30.1
35.0

-

12-22
(S)

0-200
0-500
0-1000

4.8

8.8

11.1

0.1

4.0

11.1

12-24
(S)

0-200
0-500
0-1000

23.3
38.8
45.9

45.9

_12-25(T)(S)

0-1000

42.9

42.9

12-26(I)(S)

0-1000

39.9

39.9

12-2
(S)

0-200
0-500
0-1000

16.0
26.6
31.5

36.4

12-3
(S)

0-200
0-500
0-1000

4.8
7.9
9.6

9.6

7-22
(N)

0-200
0-500
0-1000

4.4 West

8.2 West

"9.1 West

0.4 East
6.6 West
9.1 West

7-21
(N)

0-200
0-500
0-1000

18.5
28.7
31.9

7.0
21.5
31.9

7-19
(N)

0-200
0-500
0-1000

21.2
33.8
37.8

7.6
23.9
37.8

7-4
(N)

0-200
0-500
0-1000

18.7
30.2
34.3

6.3
20.9
34.3

7-11
(N)

0-200
0-500
0-1000

20.1
32.5
36.7

6.9 -

22.2

36.7

7-15
(N)

0-200
0-500
0-1000

21.2
34.9
39.8

7.0
23.2
39.8

7-24
(N)

0-200
0-500
0-1000

22.2

37.0
42.5

7.0
24.0
42.5

*Note : T-Inferred station 19 km west of the western tip of Cuba
I-Station used inferred dynamic height
S-South eddy
N-North eddy ~"

50

TABLE XVII

LOOP CURRENT GEOSTROPHIC VOLUME TRANSPORT IN SELECTED LAYERS RELATIVE
TO 1000 METERS AND TO THE BOTTOM OF THE RESPECTIVE LAYERS

(Cruise 66-A-15)

STATION
PAIRS *

LAYERS
(m)

VOLUME TRANSPORT
RELATIVE TO 1000 m (Sv)

VOLUME TRANSPORT RELATIVE
TO BOTTOM OF LAYER (Sv)

6(D-7(I)

0-1000

16.6

16.6

7(I)-10(I)

0-1000

23.0

23.0

20(I)-21

0-1000

11.6

11.6

21-22

0-200
0-500
0-1000

14.5
20.8
23.1

6.6
15.1
23.1

17-19

0-200
0-500
0-1000

18.1
28.1
32.0

7.0
18.8
32.0

4-6

0-200
0-500
0-1000

14.2
22.3
25.7

5.8

14.4
25.7

6-7

0-200
0-500
0-1000

4.5

_ 8.1

8.7

.6
6.6

8.7

7-9

0-200
0-500
0-1000

6.1
10.7
11.9

1.1

8.1
11.9

9-11

0-200
0-500
0-100

13.9
21.5
24.6

5.7
14.0
24.6

15-17

0-200
0-500
0-1000

18.2
29.4
34.2

6.4 -
18.3
34.2

22-24

0-200
0-500
0-1000

18.2
29.4
34.1

6.7
17.9
34.1

26(I)-26(J)

0-1000

32.4

32.4

l(D-3

0-1000

27.0

27.0

*Note : I or J-station used an inferred dynamic height

51

range of velocities at 200 meters of 24-26 cm/sec, a range of velo-
cities at 500 meters of 10-12 cm/sec, and a range of dynamic height

2 2

differences of 0.7-0.8 m /sec .

The relatively constant volume transport difference for the 0-200
meter and 0-500 meter layers for each individual station pair was
explained by the fact that the water in the 0-200 meter layer was
flowing through a vertical cross-section of 200 meters multiplied by
the distance between the station pair at an added velocity of 24 cm/sec,
while the water in the 0-500 meter layer was flowing through an area 2.5
times as large as that between 0-200 meters but with an added velocity
which was only 0.4 (1/2.5) times that of the water between 0-200 meters.
As explained previously in the section on procedures, station pairs with
the same difference in dynamic heights of the sea surface, relative to
1000 meters, will have the same total transport in the 0-1000 meter
layer, but the distribution of the volume transport by layers may
differ for individual station pairs. However, since the velocities
of the 200 and 500 meter water surfaces are relatively constant for all
station pairs of cruise 65-A-ll, the volume transport distribution in
the 0-200 meter and 0-500 meter layers must have been such as to pro-
duce a relatively constant volume transport difference for the 0-200
meter layer at all station pairs and the 0-500 meter layer at all
station pairs .

The differences in volume transport (Table XV) in the 0-200 meter
and 0-500 meter layers for cruise 65-A-13 were approximately 5-6 Sv
except for stations 12-13, which had a difference of approximately 2
Sv. The average velocities (relative to 1000 meters) of the 200 and
500 meter water surfaces were 25 cm/sec and 11 cm/sec respectively.,

52

for all station pairs except station pair 12-13. Station pair 12-13
had a velocity, relative to 1000 meters, of 16 cm/sec at 200 meters
and 8 cm/sec at 500 meters. The range of dynamic height differences

of the sea surface, relative to 1000 meters, for alT station pairs

2 2
except station pair 12-13, was 0.3-0.5 m /sec . This constant volume

transport difference was explained by the same reasons as given for
cruise 65-A-ll. The smaller difference was due to the smaller differ-
ences in dynamic heights of the sea surface, relative to 1000 meters.

Different velocities of the 200 and 500 meter water surfaces were
found for cruise 66-A-15, accounting for significant volume transport
differences in the 0-200 meter and 0-500 meter layers, but no rela-
tively constant difference was found as for cruises 65-A-ll and
65-A-13. For station pair 7-22 (Table XVI), Hewitt indicated an
eastward transport in the 0-200 meter layer. This was apparently
the only instance a reversal in direction of volume transport in one
layer, as compared to other layers, occurred in Hewitt's computations.
However, calculations of volume transport, relative to 1000 meters,
indicated westward transport throughout the entire column of water
from the sea surface to 1000 meters. This indicates that it is
possible to have a direction reversal in a layer when a different
reference level is chosen.

These tables also indicate that, as for cruise 67-A-6, the largest
portion of volume transport, relative to 1000 meters, for all cruises
occurred in the layer 0-200 meters as compared to the layers of 200-
500 meters and of 500-1000 meters. For the majority of station pairs,
over half of .the total transport in the layer of 0-1000 meters
occurred in the upper 200 meters of water. When the botfom of the

53

layer was used as the reference level, the volume transport in the
0-200 meter layer was much less than the 200-500 meter and 500-1000
meter layers .

The difference of 7.5 Sv between station pairs 24(I)-26 and 25-26
for cruise 65-A-ll (Table XIV) was apparently caused by a loss of
volume transport through the Florida Straits. To provide volume trans-
port continuity, the 7.5 Sv were added to the 20.3 Sv between station
pair 25(I)-25(J) providing an inferred volume transport of 27.8 Sv into
the Gulf. This value corresponds closely to the 27.5 Sv of input into
the Gulf for cruise 67-A-6. Cruise 66-A-15 indicated a volume transport
of only 19.6 Sv through the Yucatan Channel.

The location of the Loop Current in the Yucatan Channel for cruise
65-A-ll was considerably farther to the east as compared to cruises
66-A-15 and 67-A-6, and flowed along the northern coast of Cuba. The
Loop Current for cruises 66-A-15 and 67-A-6 was close to the Yucatan
Peninsula and flowed generally to the north into the eastern Gulf. The
Loop Current for cruise 66-A-15 intruded into the central Gulf and as
far north as the 1000 meter depth contour south of the Mississippi Delta
before turning toward the Florida Straits. Cruise 67-A-6 observed the
Loop Current turning toward the Florida Straits at 25 N latitude, never
intruding into the central Gulf. -

All the cruises except 66-A-15 observed at least one major eddy
which apparently had become detached from the Loop Current. Cruise
66-A-15 observed a northern and a southern loop within the Loop Current
which apparently had closed flows around their centers. The eddies for
cruises 65-A-.11 and 65-A-13 were well defined and located in the eastern
Gulf. The eddy observed by cruise 67-A-6 was in the western Gulf, but
only one section of it was observed.

54

VII . SUMMARY OF LOOP CURRENT AND EDDY VELOCITIES AND VOLUME
TRANSPORTS FOR NINE SUMMER AND WINTER CRUISES IN THE
GULF OF MEXICO FROM 1965-1968

Calculated sea surface velocities and volume transports for the
Loop Current and observed eddies of selected ALAMINOS cruises, 65-A-ll,
65-A-13, 66-A-8, 66-A-ll, 66-A-15, 67-A-l, 67-A-6, 68-A-2 , and 68-A-8
are summarized in Table XVIII and XIX. The values of volume transports
are for the entire water column from the sea surface to the chosen
reference level. _ Cruise 66-A-8 chose 1350 meters as the reference
level, while" the remaining cruises chose 1000 meters. Also, cruise
66-A-8 did not observe the Loop Current in the Yucatan Channel, so the
values indicated in Table XVIII are for a cross-section 360 km north
of the channel.

Cruise 67-A-l only took hydrographic measurements down to 300
meters, so the tables only indicate the volume transport in the upper
300 meters of water. The upper 200 to 300 meters of water usually
accounted for approximately 50 per cent of the volume transport in
previous analyses, so doubling the indicated values should be indica-
tive of the magnitude of the volume transport for the entire column of
water from the sea surface to 1000 meters.

Tables XVIII and XIX present an indication of the variations in
the calculated values for seasons and in time.

55

i

TABLE XVIII

SUMMARY OF LOOP CURRENT SEA SURFACE VELOCITIES AND VOLUME TRANSPORTS,
RELATIVE TO THE CHOSEN REFERENCE LEVEL, AT THE YUCATAN CHANNEL. FOR
SELECTED CRUISES

CRUISE

REFERENCE

LEVEL

(m)

AVERAGE

VELOCITY

(cm/sec)

MAXIMUM

VELOCITY

(cm/sec)

VOLUME
TRANSPORT
(Sv)

65-A-ll
(August)

1000

-

121

27.8

65-A-13
(Sept.)

1000

No Lo

op Current observed

66-A-8
(June)

1000

-

100

36.0

66-A-ll
(Augus t )

1350

-

102

45.0

66-A-15
(Oct-Nov)

1000

-

125

46.1

67-A-l
(April)

300

No Lo

op Current observed

67-A-6
(August)

1000

83

161*

27.5

68-A-2
(Feb -Mar)

1000

80

91

26.6

68-A-8
(August)

1000

70

107

23.3

*Note: This velocity was the result of a shallow station analysis

56

TABLE XIX

SUMMARY OF OBSERVED ANTI-CYCLONIC EDDY SEA SURFACE VELOCITIES AND
VOLUME TRANSPORTS, RELATIVE TO THE CHOSEN REFERENCE LEVEL, FOR
SELECTED CRUISES

CRUISE

REFERENCE

LEVEL

On)

LOCATION OF
EDDY CENTER

RANGE OF MAXI-
MUM VELOCITIES
AROUND THE EDDY
(cm/sec)

AVERAGE VOLUME
TRANSPORT
(Sv)

65-A-ll
(August)

1000

25°15'N
87°25'W

65-129

39.6

65-A-13
(Sept.)

1000

26°20'N
86°55'W

49-120

19.0

1000

24°19'N
87°25'W

55-73

15.1

- 1000

28°20'N
87°13!W

33-35

6.6

66-A-8
(June)

1000

25°30'N
86°30'W

100-150

45.0

66-A-ll
(August)

1350

26°00'N
88°00'W

101-116

42.7

1350

23°50'N
85°45'W

106*

41.2

66-A-15
(Oct-Nov)

1000

26°10'N
88°55'W

80-183

37.5

1000

23°45*N
85°50'W

91-130

41.0

67-A-l
(April)

300

26°35'N
91°05'W

11-33

1.5

67-A-6
(Augus t )

1000

24°24'N
88°55'W

76*

21.3

68-A-2
(Feb -Mar)

1000

24°30'N
93°42'W

21*

7.1

68-A-8
(August)

1000

25°30'N
87°00'W

55-130

35.0

*Note : Only one section of the eddy was observed

57

VIII. CONCLUSIONS

The planning of a cruise to observe and study the Loop Current is
a formidable task. The location of the current varies with time, and
no definite relation between the current's location and time has been
proposed. The current does enter the Gulf through the Yucatan Channel
and exits the Gulf through the Florida Straits, so these two areas
could be used to start a search for the current. However, even in
these two areas , the current's location varies greatly with time.
There does seem to be a correlation between the current's location in
the Yucatan Channel in the fall and winter seasons. The current seems
to be closer to the Yucatan Peninsula in the winter than in the fall.

The method of determining the current's extremities from the slope
of the 22 C isotherm or the 18 C isotherm seems to lend itself as a
plausible method to ensure that hydrographic stations are made at the
proper places to observe the current. Use of X-BT's to obtain tempera-
ture versus depth information is a rapid means of obtaining a BT cross-
section of an area. With this information the current can be located
and hydrographic stations made at the proper places.

The shallow water station (depth of the water is less than the
depth of the chosen reference level) is the most subjective part of
the analysis of the current's volume transport and velocity. When a
shallow station is required to be made, it is recommended that current
meters be used to obtain the velocity versus depth profile. Although
use of these meters would require more time on station, a much better
analysis could be made. This problem is important because the current
in the Yucatan Channel and the Florida Straits normally is flowing

58

over a shallow bottom. Also, the Campechc Bank and the Florida Shelf
are areas which should be observed so that the circulation pattern in
the Gulf can be better understood. To do this, shallow water stations
must be utilized.

The correlation of the depth of the 22 C isotherm with dynamic
height of the sea surface, relative to the chosen reference level,
seems to be a good method for determining the dynamic height for a BT
or shallow water station. A correlation curve should be made for each
individual cruise when possible. However, for cruises where the dynamic
topography was not determined, this method, using a correlation curve
from another cruise, seemed to be successful.

If the observed data are used to calculate the dynamic height and
the transport function (Q) assuming the sea surface to be a level of
no motion, Equation (1) of Appendix A may be used to calculate the
volume transport between a station pair relative to the bottom of the
layer or to a true level of no motion which may be determined after
the cruise is completed. However, the meaning of the terms in Equation
(1) change when the volume transport is not calculated relative to the
bottom of the layer. Also, when the volume transport is calculated
relative to the bottom of the layer, the layer must include the sea
surface as its top boundary. Choosing a reference level and calcu-
lating volume transport in increments from that level to the sea
surface provided the best analysis of the volume transport between
station pairs .

59

APPENDIX A

EQUATIONS UTILIZED TO COMPUTE GEOSTROPHIC VOLUME TRANSPORT AND VELOCITY

Equations (l)-(3) are given by McLellan [1965; pages 70-71].
Equation (4) is given by Fomin [1964; page 151]. The value of 10 was
used for C.

(1) T(M,N)j = - f •

%j "V " <ADNJ " Wl

T = transport between stations M and N relative to the

.th 1
j level

f = Coriolis parameter

j (AD. + AD )
i l-l

Q = £ 5 • AZ . = transport function

i=l X

AZ. = difference in depths of the (i) and (i-1) surfaces
i

AD = dynamic height anomaly

Z = chosen reference depth

C • (ADN - AD )
(2> V= flg-f

AX = distance between stations

AX • AZ • (V . + V . )
(3) AT. = y i - 1. 2, ••'

AT = increment of volume transport

(4) A = l/2-h-(cr - CL,. ,, )
T)eep Shallow

a = specific volume

h = difference in depths of stations expressed in
pressure units

60

o
oo

00

in

Ul
CO

D
Z

Z

o

<

CQ

o

8

o

z

z

° 2

o

CO

•o

0

.«/

•

/

/

1

1

-

CM

- o

00

1

i

1

J

o

o

in

8

X
O

o

«n

o
o
r*

o

m

c

1-4

u

u

a.
o
o

o

a

o

■ri
■U
O

0)
OT

CO
»

o
u
u

3
■u
CO

(-1

a»

Q,

E

CD

H

3

fn

61

LO

CD

CD

lO

^*;co

CO
LU
CO

c^

CO

C3

CO

CM

ex»

CD

o

CO

^1

3
CO

CO
(D
CO

(D

4-1
o

CO
E
o
c
<

00
•H
CO

o
•■-I
B

CO

c
Q
to

CO
>

<U
X!
4J

O

co

M

U

0
CM CO

cm ;-i

CO

4-1 4-1
O CU

E

X!

4J O

0.0
CO O

Q <H

CM

.°-. =s

C3

est

O

M

in
CM

62

o
to

in

CVJ

o

CVJ

Station Locations, Cruise 68-A-2
Figure 3

63

o
to

ID
04

O

Dynamic Topography of the Sea Surface Relative
to 1000 Meters (Cruise 68-A-2)

Figure 4

64

W&9^*,tWa, "^'

o

CM

O
C\J

Dynamic Topography of the 200 Meter Surface
Relative to 1000 Meters (Cruise 68-A-2)

Figure 5

65

Dynamic Topography of the 500 Meter Surface
Relative to 1000 Meters (Cruise 68-A-2)

Figure 6

66

Location of Loop Current (Cruise 68-A-2)
Figure 7

67

Station Locations, Cruise 67-A-6

Figure 8

68

o
ro

m

CM

O

C\J

Dynamic Topography of the Sea Surface Relative
to 1000 Meters (Cruise 67-A-6)

Figure 9

69

o

ro

10

O

Dynamic Topography of the 200 Meter Surface
Relative to 1000 Meters (Cruise 67-A-6)

Figure 10

70

o

o

CM

Dynamic Topography of the 500 Meter Surface
Relative to 1000 Meters (Cruise 67-A-6)

Figure 11

71

o

if)
CM

e

o

CM

Location of Loop Current (Cruise 67-A-6)

Figure 12

72

C THIS PROGRAM COMPUTES GEOSTROPHIC VOLUME

C TRANSPORT AND VELOCITY OF WATER LAYERSt

C RELATIVE TO 1COO ME TERS , BETWEEN PAIRS OF

C HYDROGRAPHIC STATIONS

C

IMPLICIT INTEGER*4(Z)

REAL*^ LAT1 ,LAT2,L0NG1 ,LONG?

DIMENSION SVAH19) , SVA2(19), Z ( 19 ) , V( 1° 1 , DT( 19 ) , TO ( 1Q ) ,
*SDD1(19) ,SDD2(19) ,0(19), DD (19)
C

C OMEGA IS THE ANGULAR ROTATION OF THE EARTH

C IN RADIANS/SECOND

C

0MEGA=o729E-04

C

C FACT CONVERTS DEGREES TO RADIANS

C

FACT=o017453
Fl=2o0*0MEGA

C

C READ IN THE VALUES OF THE STANDARD DEPTHS,

C IN METERS, FROM THE CHOSEN REFERENCE LEVEL

C TO THE SURFACE

C

READ(5,45) Z
45 F0RMAT119I4)

C

C READ IN STATION ONE ; NUMBER ( Nl ), LATITUDE

C IN DEGREES(XD) AND MINUTES(XM), AND

C LONGITUDE IN DEGREES(YD) AND MINUTES(YM)

C

READ(5,40) Nl ,XD,XM,YD,YM
40 FORMAT( I3,4F5o 1)
C

C READ IN THE SPECIFIC VOLUME ANOMALY TIMES

C 10E 05, AT STANDARD DEPTHS, FROM THE CHOSEN

C REFERENCE LEVEL TO THE SURFACE FOR STATION

C ONE

C

READ(5,35) SVA1
35 F0RMAT(10F8ol)

C

C READ IN STATION TWO DATA( NUMBER .LATITUDE ,

C LONGITUDE, SPECIFIC VOLUME ANOMALY) FROM THE

C CHOSEN REFERENCE LEVEL TO THE SURFACE

C

1000 READ(5,40) N2 , XXD, XXM , YYD, YYM
IF(N2oEQoO) GO TO 2000
READ(5,35) SVA2
WRITE(6,65)
65 FORMAT* '!• ,31X,« (CRUISE TITLE)')

WRITE(6,70)
70 FORMAT* '+' ,35X,« (DASHES TO UNDERLINE THE TITLF)')
WRITF(6,400)
400 FORMATCO' ,' DEPTH* ,2X, 'STATION' ,2X, 'SUM OF DELTA-D"S«
*,2X, 'STATION' ,2X,' SUM OF DELTA-D " S' , 2X , ' VEL OF SURF',
*4X, 'DELTA-T' )
WRITE(6,500)
500 FORMAT(« ', IX ,'( M) • ,4X ,' NUMBER ', 4X ,'( DYNAM IC-M )', 6X ,

♦ 'NUMBER' ,4X,' (DYNAM IC-M) ', 6X, • (CM/ SEC) ' ,4X,
*• (SVERDRUPS) ' )

WRITE(6,800)
800 FORMAT ( •+« , ■ ■ ,2X, • «,2X,« ' ,

* 2 X , » ' , 2X , ' • , 2* , • ' ,

*2X,' •)

WRITET6 , 3~0"cT~l\iT, N2
300 FORMAT( »0» , 9X , I 3 ,2^X , I 3)
SDDl(l)=0o0
SDD2(1 )=OoO
DD(l)=OoO
D(l)=OoO
V(l)=OoO

73

DT(1 ) = OoO

T0(1 )=OoO
C

C THIS SECTION UTILIZES THE POSITIONS OF THE

C STATIONS TO COMPUTE THE ACTUAL DISTANCE

C BETWEEN THE STATIONSo THE M<=AN LATITUDE

C OF THE STATIONS IS USED TO CONVERT THE

C DIFFERENCE IN LONGITUDES TO TRUE DISTANCE-,

C

LATl=XD+XM/60o0

LAT2=XXD+XXV/60oO
C

C XDIST IS THE DISTANCE, IN METERS, FOR THE

C DIFFERENCE IN LATITUDE BETWEEN STATIONS

C

XDIST=(LAT1-LAT2 )*60o0*18 52o 0
C

C PHI IS THE MEAN LATITUDE OF THE TWO

C STATIONS CONVERTED TO RADIANS

C

PHI=(LATl+LAT2)/2oO*FACT
C

C FAC CONVERTS THE DIFFERENCE IN LONGITUDE

C - TO DISTANCE IN METERS

C

FAC=111415ol3*C0S(PHI)-94o 55*COS ( 3o 0*PHI )

L0NGl=YD+YM/60oC

L0NG2=YYD+YYM/60o0

YDIST=( LONG1-LONG2K-FAC
C

C TDIST IS THE TRUE DISTANCE BETWEEN STATIONS

C

TDIST=SQRT( XDI ST** 2+YDI ST** 2 )

SLAT=SIN(PHI )
C

C F IS THE CORIOLS PARAMETER

C

F=F1*SLAT

CONST=10oO/'z

WRITE (6, 100 ) Z(l ), S-DDl(l), SDD2 ( 1 ) , V( 1 ) , DT( 1 )
100 FORMAT ( ' 0« , I4,15X,F7o4,20X,F7o4,8X,F10o5,3X,F8o4>

DO 50 1=2,19
C

C «D« IS THE DYNAMIC HEIGHT BETWEEN TWO

C ISOBARIC SURFACES AT STATION ONE

C 'DD* IS THE DYNAMIC HEIGHT BETWEEN TWO

C ISOBARIC SURFACES AT STATION TWO

C

D(IJ=(SVA1( 1-1 J + SVAKI ) )/2oO*( Z( 1-1 )-Z( I ) )*lo0E-05

DD(I) = (SVA2(I-1)+SVA2( I ) )/2oO*(Z(I-l )-Z( I) )*lo0E-0 5
C

C SDD1 AND SDD2 ARE THE SUMS OF DYNAMIC

C HEIGHTS FOR STATIONS ONE AND TWO

C RESPECTIVELY WITH RESPECT TO 1000 METERS

C

SDD1U )=SDD1( 1-1 )+DU )

SDD2U )=SDD2( 1-1 )+DD( I )
C

C "V1 IS THE VELOCITY OF A WATER SURFACE

C WITH RESPECT to THE CHOSEN REFERENCE LFVEL

C

V(I)=(SDD1 ( I)-SDD2( I) )*CONST/TDIST*loOE 0 2
C

C - •DT1 IS THE TRANSPORT I N ~A LAYER BETWEEN

C TWO STATIONS WITH RESPECT TO A CHOSEN

C REFERENCE LEVEL, IN SVERDPUPS

C TQ=TPANSPORT BETWEEN THE SURFACE AND 100C

C - METERS BETWEEN TWO STATIONS, IN SVERDRUPS

C T2=TRANSP0RT BETWEEN THE SURFACE AND 200

C METERS BETWEEN TWO STATIONS, WIIH RESPECT

C TO 1000 MFTERS, IN SVERDRUPS

C T5=TRANSP0RT BETWEEN THE SURFACE AND 500

74

C METERS BETWEEN TWO STATIONS, WITH RESPECT

C TO 1000 METERS, IN SVFRORUPS

C

DT( I l = < V(I-1) + V( I) ) /2oO*(Z U-l )-Z( I ) l*TOIST*l,OE-0?/l.
*0E 06
T0( I )=T0( 1-1 )+DT(I)

WRITE (6, 100) Z( I ) , SDD1 (I ) , SDD2 ( I ) , V ( I ) , OT ( I )
50 CONTINUE

T2=T0(19)-TQ(10)
T5=TQ(19)-TQ(6)
WRITE(6,600)
600 FORM AT ( «0« , TRANS P( C-200M) ' ,3X, 'TRANS P(0-500M)«,3X,
♦•TRANSP(O-IOOOM) ■ )
WRITE(6^oo)
700 FORMAT(» •, 1 X , M SVERDRUPS )', 6X , M SVERDRUPS )' ,7X ,
*» (SVERDRUPS) « )
WRITE<6,900)

900 FORMAT ('+• , « • , 3X , • ' ,3X,

#« • )

WRITE(6,200) T2,T5,TQU9)
200 FORM AT ( ' 0' , 3X , F9o * , 8X, P9o 4 , 8X, F9o A )

DO 60 J=l,19

SVA1 (J)=SVA2( J)
60 CONTINUE^

N1 = N2

XC=XXD

XM=XXM

YD=YYD

YM=YYM

GO TO 1000
2000 WRITE(6t30)
30 FORMATt'l1 )

STOP

END

75

BIBLIOGRAPHY

1. Cochrane, J. D. , Investigations of the Yucatan Current, Texas A&M
University Department of Oceanography report 62-14A, pp. 5-10, 1962.

2. Fomin, L. M. , The Dynamic Method in Oceanography, v. 2, pp. 149-169,
Elsevier, 1964.

3. Gordon, A. L. , "Circulation of the Caribbean Sea," Journal of Geo-
physical Research, v. 72, No. 24, pp. 6207-6223, 15 December 1967.

4. Hewitt, J. F., BT Data as a Supplement to Nansen Casts : Gulf of
Mexico August -November , 1965-1966, Master's Thesis, United States
Naval Postgraduate School, Monterey, 1970.

5. Hubertz, J. M. , A Study of the Loop Current in the Eastern Gulf of
Mexico, Master's Thesis, Texas A&M University, College Station, 1967

6. Leipper, D. F., Hydrographic Station Data, Gulf of Mexico, Texas
A&M University Department of Oceanography reports 68-14T, 68-16T,
1968.

7. Leipper, D. F. , "A Sequence of Current Patterns in the Gulf of
Mexico," Journal of Geophysical Research, v. 75, No. 3, pp. 637-657,
20 January 1970.

8. McLellan, H. J., Elements, of Physical Oceanography, pp. 70-71,
Pergamon Press, 1965.

9. Nowlin, W. D. , Jr. and McLellan, H. J., "A Characterization of the
Gulf of Mexico Waters in Winter," Journal of Marine Research, v. 25,
No. 1, pp. 29-59, January 1967.

10. Parr, A. E., "Report on Hydrographic Observations in the Gulf of
Mexico and the Adjacent Straits made During the Yale Oceanographic
Expedition on the "Mabel Taylor" in 1932," Bulletin of the Bingham
Oceanographic Collection, v. 5, pp. 1-77, 1935.

11. Schneider, J. M. , A Description of the Physical Oceanographic
Features of the Eastern Gulf of Mexico, August 1968, Master's
Thesis, Texas A&M University, College Station, 1969.

76

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26. CROUP

EPOR T TITLE

Indicated Geostrophic Velocities and Volume Transports, Central and
Eastern Gulf of Mexico, Warmest and Coldest Months

ESCRIPTIVE NOTES (Type of report and, inclusive dates)

Master's Thesis; September 1970

U THORISI (First name, middle initial, last name)

William Louis Wunderly, Jr.

EPOR T DATE

September 1970

7a. TOTAL NO. OF PACES

79

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its distribution is unlimited.

SUPPLEMENTARY NOTES

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IBSTR AC T

To make comparisons to seven similar cruises, the geostrophic method
of volume transport and velocity analysis was applied to ALAMINOS cruises
67-A-6 of 4 to 22 August 1967 and 68-A-2 of 13 February to 6 March 1968.
An average velocity of 83 cm/sec and a volume transport of 27.5 Sverdrups
was found in the Yucatan Channel in August and an average velocity of
79 cm/sec and a volume transport of 26.6 Sverdrups was found in the
channel for February to March. A subsurface westward flow occurred in
August along the southern coast of Cuba providing input into the Loop
Current north of the Yucatan Channel. The Loop Current never crossed
25°N latitude. A cold ridge extended from the Florida shelf to the
Campeche Bank.

An analysis of East-West volume transport in the central Gulf
indicated a merging of east and west Gulf waters between 87 50'W and
89°30'W longitude for the MABEL TAYLOR cruise of -1932 and the ATLANTIS
cruise of 1935. The GERONIMO cruise of February-March 1967 and cruise
68-A-2 indicated a merging of east and west Gulf waters between 89°30'W
and 91°00'W longitude.

J t NOV 68 I *"T / O

0101 -807-681 I

(PAGE 1)

79

Security Classification

a-31408

Security Classification

key wo R OS

Gulf of Mexico
Geostrophic velocities
Geostrophic transport

I!*.. 14 73 (BACK)

07-6821

80

Security Classification

A- 31 409

Thesis
W935

c.l

12193U

Wunderly

Indicated geos trophic
velocities and volume
transports, central and
eastern Gulf of Mexico,
warmest and coldest
months.

Thesis 121S30

w935 Wunderly

e.3 Indicated geos trophic
velocities and volume
transports, central and
eastern Gulf of Mexico,
warmest and coldest
months.

thesW935

lt£££2. 9eostr°Phic velocities and vol

3 2768 001 90673 8

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