OCS Study 

MMS 2004-013

Intermediate Depth Circulation 

in the Gulf of Mexico: PALACE Float 

Results for the Gulf of Mexico 

Between April 1998 and March 2002 

30oN

28o

N

26o

N

24o

N

22o

N

20o

N

18o

N

96 99o

W o

W 93o

W 90o

W 87o

W 81 84o

W o

W

U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 
U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 

OCS Study 

MMS 2004-013 

Intermediate Depth Circulation 

in the Gulf of Mexico: PALACE Float 

Results for the Gulf of Mexico 

Between April 1998 and March 2002 

Author and Principal Investigator 

George L. Weatherly 

Prepared under MMS Contract 

1435-01-00-CT-31105 

by 

Florida State University 

Department of Oceanography 

Tallahassee, FL 32306-4320 

Published by 

New Orleans

March 2004 
iii

DISCLAIMER

This report was prepared under a contract from the Minerals Management Service (MMS) to the

Florida State University. This report has been technically reviewed by the MMS and approved

for publication. Approval does not signify that the contents necessarily reflect the views and

policies of the Service, nor does mention of trade names or commercial products constitute

endorsement or recommendation for use. It is, however, exempt from review and compliance

with the MMS editorial standards.

REPORT AVAILABILITY

Extra copies of the report may be obtained from the Publication Information Office (Mail Stop

5034) at the following address:

U. S. Department of the Interior

Minerals Management Service

Gulf of Mexico OCS Region

Public Information Office (MS 5034)

1201 Elmwood Park Blvd.

New Orleans, LA 70123-2394

Telephone Number: 1-800-200-GULF

 or 504-736-2519

CITATION

Suggested citation:

Weatherly, G. Intermediate Depth Circulation in the Gulf of Mexico: PALACE Float Results

for the Gulf of Mexico Between April 1998 and March 2002. U.S. Dept. of the Interior,

Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS

Study MMS 2004-013. 51 pp.

ABOUT THE COVER

This map of the Gulf of Mexico shows all the subsurface (red or green) and surface (blue or

yellow) PALACE float trajectories. The floats were launched in the northern Gulf of Mexico and

dispersed throughout the Gulf. However, those floats launched in the eastern Gulf (yellow green)

tended to stay there and those launched in the western Gulf (red blue) tended to stay there. The

900-m isobath is shown.
iv

ACKNOWLEDGMENTS

This study analyzes PALACE float data obtained in the Gulf of Mexico by others. We gratefully

acknowledge MMS for the support and the foresight to analyze this unique data set. Dr. Carole

Current, the Contracting Officer’s Technical Representative, has been most supportive in the

technical aspects of this project. We benefited from Dr. Alexis Lugo-Fernandez’ knowledge of

the Gulf of Mexico. We are grateful to them for the manner in which they encouraged the work

reported here.

Daniel Webb of Webb Research, Falmouth, Massachusetts, archived the raw data, made it

available us and helped us to decipher it. He also maintained a web site showing results of

preliminary processing done at Webb Research which we found very useful. When this web site

was shut down he passed the information on it to us, and in turn we have put it on our web sites

which are referenced in this report. We are very grateful to him for his assistance. We are

grateful to Robert Leben for providing us with satellite altimeter maps.

Dr. Nicolas Wienders did much of the analysis reported here and for which we are very grateful.

He built upon the work done earlier by Dr. Anastasia Romanou who worked with Georges

Weatherly when an earlier phase of the PALACE float analysis was funded by the National

Oceanographic Partners Program. We are grateful for Romanou’s help in learning and building

from her experiences.

Reinard Harkema, the data analysis person of the Current Meter Facility at Florida State

University, assisted in all phases of the data analysis. Paula Tamaddoni Jahromi also actively

participated in the analysis as well as the administrative oversight. They were both invaluable in

this study.
v

TABLE OF CONTENTS

Page

Acknowledgments iv

List of Figures vi

List of Tables ix

1.0 EXECUTIVE SUMMARY 1

2.0 INTRODUCTION 2

3.0 DATA 3

3.1 Background 3

3.2 Deep Drift Data Processing 5

3.3 Temperature Data 6

4.0 RESULTS 7

4.1 Deep Currents 7

4.2 Extension of Loop Current and Loop Current Rings to a900-m Depth 12

4.3 Cold, Cyclonic Eddies and a900-m Depth Flows 14

4.4 Temperature Inversions 19

4.5 Surface Mixed Layers 23

4.6 Temperature Variability 24

4.7 Deep Current Variability 29

4.8 Evidence of Bottom Boundary Layers 29

4.9 Comparison of a900-m Depth Measured Flows with Some Numerical Model Results 32

5.0 SOME CLOSING REMARKS 35

6.0 ANNOTATED BIBLIOGRAPHY OF THE PUBLISHED PAPER AND WEB SITES 35

7.0 REFERENCES 36

8.0 APPENDIX A 38
vi

LIST OF FIGURES

Page

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Fig. 9.

Fig. 10

Fig. 11.

Fig. 12.

Fig. 13.

Fig. 14

Picture of a PALACE float.

Chart of the Gulf of Mexico showing the launch positions, the numbered open

triangles, of the 17 PALACE floats.

Timeline chart showing when the floats were working.

Surface and subsurface drifts of all 17 floats.

Chart of the Gulf of Mexico showing the number of temperature profiles taken

in 1∞ latitude by 1∞ squares.

Timeline chart showing when the floats worked and obtained deep drifts.

The 1,315 a900-m deep velocity vectors obtained from the floats.

1/2∞ by 1/2∞ averaged velocities.

Average velocity vectors in Fig. 8 with uncertainty ellipses.

Fig. 8 redrawn showing only those vectors with n ≥ 5.

Left: All the temperature profiles obtained in a 1∞ latitude by 1∞ longitude box

centered at 27.5∞N, 87.0∞W showing an example of a profile obtained in the

Loop Current; it is the right-most profile (see text Section 4.2). Also seen are

two profiles obtained in a subsurface cold eddy; the two left-most profiles (see

text Section 4.3). Right: Dashed curve is the average temperature profile of

the Gulf of Mexico from 1,481 PALACE profilers; solid curve is the average

profile of those shown in left panel.

Histogram of the velocity magnitude for measurements made under/in the

Loop Current or a Loop Current Ring and histogram of all the velocity

magnitudes obtained in the Gulf of Mexico.

Scatter plot of the vector magnitude of the current at a900-m depth when the

float was under/in the Loop Current or in a Loop Current Ring and the

corresponding surface geostrophic current.

Scatter plot of the flow direction at a900-m depth relative to that at the surface

when the float was under/in the Loop Current or in Loop Current Ring (see

Section 4.2). Positive values are counterclockwise and negative are clockwise.

About half the time the flow at depth was in the direction of the surface flow to

± 90∞, and in the other half counter to the surface flow to ± 90∞ with no

apparent pattern depending on the direction of flow at the surface (i.e., on

which side of the Loop Current or a Loop Current Ring it was on).

 3

 4

 4

 5

 6

 7

 8

 9

 9

 10

 11

 13

 13

 14
vii

Fig. 15.

Fig. 16

Fig. 17.

Fig. 18.

Fig. 19.

Fig. 20.

Fig. 21.

Fig. 22.

Fig. 23.

Fig. 24.

Fig. 25.

Fig. 26.

LIST OF FIGURES

(continued)

Location of 13 cold eddies observed from PALACE float data in the Gulf of

Mexico.

Temperature profiles used to identify and track the cold eddies in Fig. 15.

Two satellite altimeter maps with a900-m depth currents superposed.

Examples of inversions indicated by arrows: (a) and (b) show inversions

associated with a layer exposed at the surface; (c) and (d) show inversions with

a layer isolated from the surface.

Location of the profiles showing temperature inversions.

Dots show the location of profiles with inversions when the next profile from

that float also showed an inversion at the same depth to within a 100 m.

Distribution depths of inversions as a function of latitude.

Below. Dashed line is the total river discharge in mSV (1 SV = 106 m3/s) into

the Gulf of Mexico for the period 10/62-9/65 [from Hall 1967]. Solid line is

the cumulative discharge into the Gulf of Mexico from 30 U.S. rivers with the

mean of Mexican rivers added from the U.S. Geological Survey data on their

web site http://waterdata.uscs.gov. Above. Frequency of observation of

inversions observed from the floats as a function of month. The dark, light,

and clear color denote the contribution of inversions of strength DT > 0.050∞C,

0.010∞C < DT < 0.050∞C and DT < 0.010∞C, respectively.

Trajectories for combined LATEX, SCULPI and SCULPII drogued drifters

[see Ohlman et al. 2001] for (a) drifters set in the LATEX region (LouisianaTexas shelf west of mouth of Mississippi River) in June-July with tracks drawn

through the end of August and (b) drifter set in the same region in NovemberDecember with tracks drawn through the end of January.

All temperature profiles obtained in a 1∞ by 1∞ square centered at 26.5∞N,

89∞W showing an example of a very thick surface mixed layer; in the rightmost profile, which was obtained in a Loop Current Ring in the winter.

(a) Surface mixed layer depth as a function of month of the year. (b) Thickness

of surface mixed layer as a function of surface water temperature.

Location of very thick, thickness >120 m, surface mixed layers. Note that they

occur in regions where the Loop Current and Loop Current Rings are common

in the Gulf of Mexico. The 200-m and 900-m isobaths are shown.

 15

 16

 18

 19

 20

 20

 21

 22

 23

 24

 25

 25
viii

Fig. 27.

Fig. 28.

Fig. 29.

Fig. 30.

Fig. 31.

Fig. 32.

Fig. 33.

Fig. 34.

Fig. 35.

Fig. 36.

LIST OF FIGURES

(continued)

All 1,481 temperature profiles obtained in the Gulf of Mexico.

Contours of temperature at the surface, 300-m depth, 600-m depth, and 900-m

depth.

Temperature variability as a function of depth.

Depth variability of different isotherms.

Variance ellipses for the a900-m depth currents.

Four temperature profiles from a grounded float showing evidence of a bottom

boundary layer (BBL).

Output of MOM model showing temperature and currents averaged over one

year at 1,000-m depth.

Average current over a 3-year period at 1,000-m depth from the Princeton

POM.

Average current at 1000-m depth for year 2000 from the Mercator model.

Horizontally nondivergent flow field inferred from an objective analysis of the

actual flow field.

 26

 27

 28

 29

 30

 31

 32

 33

 33

 34
ix

LIST OF TABLES

Page

Table 1

Table 2

Tabulation using the figures in Appendix A of when different regions of the

Gulf of Mexico had their deep flow sampled.

Whether features seen in float data were seen in model outputs.

 11

 34
1

1.0 EXECUTIVE SUMMARY

Seventeen PALACE floats were set in the Gulf of Mexico in 1998 as part of a study of the Gulf

of Mexico by the National Oceanographic Partners Program (NOPP). The Principal Investigator

(PI) was asked to examine data after the floats were deployed. When funding for the NOPP

program was not renewed the PI successfully asked MMS for support to complete the analysis of

the float data. This is a report of that analysis.

This is a report of the first ever study of the intermediate depth circulation in the Gulf of Mexico

made using free-drifting floats. The floats were fitted with temperature sensors, and this study

also reports the first ever study of the intermediate depth thermal structure of the Gulf made by

profiling floats. The floats were set in the northern Gulf of Mexico but quickly dispersed

resulting in observations being made throughout the Gulf of Mexico.

The floats drifted at a900-m depth coming up to the surface once a week for 12 hours to be

positioned by satellite and to download their temperature profile data. Only the results of their

drift at a900-m depth are reported here. It is important to note that if the drift of the floats while

they were at the surface and transiting to and from the surface had not been eliminated as done

here a very different view of the intermediate depth circulation would have emerged. The

surface drift was often comparable to or greater than that at depth and not always in the same

direction.

About 30% of the Gulf of Mexico is continental shelf, and groundings on the shelf were a major

problem. When the floats were up at the surface once a week it was not unusual for them to drift

onto the continental shelf and consequently sink to the bottom on their next dive. Only about

50% of the profiles which could have been made were made in water of depth >900 m, the floats

drift depth.

The float deep drifts, obtained in the period April 1998 - March 2003, indicate that

900-m depth is not a level of no motion in the Gulf of Mexico. One major result of the study is

that there appears to be cyclonic flow along the border of and through out the Gulf of Mexico,

and that this flow intensifies into a a10 cm/s southward flowing current in the western Bay of

Campeche. An unexpected result was that there was a strong tendency for floats launched in the

eastern Gulf to remain there and for floats launched in the western Gulf to remain there.

The floats indicated both in their deep drifts and in their temperature profiles that the Loop

Current and Loop Current Rings extend down to at least a900-m depth. The current at a900-m

depth beneath these features was about 20% that at the surface, but the direction of flow at depth

relative to that at the surface appeared to have no clear pattern.

Subsurface cold, cyclonic eddies extending downward from a300-m depth down to at least

a900-m depth were seen both in the deep drifts and in the temperature profiles. These a100-km

diameter features were not evident in satellite altimeter maps and tended to move westward at

a3-5 cm/s and have orbital velocities at a900-m depth of a10-15 cm/s. The temperature data

obtained indicate that these subsurface cold eddies may be formed from cold core rings

originally having a surface expression.

The 1,481 temperature profiles obtained in the open Gulf in water of depths >200 m indicated

that the presence of coastal riverine water throughout the open Gulf of Mexico is common.
2

About half of the profiles showed such evidence. Their presence was relatively rare in the

summer and more common in the winter, and this seems consistent with results seen in LATEX

drifters.

The temperature profiles also revealed that surface mixed layers could be rather thick (a50 m to

a170 m) in the open Gulf in the winter. These thicker layers were generally seen in the Loop

Current and Loop Current Rings which implies considerable heat loss in the winter in these

features.

The greatest temperature variability of 14∞C was found at a150-m depth and exceeded that seen

in the surface layer. The greatest vertical excursion of isotherms was for the colder ones around

7∞C where it approached 600 m. The vertical displacement of isotherms by the presence of the

Loop Current, Loop Current and cold core rings resulted in greater thermal variability in the

thermocline than in the seasonally modulated surface layer.

When grounded the PALACE floats yielded temperature profiles which differ from those usually

obtained with CTDs in that they go to the bottom while the latter usually are terminated a10 m

above the bottom. Temperature profiles obtained from grounded floats often showed

anomalously cold bottom mixed layers of thickness a20 m.

2.0 INTRODUCTION

This report presents the results of 17 satellite-tracked deep floats set in the Northern Gulf of

Mexico. The floats were acquired and set by Dr. John Blaha of NAVOCEANO in 1988 as part

of a NOPP (National Oceanographic Partners Program) study called The Gulf of Mexico Ocean

Monitoring System (Blaha et al. 2000). The PI was asked to examine the float data after the

floats had been set; when the NOPP study was not renewed the PI sought and got MMS

(Minerals Management Services) support to bring the float study to completion. This is a report

of that study.

The floats were PALACE (Profile Autonomous Lagrangian Circulation Explorer, Davis et al.

2001) ones. They were built by Webb Research of Falmouth, Massachusetts and a PALACE

float is shown in Fig. 1. Webb Research acquired the float data and made it available to us.

The data is described later, but basically the floats measured currents at a900 m depth. They

also profiled temperature once a week when they surfaced to get positioned by satellites. While

the floats were launched in the northern Gulf of Mexico they drifted throughout the Gulf and

yielded a unique view of intermediate (a900 m) depth currents and temperature profiles (depths

<~ 900 m) over nearly a four-year period.
3

Fig. 1. Picture of a PALACE float. The PALACE float is 104 cm long excluding the 70-cm

antenna. Its diameter is 17 cm and it weighs 23 kg.

3.0 DATA

3.1 Background

The PALACE floats in this study had a seven-day cycle during which about 6.23 days were

spent at about 900-m depth, about 2.5 hours rising to the surface where they remained for about

11.5 hours transmitting data to ARGOS and getting surface fixes, and about 4.5 hours

descending back to a900-m depth. The deep drift depth was chosen to approximately match that

of expected XBT casts to be made as part of the NOPP study (Sturges 2002, personal

communication). The floats recorded in situ temperature on their way up to the surface. The

floats also recorded the depth at which they drifted for each cycle.

The floats were set between 83∞W and 94∞W in the northern Gulf of Mexico on the continental

slope and rise in water of depth about 900 m and 3,000 m (Fig. 2). Eight of them were deployed

in April 1998 and the remaining nine were set in August 1998 (Fig. 3). There is relatively more

coverage in 1998-1999; the last data from the Gulf of Mexico from these floats was in March

2002 (Fig. 3). All the surface and subsurface trajectories are shown in Fig. 4. It can be seen that

there was relatively good coverage over all of the Gulf of Mexico, that only one float escaped the

Gulf of Mexico, and that it was not unusual for a float when at the surface to drift into water

shallower than 900 m, their programmed deep-drift depths.

Five of the floats, 141, 155, 156, 157, and 162, worked for relatively long periods of time,

up to 3 years and 7 months (Fig. 3). While two of the other floats (159 and 167) failed

prematurely for unknown reasons while in deep water, the others failed prematurely after they

drifted in shallow continental shelf waters where they were not designed to operate.
4

 96oW 93oW 90oW 87oW 84oW 81oW 18o

N 

 20o

N 

 22o

N 

 24oN 

 26o

N 

 28o

N 

 30o

N 

900

3000

2000

2000

900

2000

2000

900

141

150

151

152

153

154

155

156

157

158

159

160

161

162 164

165

167

DC

FS

CB

YC

Fig. 2. Chart of the Gulf of Mexico showing the launch positions, the numbered open triangles,

of the 17 PALACE floats. The numbering system is that of Dr. John Blaha at

NAVOCEANO who acquired and launched the floats. The 900-m, 2,000-m and 3,000-

m isobaths are shown. CB denotes Campeche Bay, and the CB is positioned over the

Campeche Bump, a relic delta-like feature extending into the open Gulf of Mexico

discussed in the text. DC is DeSoto Canyon, YC is Yucatan Channel, and FS is the

Florida Straits. Jan98

167

165

164

162

161

160

159

158

157

156

155

154

153

152

151

150

141

Float number

Apr98

Jul98

Oct98

Jan99

Apr99

Jul99

Oct99

Jan00

Apr00

Jul00

Oct00

Jan01

Apr01

Jul01

Oct01

Jan02

Apr02

Fig 3. Timeline chart showing when the floats were working.
5

 99oW 96oW 93oW 90oW 87oW 84oW 81oW 

 18oN 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

Fig. 4. Surface (blue) and subsurface (red) drifts of all 17 floats. The 900-m isobath is shown.

Note the floats drifted at a900-m depth.

3.2 Deep Drift Data Processing

In order to compute the deep drift, estimates of surface drift and the drift while sinking and rising

were first made using the locations and time of the first and last satellite fixes to estimate the

surface drift. Linear extrapolation provided the positions where the floats surfaced and

submerged.

A linear decrease of current with depth from its surface value to zero at a900-m depth was used

to make an initial estimate of the drift during ascent from and descent to the maximum depth.

The deep drift was then first estimated. The drift during rising and diving was then re-estimated

using the second estimate of the deep drift and the deep drift was then re-estimated. The

procedure was repeated three more times. The deep drift speed estimates essentially did not

change after the second iteration, and the first estimate was generally within 10% of the final

estimate.

All trajectories with depth less than 830 m were then excluded. This depth is less than the initial

depth all floats descended and we assumed the float had grounded if its depth was shallower than

this value. Then depth was plotted as a function of time for each float. Relatively shallow

values were checked to see if they occurred near the 900 m isobath; if they did we assumed

grounding had then occurred and the associated deep drifts were excluded. Using these criteria

1,315 deep drifts were judged as good and are subsequently examined here. The deep velocity

data inferred from these deep drifts are available on the web at

http://ocean.fsu.edu/~georges/temps/gomvelocities.htm. We expect to maintain this web site

through 2004.

The floats drifted deeper with increasing time at a rate of about 23 m/yr ± 10 m/yr. So, towards

the beginning of the study the floats drifted around 900 m depth and towards the end of the study

they were drifting nearly 100 m deeper. The depths of the deep drift records varied from 847 m
6

to 1042 m. Because the geostrophic shear in this depth range is relatively weak (DeHaan 2002)

all the deep drifts are considered collectively here.

What are presented here are the deep drift velocities estimated from the deep drifts and when

shown in figures the mid-point of the vector is at the mid-point of the deep drift vector.

3.3 Temperature Data

The floats sampled temperature once a week every a5 m in the upper 100 m and every a10 m for

depths >100 m as they rose to the surface. The thermistors resolved temperature to about

0.002∞C - 0.004∞C (Sherman 2002, personal communication) and can be calibrated to a precision

of about 0.005∞C (Davis et al. 2001). The thermistors were not calibrated but we think their

precision is about 0.05∞C. In this report the precision is of more concern than the accuracy.

All temperature profiles made in the open Gulf of Mexico in water of depth >200 m are

considered, and there were 1,481 of them. These profiles were made throughout the Gulf of

Mexico with relatively many of them being made where the floats were launched and in the Bay

of Campeche (identified in Fig. 2) where some floats tended to aggregate (Fig. 5). These profiles

can be seen individually and collectively in 1∞ latitude by 1∞ longitude bins on the web at

http://www.ocean.fsu.edu/~georges/temps/, and we intend to maintain this site through 2004.

Towards the end of their lifetimes some temperature profiles terminated at depths a700 m with a

suspicious-looking spike; these profiles were excluded. Also, as the floats drifted deeper with

time (Section 3.2) they profiled temperature sometimes only below depth a50 m.

Fig. 5. Chart of the Gulf of Mexico showing the number of temperature profiles taken in 1∞ latitude by 1∞ squares. The 900-m isobath is shown. As the number of deep drifts is

nearly the same as that of the number of temperature profiles, this also indicates

approximately the number of deep drifts in 1∞ by 1∞ bins. The shaded area is where

temperature profiles were made in the Loop Current and in Loop Current rings.
7

4.0 RESULTS

4.1 Deep Currents

When the individual floats sampled the deep drifts is shown in Fig. 6. The gaps in this figure

compared to Fig. 3, which shows when these floats worked, are primarily due to grounding, i.e.

the floats settling on the bottom after resubmerging. A very few of the gaps are due to failure to

get a second satellite fix when at the surface; when this occurred the surface drift could not be

estimated.

All the 1,315 deep velocities are shown in Fig. 7. The vectors are color coded to show where

they were launched. The black vectors, from floats launched east of 88∞, show a tendency for

those floats to remain in the eastern Gulf of Mexico, and similarly the red vectors, from floats

launched west of 88∞W tend to remain in the western Gulf. It is interesting that some Gulf of

Mexico numerical model results tracking particles set to drift at 1,000 m depth show that there is

also little communication between the eastern and western Gulf (Welsh 2002, personal

communication).

AM J J A S O N D J F M AM J J A SO N D J F M AM J J A SO N D J F M AM J J A SO N D J F M

AM J J ASO D N J F M AM J J A S O D N J F M AM J J A S O D N J F M AM J J A S O D N J F M

167

165

164

162

161

160

159

158

157

156

155

154

153

152

151

150

141

Fig. 6. Timeline chart showing when the floats worked and obtained deep drifts. The main

difference in the timelines shown here and in Fig. 3 is due to grounding (see text).
8

Some patterns are seen in the figure:

∑ a tendency for counterclockwise, cyclonic flow on the continental margin of the

northern and western Gulf of Mexico and in the Bay of Campeche,

∑ an intensification of the above cyclonic flow in the western Bay of Campeche,

∑ a tendency for the above cyclonic flow to follow the Campeche Bump in the

southern Bay of Campeche (this bump is identified in Fig. 2’s caption and the CB

in Fig. 2 is over this topographic feature),

∑ a small a100-km diameter cyclonic gyre in the southwestern Bay of Campeche

centered near 20∞N, 95.5∞W, and two larger a300-km diameter cyclonic gyres one

in the southwestern Gulf of Mexico centered near 23∞N, 94∞W and the other

centered near 27∞N, 91∞W.

The same patterns are better evident when the same data are averaged in 0.5∞ latitude by 0.5∞

longitude bins (Fig. 8). Note that to get better coverage the above averaging in Fig. 8 is done for

boxes centered every 1/4∞ latitude by every 1/4∞ longitude.

We estimated the statistical uncertainty in the velocity values shown in Fig. 8 by assuming the

integral time scale for velocity fluctuations was 10 days and using methods outlined in Weatherly

et al. (2000). The associated uncertainty ellipses for every other velocity vector shown in Fig. 8

(the figure is messy if all are shown) are plotted in Fig. 9 provided the number of values used to

form the estimate n ≥ 5. As can be seen the velocity estimates for n ≥ 5 are generally significant

in that at least the direction of the flow is resolved. Fig. 10 is Fig. 8 redone showing only those

averages formed with for n ≥ 5. The velocity patterns noted above survive in Fig. 10.

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

900

2000

900

2000

900

900

2000

2000

2000

10 cm/s

Fig. 7. The 1,315 a900-m depth velocity vectors obtained from the floats. The floats launched

east of 89∞W are black, and those launched west of 89∞W are red. The 900-m and

2,000-m isobaths are shown.
9

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

900

900

900

10 cm/s

Fig. 8. 1/2∞ by 1/2∞ averaged velocities. The vectors are centered in the middle of the boxes,

and values are shown for boxes centered every 1/4∞ by 1/4∞. The 900-m isobath is

shown.

900

10 cm/s

18oN

20o

N

24o

N

26o

N

28o

N

30o

N

22o

N

98o

W 96oW 94oW 92oW 90oW 88oW 86o

W 84o

W

900

Fig. 9. Average velocity vectors in Fig. 8 with uncertainty ellipses (see text). Every other vector

is shown provided the number of estimates n used to form the vector was greater than 5

(see text). For n ≥ 5 generally the direction of the flow is resolved and statistically

significant.
10

To estimate when the flow features noted above were sampled during the

a3.8-year study period when deep drifts were obtained (Fig. 6) the deep velocities were drawn

for sequential six-month periods. These figures are shown in Appendix A and Table 1

summarizes the results. The cyclonic flow along the slope does appear to be a feature of the

flow during the period of study. It is least conclusive in the northeast (NE) Gulf where the

sampling was the least of all the (a) regions. The intensification of the cyclonic flow along the

western Campeche Bay (WCB) appears as a rigorous feature; every time a float went through

this region strong southerly flow was experienced at least part way through its transit of the

region. The cyclonic gyres (20, 23, and 27) appear to be there during the four-year study period,

and not due to parked, transient cyclonic eddies.

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

900

900

900

10 cm/s

Fig. 10. Fig. 8 redrawn showing only those vectors with n ≥ 5. The vectors shown here are

generally statistically significant.
11

Table 1. Tabulation using the figures in Appendix A of when different regions of the Gulf of

Mexico had their deep flow sampled. Under (a) is the continental slope region in the northeast

Gulf east of 89∞ W (NE), continental slope region in the northern Gulf between 89∞W and 95∞W

(N), continental slope region in the western Gulf north of 23∞N (NW), continental slope region in

the western Bay of Campeche (WCB) south of 23∞N, and in the Campeche Bump region of

Campeche Bay (CBB). Under (b) are the cyclonic gyres centered near 20∞N, 95.5∞W (20), near

23∞N, 94∞W (23), and near 27∞N, 91∞W (27). An “x” indicates data during the period.

Cyclonic flow along slope Cyclonic gyres

Period (a) (a) (a) (a) (a) (b) (b) (b)

NE N NW WCB CBB 20 23 27

04/98£time<10/98

10/98£time<04/99

04/99£time<10/99

10/99£time<04/00

04/00£time<10/00

10/00£time<04/01

04/01£time<10/01

10/01£time<04/02

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

0 8 16 24 32 0 8 16 24 32

Temperature (oC) Temperature (oC)

1000

800

600

400

200

0

Depth (m)

Fig. 11. Left: All the temperature profiles obtained in a 1∞ latitude by 1∞ longitude box centered

at 27.5∞N, 87.0∞W showing an example of a profile obtained in the Loop Current; it is

the right-most profile (see Section 4.2). Also seen are two profiles obtained in a

subsurface cold eddy; the two left-most profiles (see Section 4.3). Right: Dashed

curve is the average temperature profile of the Gulf of Mexico from 1,481 PALACE

profiles; solid curve is the average profile of those shown in left panel.
12

4.2 Extension of Loop Current and Loop Current Rings to a900-m Depth

We superposed velocity vectors on satellite altimeter maps to try to detect patterns between what

happened at the surface and what happened at 900-m depth. We used Bob Leben’s altimeter

maps for the comparisons (some examples are shown later). We tried two variants: monthly

satellite maps with all deep currents within 2 weeks of the map and the other bi-weekly satellite

maps with all deep currents within a week of the map. The results were inconclusive. The flow

at a900-m depth sometimes appeared to be a deep extension of what appeared to be happening at

the surface. Other times, it appeared to be flowing counter to what the satellite indicated

happened at the surface. And at other times there appeared to be no connection between the two.

We could not tell ahead what to expect. For example no pattern could be found for flow under

the Loop Current, flow under newly formed Loop Current Rings, flow under older Loop Current

Rings, and flow under cyclonic rings.

One thing that was of concern to us in the above comparison was whether the floats truly were

below the surface feature as indicated by the satellite. A slight shift in the position of the

indicated surface feature might give better agreement. Also of concern, if the surface features

were generally well positioned, there might be a vertical tilt to the features with depth. To do

away with these ambiguities we looked at deep current data when both the temperature data from

the profiling float and the altimeter indicated that the float had been under and in the Loop

Current or a Loop Current Ring. As an example, we show in Fig. 11 all temperature profiles

from one 1∞ latitude by 1∞ longitude box taken from the previously referenced web site

http://www.ocean.fsu.edu/~georges/temps/, namely the 27-28∞N, 86.5-87.5∞W box. One profile

is conspicuously warm in this box (profile 162168) indicating that this profile was made either in

the Loop Current or a Loop Current Ring. Comparing the location of this profile (27.1∞N,

86.9∞W) with the satellite map for the date of this profile (Jan. 10, 2001) found at http://wwwccar.Colorado.EDU/pub/gom/met/gif.mean shows that this temperature profile was made in this

case in the Loop Current.

We found 103 cases when the floats temperature profile and the associated altimeter map for the

appropriate day indicated that the floats was in the Loop Current or in a Loop Current Ring.

Fig. 12 shows two histograms of the magnitude of the 900-m depth currents: one is for all the

deep current data from the Gulf of Mexico and the other for the 103 values measured beneath the

Loop Current or a Loop Current Ring. The currents are indeed stronger beneath the Loop

Current and Loop Current Rings implying that in general there is a deep (to at least a900 m)

extension of these features. Scatter plotting the deep flow magnitude against that at the surface

(the latter were read from the previously referenced web site http://wwwccar.Colorado.EDU/~realtime/gom-real-time_vel/, which at the time of this writing no longer

exists) that the magnitude of the current at a900-m depth below these features a20% (with a lot

of scatter) that of the surface, geostrophic flow, and this depth is not a level of no motion (Fig.

13). Fig. 14 shows the direction of the flow at depth relative to the direction of flow at the

surface (the latter being again inferred from the previously reference web site). There are about

an equal number of cases when the flow at depth was approximately in the direction of the

surface flow and approximately counter to it. We do not see a clear pattern in the relative

direction of the deep flow depending on whether the float was on the northern, eastern or western

side of the Loop Current or a Loop Current Ring or on the southern side of a Loop Current Ring.
13

Histogram of Velocity Magnitude 

Loop Currents and Loop Current 

Rings

 1315 points used

 average: 5.03

Velocity Magnitude (cm/s)

0 5 10 15 20 25

20

16

12

8

4

0

Percentage

Fig. 12. Histogram of the velocity magnitude for measurements made under/in the Loop

Current or a Loop Current Ring (dashed curve) and histogram of all the velocity

magnitudes obtained in the Gulf of Mexico (solid curve).

Speed at Surface (cm/s)

Speed at Depth (cm/s)

0 20 40 60 80 100 120

25

20

15

10

5

0

Fig. 13. Scatter plot of the vector magnitude of the current at a900-m depth when the float was

under/in the Loop Current or in a Loop Current Ring and the corresponding surface

geostrophic current (see Section 4.2). The flow at a900-m depth was about 20% that

at the surface but with much scatter.
14

4.3 Cold, Cyclonic Eddies and a900-m Depth Flows

In Fig. 11 two cold features are seen -- the two left-most profiles. They diverge from the other,

more normal profiles at about 250 m depth and beneath this depth they are notably colder than

the other profiles down to the depth of the lowest measurements a900 m. We found 38 notably

colder profiles and where they were observed in the Gulf of Mexico is shown in Fig. 15. Our

interpretation is that these features are associated with 13 different cold eddies with 11 of these

sampled more than once. In Fig. 15 where these were first sampled is indicated by an asterisk

and subsequent sampling locations, usually a week later, are indicated by solid dotes. These

features moved generally westward at a speed of 2-4 km/day (i.e., about 3-5 cm/s) except in the

Campeche Bay where they had a southward component in the western bay and a northern

component in the eastern bay. Deep direction relative to Surface direction (+CW, -CCW)

Direction at surface (oTrue)

0 60 180 300 120 240 360

180

90

0

-90

-180

Fig. 14. Scatter plot of the flow direction at a900-m depth relative to that at the surface when

the float was under/in the Loop Current or in Loop Current Ring (see Section 4.2).

Positive values are counterclockwise and negative are clockwise. About half the time

the flow at depth was in the direction of the surface flow to ± 90∞, and in the other half

counter to the surface flow to ± 90∞ with no apparent pattern depending on the direction

of flow at the surface (i.e., on which side of the Loop Current or a Loop Current Ring it

was on).

The associated temperature profiles for Fig. 15 are shown in Fig. 16. Some of these cold eddies,

3, 5, 6, 8, 9, 11, 12, and 13, appear to extend up to the surface or at least to the base of the

surface mixed layer. Eddy 9 appears to be the one of the cyclonic eddies mentioned in ZavalaHidalgo et al. (2003) (their eddy 8), and like the others are associated with cyclonic eddy

features seen in the altimeter maps. The exception appears to be eddy 8; no clear cyclonic

feature is apparent in the associated altimeter maps. Cyclonic eddies with a surface expression

often drifting westward have previously been reported in the Gulf of Mexico (e.g., the references

in Zavala-Hidalgo et al. 2003). What may be new here is that in the Bay of Campeche they

appear to drift cyclonically about the Bay.

Other of these cold eddies, 1, 4, 7, and 10 are subsurface features; 1, 7, and 10 begin at a300 m

depth and 4 which is on the upper rise begins at a shallower a200 m depth

(Fig 16). Subsurface cyclonic eddies have been seen before in the Gulf of Mexico (Elliot 1979;
15

Hamilton 1992) what is new here is that we have quasi-simultaneous

deep velocity measurements in them clearly showing cyclonic circulation at

a900-m depth in them. Fig. 17 shows some satellite altimeter maps for the period eddy 1 was

observed. The white dashed circle in this figure shows where eddy 1 was and the velocity

vectors show clear cyclonic rotation patterns. Also, from this figure a slow drift westward over

one month period is evident. The orbital motions in this feature at 900-m depth are a10 cm/s

(Fig. 17); we suspect at the core of theses cyclonic eddies at a400 m depth these speeds are

notably faster. Note that in Fig. 17 there is no surface evidence of a cyclonic eddy. The other

satellite altimeter maps for the period of the other subsurface cold features also do not show

surface evidence of cyclonic flow overhead (not shown).

Cyclonic eddy 2 was observed in the northeastern Gulf (Fig. 15) and it is unusual in that some of

the profiles show it to have a surface expression and in another it appears as a subsurface one

(Fig. 16). Perhaps some of the subsurface cyclonic eddies start in the northeastern Gulf as

cyclonic eddies with a surface expression and the upper portion of these eddies somehow later

get clipped off leaving a subsurface eddy behind.

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

900

900

900

1

2 3

4

5

6

7

8

9

10

11

12

13

Fig. 15. Location of 13 cold eddies observed from PALACE float data in the Gulf of Mexico.

The numbered * show where the eddy was first seen and the site of subsequent

observations, when available, are shown with the symbol • .

The lines are drawn according to increasing time. Most of the cold eddies moved

westward at about 3-5 km/day, but those in the Bay of Campeche tended to drift

cyclonically. Cold eddies 1, 3, 5, 7, and 10 are subsurface rings with no surface

signature in satellite altimeter maps (see Section 4.3). The 900-m isobath is shown.
16

0

0.2

0.4

0.6

0.8

1.0

Depth (km)

0.2

0.4

0.6

0.8

1.0

0.2

0.4

0.6

0.8

1.0

0

0

Depth (km) Depth (km)

0 8 16 24 32 8 16 24 32

Temperature (oC)

Temperature Profiles

Gulf of Mexico Ceddies 1-7 21 profiles

0

Fig. 16. Temperature profiles used to identify and track the cold eddies in Fig. 15. The last two

numbers in each panel identifying each profile are the cold eddy number corresponding

to those in Fig. 15. The first three numbers are the float number and the next three the

profile number. The dashed curve in each panel is the average temperature profile for

the Gulf of Mexico as inferred from the float temperature data.
17

0

0.2

0.4

0.6

0.8

1.0

Depth (km)

0.2

0.4

0.6

0.8

1.0

0.2

0.4

0.6

0.8

1.0

0

0

Depth (km) Depth (km)

0 8 16 24 32 8 16 24 32

Temperature (oC)

Temperature Profiles

Gulf of Mexico Ceddies 8-13 17 profiles

0

Fig. 16. (continued).
18

30

20

10

0 

10 

20 

30 

15 May 1998; TP and ERS2 plus model mean altimetry(cm); Deep drifts

 96oW 93oW 90oW 87oW 84oW 81oW 18o

N 

 20o

N 

 22o

N 

 24oN 

 26oN 

 28o

N 

 30o

N 

30

20

10

0 

10 

20 

30 

 96oW 93oW 90oW 87oW 84oW 81oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

15-Jun-1998; TP and ERS2 plus model mean altimetry(cm); Deep drifts

10cms-1

10cms-1

Fig. 17. Two satellite altimeter maps with a900-m depth currents superposed. The upper

(lower) panel is for May 15, 1998 (June 15, 1998) and all deep currents obtained

within ± 2 weeks of the given date are superposed. The white dashed circle show

vectors inside cold eddy 1 in Fig. 15. Note that the vectors inside the white dashed

circles indicate cyclonic flow and that the eddy moved westward about 60 km in one

month.
19

4.4 Temperature Inversions

In examining the temperature profiles for evidence of eddies we were surprised at the number of

temperature inversions that were evident. Some examples of these inversions are found in Fig.

18. About half (48%) of the temperature profiles had inversions, and these inversions were seen

through out the Gulf (Fig. 19). About 40% of the inversions were revisited by the same float that

first sampled them (Fig. 20), and because the floats drifted tens of km between profiles we

concluded that many if not most of the inversions were stable features associated with fresher,

less saline water and were tens of km across laterally. Because the strongest inversions were

found near the mouth of the Mississippi and Atchalafaya Rivers and in the Bay of Campeche

(Fig. 20), regions which account for about 70% of the river discharge into the Gulf of Mexico,

we concluded the ultimate source of these inversions was river water because they should be

largest near their sources. Most (96%) of the inversions were found in the upper 160 m,

although some were found at essentially all depths where temperature measurements were made

(Fig. 21).

15 20 25 In situ Temperature ( C)

Profile: 092 Date: 01/26/2000 

Drifter: E Location: 21.9 N 94.7 W

*

10 20 30 In situ Temperature ( C)

Profile: 174 Date: 12/05/2001 

Drifter: H Location: 25.1 N 92.5 W

*

*

20 25

In situ Temperature ( C)

Profile: 001 Date: 04/29/1998 

Drifter: E Location: 26.8 N 90.2 W

*

20

In situ Temperature ( C)

Profile: 026 Date: 01/29/1999 

Drifter: L Location: 29.3 N 87.8 W

*

200

100

0

Depth (m)

200

100

0

Depth (m)

200

100

0

Depth (m)

200

100

0

Depth (m)

10

o o

o o

o o o o

o o o o

(a) (b)

(c) (d)

Fig. 18. Examples of inversions indicated by arrows: (a) and (b) show inversions associated

with a layer exposed at the surface; (c) and (d) show inversions with a layer isolated

from the surface.
20

200

900

900

200

200

900

900

900

*

****

*

*

* ** * *

** *

** *

*

* * *

+ ****

**

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* *

+

*

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*

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* *

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*

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**

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* *

*

* *

*

*

* + + **

**

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*

+

+

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*

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+

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*

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* * o * *

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**++

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*

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o +

* * *

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*+

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*

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++** **+

+

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*

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o

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*

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+* * ** *

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*

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+* ++

* *

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* * * * ** +

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30oN

28o

N

26oN

24o

N

22o

N

20o

N

18o

N

96o

W 94o

W 92o

W 90o

W 88o

W 86o

W 84 98o

W o

W

Fig. 19. Location of the profiles showing temperature inversions. This is about half (48%) of

the profiles. * indicate one inversion seen in the profile, + indicates two inversions

were seen in the profile, and o indicates three inversions seen in the profile.

200

900

900

200

200 900

900 . 200

. . . .

. . .

. . .

.

.

.

.

.

..

. . . .

.

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. .

.

.

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.

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.

.

.

.

.

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.

.

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.

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.

.

.

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.

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.

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.

.

.

.

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.

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o o o

o

o o

o

o o

o

o

o

o

o

o

o oooooooo o o

o

18oN

98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW

28oN

26oN

24oN

22oN

20oN

30oN

Fig. 20. Dots show the location of profiles with inversions when the next profile from that float

also showed an inversion at the same depth to within 100 m. This is 40% of the

inversion profiles. From this we conclude that the many inversions are persistent

lasting 7 days or more and have lateral scales order several tens of km (see text). The

open circles are locations of very strong inversion, DT > 0.50 ∞C. These cluster about

the base of the continental slope off the mouth of the Mississippi River (at a29∞N,

89.5∞W) and in the Bay of Campeche which is in the lower left.
21

18 20 22 24 26 28 30

Latitude ( N)

0

200

400

600

800

1000

Dm(m)

o

Fig. 21. Distribution depths of inversions as a function of latitude. Most of the inversions (96%)

were found in depths <160 m, but some were found essentially at all depths measured

by the PALACE floats.

The inversions showed a seasonal variability in that they were relatively abundant in

the late fall-early winter and relatively least abundant in the summer months

(Fig. 22). River discharge of course shows a seasonal variability, but what is of interest is that

the time of maximum river discharge is about a half-year before the time of maximum

occurrence of the inversions (Fig. 22). The explanation for this is given by results from the

LATEX, SCULP1, and SCULP2 drogued drifter study (Ohlman et al. 2001) which indicates that

in the northwestern Gulf most of the river discharge is sequestered on the shelf in the summer

months and is ejected into the open Gulf in the winter months (Fig. 23) when relatively many

inversions are seen. More details on inversions can be found in Weatherly et al. (2003) and the

profiles showing inversions and details about each inversion can be found on the web site

http://ocean.fsu.edu/~georges/temps/inversions/index.htm.
22

4

8

12

2 4 6 8 10 12

Month

Percent

20

40

R(mSv)

0

Fig. 22. Lower. Dashed line is the total river discharge in mSV (1 SV = 106 m3/s) into the Gulf

of Mexico for the period 10/62-9/65 [from Hall 1969]. Solid line is the cumulative

discharge into the Gulf of Mexico from 30 U.S. rivers with the mean of Mexican rivers

added from the U.S. Geological Survey data on their web site

http://waterdata.uscs.gov. Upper. Frequency of observation of inversions observed

from the floats as a function of month. The dark, light, and clear color denote the

contribution of inversions of strength DT > 0.050∞C, 0.010∞C < DT < 0.050∞C and DT

< 0.010∞C, respectively.
23

95oW 90oW 85oW

16oN

18oN

20oN

22oN

24oN

26oN

28oN

30oN

Origin: LATEX Shelf, Nov - Dec

Tracks Drawn Through End of Jan 

95oW 90oW 85oW

16oN

18oN

20oN

22oN

24oN

26oN

28oN

30oN

Origin: LATEX Shelf, Jun - Jul

a) Tracks Drawn Through End of Aug b)

Fig. 23. Trajectories for combined LATEX, SCULPI and SCULPII drogued drifters [see

Ohlman et al. 2001] for (a) drifters set in the LATEX region (Louisiana-Texas shelf

west of mouth of Mississippi River) in June-July with tracks drawn through the end of

August and (b) drifter set in the same region in November-December with tracks

drawn through the end of January. These figures indicate a much higher probability of

finding coastal water in the open Gulf of Mexico in the winter. The 200-m, 1,000-m

and 2,000-m isobaths are drawn. Figure is from Morey et al. (2002).

4.5 Surface Mixed Layers

In Fig. 11 examples of surface mixed layers (SMLs) can be found, and most are unremarkable in

that they are a few tens of m thick as we would expect. The SML thickness for the profile in the

Loop Current (the unusually warm one discussed earlier) in this figure is very thick at a75 m.

We were a little surprised how thick the surface layer can get in the Gulf of Mexico, and an

example of one of the thicker ones, about a170 m thick, can be seen in Fig. 24. In this section

results using the criterion that the temperature in the SML was uniform to within 0.1∞C are

presented. Other criteria were tried but the results presented here are not overly sensitive to the

choice made. Fig. 25 shows that SMLs thicker than 50 m were not that unusual, that (as

expected) thicker layers occur in the winter months, and that the deepest ones occur not where

the surface waters are coldest. Fig. 26 shows that the deepest SMLs occur in the eastern and

central Gulf of Mexico where the Loop Current and newly formed Loop Current Rings are more

common, and not in the northern Gulf where the surface temperatures are lowest in the winter.

In fact the deepest ones were indeed formed in the Loop Current or in newly formed Loop

Current Rings in the winter. Apparently there is sufficient heat loss from the Loop Current and

Loop Current Rings in the winter to cause rather deep SMLs.
24

4.6 Temperature Variability

The 1,481 temperature profiles obtained in the Gulf of Mexico when examined simultaneously

show that there is much variability in the temperature at all depths and much variability in the

depths of many isotherms (Fig. 27). In this section some of this variability is examined and

quantified.

The temperature data contoured at the surface, 300-m depth, 600-m depth, and 900-m depth

indicates that the Loop Current and Loop Current Rings extend down to at least 900-m depth

(Fig. 28). This supports the conclusion reached in Section 4.2 that the Loop Current and Loop

Current Rings extend to 900-m depth. Fig. 28 suggests that Loop Current below 300 m depth is

shifted about 100 km to the west of the Loop Current at the surface. This apparent shift is

artificial and due to the measurements made in the eastern portion of the Loop Current being

taken in summer months while those made in the western portion of the Loop Current were made

in winter months. No apparent lateral shift in the Loop Current below 300 m depth. It was noted

in Section 4.5 that the temperature profiles indicate there is much heat loss in the Loop Current

in winter.

depth (m)

Temperature (oC)

0

200

400

600

800

1000

0 8 16 24 32

Fig. 24. All temperature profiles obtained in a 1∞ by 1∞ square centered at 26.5∞N, 89∞W

showing an example of a very thick surface mixed layer; in the

right-most profile which was obtained in a Loop Current Ring in the winter.
25

Criteria .1

h(m)

Months Surface temperature (o

C)

(a) (b)

Criteria .1

Fig. 25. (a) Surface mixed layer depth as a function of month of the year. Note that rather

thick mixed layers, deeper than a50 m, are usual in the winter months. (b) Thickness

of surface mixed layer as a function of surface water temperature. Note that the

thickest layers do not occur where the surface water is coolest.

200

900

200

900

900

200

200

200

200

900

900

900

900

+

+

+

+

++

+ +

+

+

+

+

+ + + +

30oN

28o

N

26o

N

24o

N

22o

N

20o

N

18o

N

98o

W 96o

W 94oW 92o

W 90o

W 88o

W 86o

W 84oW

Fig. 26. Location of very thick, thickness >120 m, surface mixed layers. Note that they occur

in regions where the Loop Current and Loop Current Rings are common in the Gulf of

Mexico. The 200-m and 900-m isobaths are shown.
26

5 10 15 20 25 30

1200

1000

800

600

400

200

Surf

Temperature (oC)

depth (m)

Fig. 27. All 1,481 temperature profiles obtained in the Gulf of Mexico.
27

surface 300 m

600 m 900 m

Fig. 28. Contours of temperature at the surface (upper left), 300-m depth (upper right),

600-m depth (lower left), and 900-m depth (lower right). These figures reinforce the

conclusion that the Loop Current and Loop Current Rings extend down to at least 900-

m depth (Section 4.2). Comparing the panel for the surface with those at depth

suggests that the Loop Current shifts westward about 100 km between the surface and

300 m. This is an artificial shift due to when different parts of the surface Loop

Current was sampled (see Section 4.6). No lateral shifts with depth for depths ≥ 300 m

for the Loop Current and Loop Current are apparent.

It is interesting that in Fig. 27 the depth of greatest temperature variability is 149 m and that the

variability there is 14.1∞C (Fig. 29). Thus according to the data set examined here the greatest

temperature variability in the Gulf of Mexico occurs at a150 m depth and it is appreciably

greater than that seen at the surface and in the surface layer. It is further interesting that the

temperature variability at 300 to 400-m depths, way below the depth of seasonal changes, is

comparable to that seen in the surface layer.

Dividing the Gulf into an eastern region, east of 88∞W, a northwestern region, west of 88∞W and

north of 22∞N, and a Campeche Bay region, west of 88∞W and south of 22∞N, the temperature

variability is comparable to that of the shown in Fig. 29. For the eastern Gulf the maximum

temperature variability is 12.7∞C and occurs at 188-m depth, for the northwestern Gulf it is

12.2∞C and occurs at 168-m depth, and in the Campeche Bay it is 10.0∞C at 157-m depth; the

figures for these regions comparable to Fig. 29 and from which these values are taken are not

shown.
28

From Fig. 27 it is evident that the range of depth various temperatures were measured could be

large. For example, the 10∞C isotherm was measured between depths of a220 m and a710 m

depth, a depth variation of a500 m. The largest depth variation was for the 7∞C isotherm and is

a580 m (Fig. 30).

0

400

600

800

1000

200

0 4 8 12 16

Temperature (oC)

entire gulf min-max

Depth (m)

Fig. 29. Temperature variability as a function of depth. The greatest variability is not at the

surface or in the seasonally modulated surface layer but rather at a150-m depth. It is

interesting that at 300-m to 400-m depths, well below the seasonally modulated

regions, the variability is as great as in the seasonally modulated surface region.

Below a150-m depth the temperature variability is likely due in large part to vertical

isotherm displacements of transiting Loop Current, Loop Current Ring, and cold core

rings.
29

Temperature (oC)

0 8 16 24 32

Depth variability (m)

0

100

200

300

400

500

600

entire gulf min - max

Fig. 30. Depth variability of different isotherms. The greatest variability was for the 7∞C

isotherm; the depth it was measured varied by 580 m.

4.7 Deep Current Variability

A measure of the variability of the measured a900-m depth currents is their variance. The

variance of the currents measured in each 0.5∞ latitude by 0.5∞ longitude bins is shown in

Fig. 31. As for the mean velocities shown in Fig. 9 values are shown every 1/2 degree and only

if n > 5. Generally along the continental rise and margin these ellipses are aligned with their

major axes parallel to isobaths corroborating evidence of topographic Rossby waves there

reported in Hamilton (1990) and Hamilton and Lugo-Fernandez (2001). The ellipses are more

circular in the north-central Gulf in the region 26∞N-28∞N, 91∞W-93∞W; this may be due to

additional variability associated with passing Loop Current Rings in this region. In the central

northwestern Gulf, in the region a24∞N - 26∞N, a92∞W - 95∞W, the ellipses are relatively larger

and may be due to the previously mentioned deep extension of Loop Current Rings which passed

through the region.

4.8 Evidence of Bottom Boundary Layers

By way of background it is rare to see a temperature profile which completely scans the bottom

boundary layer (BBL). That is because when CTDs are profiled one usually stops a10 m above

the bottom for fear of driving the CTD into the bottom and damaging the CTD. Hence the

lowest a10 m of the BBL are rarely profiled.
30

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

900

900

900

40 (cm/s)2

Fig. 31. Variance ellipses for the a900-m depth currents. Currents were averaged, as for Fig. 8,

in 1/2∞ by 1/2∞ bins shifted every 1/4∞ and for n ≥ 5, where n is again the number of

data used to form averages. Along the continental margin the ellipses are oriented

with their major axes parallel to isobaths suggesting topographic Rossby wave activity

there as suggested in earlier studies. The variance is larger in the interior where Loop

Current Rings transit and in the western Campeche Bay where the southward flowing

current there intensifies.

Those temperature profiles obtained from the PALACE floats in 200 m < water depths

< 800 m completely profiled the BBL because in these cases the floats started from the bottom

where they were grounded. We present a few sample BBL profiles obtained from grounded

PALACE floats because they are unusual in that they completely profile the BBL. Of course

these float profiles yielded no direct deep current information associated with the temperature

profiles because they were grounded. However, from the BBL temperature profile one can infer

whether the near-bottom current flowed with deep water to the left looking downstream or deep

water to the right looking downstream at the time the profile was made (e.g., Weatherly and

Martin 1978). In the first case the BBL will appear as a cold bottom layer, and in the second

case as a warm bottom layer.

Some BBL temperature profiles obtained in a 1∞ latitude by 1∞ longitude box centered at 28.5∞N,

86∞W are shown in Fig. 32. From these profiles it appears that the flow immediately above the

bottom on the rise in water deeper than a300 m was to the east (deep water to the right looking

downstream) and at shallower depths on the rise it flowed to the west. We am unaware of any

current measurements made in this region at the time of the profiles were made.
31

Depth (km)

Temperature profiles

Temperature (oC)

∆T=-1.6o

h=70m

∆T=-1.1o

h=40m

∆T=-1.6o

h=40m

∆T=+0.9o

h=60m

Fig. 32. Four temperature profiles from a grounded float showing evidence of a bottom

boundary layer (BBL). The solid curved line is the temperature, the horizontal

straight line denotes bottom, the doted and dashed lines are drawn to estimate DT, the

temperature anomaly of the BBL, and h the BBL thicknesses. For water depth

< 300 m (upper left panel) the BBL is anomalously warm (DT > 0) indicating here

flow to the east above the BBL. For water depths > 300 m (remaining panels) the

BBL is anomalously cold (DT < 0) indicating here flow to the west above the BBL.
32

4.9 Comparison of a900-m Depth Measured Flows with Some Numerical Model Results

Figs. 33 and 34 and 35 show the average velocity and temperature at 1000 m depth for a one

year MOM simulation (courtesy of Susan Welsh), the 3-year average velocity at 1000 m depth

from the North Atlantic Sigma Princeton Ocean Model (courtesy of Tal Ezer), and the year 2000

velocity field from the Mercator model (Groupment Mercator) applied to the Atlantic Ocean,

respectively. Table 2 indicates that there is general good agreement with the data.

A feature seen in the model outputs but not seen in Figs. 8 and 10 is an anticyclonic gyre in the

northwest corner of the Gulf of Mexico centered near 27∞N, 95∞W. This region of the Gulf of

Mexico was relatively poorly sampled (Fig. 5). Results of an objective analysis we did which

produced a non-divergent flow (Gille 2003) indicate an anticyclonic eddy in this region

(Fig. 36). In this figure the red arrows are the results of the objective analysis and the blue

arrows are the float data.

Fig. 33. Output of MOM model showing temperature and currents averaged over one year at

1,000-m depth (see Section 4.9). For information about MOM see Welsh and Inoue

(2001).
33

Fig. 34. Average current over a 3-year period at 1,000-m depth from the Princeton POM (see

Section 4.8).

10cms 1

 99oW 96oW 93oW 90oW 87oW 84oW 81oW 78oW 

 18oN 

 21o

N 

 24o

N 

 27o

N 

 30o

N 

 33o

N 

10cms 1

Fig. 35. Average current at 1,000-m depth for year 2000 from the Mercator model (see

Section 4.8).
34

10cms 1

e=0.5, L1=L2=100 km, P=1000 km

10cms 1

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24oN 

 26o

N 

 28o

N 

 30o

N 

10cms 1

10cms 1

Fig. 36. Horizontally nondivergent flow field (red arrows) inferred from an objective analysis of

the actual flow field (blue arrows). The objectively inferred flow field has an

anticyclonic eddy in the northwest corner of the Gulf of Mexico as do all the computer

model simulations in Figs. 32-35.

Table 2. Whether features seen in float data were seen in model outputs. Under (a) is the

continental slope region in the northeast Gulf east of 89∞W (NE), continental slope region in the

northern Gulf between 89∞W and 95∞W (N), continental slope region in the western Gulf north

of 23∞N (NW), continental slope region in the western Bay of Campeche (WCB) south of 23∞N,

and in the Campeche Bump region of Campeche Bay (CBB). Under (b) are the cyclonic gyres

centered near 20∞N, 95.5∞W (20), near 23∞N, 94∞W (23), and near 27∞N, 91∞W (27). An ”x”

indicates seen in the model output.

Cyclonic flow along slope Cyclonic gyres

Model (a) (a) (a) (a) (a) (b) (b) (b)

NE N NW WCB CBB 20 23 27

MOM

Princeton POM

Mercator

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x
35

5.0 SOME CLOSING REMARKS

The PI would not have chosen 900 m as the drift depth of the PALACE floats. It is too close to

the so-called level-of-no-motion (e.g., Stommel 1965). It is also close to the transition depth

between an upper layer regime where the flow is dominated by the Loop Current and winddriven flows, and a deeper abyssal layer as yet poorly understood (e.g., Hamilton and LugoFernandez 2001). Finally, this is also near the sill depth for the Straits of Florida through which

the Loop Current exits the Gulf of Mexico. For these reasons the PI expected the mean flow to

be rather weak at this depth, as indeed the PALACE floats generally indicated. The only place

their data indicated that there was a moderately strong (a 10 cm/s) mean flow was along the

continental margin in the western Bay of Campeche.

Nonetheless the PALACE data indicated there was a general cyclonic flow around the

continental margin of the Gulf of Mexico at about 900-m depth. Whether this is due to upper

layer effects or lower layer effects is unknown. Also, the PALACE float data indicated that

when there was enough data to estimate a mean current, the mean current was not zero within the

uncertainty estimates. So the PALACE data examined do indeed suggest that 900-m depth is not

a level of no motion but a region of mean generally weak cyclonic flow about the Gulf of

Mexico. This seems an important feature which numerical models should be able to reproduce.

The submerged time of the floats, 6.5 days, is such that the intermediate depth flows they

experienced were influenced by strong, transitory flows associated with planetary waves and

mesoscale eddies because these flows have time integral time scales also of this length. This

study indicates that, for PALACE floats drifting at intermediate depths in the Gulf of Mexico

with a weekly cycle times, about five samples are needed to average out these transitory flows.

About half of the temperature profiles obtained showed cold layers above warmer layers, which

here were called (temperature) inversions. Of these inversions about half were revisited again

miles away on the float’s next profile. It was concluded this could be only if the resampled cold

layers were also less saline layers too, and that they were riverine and coastal waters ejected into

the open Gulf of Mexico. Does that mean that the other half of the inversions were due to some

other process? Perhaps some were, but it is thought most were not. This is because the tally did

not include the many inversions which were sampled on the next-but-one profile. Thus it is

thought that most of the inversions are indicators of the presence of riverine and coastal waters

rather than mixing processes associate with breaking internal waves and/or double-diffusive

processes. If correct, it is unclear what implications the common occurrence of fresh, cool

coastal waters far offshore have on the physics, biology and chemistry of the Gulf of Mexico.

6.0 ANNOTATED BIBLIOGRAPHY OF THE PUBLISHED PAPER AND WEB SITES

Weatherly, G., N. Winders, and R. Harkema, 2003. Temperature inversions in the open Gulf of

Mexico. J. Geophys. Res. 108, doi:10.1029/2002JC001680.

This paper reports on evidence of riverine water found in the open Gulf of Mexico from the

temperature profile data obtained by the PALACE floats. Half of the profiles showed evidence

of riverine water throughout the Gulf of Mexico and at all times. It was an unexpected result to
36

see so many and to see them everywhere. We document that the major sources of these fresh

layers are the Mississippi and Atchalafaya Rivers, which account for a60% of the freshwater

inflow to the Gulf of Mexico, and 19 Mexican rivers discharging into the Campeche Bay, which

account for another a20% of the fresh water inflow. Most of the layers were found near the

surface but a few were found at essentially all depths. It is unclear if the deeper ones are due to

possible downwelling of the surface fresh layers, double diffusion processes acting to sink the

fresh surface layers, and/or lateral advection from the Caribbean Sea of deep waters rich in

Antarctic intermediate water.

Note that the following web sites are all linked. We expect to maintain them through the end of

2004.

The web site http://ocean.fsu.edu/~georges/temps/index.htm lists the velocity values inferred at

a900-m depth for each PALACE float. It is intended as an archive of this data and to make it

available to those who might want to access it. The data is also listed in MATLAB form for

those wishing to look at these data using MATLAB.

The web site http://ocean.fsu.edu/~georges/temps/index.htm is concerned with the temperature

data from the PALACE floats. In this site we have made use of material that was on a now

discontinued web site which Webb Research use to maintain. In so doing all the material

previously on that web page about the PALACE floats is now on this page.

The temperature profiles sorted in 1∞ latitude by 1∞ longitude bins are shown as well as the

average profile for each bin compared to the average profile for the whole Gulf. It is possible to

see individual profiles by appropriately clicking.

Plots of the average over 1∞ by 1∞ temperature contour plots at three depths are shown.

The web site http://ocean.fsu.edu/~georges/temps/inversions/index.htm is concerned with the

profiles showing temperature inversions discussed in Weatherly et al. (2003). How they were

geographically distributed in 1∞ by 1∞ bins is shown, and by appropriately clicking the

individual profiles with temperature inversions can be viewed. Also, where to look for the

inversion on the profile and how strong it is are given.

7.0 REFERENCES

Blaha, J., G.H. Born, N.L. Guinasso Jr., H.J. Herring, G. A. Jacobs, F.J. Kelly, R.R. Beden, R.D.

Martin Jr., G.L. Mellor, P.P. Niiler, M.R. Parks, R.C. Patchen, K. Schaudt, N.W. Scheffner,

C.K. Shum, C. Ohlmann, W. Sturges III, G.L. Weatherly, D. Webb, and H.J. White. 2000.

Gulf of Mexico ocean monitoring system, Oceanography, 13:10-17.

Davis, R.E., J.T. Sherman, and J. Dufour. 2001. Profiling ALACEs and other advances in

autonomous subsurface floats, J. Atmos. and Oceanic Tech., 18:982-993.

DeHaan, C.J. 2002. Determining the deep current structure in the Gulf of Mexico, Ph.D. Thesis,

Florida St. Univ., Tallahassee, FL. 80 pp.
37

Elliot, B.A. 1979. Anticyclonic rings and the energetics of the circulation of the Gulf of Mexico,

Ph.D. Thesis, Texas A&M Univ., College Station, TX. 188 pp.

Gille, S. T. 2003. Float observations of the southern ocean: Part 1, Estimating mean fields,

bottom velocities, and topographic steering, J. Phys. Oceanogr, 33:1,167-1,181.

Hall, C.S. 1969. An investigation of the water balance of the basin of the Gulf of Mexico. M.S.

Thesis, Texas A & M Univ., College Station, TX. 39 pp.

Hamilton, P. 1990. Deep currents in the Gulf of Mexico, J. Phys. Oceanogr., 20:1,087-1,104.

Hamilton, P. 1992. Lower continental slope cyclonic eddies in the Gulf of Mexico, J. Geophys.

Res., 97:2,185-2,200

Hamilton, P. and A. Lugo-Fernandez. 2001. Observations of high-speed currents in the northern

Gulf of Mexico, Geophys. Res. Lett., 28:2,867-2,870.

Morey, S.L., J.J. O’Brien, W.W. Schroeder, and J. Zavala-Hidalgo. 2002. Seasonal variability of

the export of river discharged fresh water in the northern Gulf of Mexico, Proceedings:

Oceans 2002 MTS/IEEE Conference Biloxi, MS. pp. 1,480-1,484.

Ohlman, J.C., P.P. Niiler, C.A. Fox, and R.R. Leben. 2001. Eddy energy and shelf interactions in

the Gulf of Mexico, J. Geophys. Res., 106:2,605-2,620.

Stommel, H. 1965. The Gulf Stream, Cambridge University Press, London, 2nd edition, 248 pp.

Welsh, S. and M. Inoue. 2000. Loop current rings and the deep circulation in the Gulf of Mexico,

J. Geophy. Res., 105:16,951-16,959.

Weatherly, G. and P. Martin. 1978. On the structure and dynamics of the ocean bottom boundary

layer, J. Phys. Oceanogr., 8:557-570.

Weatherly, G., N. Wienders, and R. Harkema. 2003. Temperature inversions in the open Gulf of

Mexico, J. Geophys. Res. 108(C6 ):10.1029/2002JC001680.

Weatherly, G.L., Y.Y. Kim, and E. Kontar. 2000. Eulerian measurements of the North Atlantic

deep water deep western boundary current at 18∞S. J. Phys. Oceanogr., 30:971-986.

Zavala-Hidalgo, J., S.L. Morey, and J.J. O’Brien. 2003. Cyclonic eddies northeast of the

Campeche Bank from altimetry data, J. Phys. Oceanogr., 33:623-629.
38

8.0 APPENDIX A

The following figures are the PALACE velocity vectors in 6-month intervals and were used in

determining Table 1. The first is for 04/98 £ time < 10/98, the second for 10/98 £ time < 04/99, the

third for 04/99 £ time < 10/99, the fourth for 10/99 £ time < 04/00, the fifth for 04/00 £ time

< 10/00, the sixth for 10/00 £ time < 04/01, the seventh for 04/01 £ time < 10/01, and the seventh for

10/01 £ time < 04/02.

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

900

2000

900

2000

900

2000

10 cm/s

04 1998 <= date < 10 1998

Fig. A1.

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

900

900

900

900

2000

2000

2000

10 cm/s

10

1998 <= date < 04

1999

Fig. A2.
39

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

900

900

900

2000

2000

2000

10 cm/s

04

1999 <= date < 10

1999

Fig. A3.

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

900

900

900

900

2000

2000

2000

10 cm/s

10

1999 <= date < 04

2000

Fig. A4.
40

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

900

900

900

2000

2000

2000

10 cm/s

04

2000 <= date < 10

2000

Fig. A5.

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

900

900

900

2000

2000

2000

10 cm/s

10

2000 <= date < 04

2001

Fig. A6.
41

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24o

N 

 26o

N 

 28o

N 

 30o

N 

2000

2000

900

900

900

10 cm/s

04

2001 <= date < 10

2001

Fig. A7.

 98oW 96oW 94oW 92oW 90oW 88oW 86oW 84oW 18o

N 

 20o

N 

 22o

N 

 24oN 

 26o

N 

 28o

N 

 30o

N 

2000

2000

2000

2000

900

900

900

900

10 cm/s

10

2001 <= date < 04

2002

Fig. A8.