OCS Study 

MMS 2004-040 

 

Strong Mid-Depth Currents and a 

Deep Cyclonic Gyre in the Gulf of Mexico 

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 2004-040 

Strong Mid-Depth Currents and a 

Deep Cyclonic Gyre in the Gulf of Mexico 

Authors 

Wilton Sturges 

Department of Oceanography 

Florida State University 

Tallahassee, Florida 

Eric Chassignet 

RSMAS 

University of Miami 

Miami, Florida 

Tal Ezer 

Atmospheric & Oceanic Sciences 

Princeton University 

Princeton, New Jersey 

Prepared under MMS Contract 

14-35-01-99-CT-31027 

by 

Florida State University 

Department of Oceanography 

Tallahassee, Florida 

Published by 

New Orleans

May 2004 
iii

DISCLAIMER 

This report was prepared under contract between the Minerals Management 

Service (MMS) and Florida State University (FSU). This report has been 

technically reviewed by MMS, and it has been approved for publication. Approval 

does not signify that the contents necessarily reflect the views and policies of 

MMS, nor does mention of trade names or commercial products constitute 

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

compliance with MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of the report may be obtained from the Public 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 Boulevard 

New Orleans, LA 70123-2394 

Telephone: 1-800-200-GULF or 

504-736-2519 

CITATION 

Suggested citation: 

Sturges, W., E. Chassignet, and T. Ezer. 2004. Strong mid-depth currents and a 

deep cyclonic gyre in the Gulf of Mexico: Final report. U.S. Dept. of the 

Interior, Minerals Management Service, Gulf of Mexico OCS Region, New 

Orleans, LA. OCS Study MMS 2004-040. 89 pp. 

ABOUT THE COVER 

The cover figure shows a vertical section at 90°W in the Gulf of Mexico; the 

contours are the mean E-W velocity from the Princeton model. The deep mean 

flow along the northern (U.S.) side is now well documented. The prominent feature 

along the southern (Mexican) side shows deep flow to the east. That eastwardflowing current has not been observed but is predicted by the studies in this report.
v

ACKNOWLEDGMENTS 

We thank Evans Waddell and Peter Hamilton, SAIC, Inc., Raleigh NC for 

generously providing us with data of various kinds and on many occasions. We 

thank our colleagues G. L. Weatherly and N. Wienders (F.S.U.) for allowing us the 

use of the early, unpublished results of their corrected float data from the NOPP 

program early on in our work. We thank David Szabo of Fugro GEOS for 

allowing us limited access to proprietary data from the GULL program. We thank 

Ms. Jane Jimeian for help with the data analysis and Mrs. E. Shargel for editorial 

assistance and advice. 

During the course of this work, Chassignet was assisted by Dr. Anastasia Romanou 

and Dr. Laurent Cherubin; Sturges was assisted by Dr. Christopher J. DeHaan. We 

have had the good fortune to have the excellent assistance of both Dr. Carole 

Current and Dr. Alexis Lugo-Fernandez as the MMS Contracting Officer's 

Technical Representatives. It is a pleasure to acknowledge our gratitude for their 

cooperation and assistance.
vii

TABLE OF CONTENTS 

 Page

LIST OF FIGURES...........................................................................ix 

LIST OF TABLES..........................................................................xiii 

1. EXECUTIVE SUMMARY...............................................................1 

2. INTRODUCTION........................................................................5 

 2.1 Introduction............................................................................5 

2.2 Why We Expect the Deep Flow to be Cyclonic.......................................9 

3. ANALYSES OF AVAILABLE OBSERVATIONS ...............................11 

 3.1 Deep Temperature Structure and Vertical Shear..................................11 

3.1.1 Eastern Gulf...............................................................................11 

3.1.2 Central Gulf...............................................................................14 

3.1.3 Western Gulf ............................................................................18 

3.1.4 Variability of the Geostrophic Shear.........................................18 

3.2 Resolving Reference-Level Issue: Observations of Deep Flow.............22 

3.2.1 Observations from Deep Current-Meter Moorings ...................22 

3.2.2 Observations from Deep Drifting Floats....................................28 

3.3 Path of Cold Renewal Water ............................................................29 

4. RESULTS FROM NUMERICAL MODELING: 

STUDIES WITH THE PRINCETON MODEL ................................... 37 

4.1 Description of the Princeton Ocean Model........................................37 

4.2 General Circulation in the Gulf of Mexico and Comparisons 

Between POM and MICOM...............................................................38 

4.3 Effects of Propagating Loop Current Eddies on the Deep 

Circulation............................................................................................47 

4.4 Variability of Flow in the Yucatan Channel and its Relation to 

 Variations in the Loop Current............................................................49 

5. RESULTS FROM NUMERICAL MODELING: 

 STUDIES USING MICOM................................................................53 

 5.1 Miami Isopycnic-Coordinate Ocean Model (MICOM)........................53 

5.2 General Circulation Patterns in the Gulf of Mexico from MICOM........53 

5.3 Strong Current Events Near the Bottom of the Gulf ........................59 

 5.4 Meridional Distribution of the Zonal Transport at 90°W .......................59 

 5.5 Lagrangian Trajectories in the Deep Gulf Circulation...........................64 

6. ANNOTATED BIBLIOGAPHY .................................................................69 

7. REFERENCES...............................................................................................73 
ix

LIST OF FIGURES 

 Page

Figure 1.1. Mean E-W (U) velocity component along a N-S section at 

90° W from the Princeton Ocean Model............................................3 

Figure 2.1. Time-mean temperature distribution at 400 m, based on the full 

historical database from NODC. ......................................................6 

Figure 2.2. Time-mean distribution of potential temperature at 1,250 m relative to 

the sea surface, or 0 db ........................................................8 

Figure 3.1. Location of the regions used to form data means for constructing 

shear profiles..................................................................................12 

Figure 3.2. Mean vertical temperature distributions in the eastern Gulf of 

Mexico....................................................................................13 

Figure 3.3. Mean N-S geostrophic speeds relative to 1,000 db computed 

from the temperature profiles of Figure 3.2 and using a mean T-S 

curve ...........................................................................15 

Figure 3.4. Histogram of the number of hydrographic observations at each 

depth for the eastern region (shown in Figure 3.1) of the eastern 

Gulf for these calculations. .........................................................16 

Figure 3.5. Mean E-W speeds, relative to 1,000 db, computed from the 

temperature signal in the central Gulf. ..........................................17 

Figure 3.6. Mean N-S geostrophic speeds relative to 1,000 m in the 

Western Gulf of Mexico. ...................................... .......................19 

Figure 3.7. Locations of station pairs (from single cruises) from which 

geostrophic velocities were computed. .......................................20 

Figure 3.8. Locations of current meter Moorings A and G................................24 

Figure 3.9. Velocity records at the deepest instruments on Mooring A...............25 

Figure 3.10. Mean velocity profile for speeds at Mooring A...............................26 
x

LIST OF FIGURES 

(continued) 

Figure 3.11. Mean velocity of drifters at 900 m over the course of several 

years. .................................................................................30 

Figure 3.12. Time-mean potential temperature around the edge of the Gulf of 

Mexico at 2,000 m, averaged in one-degree boxes, plotted as a 

function of distance from the sill in Yucatan Channel. ....................32 

Figure 3.13. Mean temperature at 2,000 m........................................................34 

Figure 3.14. Potential temperature-salinity properties of the deep 

Gulf.................................................................................................35 

Figure 4.1. Trajectory plot based on the 5-year mean velocity at 400-m 

depth from POM forced by 6-hourly ECMWF winds but 

without data assimilation......................................................39 

Figure 4.2. Trajectory plot based on the 5-year mean velocity at 500-m 

depth from POM forced by 6-hourly ECMWF winds but 

without data assimilation. ..................................................................40 

Figure 4.3. Trajectory plot based on the 5-year mean velocity at 1,500 m from 

POM forced by 6-hourly ECMWF winds but without data 

assimilation....................................................................41 

Figure 4.4. Trajectory plot based on the 5-year mean velocity at 2,000 m from 

POM forced by 6-hourly ECMWF winds but without data 

assimilation. ..................................................................42 

Figure 4.5. Trajectory plot based on the 5-year mean velocity at 2,500 m from 

POM forced by 6-hourly ECMWF winds but without data 

assimilation.................................................................................. ......43 

Figure 4.6. Mean E-W (U) velocity component along a N-S section at 90°W 

from POM. .........................................................................44 

Figure 4.7. Instantaneous flow pattern along the 90°W section at four phases 

of a ring passage (from POM). ................................................46 
xi

LIST OF FIGURES 

(continued) 

Figure 4.8. Mean surface elevation across 90oW as a function of time indicates 

eddies (positive peaks) crossing the section. ..............................48 

Figure 4.9. Total inflow transport in the upper Yucatan Channel from the 

Caribbean Sea into the GOM, the time evolution of EOF mode 2, 

and eddy shedding events ......................................................50 

Figure 4.10. Deep outflow (in the POM) from the Gulf below 800 m (x-axis) 

and the change in sea level in the Loop Current region (positive 

y-axis values indicate times when the Loop Current is growing in 

area)................................................................................51 

Figure 5.1. Mean velocity from MICOM at a depth of 400 m...........................54 

Figure 5.2. Mean velocity from MICOM at a depth of 900 m...........................55 

Figure 5.3. Mean velocity from MICOM at a depth of 1,250 m........................56 

Figure 5.4. Mean velocity from MICOM at a depth of 1,500 m.........................57 

Figure 5.5. Mean velocity from MICOM at a depth of 2,000 m.........................58 

Figure 5.6. Standard deviation of the velocity field at 900 m............................60 

Figure 5.7. Standard deviation of the velocity field at 1,500 m.........................61 

Figure 5.8. Amplitude of current bursts in layer 10, in a latitude-time diagram 

and the location of the burst (latitude)......................................62 

Figure 5.9. Longitude-time plots of the amplitude and the location of strong 

bursts in layer 13. ................................................................63 

Figure 5.10. Time series of transports across a N-S section at 90° W.................65 

Figure 5.11. Lagrangian trajectories of particles in layer 13, originating in the 

northern and central regions of the Gulf. ......................................66 

Figure 5.12. Lagrangian trajectories in layer 12 for two time intervals..............67 
xiii

LIST OF TABLES 

 Page

Table 2.1. Mean potential temperature, °C, in the centers and edges of the 

eastern and central Gulf of Mexico. ..............................................7 

 

Table 3.1. Mean velocity shear and standard deviation relative to 2,000 db....21 

Table 3.2. Observations of deep currents near the edges of the Gulf of 

Mexico...........................................................................................23 
1 

1. EXECUTIVE SUMMARY 

The main purpose of this work was to explore the possibility that the deep flow 

(~2000 m) around the edges of the Gulf circulates in a cyclonic, or counterclockwise direction. The existence of such flow was proposed on theoretical 

grounds but had not been previously documented. Our results are quite clear that 

such flow is reliably observed. 

Recall that the surface flow in the eastern Gulf is strongly anticyclonic as a result 

of the Loop Current. The surface flow in the western Gulf is also largely 

anticyclonic as a result of both wind forcing and the presence of large eddies that 

separate from the Loop Current. So a cyclonic deep flow might appear at first 

glance to be counter-intuitive. One tacit over-simplification we have made is that 

the deep flow in both the eastern and western Gulf can be adequately described as 

a simple large gyre. We suspect that better measurements in the future will lead to 

an awareness of a richer mean field with more detail, but at the moment the 

database is not sufficient to allow better resolution. It should be clear that, being 

strictly a boundary flow, there is ample space for a much more detailed flow in the 

interior. 

Surprisingly, the warm-core temperature and density patterns associated with these 

surface flows extend to at least 2000 m. There are reasons, however, to suspect 

that the deep mean flow should actually be cyclonic. Topographic wave 

rectification and vortex stretching contribute to this cyclonic tendency, as will the 

supply of cold incoming deep water at the edges of the basin. We find that the 

deep mean flow is cyclonic both in the eastern and western Gulf, with speeds on 

the order of 2 to 4 cm/s at 2000 m. Recent observations at deep moorings in the 

northern Gulf reveal even stronger mean speeds. 

This report is based on observational and numerical modeling studies and it deals 

with three separate but related topics. The observations include four separate 

groups of current-meter moorings at different locations and in different years. The 

observations range in duration from nearly one year to three years. There was also 

a major deep-drifting buoy project in the western Gulf that gave supportive results. 

While this was hypothesized from a theoretical point of view, the conclusive 

evidence is completely observational. This cyclonic mean deep flow takes place 

around the edges of the basin, rather like the well-known deep western boundary 

current off the east coast of the U.S. We are not aware of any long-term data set 

that offers conflicting evidence. 
2 

The temperature around the edges of the basin at 2000 m is coldest near Yucatan 

Channel, where Caribbean Sea water is colder by ~0.1°C. The temperature 

increases steadily with distance from Yucatan in the counter-clockwise direction, 

consistent with a deep mean cyclonic boundary flow. 

The second purpose of the work was to try to explain the rare but persistent 

findings of strong bursts of flow at intermediate depths, as reported by drilling 

platform operators. It is possible to search for such bursts of flow in the results 

from numerical models. The models used here are two of the best-known models 

in oceanography: The Princeton Ocean Model, which is an outgrowth of the 

Mellor-Yamada model, and the Miami Isopycnic Coordinate Model, or MICOM. 

Some strong bursts of flow are found, as reported herein, but while the results are 

informative, they are not compelling. One fundamental difficulty is that the 

reported observations of these strong flows continue to be remarkably elusive, so it 

is difficult to know exactly what one should be looking for in the models and under 

what conditions. 

The third part of our results deals with comparisons between the observations and 

the numerical modeling results. At this stage in numerical model development, 

results tend to capture most of the major time-dependent features of ocean 

circulation rather well. However the mean flows over many years are perhaps not 

the most reliable part of the results. The interesting result here is that, when two 

"top of the line" numerical models were run by experienced practitioners, one of 

the models found a deep mean flow that is consistent with the observations, and the 

other model did not. And the reasons for the differences are the subject of 

considerable debate. It obviously will require further study to help unravel this 

mystery. Figure 1.1, which also appears on the cover of this report, is a N-S 

section through the Gulf at 90° W. Mexico is on the left, the U.S. on the right. 

The region of flow of ~4 cm/sec (to the west) along the steep slope off the U.S. 

coast, at depths of 1500-2500 m, was found to be quite consistent with two 

separate long-term current-meter mooring arrays. The observations of this flow 

are discussed in Section 3 and the numerical modeling in Section 4. 

It is appropriate to point out the significance of the primary finding of this work. 

The deep mean cyclonic flow in the Gulf appears to be analogous in many ways to 

the deep western boundary current of North Atlantic Deep Water flowing to the 

south along the east coasts of North and South America. The deep flow in the Gulf 

is strongly time-dependent; its cyclonic nature emerges primarily in the time mean, 

as the instantaneous velocities have a large standard deviation. Our understanding 

of deep western boundary flows in general is that the source regions 
3 

Figure 1.1. Mean East-West velocity component along a North-South section at 

90° W from the Princeton Ocean Model. The mean speed over 5 

years is shown in cm/sec. 
4 

of cold deep water are in the far north and the cold water flows generally to the 

south. Because there is such large eddy variability, the cold source waters slowly 

bleed off into the interior by eddy fluxes as the main body of flow continues south. 

The cold signal is continually but slowly eroded away. These eddy processes have 

not been studied extensively in the Gulf, and are not the subject of our study here. 

The main point of course is that these results suggest both a mechanism and a 

pathway whereby the cold deep renewal water from the Caribbean is supplied to 

the interior of the Gulf. These added features are based on the analogy with deep 

western boundary currents. They have not yet been observed or studied in the Gulf 

and remain to be explored further.
5 

2. INTRODUCTION 

2.1. Introduction 

The Loop Current and the warm-core rings that detach from it dominate the upperlayer flow in the Gulf of Mexico. The well-known primary flow enters the 

Caribbean Sea from the open Atlantic, flows into the Gulf of Mexico through 

Yucatan Channel and leaves the Gulf to form the Florida Current and Gulf Stream 

(see, for example, Schmitz and Richardson, 1991; Niiler and Richardson, 1973). 

There have been several earlier attempts to summarize the flow in the Gulf using 

the historical data (see Hoffman and Worley, 1986; Molinari et al., 1978). The 

upper-layer flow has been observed fairly well in recent years by satellite 

observations of temperature and sea-surface height. The main flow is restricted to 

approximately 850 m as a result of the limiting depth of the Florida Straits. 

The deep flow has been described by Hamilton (1990). A magnificent new set of 

mooring data in the Yucatan Channel has now been reported (Bunge et al., 2002; 

Ochoa et al., 2001; Sheinbaum et al. 2002). The mean upper-layer flow is well 

described by almost any temperature or density surface near or in the main 

thermocline. Figure 2.1 shows an attempt to determine the (long-term) mean 

temperature at 400 m. The mean depth of the 27.0-sigma-t surface shows a similar 

structure. The dots that appear to be data points are local means of essentially 

random concentrations of ~5-10 hydro stations. Using data from many years, these 

were selected from the full historical NODC data base (Conkright et al., 2000) in 

an attempt to suppress the very great time variability of warm-core rings 

propagating to the west. 

Two individual warm-core features are evident in Figure 2.1. Although the Loop 

Current position is notoriously variable, it is well known that a clear mean emerges 

in the east . In the central and western Gulf the anticyclonic pattern is maintained 

both by the mean wind field and by the passage of large warm-core rings that have 

separated from the Loop Current. The relative importance of these two forcing 

mechanisms remains an open question. The western gyre appears to have 

southerly flow from ~90°W to ~95°W; the models used here generally show this 

flow (plus details; see for example the maps of velocity at 400 m, Figure 4.1, et

seq., later in this report). At the far western edge, however, the strong core of 

northerly flow found in the models is not apparent in the data as a result of poor 

data resolution. One problem has been that the majority of the older hydro 

 
6 

Figure 2.1. Time-mean temperature distribution at 400 m, based on the full 

historical database from NODC. The individual dots that appear to be 

data points are local means computed from ~5-10 hydro stations 

concentrated near the point. The 1,000-m isobath is shown. The flow 

between Cuba and Florida is not resolved in these data. 
7 

sections run N-S. Though we know the upper-layer flow fairly well, we do not 

know the deep flow very well at all. Several numerical models include the Gulf of 

Mexico, but good observations at depth are scarce. The intriguing fact is that the 

upper-layer flow, so obviously anticyclonic, appears to extend—in the density 

field—to depths of ~1,500 m to 2,000 m. Because we know that the flow in the 

Florida Current penetrates only to ~850 m we might expect the warm-core 

structure of Figure 2.1 to extend only to that depth, whereas observations show that 

it goes much deeper. 

Figure 2.2 shows a map of temperature at 1,250 m. The warm-core patterns are 

still evident at this depth. They remain evident, if not as clearly so, as deep as 

2,000 m. In the eastern Gulf, the horizontal temperature difference at 1,500 m is 

~0.06° C between the central region and the eastern edge. In the central Gulf the 

difference has been eroded to ~0.04° C between the central region and the northern 

edge. This “temperature difference” signal is still evident at depths of 2,000 m. 

Table 2.1, adapted from DeHaan and Sturges (2004), shows the essential 

information. 

_________________________________________________________________ 

Table 2.1 

Mean Potential Temperature, °C, in the Centers and Edges of the 

Eastern and Central Gulf of Mexico 

(The first value is potential temperature; the second is the number of samples.) 

 Central Warm Outer Cool Difference 

 Region Region ∆ T, °C

E. Gulf, 1,500 m 4.185, 44 4.130, 123 0.045 

Central, 1,500 m 4.142, 32 4.105, 45 0.037 

E. Gulf, 2,000 m 4.050, 19 4.016, 34 0.034 

Central, 2,000 m 4.053, 21 4.024, 19 0.029 

__________________________________________________________________ 
8 

Figure 2.2. Time-mean distribution of potential temperature at 1,250 m relative to 

the sea surface, or 0 db. The original data set is the same as for Figure 

2.1. The isobath shown is 1,250 m. 
9 

A “warm-core ring” in the upper ocean suggests anticyclonic flow, but (as is well 

known) in deep water this signal tells us only about vertical shear. For reasons put 

forth in the next section, it seems reasonable to expect that the deep mean flow 

should be cyclonic. And so we ask, is there evidence for a reversal of the mean 

flow between ~800 m and 1,500 m? 

Some readers may raise the additional question: Why do we care about the 

direction of deep mean flow? Or, to put it another way, what is the significance of 

these results? The Caribbean Sea is the only source of deep water for the Gulf of 

Mexico, for no deep water is formed locally. The Caribbean source of cold deep 

water (e.g., Bunge et al., 2002) supplies oxygen and nutrients. The return flow 

flushes the deep Gulf. The deep mean cyclonic flow in the Gulf is analogous in 

many ways to the Deep Western Boundary Current of North Atlantic Deep Water 

flowing to the south along the east coasts of North and South America. The deep 

flow in the Gulf is strongly time-dependent; its cyclonic nature emerges primarily 

in the time mean, as the instantaneous velocities have a large standard deviation. 

Our understanding of deep western boundary flows in general is that the source 

regions of cold deep water are in high latitudes. In the case of NADW, the cold 

water flows generally to the south. Because there is such large eddy variability, 

the cold source waters slowly bleed off into the interior by eddy fluxes as the main 

body of flow continues south. The cold signal is continually but slowly eroded 

away. These eddy processes have not been studied extensively in the Gulf, and are 

not explored here. The main point of course is that these results point to a 

mechanism and a pathway whereby the cold deep renewal water from the 

Caribbean is supplied to the interior of the Gulf. These added features are based 

on the analogy with deep western boundary currents but have not been observed in 

the Gulf. These effects are obvious targets for further study if we are to understand 

the mixing of deep water and the residence times in the deep Gulf. Examples of 

such further questions would be, If a massive deep spill should occur, can we 

estimate the flushing time of the waters between 1,000 m and 2,000 m? Or at any 

depths? 

2.2. Why We Expect the Deep Flow to be Cyclonic 

There are three mechanisms here that could generate cyclonic deep flow. First, it 

is well known that there is a substantial amount of eddy-like activity over the entire 

basin at depth, composed mostly of topographic Rossby waves (Hamilton, 1990; 

Oey and Lee, 2002). Topographic rectification of these waves would contribute to 

cyclonic mean flow. Second, there is the introduction of cold deep water from the 

Caribbean, through the Yucatan Channel. At 2,000 m, the Caribbean is ~0.1° C 

cooler than the Gulf. Recent observations (e.g., Bunge et al., 2002) show that for 
10 

intervals of many months the deep exchange can be as large as 5–10 Sv while the 

Loop Current goes through a ring-shedding cycle. Parcels of water would, of 

course, gradually lose their temperature deficit as they mix downstream from 

Yucatan. But we know from observations elsewhere (Mediterranean outflow, 

Denmark Straits overflow, Caribbean Sea inflow) that cold, dense incoming 

parcels hug the right-hand slope even after some mixing and sinking. This supply 

of cold dense water is analogous to, even if weaker than, the supply of North 

Atlantic Deep Water to the Deep Western Boundary current along the East coast of 

the United States. This effect also introduces colder water around the periphery, 

enhancing the existing “warm-core” shear structure. 

A third mechanism, we suspect, operates only in the eastern Gulf. During one 

phase of the Loop Current cycle, as deep Caribbean waters flow over the sill into 

the Gulf, the “bottom falls away” in the downstream direction. Denser water that 

enters near 2,000 m flows into a weakly-stratified region where the bottom drops 

abruptly to ~3,500 m. The resultant vortex stretching should induce a cyclonic 

spin of the entering fluid, consistent with similar findings of Spall and Price 

(1998). By contrast, when water leaves the Gulf during the reverse phase of the 

Loop Current cycle, the ambient stratification in the Gulf, even though it is weak, 

tends to restrict the source of outgoing fluid to depths above the sill. So we expect 

much less vortex compression on the other half of the deep flow cycle. 
11 

3. ANALYSES OF AVAILABLE OBSERVATIONS 

3.1. Deep Temperature Structure and Vertical Shear 

3.1.1. Eastern Gulf 

We have computed the vertical shear associated with these anticyclonic patterns. 

Figure 3.1 shows ten regions in which we computed the mean hydrographic 

conditions based on the available NODC database. 

Because the variability is so great, we wished to use observations over as great a 

time span as possible and over as great a horizontal extent as seemed appropriate. 

From a construction of composite temperature-salinity curves we concluded that 

observations of temperature have much less apparent error, or scatter, than 

salinities. We also compared the temperature-salinity curves in various regions of 

the deep Gulf and could not convince ourselves that the t,S properties are 

significantly different from one region to another. For these reasons and because 

the salinity gradients are so small in the deep Gulf, we have chosen to compute 

density from a mean temperature-salinity curve, using the temperature data alone 

for gradient information. (For the reader interested in the details, Figure 3.14, at 

the end of this section, shows a plot of potential temperature against salinity for a 

collection of deep Gulf hydro stations.) 

In the eastern Gulf, it turns out that the shear profile on the eastern side has a much 

better signal-to-noise ratio than that on the western side. The western side has 

sparse observations, contributing to the poor signal-to-noise ratio. The position of 

the Loop Current is constrained by the Florida shelf on the Eastern side, but not on 

the Western side, which may also contribute to the greater variability on the 

Western side. 

Figure 3.2 shows the resulting mean temperature signal. Below 1,500 m, the 

standard deviation of the temperature variability is ~0.02°C in the central and 

eastern boxes; Figure 3.2 shows that the temperature differences are on the same 

order. (The standard error of the means, of course, is smaller.) We computed error 

estimates in the following way. Because the standard deviations of the temperature 

data values are as large as the signal, we estimated a standard error of the mean by 

computing the composite mean values between 1,000 and 2,000 m. The 

temperature difference on the eastern side was 0.13°C, with a standard error of the 

mean of 0.03°C, based on 106 observations in the east and 163 in the center. 
12 

Figure 3.1. Location of the regions used to form data means for constructing 

shear profiles. 
13 

Figure 3.2. Mean vertical temperature distributions in the eastern Gulf of Mexico 

(an expanded scale is shown in the lower box). The regions used are 

shown in Figure 3.1. 
14 

We are not able to compute the details of the vertical shear with great accuracy, 

although the mean difference is reliably greater than the standard error. It should 

be emphasized, however, that the biggest variability in these data is not in the usual 

sense of “error,” but in time. Multiple data points on a single hydro cast are not 

independent in the way we would like. The vertical averaging, however, will 

reduce problems from instrumental errors, internal waves, and other such sources. 

Therefore, the significance level is not nearly as high as one would like. 

Nevertheless, the same result holds true for all the individual calculations: the 

signals are small but consistently of the sign appropriate to support the idea of 

cyclonic flow. The geostrophic velocity computed from this temperature-density 

distribution is in Figure 3.3. 

The flow at 2,000 m relative to 1,000 m is ~1 cm s-1 to the north. It is weak but 

reliably above the noise level. It remains, of course, to determine an appropriate 

but believable reference level for the geostrophic calculation. To anticipate the 

results of the next sections, however, we point out here that a 1,000-m reference 

level turns out to be a remarkably good choice. 

The signal-to-noise ratio in our calculation is barely adequate on the east side 

beneath the Loop Current, but is even worse on the western side. The mean 

temperature difference there (between the central box and the western box, beneath 

the Loop Current – see Figure 3.1) is only 0.05°C and has a standard error of 

0.04°C. There are a number of reasons why the computed signal could be so 

small. We are unable to offer any definitive explanation; beyond noting the fact 

that the data are quite sparse on the western side. The distribution of the number of 

stations in the eastern region (the region with more data), as shown in Figure 3.4, 

illustrates this problem nicely. 

3.1.2. Central Gulf 

In a fashion similar to our methods in the eastern Gulf, we have computed the 

mean geostrophic vertical shear in the central area of the Gulf using the regions 

shown in Figure 3.1. Figure 3.5 shows the velocity structure relative to 1000 m. 

The conventional velocity sign convention is used; “u” is positive east. 

The deep velocity profile is shown for only the northern side of the Gulf because 

the signal-to-noise ratio is below the noise level on the southern side. 

The flow at 2,000 m, relative to 1,000 m, is to the west in Figure 3.5, again 

suggesting a cyclonic flow pattern. (As before, the issue of justifying the choice of 

reference level is postponed to the next section.) The velocity at depth is only a 
15 

Figure 3.3. Mean N-S geostrophic speeds relative to 1,000 db computed from the 

temperature profiles of Figure 3.2 and using a mean T-S curve. 
16 

Figure 3.4. Histogram of the number of hydrographic observations at each depth 

for the eastern region (shown in Figure 3.1) of the eastern Gulf for 

these calculations. 
17 

Figure 3.5. Mean E-W speeds, relative to 1,000 db, computed from the 

temperature signal in the central Gulf. The result is shown for the 

deeper section only for the northern half, where the signal-to-noise 

ratio allows a significant result. 
18 

few tenths of a cm s-1. This value seems almost ridiculously small, but we note 

that, first, it is to the west, consistent with theoretical expectations. Second, the 

shear profile at depth is monotonic almost to the bottom. (We would not bet 

heavily on the small bend at the bottom.) The actual velocity signal of the 

expected flow, if it is concentrated in a fairly narrow region near the boundary, will 

of course not be sampled closely in these average data. Thus high speeds would be 

almost impossible to see by this calculation, although we should certainly get the 

sign right. These factors all support the cyclonic hypothesis we have put forward. 

3.1.3. Western Gulf 

The averaging areas, or boxes, shown in Figure 3.1 also show areas in the western 

Gulf where we made calculations similar to those shown previously. We formed 

two pairs of regions there, north and south of 24° N: a north-western pair and a 

southwestern pair. Figure 3.6 shows the velocity profile from the southwestern 

pair. There we see speeds (relative to 1,000 m) of order 0.5 cm s-1. The flow is to 

the south. This result, too, based on data completely independent from that of the 

previous sections, is consistent with the assumption of cyclonic flow. For the 

northwestern region, in the western Gulf, however, the region is “data poor” and 

the signal does not emerge above the noise. 

3.1.4. Variability of the Geostrophic Shear 

The geostrophic results presented in the preceding sections show our best attempts 

at determining mean values. To what extent, one wonders, is it possible to 

estimate the variability of the geostrophic velocity? 

Figure 3.7 shows a collection of 46 station pairs from which we computed 

individual velocity shear patterns. The stations in each pair are from a single 

cruise. The data are from the same dataset as those in the previous section, but are 

treated quite differently. The reason for this additional calculation, obviously, is 

that we assumed (or at least hoped) that using pairs of stations from individual 

cruises might improve the accuracy of the individual calculations because 

calibration issues would be minimized. 

As would be expected, the means from these calculations are essentially the same 

as from the previous calculations. There are many reversals in sign, but similar 

means emerge. In an attempt to construct the most accurate values, we have 

computed an absolute mean at each level from which to compute the standard 

deviations. Table 3.1 shows these results. In this case only, the values are 
19 

Figure 3.6. Mean N-S geostrophic speeds relative to 1,000 m in the Western Gulf 

of Mexico. 
20 

Figure 3.7. Locations of station pairs (from single cruises) from which geostrophic 

velocities were computed. Results are shown in Table 3.1 (see text). 
21 

______________________________________________________________________________ 

Table 3.1 

Mean Velocity Shear and Standard Deviation Relative to 2,000 db 

Depth Mean Velocity Magnitude Standard Deviation 

 cm/sec of Velocity

 100 16.7 15.0 

 500 5.7 5.1 

1,000 2.1 2.1 

1,500 0.5 0.5 

(These values were calculated by finding the absolute magnitude of all the 

individual station pairs and taking the mean and standard deviations.) 

__________________________________________________________________ 

computed relative to 2,000 m to allow consistent comparisons at varying depths. 

The standard deviations of the values are similar to the speeds themselves, in part 

because the means are so near zero. Because these are determined from 46 

individual calculations, the standard errors of the mean would be reduced by ~6.7 

(i.e., 45 ), suggesting that the mean values are indeed significantly different from 

zero. 

There is an important point that perhaps should be emphasized. The mean speeds 

are small, but the individual velocity values are strong. The fact that the individual 

velocity values are so large (i.e., the variance is large), we suspect, is why the 

signal-to-noise-ratio is so small. 
22 

3.2. Resolving the Reference-Level Issue: Observations of Deep Flow 

3.2.1. Observations from Deep Current-Meter Moorings 

Long-term current-meter moorings that are well suited to provide a check on 

absolute deep velocities are scarce. We have found four sets of current-meter 

mooring data that are appropriate as a reference for these geostrophic calculations; 

two older arrays in the eastern Gulf and two recent ones in the northern Gulf. 

They are summarized in Table 3.2. We deal first with the data farthest south, 

along the Florida escarpment, taken in an MMS-sponsored program in the early 

1980s (Hamilton, 1990). 

Figure 3.8 shows the locations of Moorings A and G; plots of the north-south 

(along-isobath) velocity from Mooring A are in Figure 3.9. Figure 3.10 shows the 

mean speeds as a function of depth. 

The variability is similar at the other mooring, and the mean speeds at depth are 

also to the north, but the values are only barely greater than zero. That mooring, 

however, is farther away from the steeply-sloping face of the West Florida 

Escarpment. 

The spectrum of the N-S velocity component at the deepest instrument shows that 

the energy peaks at periods of 20-30 days. (This frequency band is typical of 

topographic Rossby waves and will be found at other moorings as discussed later.) 

There are ~500 days of total record at the lower instrument, capturing ~20 

“periods.” Because the correlation coefficient falls to zero at one fourth of a 

period for a narrow-band signal, we estimate that there are approximately 80 

independent observations. This conclusion leads to an estimated uncertainty 

(standard error of the mean) of ~0.5 cm s-1. Thus we conclude that the mean value 

at 1600 m (Figure 3.10) of ~4 cm s-1 (to the north) is significantly different from 

zero by 8 times (!) the standard error of the mean. (Many probability tables do not 

show results for values greater than 4 times the standard deviation; one's 

assumptions about the shape of the distribution become crucial.) 

 

Perhaps we should emphasize the main point here: the observed mean speed over 

nearly 3 years is several cm s-1 to the north and significantly different from zero 

during this time interval. However, considering the large variability of such 

signals on decadal time scales, we would not assume that this mean value or error 

estimate is valid for all time. 
23 

__________________________________________________________________ 

Table 3.2 

Observations of Deep Currents Near the Edges of the Gulf of Mexico 

Agency Method Location Depths Dates

 (m)

NOAA C-M W. Fla. Escarp, 27.5°N, 950 6/78-5/79 

(Molinari) 85.5°W 

SAIC C-M W. Fla. Escarp., 25.6°N, 1,100, 1/83-1/86 

 84.6°W 1,600 

NOPP Deep Western Gulf ~900 3/98-8/02 

 Floats 

Ensenada C-M Yucatan Channel Full 9/99-5/01 

Group Water Column 

GULL C-M Northern Gulf, 27°N 1,500- 9/00-11/01 

(Szabo) 89°W-92°W 3,300 

SAIC C-M Northern Gulf, 32- 8/99-9/01 

 ~90°W 2,164 

__________________________________________________________________ 

It seems plausible, therefore, that our choice of 1,000 m as a reference level is an 

effective choice for the purpose at hand. Our estimated mean speed, relative to 

1,000 m, was found to be ~ 1.1 cm/sec (Figure 3.3). To this we may add ~ 3 cm s-1

(Figure 3.10), suggesting a deep cyclonic speed in the eastern Gulf of order 3-4 cm 

s

-1. Since there probably is not a single level of “no motion” in the Gulf, 1,000 m 

is used, merely to be able to make comparisons between the geostrophic shear 

profiles; the current meter moorings suggest that this choice is eminently 

reasonable. 
24 

Figure 3.8. Locations of current meter Moorings A and G. 
25 

Figure 3.9. Velocity records at the deepest instruments on Mooring A (the location 

of Mooring A is shown in Figure 3.8). 
26 

Figure 3.10. Mean velocity profile for speeds at Mooring A. 
27 

We also explored the possible coherence between temperature and velocity at these 

two moorings. If cold bursts of incoming deep water from the Caribbean Sea 

retain their temperature deficit this far into the Gulf, it would be an interesting 

finding. Unfortunately, the temperature signals were barely resolved at each 

instrument, and there was no clear coherence between velocity and temperature. 

At depths near 1,600 m, the deepest instrument at Mooring A, the temperature 

difference between the Caribbean and Gulf water is very small, so lack of 

coherence there is not surprising. Near sill depth (2,000 m), the Caribbean is 

roughly 0.1°C cooler. However, at mooring G the nearest instrument was at 

~2,360 m; temperature record was composed largely of “background temperature,” 

with occasional departures of a few hundredths of a degree. It seems likely that the 

instrument was deeper than the incoming bursts of cooler water. Because there 

were so many “background temperature” data points, which appear as “zero” in a 

calculation of the spectrum, we considered the calculations of cross-spectral 

coherence to be of questionable validity. 

It is instructive to estimate the transports associated with these deep flows. At 

Mooring A, the inshore mooring, the mean nearly-barotropic N-S velocity is 

~3 cm/sec, over a depth span of at least 1,000 m and possibly twice that. If we 

take the horizontal extent to be only 75 km (the distance to Mooring G, farther 

offshore) the mean transport is thus estimated to be ~3 to 6 Sv. This mean value 

applies to a narrow boundary current. 

For the stronger bursts of flow, however, the speeds are easily 25 cm/sec at 

Mooring G, and clearly are barotropic to ~3,200 m. It is hard to escape the 

conclusion that the transports in these bursts (to the north) are much larger than our 

estimate for the boundary flow. The estimated transports for brief periods in the 

deeper layers in Yucatan (see, for example, Bunge et al. 2002) from the recent 

moorings there are consistent with these surprisingly large values. 

Additional evidence for northerly flow along the steep slopes off west Florida, 

from deep long-term moorings, is found in the work of Molinari and Mayer (1980). 

They measured the flow at ~1,000 m offshore of Tampa, FL (near 27.5°N; see 

Figure 3.8 above). They show (their Figure 33) that at the uppermost mooring (at 

150 m) there was almost no net along-isobath flow for the whole year (June 1978 

to May 1979). At the deepest mooring (950 m, 100 m above the bottom) there was 

flow to the north, along the isobaths, of the order of 5 cm s-1 in 9 out of 11 months. 

In the other two months (November and December) the flow was essentially in the 

noise level. The mean flow at the deepest mooring over the full record was 
28 

~3.3 cm/sec to the northwest, the orientation of the local isobaths. Thus their 

measurements, although taken in different years, are remarkably consistent with 

the results of the SAIC moorings shown in Figure 3.10. 

Several current-meter mooring arrays have been installed in the north-central Gulf 

in recent years. One set of moorings that is relevant to our present discussion was 

installed by Fugro GEOS in their GULL program. Their array was composed of 

nineteen moorings on which instruments were placed approximately 100 and 300 

m from the bottom, in water depths of ~ 1,600 m to ~3,400 m. Several groups of 

moorings were located between 89°W and 91°W, from late August 2000 through 

November 2001. The major difference between the results from these moorings 

and the data from the West Florida Escarpment is that the flow near steep bottom 

topography in the northern Gulf near 90° W (i.e., the Sigsbee Escarpment) is 

somewhat more energetic; the speeds are greater than those shown in Figure 3.9. 

With these moorings it is possible to see that, in the mean, the deeper flow is 

strong and toward the west, again consistent with the cyclonic deep flow 

hypothesis. This dataset remains proprietary at the time of this writing. 

The final set of moorings that is relevant to our discussion has been reported 

recently by Hamilton and Lugo-Fernandez (2001). There were 4 moorings in an 

array near 27°N, 90°W, measuring flow especially near the bottom (as well as full 

water column) at depths near 2,000 m. 

The mean speeds near the bottom, at 2,000 m, ranged from ~2 to 4.2 cm/sec, all to 

the west (along the local topography). The highest speeds were seen at 

mooring I2; this mooring is near the center of the array and near the steepest 

bottom slopes. At this mooring, which had a 2-year long record, the mean speed at 

~11 m above the bottom was 4.2 cm/s, with a standard deviation of 16.75 cm/sec. 

The TRW energy has a broad peak at ~ 20 days. (Recall that, for relatively 

narrow-band signals, there are essentially 4 observations per cycle.) For a twoyear record the data length suggests a standard error of the mean of ~1.3 cm/sec. 

Thus the mean speed is more than 3 times the standard error, even in a situation 

with large variance. This is indeed significant, as were the results from the other 

mooring areas. The deep mean speeds at all 4 moorings were to the west, so if 

they were grouped, the significance level would be even higher. 

3.2.2. Observations from Deep Drifting Floats 

The best data set we have found that covers a broad region of the Gulf is composed 

of velocities from an experiment using PALACE (Profiling Autonomous 

LAgrangian Circulation Experiment) floats. These are a profiling version of the 
29 

original ALACE floats (Davis, et al., 1991; see also the web site of the 

manufacturer, http://www.webbresearch.com). The National Ocean Partnership 

Program (NOPP) sponsored this experiment in the Gulf. Approximately four dozen 

floats were tracked from 1998-2002. A potentially serious problem is that the 

floats are at the surface for several hours to transmit data. The surface velocities 

are usually much greater than those at depth. If we were to assume that all the 

motion takes place at depth, there could be serious contamination of results by 

surface drift. But navigation fixes at the surface allow the surface drift to be 

estimated so that the deep float velocities can be corrected for surface motion. We 

are greatly indebted to our colleagues Georges Weatherly and Nicolas Wienders 

for allowing us to use their prepublication results (Wienders et al., 2002; 

Weatherly and Wienders, 2003). 

Figure 3.11 shows the computed means and variance ellipses after the corrections 

have been applied. The mean velocity vectors for this figure were obtained by 

averaging all the float velocities in 0.5° bins. For these results, only bins with 

more than 5 float values were used (an admittedly arbitrary choice). While the 

mean values are small, there is a robust tendency toward a cyclonic pattern at 900 

m. While some regions are sampled poorly (and some not at all), the tendency for 

cyclonic flow, just above 1,000 m, is evident. (In case it is “hard to see,” the small 

vectors have arrowheads pointing in the direction of mean flow.) 

The major purpose here is not, strictly speaking, to determine exactly the velocity 

at 1,000 m, but to determine the validity of our choice of 1,000 m as an adequate 

reference level for the geostrophic calculations. Careful examination of Figure 

3.11 shows that the flow near the edges of the basin has become cyclonic, albeit 

weakly, at 900 m. Our calculations suggest, of course, that the speeds increase, by 

roughly ~1 to 2 cm/sec, down to ~2,000 m (see Table 2). 

3.3. Path of Cold Renewal Water 

At depths of 2,000 m, the Caribbean Sea is colder by ~ 0.1°C than the Gulf. The 

results of the Mexican mooring experiment in Yucatan (Bunge et al., 2002) show 

that cold Caribbean water pours in at Yucatan Channel during every Loop Current 

cycle. So if there is a deep cyclonic boundary flow, we expect that the temperature 

around the edges of the deep Gulf should be coldest near Yucatan where the “new” 

renewal water enters, . We would expect that the cold signal, or temperature 

deficit, would decay with distance “downstream” from Yucatan, as is observed in 

all such overflow situations (in the Mediterranean outflow, Denmark Strait 

overflow, etc.; see for example Girton and Sanford 2003). 
30 

Figure 3.11. Mean velocity of drifters at 900 m over the course of several years. 

The freely-drifting floats surfaced every 7 days; velocity values have 

been corrected for surface drift effects. These are averaged in 1⁄2-

degree bins (courtesy of Weatherly and Wienders, 2003). Values are 

shown only for bins that have 5 or more observations. 
31 

To examine the possibility of such an effect, Figure 3.12 shows the potential 

temperature at 2,000 m, averaged in one-degree boxes around the edge, plotted as a 

function of distance from Yucatan. Because the path is irregular, the x-axis is only 

very roughly a measure of the distance from the source of cold water entering at 

Yucatan. The individual mean data values are shown, with a smoothed curve 

through the scattered data points. The increase in temperature with distance from 

Yucatan is strikingly clear. The sharpest gradient is nearest the entrance at 

Yucatan, which is consistent with strong initial mixing. Apart from this strong 

gradient in the first half-dozen data points, a linear fit through the other data would 

be adequate. The total increase in temperature is ~0.07°C, which is surprisingly 

consistent with the net difference in temperature between the two basins. This 

effect could be interpreted as an indication of the cyclonic boundary flow. 

However, it is entirely possible that the supply of cold water at Yucatan is one 

possible mechanism for causing the boundary flow, in a manner similar to that of 

the Deep Western Boundary Current along the U.S. east coast. The historical data 

set is adequate at 2,000 m to see the changes in Figure 3.12, but at greater depths 

the sampling is strongly concentrated near 2,500 m and 3,000 m. The mean 

(potential) temperature in the Gulf continues to decrease only by ~ 0.02°C between 

2,000 m and 3,000 m. A better understanding of the mechanisms and details of the 

deep-water renewal remains for future work.
32 

Figure 3.12. Time-mean potential temperature around the edge of the Gulf of 

Mexico at 2,000 m, averaged in one-degree boxes, plotted as a 

function of distance from the sill in Yucatan Channel. The x-axis is 

roughly a measure of distance. The individual mean data points are 

shown (x); if a one-degree box had no data, linear interpolation was 

used for this figure, and the curve connecting the points was 

smoothed slightly. Except for the data points nearest Yucatan, a 

least-squares straight-line fit would probably be adequate. Note that 

the potential temperature values have had 4.0 subtracted from them. 
33 

For completeness, Figure 3.13 shows the details of the mean potential temperature 

distribution, averaged in one-degree boxes, at 2,000 m. In each one-degree box, 

the upper number shows the number of samples from the NODC data base. The 

lowest figure is the standard error of the mean in that box. From each mean 

temperature, the value 4.00 has been subtracted; thus a value of “7.3” implies a 

true potential temperature of 4.073° C. (Some people use the term “anomalies” for 

such values.) The data in Figure 3.12 are of course completely consistent with this 

figure, as they are from the same data base. Although this figure represents the 

entire existing historical data base from U.S. ships, we note that there remain 14 

one-degree boxes with no data (including boxes with partial areas deeper than 

2,000 m). 

 

Our calculations of density in the deep water were based on a mean t,S curve. 

Figure 3.14 presents the over-all potential temperature-salinity properties of the 

deep Gulf. Salinity is plotted as a function of potential temperature for a collection 

of all deep U.S. hydro stations on file at NODC. We show only data colder than 

6.0 C, an isotherm that lies well above 1,000 m. The choice of U.S. data only was 

in an attempt to reduce the uncertainties in salinity calibrations. The individual 

data points show a series of mean potential temperature and salinity points, 

together with the standard deviation in salinity. This plot shows the results after a 

“pruning” to remove points that fell outside 3 standard deviations. 
34 

Figure 3.13. Mean temperature at 2,000 m. In each box all available U.S. data 

were averaged. The number of samples, the mean potential 

temperature and standard error are shown. From each mean 

temperature, 4.00°C has been subtracted; thus a value of 7.3 implies a 

true temperature of 4.073°C. 
35 

Figure 3.14. Potential temperature-salinity properties of the deep Gulf. Salinity 

(vertical axis) is plotted as a function of potential temperature for all 

deep U.S. hydro stations on file at NODC for data colder than 6.0° C. 

The individual data points (red dots and bars) show a series of mean 

potential temperature and salinity points and the standard deviation in 

salinity. This plot shows the results after a “pruning” to remove 

points that fell outside 3 standard deviations. 
37 

4. RESULTS FROM NUMERICAL MODELING: STUDIES USING 

 THE PRINCETON MODEL 

4.1. Description of the Princeton Ocean Model 

The numerical model is based on the sigma-coordinate (or terrain-following) 

Princeton Ocean Model (POM) (Blumberg and Mellor, 1987). For a recent 

description, please see the web page for this model located at: 

http://www.aos.princeton.edu/WWWPUBLIC/htdocs.pom. The northwest Atlantic 

Ocean implementation of the model, known as the Princeton Regional Ocean 

Forecast System (PROFS), is described in detail by Oey and Lee (2002; 

http://www.aos.princeton.edu/WWWPUBLIC/PROFS) and by Ezer et al. (2003). 

The model uses 25 layers in the vertical, with finer resolution near the surface and 

near the bottom. A curvilinear, orthogonal, horizontal grid is used with resolution 

that varies from ~5 km in the northern Gulf of Mexico to ~10 k in the Yucatan 

Channel and ~20 km in the open subtropical Atlantic. The model domain, 55o W to 

98o W and 5o N to 50o N, includes the Gulf Stream, the Gulf of Mexico and the 

Caribbean Sea. Across the eastern boundary, at 55o W, constant inflow and 

outflow transports are specified to account for the large-scale ocean circulation 

component, while temperature and salinity on the boundary are based on the 

monthly climatology. The 6-hourly ECMWF winds are used to force the model, as 

are surface salt and heat fluxes and river runoff. Data assimilation is sometimes 

included in this model system (Wang et al., 2003), but the analyses done in this 

project are for runs without data assimilation, so as to account only for model 

dynamics without data-based corrections. 

Oey et al. (2003a) conducted studies to evaluate the effect of the open boundary 

conditions and the “local versus remote forcing” effects on the 

variability of the Caribbean Sea and the Gulf of Mexico. Experiments with nested 

grids with doubling the resolution were done as well, but they are reported 

elsewhere by Oey and his group. Most of the main circulation features of the mean 

flows remain the same in the case of higher resolution models, though the 

mesoscale variability is larger and closer to the observed variability. In addition to 

their material covered in this report, this work has been widely summarized in the 

published literature. See, for example, Oey and Lee (2002); Oey et al. (2003b); 

Ezer et al. (2002, 2003); and Wang et al. (2003). 
38 

4.2. General Circulation in the Gulf of Mexico and Comparisons Between 

 POM and MICOM 

The circulation in the Gulf of Mexico in this model shows a complicated structure 

with several semi-permanent gyres. Figures 4.1– 4.5 show the 5-year mean 

trajectories at 400 m, 500 m, 1,500 m, 2,000 m and 2,500 m, respectively. These 

are trajectories of particles launched at every second model grid point and 

propagated by the (constant) mean flow for a period of 20 days. The scale vector 

(in cm/s) represents the distance that a particle with that mean speed would have 

traveled during this period. These plots can be compared with velocity plots at 

similar depths from the MICOM model in Section 5 of this report. These semipermanent gyres are affected by the propagating Loop Current eddies (i.e., 

averaging the effect of several eddies) and by topographical features. Some gyres 

are quite similar to those found in the MICOM simulations, but some are different, 

as would be expected. Both models show the Loop Current quite clearly, of 

course. The POM and the MICOM indicate an anticyclonic gyre in the western 

Gulf and a cyclonic gyre in Campeche Bay, in the southwest corner. These are 

robust features and well known. 

At depths below ~1,500 m, however, the deep circulation in POM is characterized 

by a strong boundary-current-like cyclonic circulation, hugging the slopes, 

consistent with the theory and observational results put forward in Section III of 

this report, whereas in MICOM a weak anticyclonic circulation is found at this 

depth along the boundary. In higher resolution POM calculations this boundary 

current has larger variability and weaker mean, but its main direction remains 

cyclonic. At 2,500 m (2,000 m in MICOM) both models indicate a cyclonic gyre 

around 92°-94oW and 24oN. Figures 4.3- 4.5 show that the flow near the 

boundaries is cyclonic—as found in Section III and here—but the interior flow is 

much more complex than a mere single gyre. 

Vertical sections of the flow show us another view of the model results. Figure 4.6 

shows a N-S section at 90o W in the central Gulf. There is a good bit of detail in 

the interior, but the point of the figure here is to show the strong flows near the 

boundaries. The flow on the steep slopes along the coast of Mexico is striking. 

The horizontal maps of Figures 4.1-4.5 show some of the same information of 

course, but in this presentation it shows up differently. So far as we are aware 

there are no direct observations of this flow along the Mexican continental rise. 
39 

Figure 4.1. Trajectory plot based on the 5-year mean velocity at 400-m depth from 

POM forced by 6-hourly ECMWF winds but without data 

assimilation. These are trajectories of particles launched at every 

second model grid point and propagated by the (constant) mean flow 

for a period of 20 days. The scale vector (in cm/s) represents the 

distance that a particle with that mean speed would have traveled 

during this period. 
40 

Figure 4.2. Trajectory plot based on the 5-year mean velocity at 500-m depth from 

POM forced by 6-hourly ECMWF winds but without data 

assimilation. These are trajectories of particles launched at every 

second model grid point and propagated by the (constant) mean flow 

for a period of 20 days. The scale vector (in cm/s) represents the 

distance that a particle with that mean speed would have traveled 

during this period. 
41 

Figure 4.3. Trajectory plot based on the 5-year mean velocity at 1,500-m depth 

from POM forced by 6-hourly ECMWF winds but without data 

assimilation. These are trajectories of particles launched at every 

second model grid point and propagated by the (constant) mean flow 

for a period of 20 days. The scale vector (in cm/s) represents the 

distance that a particle with that mean speed would have traveled 

during this period. 
42 

Figure 4.4. Trajectory plot based on the 5-year mean velocity at 2,000-m depth 

from POM forced by 6-hourly ECMWF winds but without data 

assimilation. These are trajectories of particles launched at every 

second model grid point and propagated by the (constant) mean flow 

for a period of 20 days. The scale vector (in cm/s) represents the 

distance that a particle with that mean speed would have traveled 

during this period. 
43 

Figure 4.5. Trajectory plot based on the 5-year mean velocity at 2,500-m depth 

from POM forced by 6-hourly ECMWF winds but without data 

assimilation. These are trajectories of particles launched at every 

second model grid point and propagated by the (constant) mean flow 

for a period of 20 days. The scale vector (in cm/s) represents the 

distance that a particle with that mean speed would have traveled 

during this period. 
44

Figure 4.6. Mean E-W (U) velocity component along a N-S section at 90° W 

from POM. The upper figure shows the mean speed and the lower 

figure shows the standard deviation. 
45 

Examining this N-S section in some detail, the model allows us to see the different 

phases of the flow as a large ring passes. Figure 4.7 shows four cross sections of 

the flow before, during, and after a ring drifts to the west. The schematic in the 

lower right corner of each figure shows the position of the Loop Current and the 

ring. The sequence is upper left to lower left, then upper right to lower right. In 

the third frame, as the ring is centered on the section, a "true believer" might be 

willing to see hints of a Taylor column of fluid being carried beneath the center of 

the ring. In such an interpretation the return flow to compensate for mass balance 

is on either side. 

Another difference between the POM and MICOM models is found in the deep 

circulation in the eastern Gulf, under the Loop Current. Whereas a seemingly 

barotropic anticyclonic circulation remains in MICOM throughout the water 

column, a reverse, cyclonic circulation is seen in POM (as well as in Welsh’s 

calculations using the modular version of the Bryan-Cox-Semptner, or MOM, 

model). 

Comparisons (not shown here) of velocity and temperature cross sections at 26oN 

and 90oW in the two models support the above results and indicates that the 

MICOM flow may be more barotropic, while the POM flows are more baroclinic 

and more strongly affected by bottom topography, creating stronger deep boundary 

currents. To explain the differences between the results of the two models, one 

must understand the differences between the two model types. The vertical grid in 

MICOM follows isopycnal surfaces. Thus in the deep ocean, where stratification 

is very weak, the resolution can possibly be reduced. The terrain-following 

vertical grid in POM, on the other hand, as well as the manner in which it has been 

implemented here, has finer resolution and thus supports intense mixing. This is 

true both near the bottom, resolving the Bottom Boundary Layer (BBL), and near 

the surface, resolving the Surface Mixed Layer (SML). Oey and Lee (2002) have 

shown that the deep boundary current along the slopes of the Gulf is, to a large 

extent, the result of Topographic Rossby Waves (TRW) induced by fluctuations in 

the Loop Current and its associated eddies. Adequate resolution near the bottom is 

necessary to resolve the TRWs. In both models and observations, cross sections of 

temperature and salinity near the deep boundary current show very weak density 

gradients, hence the difficulty of calculating these currents from hydrographic 

measurements as reported in Section III above. These boundary currents are much 

more pronounced in observations based on floats or current meters, as one would 

expect. Direct comparisons of POM currents and observations are in fact quite 

good (e.g., Wang et al., 2003). 
46 

Figure 4.7. The instantaneous flow pattern along the 90° W section at four phases 

of a ring passage (from POM). The schematic in the lower right 

corner shows the positions of the ring and of the Loop Current. The 

sequence begins in the upper left, then to the lower left; then from the 

upper right to lower right. 
47 

 

4.3. Effects of Propagating Loop Current Eddies on the Deep Circulation 

The Gulf is dominated by Loop Current eddy shedding events and the westward 

propagation of those eddies. One of the basic questions yet to be fully investigated 

is the effect of the eddies on the mean deep circulation. Oey and Lee (2002) have 

been able to demonstrate the indirect influence of eddies through generation of the 

TRWs. For example, as eddies propagate to the west they carry fluid with them in 

the upper ocean, so a return transport to the east is expected. However, the mean 

horizontal flow is dominated by the much stronger boundary currents and the semipermanent gyres, so there is no clear evidence as to where this weak return 

transport occurs; see Figures 4.1-4.5. 

To investigate this interesting question, we show (in Figure 4.8) the net transport 

below 800 m across 90oW (blue line). The average surface elevation across the 

section (green dashed line) indicates that whenever an eddy passes this section 

(peak in elevation) there is a peak in return transport (below 800 m) with net flow 

to the east. The correlation between the two curves is striking. By contrast, the net 

return deep transport, averaged over many eddies, is less than 1 Sv. In comparison 

with the much larger transports associated with Loop Current Rings, this small 

transport could easily be lost in the noise and its statistical significance would 

appear to be questionable. Yet we can be sure that it is a real feature of the 

circulation, and that the return transport must appear. 

One might suspect, a priori, that there should be a Taylor column of fluid being 

carried along beneath the ring as it travels to the west. The fact that the model 

indicates increased flow back to the east when a ring passes by seems to suggest 

that any Taylor column effects, as mentioned in connection with Figure 4.7, are 

overwhelmed by other parts of the flow field. It may be that the Gulf is simply not 

big enough in E-W extent to allow the classic effects one might expect. 
48 

Figure 4.8. Mean surface elevation across 90oW (green dashed line) as a function 

of time indicates eddies (positive peaks) crossing the section. The net 

deep transport below 800 m across 90oW (blue solid line) indicates 

eastward (positive) deep transport when eddies drift westward 

overhead. 

 
49 

4.4. Variability of Flow in the Yucatan Channel and Its Relation to Variations 

in the Loop Current 

Analysis of the model flow variability in the Yucatan Channel has three purposes. 

The first is to compare model results with past observations (Maul et al., 1985) as 

well as more recent ones (Ochoa et al., 2001; Bunge et al., 2002). The second is to 

understand the forcing mechanisms of the flow; the third, to determine the relation 

between the Yucatan Channel flow and variations of the Loop Current. For details 

see Ezer et al. (2002, 2003) and Oey et al. (2003b). The structure of the mean flow 

in the Yucatan Channel as obtained from the POM calculations seems to agree 

with the main features obtained from the MICOM model. Both models confirm the 

new finding of the existence of deep return outflows along the slopes of the 

Channel (Ochoa et al., 2001) and not at the center of the sill as previously thought. 

The net mean transport in the Yucatan Channel in our model, ~25 Sv, and the large 

range, ~16-32 Sv (but with no clear seasonal signal), all agree very well with the 

new observations of the Mexican group. EOF analysis identified the main 

mechanisms and forcing of the flow variability (see Ezer et al., 2003, and the 

description of publications supported by MMS below). For example, Figure 4.9 

shows that the long-term fluctuations of the inflow from the Caribbean Sea into the 

Gulf (solid line) is correlated with EOF mode 2 (dashed line) of the inflow velocity 

across Yucatan Channel. Moreover, almost every event of eddy shedding from the 

Loop Current (shown by “E”) is associated with a peak in the inflow transport. 

Such events are also associated with peaks in the return deep transport below 800 

m. 

Figure 4.10 is something of an unconventional plot and requires a bit of study. It 

shows the correlation between the changes in the deep flow (positive values in the 

x-axis represent southward transport below 800 m) and changes in the area of the 

Loop Current (positive values in the y-axis represent Loop Current growing in 

area). Data were taken from the passage of three eddies. We see that the deep 

return transport and the Loop Current extension both grow before an eddy is shed 

(triangles), and start decreasing afterwards (circles). Using only data points from a 

two-week interval before and after the ring passes, the correlation increases 

dramatically to 0.7. This process is consistent with the theory as proposed by 

Maul. et al, (1985) and recently confirmed from the new Mexican observations 

(Bunge et al., 2002). 
50 

Figure 4.9. Total inflow transport in the upper Yucatan Channel from the 

Caribbean Sea into the GOM (solid line), the time evolution of EOF 

mode 2 (dashed line), and eddy shedding events “E” (From Ezer et al., 

2003). 
51 

Figure 4.10. Deep outflow (in the POM) from the Gulf below 800 m (x-axis) and 

the change in sea level in the Loop Current region (positive y-axis 

values indicate times when the Loop Current is growing in area). The 

changes in these parameters for three eddies are indicated: blue 

triangles are for the two weeks before eddy shedding events; circles 

are for the two weeks afterward. The linear regression coefficient 

between deep transport and Loop Current change is 0.4 for all daily 

values, but is 0.7 if only periods around eddy shedding events are 

considered (from Ezer et al., 2002). 
53 

5. RESULTS FROM NUMERICAL MODELING STUDIES USING 

 MICOM 

5.1. Miami Isopycnic-Coordinate Ocean Model (MICOM) 

Almost all mainstream ocean models in use today are based on the same equations 

of motion. Different models, however, formulate the solutions in different ways. 

The Miami model is designed so that the "horizontal" coordinate system follows 

the slope of the density field of the ocean. This choice does not automatically 

solve all problems, but its purpose is to 

• avoid inconsistencies between vertical and horizontal transport terms that 

 cause (among other things) false diapycnal mixing; 

• hide truncation errors associated with horizontal transport behind the smoke 

 screen of isopycnal mixing. 

The model is in widespread use in the ocean modeling community and is well 

described on their home page: 

http://oceanmodeling.rsmas.miami.edu/micom/

Some of the results of this study have already been submitted for publication. See, 

for example, Cherubin et al. (2004) and Romanou et al. (2004). 

5.2. General Circulation Patterns in the Gulf of Mexico from MICOM 

The upper-layer circulation in the Gulf is, of course, anticyclonic. The main result 

from the analysis of the MICOM simulation of the horizontal circulation in the 

Gulf of Mexico is that the circulation is reversed in the deep waters. Figure 5.1 

shows the 6-year averaged Eulerian circulation at 400 m; an anticyclonic 

(clockwise) gyre appears, with two circulation cells in the west. The northern cell 

is anticyclonic, in part because most of the Loop Current rings end their lifetime in 

this region. An intense jet is observed on the western side of the anticyclonic gyre. 

South of this region, the model shows a cylconic circulation cell. Deeper in the 

Gulf, the dominant feature remains the western jet of the anticyclonic gyre flowing 

along the shelf slope of the Gulf of Mexico. Figure 5.2 shows a similar view at 

900 m. 
54 

Figure 5.1. Mean velocity field from the MICOM model at a depth of 400 m. 
55 

Figure 5.2. Mean velocity field from the MICOM model at a depth of 900 m. 

 
56 

Figure 5.3. Mean velocity field from the MICOM model at a depth of 1,250 m. 
57 

Figure 5.4. Mean velocity field from the MICOM model at a depth of 1,500 m. 
58 

Figure 5.5. Mean velocity field from the MICOM model at a depth of 2,000 m. 
59 

On the other hand, the cyclonic circulation cell becomes weaker with depth and 

has shifted slightly to the east of the south corner of the western Gulf. Figure 5.3 

shows the mean flow at 1,250 m, and Figure 5.4 shows the mean flow at 1,500 m. 

Finally, Figure 5.5 shows that at 2,000 m the dominant circulation is cyclonic. The 

current is to the west in the deepest part of the Gulf, veers to the east in the western 

Gulf and flows along the southern shelf slope. North of the cyclonic gyre, a weak 

anticyclone is still present. In the Loop Current area, the horizontal circulation in 

these model results is always anticyclonic. This pattern is not the same as found in 

the Princeton model nor does it agree with observations. We are continuing to 

study this feature. 

While this project has focused on the mean flow around the edges of the deep Gulf, 

it remains true that the mean flow is not always the topic of greatest interest to 

some readers (nor one of the most robust features of model results). To show a 

measure of the energy of the flow, Figure 5.6 shows the standard deviation of the 

flow at 900 m. Figure 5.7 shows the equivalent standard deviation at 1,500 m. At 

900 m the standard deviation ranges from ~2cm/sec in the west to 6 cm/sec 

beneath the Loop Current. It is clear that these values are as large as the mean 

flow itself. The means and standard deviation decrease only slightly at 1,500 m 

and the general conclusion remains the same. 

5.3. Strong Current Events Near the Bottom of the Gulf of Mexico 

In MICOM simulation, we followed the occurrence of strong current events in the 

depth range 500-1,500 m on the northern shelf slope of the Gulf of Mexico over 

six years. The peak speeds in the model are about 28 cm/s between 500 and 600 m 

and 20 cm/s between 1,300 and 1,600 m. Those events are strongly associated with 

the generation of Loop Current rings in the eastern Gulf and with its westward 

motion. This finding is especially clear when these events occur on the northern 

shelf of the Gulf of Mexico between 90°W and the western gulf. Figures 5.8 and 

5.9 show the current bursts using longitude-versus-time diagrams. These plots 

show their propagation to the west as well as their intensity. 

5.4. Meridional Distribution of the Zonal Transport at 90°W 

In this section we address the transport budget across a meridional section at 

90°W. Somewhat arbitrarily, we divided the section through the middle of the Gulf 

of Mexico, resulting in a northern and a southern half. The transport budget reveals 

an anticylonic pattern with more flow going to the east in the northern half and 

more going to the west in the southern half. All the increases of current transport 
60 

Figure 5.6. Standard deviation of the velocity field at 900 m (see for comparison 

Figure 5.2). 
61 

Figure 5.7. Standard deviation of the velocity field at 1,500 m. 
62 

Figure 5.8. Amplitude of current bursts in layer 10, in a latitude-time diagram 

(upper panel) and the location of the burst (latitude) (lower panel). 
63 

Figure 5.9. Longitude-time plots of the amplitude and the location of strong bursts 

in layer 13. 
64 

in both directions are associated with a Loop current ring passage. During those 

events, the transport in the deep layers also increases, as shown in Figure 5.10. 

The budget result gives a weak eastward transport of 0.08 Sv (1 Sv = 106

 m3/s), 

with a weak eastward component above 1,300 m (0.26 Sv) and a westward one 

below (-0.17 Sv). The paths followed by the water parcels are given by the 

Lagrangian trajectories as described in the next section. 

5.5. Lagrangian Trajectories in the Deep Gulf Circulation 

One of the goals of this study was to determine whether some of the Gulf of 

Mexico waters have a net transport returning to the Caribbean Sea below the depth 

of the sill in the Florida Strait. The previous results showed a strong eastward 

transport in the northern half of the Gulf; the Eulerian currents between 1,000 and 

2,000 m suggest a return flow from the north to the Caribbean Sea (Figures 5.3 and 

5.4). Figure 5.11 shows a collection of Lagrangian trajectories in layer 13 of the 

model. Layer 13 occupies the depth range ~900-1,400 m in the Yucatan Channel. 

We see that the water parcels that return to the Caribbean Sea come from the 

northern Gulf in a region located east of a ring that has recently separated. These 

trajectories were obtained as the Loop Current ring was moving to the west and the 

path of the Loop Current was going straight from the Caribbean Sea out through 

the Florida Straits. In the model no flow beneath layer 13 was observed to return 

to the Caribbean Sea just after a Loop Current ring formation. 

Finally, we examine the model Lagrangian trajectories in the intermediate layers, 

between 400 and 1,000 m. These trajectories reveal new patterns of the circulation 

not visible in the above circulation vector plots. Because of the cyclones generated 

during the formation of a Loop Current ring, some water parcels can be entrained 

against the main stream. Therefore, instead of following the anticyclonic gyre 

described above, the water parcels undergo a cyclonic motion initiated east of a 

Loop Current ring in the eastern Gulf. Figure 5.12 shows particles that drift to the 

southwest and then return to the east. Those very surprising motions are similar to 

those that have been observed in actual float trajectories. 
65 

Figure 5.10. Time series of transports across a N-S section at 90°W. Top panel 

shows transports above and below layer 12 (approximately 1,300 m). 

Lower two panels show transports to the east and to the west, in the 

northern and southern halves of the section. 
66 

Figure 5.11. Lagrangian trajectories of particles in layer 13, originating in the 

northern and central regions of the Gulf. The particles are launched 

along the red N-S line at the squares; the end points of the paths are 

shown by the stars. 
67 

Figure 5.12. Lagrangian trajectories in layer 12 for two different time intervals. 

The particles are launched along the red N-S line at the squares; the 

end points of the paths are shown by the stars; see text.
69

6. ANNOTATED BIBLIOGRAPHY 

The following is a brief discussion of publications resulting from MMS support of 

this project. 

Cherubin, L.M, W. Sturges, and E. Chassignet, 2004. Deep flow variability in 

the vicinity of the Yucatan Straits from a high resolution MICOM simulation. 

J. Geophys Res., submitted. 

The deep flow variability in the vicinity of the Yucatan Channel between the 

Caribbean Sea and the Gulf of Mexico is examined within a high resolution 

numerical simulation of the North Atlantic Ocean. First, since the model is forced 

with daily ECMWF forcing, the circulation in the vicinity of the Yucatan channel 

presents a high variability in the flow regimes and in the shedding period which is 

in good agreement with earlier observations. The outflow (towards the Carribean 

Sea) in the Yucatan Channel is shown to be in part controlled by the regular eastwest shift of the core of the Loop Current in phase with the transport variations. 

The outflow is shown to compensate the excess of inflow whatever the Loop 

Current extension. The analysis of the growth of the loop is also shown to be in 

good agreement with the retroflexion paradox and with the ballooning process 

proposed by Pichevin and Nof (1997) and Nof and Pichevin (2001) to explain the 

Loop Current Ring formation. Moreover, at the end of each cycle of Loop Current 

ring formation a sudden deepening of the loop occurs together with an 

intensification of the transport and the currents in the deep layers underneath the 

Loop Current. This process is shown to be connected with the growth of an 

instability, probably barotropic, as originally proposed by Hurlburt and Thompson. 

Cherubin, L.M., Y. Morel and E. Chassignet, 2004. Loop Current ring 

formation: a new mechanism. To be submitted to J. Phys Oceanogr. 

The formation of the Loop Current rings is studied by comparing the growth of 

cyclones around the Loop Current ring, during its formation stage, in the very high 

resolution MICOM simulation with a case study model. In this case study ring; and 

those cyclones are responsible for the cleavage between the Loop Current ring and 

the Loop Current itself. Secondly, the Campeche bank forces the mode 3 instability 

(3 growing cyclones) of the Loop Current ring. The steady state is a pentapole, 

which consists in an anticyclone surrounded by four cyclones. Thirdly, the slope of 

the northern Gulf compensates the beta effect what prevents the instability to grow 

as a mode 1 which produces only one big cyclone north of the anticyclone. 
70

DeHaan, Christopher J., 2002. Determining the Deep Current Structure in 

the Gulf of Mexico and the Yucatan Strait from Multiple Data Types. 

Doctoral Dissertation, Dept. of Oceanography, Florida State University, 

Tallahassee. October, 2002. This Dissertation is the basis of the journal article 

described below. Dr. DeHaan is now an oceanographer at the U.S. Navy 

Oceanographic Office, Stennis MS. 

 

DeHaan, C.J., and W. Sturges, 2004. Deep cyclonic circulation in the Gulf of 

Mexico. J. Phys Oceanogr. in press. 

This paper examines the historical density field of the Gulf as well as all the 

available (non-proprietary) direct current observations. Using moored current 

meter data and deep drifting floats, they conclude that the deep (~2000 m) flow 

around the edges of the Gulf is cyclonic. Section III of this report is based heavily 

on this paper. 

Ezer, T., L.-Y. Oey. and H.-C. Lee, 2002. Simulation of velocities in the 

Yucatan Channel, In: Proc., Oceans 2002, MTS/IEEE Publ., 1467-1471. 

Results from the POM numerical simulations of the Gulf of Mexico are shown to 

compare very well with past and recent observations of velocities and transports in 

the Yucatan Channel. The main model inflow into the Gulf is found near the 

surface in the western part of the Channel, while return flows back into the 

Caribbean Sea are found near the surface on the eastern side of the Channel and 

along the eastern and western slopes around 1500 m depth. The location and 

transport of each one of these flows are in good agreement with recent 

observations (Ochoa et al., 2001). Variations in the upper inflow and deep outflow 

transports seem to correlate with variations in the extension of the Loop Current, as 

suggested by analyses of observations (Bunge et al., 2002) . Such correlations were 

found to be especially high near the time when Loop Current eddies are shed into 

the Gulf of Mexico and return deep transports out of the Gulf are significantly 

larger. 
71

Ezer, T., L.-Y. Oey, H.-C. Lee, and W. Sturges, 2003. The variability of 

currents in the Yucatan Channel: Analysis of results from a numerical ocean 

model, J. Geophys. Res., 108, 3012, 10.1029/2002JC001509. 

An Empirical Orthogonal Function (EOF) analysis helps to identify, possibly for 

the first time, the physical parameters responsible for the dominant modal 

fluctuations in the Yucatan Channel and the likely reason for the model eddy 

shedding periods. The EOF mode 1 and mode 2 represent the variations in crosschannel oscillations and in inflow transport into the Gulf, respectively. The third 

and fourth EOF modes together, represent variations in the deep current over the 

sill, with long periods similar to those observed by Maul et al. (1985). The 

dominant EOF modal periods, around 6, 8 and 11 months are almost identical to 

the most dominant observed periods of Loop Current eddy shedding found by 

Sturges and Leben (2000). A follow up study by Oey et al. (2003b) provides 

further analysis of the effect of wind variations and Caribbean eddies on the eddy 

shedding frequency and mechanism. 

Romanou, Anastasia, Eric P. Chassignet, and Wilton Sturges, 2004. The Gulf 

of Mexico circulation within a high-resolution numerical simulation of the 

North Atlantic. J. Geophys. Res., 109, C01003, doi:10.1029/2003JC001770. 

This paper was written while Ms. Romanou worked with Chassignet. It reports the 

results of the MICOM studies over the full Gulf. The characteristics of rings (size, 

lifetimes, etc.) are compared with observations. There is also a discussion of 

comparisons of flow in Yucatan Channel with the observations of the Ensenada 

group. Ms. Romanou then went to a post-doctoral position at N.Y.U. 

Wang, D.-P., L.-Y. Oey, T. Ezer, and P. Hamilton, 2003. Near-surface 

currents in DeSoto Canyon (1997-99): Comparison of current meters, satellite 

observation and model simulation, J. Phys. Oceanogr., 33(1), 313-326. 

This paper was the result of several MMS projects, including the Gulf of Mexico 

deep flows collaborative study by Ezer-Sturges, the Gulf of Mexico deep flows and 

energetics study by Oey and collaborators, and the GOM hindcast system 

development efforts by Oey-Ezer. In this paper, the data assimilation methodology, 

previously developed for the Gulf Stream region (Ezer and Mellor, 1994, 1997), 

has been evaluated for the Gulf. The model’s hindcasts are compared with 

analysis of moored current meter data in DeSoto Canyon and with analysis of 

altimeter data, using the Singular Value Decomposition (SVD) method. The model 

results were found to compare well with the mean and variance of the currents in 

the Canyon, which seem to be largely influenced by the local topography. 
73

7. REFERENCES 

Bunge, L., J. Ochoa, A. Badan, J. Candela, and J. Sheinbaum. 2002. Deep flows in 

the Yucatan Channel and their relation to changes in the Loop Current 

extension. J. Geophys. Res., 107: 10.1029/2001 JC001256. 

Blumberg, A. F. and G. L. Mellor. 1987. A description of a three-dimensional 

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Amer. Geophys. U., Washington, D.C. 

Cherubin, L.M., W. Sturges, and E. Chassignet. 2004. Deep flow variability in the 

vicinity of the Yucatan Straits from a high resolution MICOM simulation. J. 

Geophys Res. Submitted. 

Conkright, M. E., S. Levitus, T. O'Brien, T. P. Boyer, C. Stephens, D. Johnson, O. 

Baranova, J. Antonov, R. Gelfield, J. Rochester, and C. Forgy. 2000. World 

ocean database 1998. Technical Report, Ocean Climate Laboratory, 

National Oceanographic Data Center, NOAA, Washington D.C. 

Davis, R. E., D. C. Webb, L. A. Regier, and J. Dufour. 1991. The Autonomous 

Lagrangian Circulation Explorer (ALACE). J. Atmos. Ocean. Techn., 

9:264-285. 

DeHaan, C.J. and W. Sturges. 2004. Deep cyclonic circulation in the Gulf of 

Mexico. J. Phys Oceanogr. In press. 

Ezer, T. and G. L. Mellor. 1994. Continuous assimilation of GEOSAT altimeter 

data into a three-dimensional primitive equation Gulf Stream model, J. Phys. 

Oceanogr., 24: 832-847. 

Ezer,T. and G. L. Mellor. 1997. Data assimilation experiments in the Gulf Stream 

region: How useful are satellite-derived surface data for nowcasting the 

subsurface fields?, J. Atmos. Ocean Tech., 14(6): 1379-1391. 

Ezer, T., L.-Y. Oey, and H.-C. Lee. 2002. Simulation of velocities in the Yucatan 

Channel, In: Proc., Oceans 2002, MTS/IEEE Publ., 1467-1471. 

Ezer, T., L.-Y. Oey, H.-C. Lee, and W. Sturges. 2003. The variability of currents 

in the Yucatan Channel: Analysis of results from a numerical ocean model, 

J. Geophys. Res., 108(C1): 3012, 10.1029/2002JC001509. 
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Girton, J. B. and T. B. Sanford. 2003. Descent and modification of the overflow 

plume in the Denmark Strait. J. Phys. Oceanogr., 33: 1351-1364. 

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

1087-1104. 

Hamilton, P. and A. Lugo-Fernandez. 2001. Observations of high-speed deep 

currents in the northern Gulf of Mexico. Geophys. Res. Lett, 28: 2867-2870. 

Hofmann, E. E. and S. J. Worley. 1986. An investigation of the circulation of the 

Gulf of Mexico. J. Geophys. Res., 91(C12): 14,221-14,236. 

Maul, G. A., D. A. Mayer, and S. R. Baig. 1985. Comparisons between a 

continuous 3-year current-meter observation at the sill of the Yucatan Strait, 

satellite measurements of Loop Current area, and regional sea level. J. 

Geophys. Res., 90: 9089-9096. 

Molinari, R. L. and D. Mayer. 1980. Physical oceanographic conditions at a 

potential OTEC site in the Gulf of Mexico; 27.5◦

N, 85.5◦W, NOAA Tech. 

Memo. ERL-AOML - 42; AOML, NOAA, Miami, FL, May 1980. 

Molinari, R. L., J. F. Festa, and D. W. Behringer. 1978. The circulation in the Gulf 

of Mexico derived from estimated dynamic height fields. J. Phys. 

Oceanogr., 8: 987-996. 

Nof, D. and T. Pichevin. 2001. The ballooning of outflows. J. Phys Oceanogr., 31: 

3045-3058. 

Niiler, P. P. and W. S. Richardson. 1973. Seasonal variability of the Florida 

Current. J. Mar. Res., 31(3): 144-167. 

Ochoa, J., J. Sheinbaum, A. Badan, J. Candela, and D. Wilson. 2001. Geostrophy 

via potential vorticity inversion in the in the Yucatan Channel. J. Mar. Res., 

59:725-747. 

Oey, L.-Y., H.C. Lee, T. Fan, T. Ezer, H. Wei, and P. Hamilton. 2004. A model 

hindcast of the circulation in the Gulf of Mexico. Manuscript in preparation. 
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Oey, L.-Y., P. Hamilton, and H.-C. Lee. 2003a. Modeling and data analyses of 

circulation processes in the Gulf of Mexico: Final report. U.S. Dept. of the 

Interior, Minerals Management Service, Gulf of Mexico OCS Region, New 

Orleans, LA. OCS Study MMS 2003-074. 

Oey, L.-Y., H. C. Lee, and W.J. Schmitz, Jr. 2003b. Effects of winds and 

Caribbean eddies on the frequency of Loop Current eddy shedding: A 

numerical model study. J. Geophy. Res. 108(C10): 3324, 

doi:10.1029/2002JC001698. 

Oey, L.-Y. and H.-C. Lee. 2002. Deep eddy energy and topographic Rossby 

waves in the Gulf of Mexico. J. Phys. Oceanogr., 32: 2499-3527. 

Pichevin, T. and D. Nof. 1997. The momentum imbalance paradox. Tellus, 

49A:298-319. 

Romanou, A., E. P. Chassignet, and W. Sturges. 2004. The Gulf of Mexico 

circulation within a high-resolution numerical simulation of the North 

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Current, Deep-Sea Res., 38 (Suppl. 1): S379-S409. 

Sheinbaum, J., J. Candela, A. Badan, and J. Ochoa. 2002. Flow structure and 

transport in the Yucatan Channel. Geophys. Res. Lett., 29: 1-4. 

Spall, M. A. and J. E. Price. 1998. Mesoscale variability in Denmark Strait: The 

PV outflow hypothesis. J. of Phys. Oceanogr., 28: 1598-1623. 

Sturges, W. and R. Leben. 2000. Frequency of ring separations from the Loop 

Current in the Gulf of Mexico: A revised estimate. J. Phys. Oceanogr., 

30: 1,814-1,818. 

Wang, D.-P., L.-Y. Oey, T. Ezer, and P. Hamilton. 2003. Near-surface currents in 

DeSoto Canyon (1997-99): Comparison of current meters, satellite 

observation and model simulation. J. Phys. Oceanogr., 33(1): 313-326. 

Weatherly, G. L. and N. Wienders. 2004. Mean circulation in the Gulf of Mexico 

at 900 m from PALACE floats. J. Geophys. Res. Submitted.
76

Wienders, N., G. L. Weatherly, and S. E. Welsh. 2002. Gulf of Mexico 900-meter 

circulation from PALACE floats. Abstract, EOS, Proceedings Amer. 

Geophys. Union. 
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