iii

DISCLAIMER

This report was prepared under contract between the Minerals Management Service 

(MMS) and Louisiana State University. This report has been technically reviewed by the MMS 

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

view and policies of the Service nor does mention of trade names or commercial products 

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

compliance with MMS editorial standards.

REPORT AVAILABILITY

Extra copies of the report may be obtained from the Public Information Office (MS 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, Louisiana 70123-2394

Telephone Number: (504) 736-2519

 1-800-200-GULF

CITATION

Suggested citation:

S.E. Welsh and M. Inoue. 2002. Langrangian study of the circulation, transport, and vertical 

exchange in the Gulf of Mexico: Final report. OCS Study MMS 2002-064. U.S. Dept. of 

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

La. 51 pp.
v

ABSTRACT

The recent increased activity associated with offshore oil and gas exploration and 

production in the outer continental shelf and slope regions of the northern Gulf of Mexico 

(GOM) calls for a better understanding of the ocean circulation in these regions and the potential 

environmental effects of those activities. Safe operation of offshore facilities during exploration 

and production is required for the protection of human life and the marine environment. Moving 

into deep water places the oil and gas operations closer to strong surface-intensified currents 

associated with the Loop Current (LC) and LC rings. Sub-surface current measurements over the 

northern continental slope provide evidence of vigorous currents and Topographic Rossby 

Waves. In the deep GOM below 1000 m, vertically homogeneous currents are observed to 

intensify near the bottom. Understanding how these deep energetic currents and eddies interact 

with the bottom topography is important to ensure that deepwater activities are safe and 

environmentally sound. Three other considerations in the mining of deep sea mineral resources 

are (1) the transport of material by deep currents in both the horizontal and vertical; (2) mixing 

and ventilation of deep water; and (3) the residence time of water in the deep GOM. 

The first goal of this project was to realistically reproduce the observed upper-layer 

circulation features of the GOM using a primitive-equation, numerical ocean model. The model 

features high vertical resolution and seasonally-varying inflow through the Yucatan Channel. 

Special attention was given to simulating the 3-dimensional structure of the LC and LC rings, 

which dominate the circulation in the eastern GOM. Lagrangian methods were used to analyze 

the trajectories of tens of thousands of inert tracer particles in an effort to characterize the threedimensional circulation and transport pathways in the GOM. Specific goals were to identify 

regions where vertical exchanges may occur and what processes are responsible for ventilation 

of the deep water. The exchange of particles across density surfaces occurs mainly beneath the 

LC in the eastern GOM, in the northwestern region where LC rings spin-down, and near steep 

slopes. During the process of ring separation, particles were observed to slowly spiral upward in 

the water column within the lower layer. The descent of particles occurs in the eastern GOM as 

cold, fresh water cascades over the sill beneath the Yucatan Current and flows northward along 

the Florida Escarpment before turning westward. There was little communication between the 

eastern and central regions of the GOM across 88oW below the surface layer. Tracer particles in 

the deep GOM to the west of 88oW are observed to continually circulate in a generally cyclonic 

direction, indicating a mean cyclonic drift in the deep water. Analysis of individual particle 

paths and comparison of those paths to the model circulation has provided information on the 

possible mechanisms for exchange of surface and deep water.
vii

TABLE OF CONTENTS

 PAGE

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

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

CHAPTER 

1 INTRODUCTION .................................................................................................. 1

1.1 Deep Water Research ............................................................................... 1

1.2 The General Circulation............................................................................. 2

1.3 Numerical Simulation of Gulf of Mexico Circulation................................. 3

2 OVERVIEW OF MODEL SIMULATIONS ............................................................ 5

 2.1 Numerical Model Configuration ............................................................... 5

2.2 Particle Tracing Method ........................................................................... 7

2.3 Summary of Model Simulations................................................................. 8

2.4 Model General Circulation ........................................................................ 9

3 RESULTS OF PARTICLE TRACING EXPERIMENTS........................................ 17

3.1 Experiment 1: Bottom Water Tracer Particle Release ............................. 17

3.2 Experiment 2: Surface Water Tracer Particle Release ............................. 24

3.3 Experiment 3: Base of Mixed Layer Tracer Particle Release .................. 26

3.4 Experiment 4: Level 11 (1100 m) Particle Release .................................. 31

3.5 Experiment 5: Eulerian Framework Study at 2200 m............................... 33

4 MEAN CIRCULATION ........................................................................................ 38

4.1 Deep Mean Hydrographic and Velocity Fields......................................... 38

4.2 Kinetic Energy Distribution ..................................................................... 41

5 DISCUSSION AND SUMMARY ........................................................................ 45

5.1 Particle Paths in the Deep Water ............................................................ 45

5.2 Particle Paths in the Upper Layer ........................................................... 46

5.3 Mixing and Ventilation of the Deep Water............................................. 47

REFERENCES ....................................................................................................................... 49
ix

LIST OF FIGURES

FIGURE PAGE

1 Model grid of the Gulf of Mexico ............................................................................ 7

2 12-year mean u- and v-components of velocity through the Yucatan Channel .......... 8

3A Temperature and velocity fields at level 1 (12.5 m) during the shedding 

of Ring S3 on year 25, day 260 .............................................................................. 11

3B Temperature and velocity fields at level 1 (12.5 m) during the shedding 

of Ring S4 on year 26, day 150 .............................................................................. 12

4 Histogram of 27 model LC ring separation intervals .............................................. 13

5 Power spectral density of v-component of velocity at 87°W, 25.5°N, and

12.5 m for model years 24 through 28.................................................................... 14

6 Paths of 16 LC rings observed during model years 24 through 32 .......................... 15

7 Plan view of initial starting locations at 2200 m for tracer particles in 

Experiment 1 ......................................................................................................... 17

8 Percentages of particles to ascend above specified depth levels during

the 16-year model simulation ................................................................................. 18

9 Percentages of all particles to ascend to (a) 2000 m, (b) 1000 m, 

(c) 500 m, and (d) 200 m according to starting location.......................................... 18

10 X-Z projection of particles according to starting location ....................................... 20

11 Path of particle 16019: (a) plan view for years 24-30, (b) latitude-depth

projection for years 24-30, (c) longitude-depth projection while 16019 

was in the eastern basin, years 24-28, and (d) longitude-depth projection

after 16019 became entrained in Ring S9 ............................................................... 21

12 Time series of particle 16019 latitude (dotted) and temperature at 

87°W, 25.5°N, 128 m............................................................................................. 21

13 Path of particle 17976: (a) plan view for years 24-38, (b) latitudedepth projection for years 24-38, (c) longitude-depth projection while

17976 was in the eastern basin, years 24-32, and (d) longitude-depth

projection after 17976 became entrained in Ring S17 in year 33 ............................ 22
x

FIGURE PAGE

14 Paths of two particles in the deep GOM during years 24-38: (a) plan

view of particle 1007, (b) longitude-depth projection of particle 1007, 

(c) plan view of particle 4005, and (d) longitude-depth projection of 

particle 4005 ............................................................................................................23

15 Percentage of particles found at specified depth ranges during 

Experiment 2 ...........................................................................................................24

16 Path of particle 19829 during years 24-29 in plan view and crosssectional view ..........................................................................................................25

17 Path of particle 7453 during years 24-29 in plan view and crosssectional view ..........................................................................................................25

18 Percentages of particles found at specified depth ranges during 

Experiment 3 ...........................................................................................................27

19 Plan view and longitude-depth projection of particles (a) 14737, 

(b) 14892, and (c) 15049. The year of observation is indicated................................28

19 Plan view and longitude-depth projection of particles (d) 14579, 

(e) 14727, and (f) 15735. The year of observation is indicated ................................29

20 Velocity (cm s-1) and temperature (°C) at 181.125 m on model 

year 25, day 240.......................................................................................................30

21 Velocity (cm s-1) and temperature (°C) at 181.125 m on model 

year 25, day 300.......................................................................................................30

22 Particles (a) 1500 and (b) 2500 in the Campeche Bay during model

years 24-26 ..............................................................................................................32

23 Particles 1100 and 3100 in the Campeche Bay during model

years 24-26 ..............................................................................................................33

24 30-day particle displacements for each month in model year 25 ...............................34

25 Vertical displacements of particles during model years 24 and 25

of Experiment 5: (a) all particles between 22°N and 25°N, (b) all 

particles between 25°N and 28°N, and (c) all particles west of 94.5°W....................35
xi

FIGURE PAGE

26A 30-day longitude displacement vectors for particle 112 at 84.35°W, 

23.45°N ...................................................................................................................36

26B 30-day longitude displacement vectors for particle 77 at 87.95°W,

26.25°N ...................................................................................................................37

26C 30-day longitude displacement vectors for particle 54 at 91.15°W, 

25.85°N ...................................................................................................................37

27A 12-year average model temperature (°C) and velocity vectors (cm s-1) 

at 12.5 m..................................................................................................................39

27B 12-year average model temperature (°C) and velocity vectors (cm s-1) 

at 1000 m.................................................................................................................39

27C 12-year average model temperature ((T-4)*100°C) and velocity 

vectors (cm s-1) at 2200 m........................................................................................40

27D 12-year average model temperature ((T-4)*100°C) and velocity 

vectors (cm s-1) at 3100 m........................................................................................40

28 3-year average eddy kinetic energy at 2500 m and 3400 m.......................................42

29 Low-pass filtered (180 day cut-off) kinetic energy at 2800 m 

averaged over 3 years...............................................................................................43

30 Band-pass filtered (40 to 10 days) kinetic energy at 2800 m

averaged over 3 years...............................................................................................44
xiii

LIST OF TABLES

TABLE PAGE

1 Depths of Grid Levels in Meters ................................................................................ 6

2 Life Histories of Nine LC Rings that Separated during Model 

Years 24 through 28 ................................................................................................. 10
1

CHAPTER 1

INTRODUCTION

1.1 Deep Water Research

Offshore oil and gas exploration and production in the Gulf of Mexico (GOM) has moved 

seaward into water depths greater than 300 m, creating a need for new technologies. The recent 

success of the deepwater projects has resulted in a commitment by industry to continue exploring 

the outer continental shelf and slope for mineral resources and to develop and improve 

technology for safe operation of deepwater facilities. Detailed understanding of the ocean 

environment on the slope region is essential in order to assess the adequacy of the existing 

regulations and environmental impacts of all facets of deepwater exploration and production. 

The deep GOM is also ecologically interesting. Along the continental slope there is an 

abundance of benthic communities associated with numerous hydro-carbon seeps. Moreover, 

there may be distinct habitats that are zoned according to depth and it is not understood what 

controls the diversity in the deep sea (Carney 1997). The goal of this study was to characterize 

the transport, mixing, and ventilation processes in deep water in order to assess the 

environmental impacts of exploration and production on deep-sea communities.

The following MMS field programs have provided information about the general 

circulation and eddy fields along the shelf and slope from the U.S.-Mexican border to the west 

Florida Shelf: Louisiana/Texas Physical Oceanography Program (LATEX), the Northeastern 

Gulf of Mexico Physical Oceanography Program, the Northeastern Gulf of Mexico Coastal and 

Marine Ecosystem Program, GulfCet I & II, and the DeSoto Eddy Intrusion Study. The 

relevance of these observational programs to a modeling study of deep water is in the model 

verification process. The GOM can be considered as a two-layer system in which the behavior 

of the upper layer anticyclones and lower layer eddies are coupled and the bottom bathymetry 

can influence the path of the ring in the upper layer through feedback from the lower layer 

(Hurlburt and Thompson 1982). A modeling study that aims to describe vertical exchanges and 

horizontal advection should accurately represent the upper layer processes, as well as the deep 

oceanographic environment.

The GOM is a semi-enclosed sea with maximum depths of approximately 3400 m in the 

eastern portion and 3700 m in the western portion. The surface waters of the GOM have been 

studied in great detail. In comparison, there is very little information about the circulation below 

1000 m. Direct current measurements were rare (e. g., Pequegnat 1972; Hamilton 1990) until 

recently (Wiseman and Walker, 2001, unpublished data; Inoue et al. 2002; Hamilton and LugoFernandez 2001). Also limited attention has been focused on the deep circulation in the GOM by 

the modeling community. Although the deep water of the GOM below the deepest sill depth is 

completely isolated from outside, it is well-ventilated (Buerkert 1997; Berberian and Cantillo 

1999). Renewal of deep water should take place only via vertical exchange of water with the 

water above the sill depth. Several modeling studies of the GOM have indicated the presence of 

energetic deep vortices (e.g., Hurlburt and Thompson 1982; Indest 1992; Sturges et al. 1993; 

Welsh and Inoue 2000). Generation of deep vortices, interaction of deep vortices with bottom 
2

topography (Vidal et al. 1994), and the spin down of upper layer anticyclones (Flierl and Mied 

1985; Schmitz and Vastano 1976) appear to play an important role in the vertical exchange of 

water in the GOM.

1.2 The General Circulation

The Loop Current (LC) enters the GOM from the Caribbean Sea through the Yucatan 

Channel (sill depth of approximately 1900 m) and exits through the southern Straits of Florida 

(sill depth of about 800 m). The upper-layer circulation in the eastern GOM is dominated by 

boundary fluctuations of the LC and the separation of anticyclonic eddies, referred to herein as 

LC rings. Estimates of the ring separation period have been changing as more data has become 

available. The previously published range was between 6 and 17 months (Sturges 1994; 

Vukovich 1995), and estimates of the primary and secondary separation modes are 8-9 months 

and 13-14 months, respectively. The most recent analysis of 34 rings from 1973 to 1999 includes 

new or revised data with ring separation periods as 3 and 5 months. Peaks in a histogram of this 

data occur at 6 months and 9 months with a broad peak at 11 months (Sturges and Leben 2000). 

The mean separation period during this 27-year record is close to 9.5 months, which is somewhat 

shorter than an earlier estimate of 11 months (Vukovich 1995).

The three-dimensional structure of LC rings has been reported by several authors (Elliot 

1982; Vukovich and Crissman 1986; Lewis and Kirwan 1987; and Cooper et al. 1990). LC rings 

have average diameters in the eastern GOM of 300-400 km and surface current speeds can reach 

2 ms-1. The strong currents associated with the rings are observed to penetrate to depths of 700-

800m. A core of high salinity Subtropical Underwater (>36.6 PSU at 22.5°C) is found in the LC 

rings at depths of 150-250 m. LC rings migrate westward with an average translation speed of 4 

km/day and have a typical life span of one year. As a ring migrates westward, the following 

changes are observed: there is a shoaling of the isotherms relative to the surrounding water, the 

subsurface salinity maximum degrades, the lateral extent of the rings decreases, and there is a 

substantial decrease in peak swirl velocities.

Vukovich and Crissman (1986) analyzed GOES and ocean infrared satellite images 

collected over a 12-year period and concluded that there were three main LC ring paths. The LC 

rings following the predominant path (path 1) reach the western wall at around 22°N and then 

move northward. All ring paths in this 12-year study terminate in the northwestern corner, 

sometimes referred to as the “eddy graveyard”. Kirwan et al. (1988) described the westward 

migration of two LC rings using satellite-tracked drifters. Both of these rings migrated toward 

the southwest and reached the western boundary near 22.8°N and eventually moved northward.

After reaching the Mexican-Texas slope there are several scenarios as to the fate of LC 

rings. Hamilton et al. (1999) follow the paths of ten eddies in the western Gulf using ARGOStracked drifters and hydrographic surveys. The authors conclude that there are no preferred LC 

ring paths in the main basin or that LC rings move in a preferred direction once reaching the 

western slope. Sturges et al. (1993) estimated that LC rings decay rapidly once they reach the 

western boundary (~70 days) and do not contribute to a western boundary current, whereas 

(Vidal et al. 1999) state that the contribution of anticyclonic vorticity from rings is a major factor 

in the maintenance of a western boundary current. The stalling of LC eddies in the far 
3

northwestern portion of the GOM contributes to a cyclonic gyre on the Louisiana-Texas shelf as 

well as the existence of an eastward current that runs along the shelf break to the Mississippi 

Delta (Oey 1995).

1.3 Numerical Simulation of Gulf of Mexico Circulation

Several different types of numerical models have been used to model circulation in 

GOM. Since the pioneering modeling study of circulation in GOM by Hurlburt and Thompson 

(1980, 1982) the following models have been implemented: Princeton Ocean Model, Modular 

Ocean Model, Naval Research Laboratory global layer-model, Miami Isopycnic Coordinate 

Model. Those models use either a limited-domain or a larger-domain. The use of a limiteddomain usually requires the specification of inflow as well as outflow representing the LC and 

the Florida Current, respectively. Under a larger-domain, circulation within GOM is modeled as 

part of a much larger model domain, such as the entire North Atlantic, or even a global 

circulation model. The use of a larger-domain alleviates the need to specify flow into and out of 

GOM. However, the model resolution suffers due to the increased computational efforts 

required.

Welsh (1996) and Inoue and Welsh (1997) used the Modular Ocean Model to describe 

both the surface and deep circulation in the GOM using a 0.1° grid. The model was able to 

simulate many features of the observed upper-layer seasonal circulation including the volume 

transport through the Yucatan Channel, the western boundary current in the GOM, upwelling on 

the Campeche Bank, and the seasonal cyclonic circulation in the Campeche Bay. In addition, 

many of the observed features of LC rings were simulated in the model, including their initial 

size, mean swirl speed, migration speed, characteristic migration paths (Vukovich and Crissman 

1986), and life span.

Model studies of Sturges et al. (1993) and Welsh and Inoue (2000) indicate that deep 

motions in the GOM are highly coherent in the vertical from nearly 1300 m all the way to the 

bottom, as recorded by deep current meter moorings (Hamilton 1990). Welsh and Inoue (2000) 

observed that an anticyclone-cyclone pair forms in the eastern GOM beneath the LC as a ring is 

separating from the LC. Deep cyclones and deep anticyclone-cyclone pairs have also been 

identified in other modeling studies of the GOM circulation (Hurlburt and Thompson 1982; 

Indest 1992; Sturges et al. 1993). The following similarities are noted for these three studies: (1) 

an anticyclone-cyclone pair, which Hurlburt and Thompson (1982) refer to as a 'modon (Stern 

1975)', is generated in the lower layer during the formation of LC rings; (2) during westward 

migration, the axis of the modon is oriented close to the direction of propagation of the ring with 

the anticyclone leading; and (3) the leading lower-layer anticyclone weakens relative to the 

trailing cyclone as the modon propagates westward (except for experiment 2 of Hurlburt and 

Thompson (1982) which featured an upper layer inflow directed 27° west of normal and no 

lower layer inflow). Hurlburt and Thompson (1982) reported that in the case with idealized 

bathymetry, the modon is confined to the abyssal plain and is steered by the bathymetry, which is 

also evident in Welsh and Inoue (2000).

The connection between the migration of LC rings in the upper layer and the deep 

circulation of the GOM has been observed in several numerical models (Hurlburt and Thompson 

1982; Sturges et al. 1993; and Welsh and Inoue 2000). Cushman-Roisin et al. (1990) investigated 
4

the generation of deep cyclone-anticyclone pairs in conjunction with the westward migration of 

surface anticyclones for a two-layer system with a flat bottom and a rigid lid. A depression 

forms in the vertical interface between an anticyclonic eddy in the surface layer and the deeper 

layer with respect to the surrounding fluid. The relative vorticity of the lower layer is higher 

beneath the eddy than beneath the surrounding fluid, and as the eddy migrates westward on a âplane, the lower layer acquires anticyclonic relative vorticity beneath the eddy and cyclonic 

relative vorticity behind the eddy. The result is an eddy pair in the lower layer with the leading 

anticyclone beneath the surface vortex and a trailing cyclone (Cushman-Roisin et al. 1990).

The existence and fate of lower layer modons coupled to upper layer surface intensified 

vortices over variable topography has recently been examined using analytical models (Reznik 

and Sutyrin 2001; Sutyrin 2001). The effect of the cross-slope drift of an upper layer vortex is to 

create dipolar gyres in the lower layer. Feedback from the lower layer modon on the upper layer 

vortex creates additional westward propagation along the isobaths (Sutyrin 2001). If the slope of 

the interface between the upper and lower layer vortices is comparable to the slope of the 

topography, then the gyres in the lower layer lose their mirror symmetry and the cyclonic gyre 

strengthens relative to the anticyclonic gyre (Sutyrin 2001). 
5

CHAPTER 2

OVERVIEW OF NUMERICAL SIMULATIONS

2.1 Numerical Model Configuration

The Modular Ocean Model version 1.1 (Pacanowski et al. 1991), a three-dimensional, 

primitive-equation, numerical model, which has evolved from the Bryan-Cox model (Bryan 

1969; Cox 1984), was chosen for this study. This model is ideal for this project because the 

effects of bottom topography and the resolution of eddy dynamics are both important in the 

GOM. The model grid is derived from the ETOP05 world topography data set. The bathymetric 

values are linearly interpolated to 0.1° and smoothed to prevent topographically induced 2Äx 

noise in the numerical solution. The model domain extends from 98.0°W to 72.0°W and from 

15.0°N to 31.0°N. The model grid has lateral dimensions of 280 by 161. There are 20 levels in 

the vertical with a maximum depth of 3850 m. The use of evenly spaced vertical levels is 

preferred to reduce the error in the finite difference formulation of the vertical velocity. Greater 

resolution is needed near the surface for more accurate representation of the shelf bathymetry 

and to resolve the thermocline. Therefore the thickness of each of the upper 3 levels is 25 m, the 

lowest 10 levels are each 300-m thick, and the transition region represented by levels 4 through 

10 gradually increases in depth. These depths are summarized in Table 1.

In order to achieve a realistic inflow condition, this model domain extends outside the 

GOM into a synthetic return flow region that links the Straits of Florida with the western 

Caribbean (Figure 1). The bathymetry in the return flow region is altered to allow flow exiting 

the GOM to re-circulate around Cuba and enter the GOM through the western Caribbean. The 

method of forcing the inflow is the same as has been implemented in previous modeling studies 

by Welsh (Inoue and Welsh 1997; Welsh and Inoue 2000).

The annual averages of the 3-D temperature and salinity fields used to initialize the 

model are derived from the Global Ocean Temperature and Salinity Data (NODC 1994). Level 1 

temperature and salinity fields (vertically centered at a depth of 12.5 m) are restored to seasonal 

climatology with a relaxation time scale of 6 weeks. This surface boundary condition prevents 

the temperature and salinity in the surface layer from drifting away from the observed 

climatology. The surface wind stress field is derived from the Hellerman and Rosenstein (1983) 

normal monthly wind stress climatology. Wind stress is applied over the entire GOM and in the 

Caribbean to the west of the geostrophic forcing region, but no wind stress is applied over the 

return flow region. The wind stress field changes smoothly in time by linear interpolation at 

each time step.

A target value of 28 Sv is chosen to represent the annual-mean volume transport through 

the southern Straits of Florida. This target volume transport is based on published analyses of 

current meter measurements and cable-based transport measurements across each of the various 

channels that feed the Florida Current. Scientific Applications International Corporation (SAIC) 

(1992) estimated the annual-mean transport through the Florida Straits at 27°N to be 31.1 Sv and 

the combined annual-mean transport through the Old Bahama Channel and the Northwest 

Providence Channel to be 2.8 Sv. Subtracting the combined transports through these two 
6

channels from the transport at 27°N results in an annual-mean transport through the southern 

Straits of Florida of 28.3 Sv. The widely accepted value for the volume transport of the Florida 

Current at 24°N is 30 Sv (Schmitz et al. 1992), which includes the 1.9 Sv flowing through the 

Old Bahama Channel measured by SAIC (1992).

Table 1

Depths of the Grid Levels in Meters

 Grid level DZ DZT DZW DW

1 25. 12.5 25. 25.

2 25. 37.5 25. 50.

3 25. 62.5 25. 75.

4 31.25 87.5 37.5 106.25

5 43.75 125. 50. 150.

6 62.50 175. 75. 212.50

7 87.50 250. 100. 300.

8 125. 350. 150. 425.

9 175. 500. 200. 600.

10 250. 700. 300. 850.

11 300. 1000. 300. 1150.

12 300. 1300. 300. 1450.

13 300. 1600. 300. 1750.

14 300. 1900. 300. 2050.

15 300. 2200. 300. 2350.

16 300. 2500. 300. 2650.

17 300. 2800. 300. 2950.

18 300. 3100. 300. 3250.

19 300. 3400. 300. 3550.

20 300. 3700. 150. 3850.

_______________________________________________________________

DZ=height of T-grid boxes. 

DZT=depth at the center of each T-grid box. 

DZW=the distance between centers of T-grid boxes.

DW=depth at the bottom of each T-grid box.

___________________________________________________________

The total volume flux through the Yucatan Channel is controlled by adding a constant to 

the u-component of velocity at each grid point within the geostrophic forcing region. During 

spin-up, this constant is increased by small increments until the annual average of the volume 

transport through the Yucatan Channel is equal to the target value. At least one year of spin-up 

time is needed for the model to adjust to changes in the target volume transport. The vertical 

shear of the Caribbean Current is created by relaxing the temperature and salinity along a north-
7

south transect centered at 82.0°W in the geostrophic forcing region to the monthly climatology. 

A relaxation time scale of 6 weeks was used to produce flow through this transect that best 

approximated the observed geostrophic shear.

 Figure 1. Model grid of the Gulf of Mexico.

Twelve-year cross-sectional averages of the u- and v-components of the inflow through 

the Yucatan Channel are presented in Figure 2. The core of the flow is concentrated near the 

surface on the western portion of the Channel. There is about 15 Sv of southward flow directed 

toward the Caribbean near the western Cuban Coast, as well as regions of southward flow 

(maximum 5 Sv) in the center of the channel near the bottom.

2.2 Particle Tracing Method

The Lagrangian technique of seeding and tracking tracer particles in the model is used to 

examine detailed transport and mixing processes and to identify the processes responsible for 

ventilation of the deep water. The model is seeded with large numbers of inert particles that drift 

freely within the model domain in response to the velocity field. Several different experiments 

were designed that feature different starting positions of the tracer particles that depend on the 

region or process that we are interested in. The positions of the tracers were recorded and later 

analyzed to identify characteristic flow paths and assess the effects of physical processes on the 

individual particles.
8

Figure 2. 12-year mean u- and v-components of velocity through the Yucatan Channel.

The method used to track the particles (Pacanowski 1996) features three-dimensional 

interpolation of each particle position at each time step followed by integration of the particle 

position in space using the three-dimensional components of velocity. Each particle is bounded 

within the space defined by the vertices given by the eight nearest surrounding model grid points. 

The indices of the deepest northeast corner of this bounding volume are recorded. The volume 

weights for linear interpolation of velocity at the vertices of the bounding volume to the particle 

position are computed. The velocity at the position of a particle is constructed by 3-D linear 

interpolation. The particle trajectory is integrated forward for one time and the bottom most 

northeast index of bounding volume is then updated. Any component of the trajectory is reset to 

its previous value if it crosses over to land, thereby simulating free slip conditions.

In addition to recording locations of particle positions, the model temperature, salinity, 

horizontal velocity, and stream function are saved at selected levels every 10 days and at every 

grid point every 30 days. Animations of the particle positions superimposed on the model flow 

fields were made to qualitatively assess the ability of the model to transport the tracer particles 

within oceanographic features. The method of integrating the particle positions in response to 

the model flow field appears to work extremely well.

2.3 Summary of Model Simulations

The model development and verification took place during the first 23 years of model 

integration. Changes and improvements were made to the model forcing, the variable viscosity, 

and the particle tracing methods. Five separate modeling experiments were made with different 
9

starting regions for the tracer particles. In each of these experiments the same restart file for the 

end of model year 23 is used, and the starting location of the tracers is changed. The first 

experiment was designed to provide information about how the deep water (below 2000 m) 

circulates and where the vertical exchange of fluid takes place. The model was seeded with 

19,105 tracer particles at the center of all grid boxes from 2200 m to 3700 m. The particles were 

tracked for 12 model years (year 24 through year 35). The results of Experiment 1 were very 

insightful and are described in Section 4.2.

The next three experiments were designed to identify the regions and processes 

associated with the vertical exchange of fluid within the upper layer (0-1000 m). In Experiment 

2, the top two layers (centered at 12.5 m and 37.5 m) were seeded with 28,294 tracer particles, 

which were tracked for six model years (year 24 through 29). A shortcoming of this experiment 

was that many particles were quickly flushed-out through the Florida Straits or into the 

Caribbean, and some particles became stranded in shallow water. Although many particles were 

eliminated from the system due to flushing or grounding, enough particles remained buoyant to 

make some interesting observations. The results of Experiment 2 are presented in section 4.3.

Experiment 3 was designed to see how much vertical exchange is taking place at the base 

of the mixed layer. This experiment featured a repeat of model years 24 through 26 with 18184 

particles initially placed at the centers of levels 5 (128.125 m) and 6 (181.250 m). The results of 

Experiment 3 are described in section 4.4. Experiment 4 was designed to look at lateral 

circulation and vertical exchange processes at a depth of 1000 m (level 11). The model was 

initialized with 7606 tracer particles at the center of level 11 and model years 24 through 26 were 

again repeated. The results of Experiment 4 are described in Section 4.5.

The purpose of the final experiment, Experiment 5, was to learn about flow at fixed 

locations over time. The model was seeded with 116 particles at 2200 m (level 15), but only in 

water depths less than or equal to 2800 m (level 17). Every 4th grid box in both the north-south 

and east-west direction was initialized. We were primarily interested in particle trajectories 

directly over the slope. The particles were reset to their initial positions at the start of each 

month and their movement was recorded every 10 days for 30-day periods. For this experiment 

only model years 24 and 25 were repeated and the results are described in Section 4.6.

2.4 Model General Circulation

Many aspects of the general circulation of the GOM are reproduced in the output of this 

model. All of the features of the seasonal circulation that were generated in the previous 1/8°

modeling study (Inoue and Welsh, 1997; Welsh and Inoue, 2000) are again observed in this 

study. Features such as the spring/summer upwelling on the Campeche Bank and the late 

summer/fall cyclonic circulation in the Campeche Bay are observed, but are not discussed in this 

report. A description of the general behavior of the LC, LC ring separation, and LC ring 

westward migration are included because the vertical motions observed in the model are highly 

correlated with the LC and LC ring behavior. The separation, life span, and disappearance of 

rings S1 through S8 were described by analyzing plots of level 1 temperature contours and 

velocity vectors at 10-day intervals. A summary of these observations is presented in Table 2.
10

Year 24 begins with a ring (S0) in the western GOM centered near 25°N and 94°W and 

the LC is centered at 86.6°W and 25.2°N. By day 90 a new ring (S1) has completely separated 

from the LC and is centered at 89°W and 26.2°N. Ring S0 reaches the northwest corner and is 

centered at 26°N, 95.25°W on day 150. On day 170 rings S0 and S1 begin to exchange fluid 

around their peripheries and by day 200 the two rings appear to have completely merged.

Table 2

Life Histories of Nine LC Rings that Separated during Model Years 24 through 28

______________________________________________________________________________

Ring

 S1

 S2

 S3

 S4

 S5

 S6

 S7

 S8

 S9

Separation Time

between

 separations

(year/day) (weeks)

 

 24 / 90

 24 / 240 [150]

 25 / 120 [240]

 25 / 260 [140]

 26 / 150 [250]

 26 / 350 [200]

 27 / 160 [170]

 28 / 50 [250]

 28 / 220 [170]

Reach 91oW Time to reach 

91oW

(year/day) (weeks)

 24 / 140 [180]

 24 / 320 [210]

 25 / 170 [190]

 25 / 360 [220]

 26 / 220 [200]

 27 / 60 [180]

 27 / 240 [220]

 28 / 100 [200]

 28 / 300 [200]

Reach Eddy 

Graveyard

(year/day)

 24 / 270

 25 / 160

 25 / 280

 

 26 / 330

 27 / 200

 27 / 340

 28 / 190

 Life Span

(weeks)

 320 

 490 

 400 

 340

 300 

 340

 310

 400

average life span = 346 days; average time to reach 91oW = 200 days

minimum separation interval = 4.6 months; maximum separation interval = 8.3 months

average separation interval = 196.25 days

______________________________________________________________________________

Ring S2 has completely separated from the LC by day 240 and ring S1 has reached the 

northwest corner by day 270. Ring S2 becomes very elongated in a northeast-southwest 

direction at 92.6°W and 24.5°N near day 360 and has split apart by day 10 of year 25. The 

southern portion of S2 moves toward the southwest until it reaches the slope and then moves 

rapidly northward. Meanwhile the northern portion moves westward and eventually merges with 

its other half on day 70 of year 25. Ring S3 separates on day 120 of year 25. Ring S2 reaches the 

northwest corner by day 160. Rings S2 and S3 begin to merge on day 170 of year 25 and the 

surface velocity signature of ring S2 vanishes by day 210.

The LC stays confined to the southeastern GOM during the formation of ring S4. The 

center of circulation of ring S4 at the time of separation on day 260 of year 25 is located at 

87.2°W and 25.5°N. By comparison, ring S3 had a center of circulation near 89°W and 26.2°N 

at the time of separation. The LC reformed much deeper into the northeastern GOM after 

separation of ring S3 than after the separation of ring S4 (Figure 3). The interesting thing to note 
11

is that ring S4 separated only 140 days after ring S3, while ring S5 separated 250 days after ring 

S3. The length of time between consecutive shedding events is positively correlated with the 

position of the LC after the first ring has shed. If the LC reforms far to the south, then the 

interval before the next eddy shedding is relatively long. Whereas if the LC extends far into the 

GOM after a ring has separated, then the interval between shedding events is shorter.

Figure 3A. Temperature and velocity fields at level 1 (12.5 m) during the shedding of Ring S3

 on year 25, day 260.
12

Figure 3B. Temperature and velocity fields at level 1 (12.5 m) during the shedding of Ring S4

 on year 26, day 150.

The time at which model LC rings become detached from the LC is determined by visual 

inspection of the surface velocity fields, as well as time series analysis of model temperature and 

velocity at selected locations. A histogram of separation intervals was constructed from the 27 

shedding events over a 15-year model simulation (Figure 4). The criterion for a LC rings to have 

separated is for the circulation around the rings at the surface to be all or mostly closed. 
13

Therefore, the moment that a ring has just separated is somewhat subjective, but the error is 

within ~15 model days. The minimum, maximum, and average separation interval that were 

observed during years 24 through 38 were 4.7 months, 8.3 months and 6.5 months, respectively. 

A time series of model velocity at 12.5 m, 25.5°N, 87°W for years 24 through 28 was compared 

to the timing of separation of rings S1 through S9. The minimum in north-south velocity occurs 

at the time of each ring separation to within ~20 days. A power spectrum was computed for this

15-year velocity time series (Figure 5). The peaks found at 4.5, 4.75, 6.5, and 8.3 are within the 

range of the model separation intervals. Smaller peaks are found at 9.5 and 14 months, but there 

is very little energy at 12 months (.055). The spectrum at lower frequencies (longer than 8.3 

months/cycle) varies somewhat depending on the geographic location of the time series. 

Figure 4. Histogram of 27 model LC ring separation intervals.
14

Figure 5. Power spectral density of v-component of velocity at 87oW, 25.5oN, and 12.5 m for

 model years 24 through 38.

The observed mean eddy shedding interval of 9.5 months [Sturges and Leben 2000] is 

larger than the mean model ring separation by 3 months. The model LC sheds eddies at the high

frequency end of the observed spectrum, but the model LC does not shed eddies on time scales 

longer than 250 days. A difference between the behavior of LC ring formation in the model and 

the real world is that each time the model LC penetrates into the northeastern GOM, a ring 

separation takes place. In actuality, the LC can penetrate far into the eastern and sometimes the 

central GOM without shedding an eddy for many months. The LC penetrated far into the GOM 

and nearly shed an eddy in late fall 2000 and again in January 2001 before Millenium eddy 

completely separated and began to migrate westward in March/April 2001. LC rings have also 

been observed to separate and then rejoin the LC, which never occurs in the model.

The paths of the 16 model LC rings to separate during model years 24 through 32 are 

presented in Figure 6. These paths are determined by plotting the center of circulation of each 

ring in the top level (centered at 12.5m) every 10-30 days after the ring has completely detached 

from the LC. The center of each ring passes between 25°N and 26°N at 88°W, after which their 
15

paths diverge. The majority of rings migrated almost directly westward, although five of the 16 

rings migrated toward the southwest and were south of 24oN when they reached 95oW. Once in 

the far western GOM, most rings then moved northward along the slope into the ‘eddy 

graveyard’, which has its center at approximately 26.5oand 95oW. The eddy graveyard is the 

termination point for nearly all ring paths in this modeling study, other GOM modeling studies 

(Hurlburt and Thompson, 1982; Sturges et al., 1993; Oey, 1995), and observational studies of LC 

rings (Vukovich and Crissman, 1986). After LC rings enter the eddy graveyard, they continue to 

decrease in lateral size and swirl velocities. Also as the rings spin-down, the vertical 

displacement of the isotherms decreases and the sub-surface salinity maxima erodes. Only two of 

these16 model rings had completely dissipated before a new ring arrived, whereas the rest of the 

older rings were eventually absorbed by new rings.

Figure 6. Paths of 16 LC rings observed during model years 24 through 32.
16

Vukovich and Crissman (1986) examined 12-years of National Oceanic and Atmospheric 

Administration (NOAA) infrared imagery and Geostationary Operational Environmental 

Satellite (GOES) data for the purpose of tracking the centers of warm core rings in the GOM. 

Nine long-term ring paths, a few short-term paths, and several isolated locations of ring centers 

were compared to determine the characteristic migration paths of LC rings. Six of the long-term 

paths were directed toward the southwest, two were directed toward the west, and one was 

observed to remain in the eddy graveyard for eight months in a region roughly centered at 

94.5oW and 26.5oN. Several of the isolated ring centers also indicated a more generally 

westward migration. The model ring paths were in very good agreement with the observations. 

Most of the model rings followed a generally westward path, oriented along the 3000-m isobath, 

although five model rings were observed to follow a southwestward path. None of the model 

rings moved to the south after reaching the western slope, in agreement with the characteristic 

LC ring paths described by Vukovich and Crissman (1986) which converged in the northwestern 

GOM.

Hamilton et al. (1999) analyzed the paths of ten LC rings in the western GOM and 

determined that LC rings do not follow any preferred paths as they migrate westward and can 

move north or south after reaching the western slope. At least six of the ten rings analyzed by 

Hamilton et al. (1999) (C6, T, U, V, W and Y) were observed in the northwestern GOM (north 

of 25°N and west of 95°W) at some time during their life span. Hamilton et al. (1999) noted that 

the presence of lower slope cyclones in the northern GOM and the absorption of older, fossil LC 

rings influence the westward migration of LC rings.
17

CHAPTER 3

RESULTS OF PARTICLE TRACING EXPERIMENTS

3.1 Experiment 1: Bottom Water Tracer Particle Release

The model was seeded below 2000 m with 19,105 inert particles that drift freely within 

the model domain in response to the velocity field (Figure 7). Every ocean grid box at levels 15 

through 20 (depths 2200 m through 3700 m) was seeded with a single particle. The positions of 

the tracers were recorded every 10 days for 16 model years (from the beginning of model year 24 

through the end of model year 39). Groups of particles are referred to by their initial geographic 

locations: west, central and east.

Figure 7. Plan view of initial starting locations at 2200 m for tracer particles in Experiment 1.

 The bold letters identify the starting regions as west (W), central (C), and east (E).

The percentages of all particles to rise above specified levels over a 16-year simulation 

are presented in Figure 8. Particles rose above 2000 m within the first year after initialization, 

1000 m in year 27, and 500 m in year 30. The percentages of particles to rise above 2000 m, 

1000 m, and 500 m according to starting locations are presented in Figure 9. Particles from the 
18

east group rose most rapidly to all levels. Particles from the central and west groups rose above 

2000 m at nearly the same rate, but the number of particles to rise above 1000 m from the west 

group was very small. The curves appear to level off because the supply of particles decreases in 

each region with time.

Figure 8. Percentages of particles to ascend above specified depth levels during the 16-year 

 model simulation.

Figure 9. Percentages of all particles to ascend to (a) 2000 m, (b) 1000 m, (c) 500 m, and 

 (d) 200 m according to starting location.
19

Although the particle tracing method implemented in the model prevented the particles 

from entering the land using a free slip condition, the particles were not prevented from 

occasionally becoming stranded. If a particle became trapped in a grid box that was bounded on 

the two adjacent sides and the bottom by land, then the particle may remain there indefinitely or 

may become free again. The stranding of particles was observed in the visual analysis of particle 

tracks. The actual percentage of particles that became stranded in these experiments was not 

computed, but appeared to be very small.

The lateral movement of the particles according to starting region over the 16-year 

simulation was also recorded. The longitude-depth projection of all particles at the end of the 

15-year simulation and the projection of just those particles that were initially at 2200 m is 

presented in Figure 10. The particles tend to be segregated into regions east and west of 88°W. 

Particles mixed freely in the western and central portions, but were cut-off from mixing with the 

eastern portion. 9.11% of particles initially east of 88°W (blue) were located west of 88°W after 

15 years and 8.71% of particles initially west of 88°W (red and green) were located to the east of 

88°W after 15 years. 90.49% of all particles initially in the west and central portions (red and 

green) remained west of 88°W at depths below 1500m. Only 77.0% of eastern particles (blue) 

remained east of 88°W at depths below 1500m. These percentages indicate that there is little 

exchange of water across 88°W in the deep layer and that vertical exchange with the upper layer 

is occurring predominantly in the eastern basin. The lack of exchange between basins has been 

observed by Weatherly using PALACE floats at 1000 m depth (personnel communication, 2001) 

and by Lugo-Fernandez et al. (2001) from analysis of surface drifters in the GOM. The 

geographic restriction of tracer particles to the deep western basin is exemplified by the paths of 

particles 1007 and 4005, which is discussed in detail later in this chapter. 

By tracking individual particles, it can be seen that the upward motion of particles from 

below 2000 m to above 1000 m occurs almost exclusively in the ring separation region in the 

eastern GOM. Individual particles are observed to spiral upward with each separation of a new 

ring from the LC. Particle 16019 moves upward in a spiral that covers a lateral area of 

approximately 2° by 2° until the particle reaches 1000 m in year 27 (Figure 11). The circulation 

directly beneath the LC fluctuates between anticyclonic and cyclonic (Welsh and Inoue 2000) 

therefore the direction of rotation of the particle can change. Particle 19106 completed one and a 

half cyclonic rotations during year 25, and during year 26 particle 16109 completed two 

anticyclonic rotations. In year 28, particle 16019 was entrained in the LC at a depth of about 

1000 m and the lateral excursion of the particle increased as the particle completed nearly two 

anticyclonic rotations. There is a direct correlation between the north-south excursion of particle 

16109 and the ring separation index, which is defined as the water temperature at 87°W, 25.5°N 

and 125 m depth (Figure 12). In Year 27 the positive correlation of the particle’s latitude with 

the ring separation index shifts and becomes negative in years 28 and 29 when the particle is

entrained in the Loop Current. In the second half of year 29, a ring separates from the Loop 

Current and transports particle 16019 into the western GOM where it stagnates on the Mexican 

slope in year 30.
20

Figure 10. X-Z projection of particles according to starting location. Blue for particles initially

 east of 88oW, green for particles initially between 88oW and 92oW, and red for 

 particles initially west of 92oW.
21

Figure 11. Path of particle 16019: (a) plan view for years 24-30, (b) latitude-depth projection

 for years 24-30, (c) longitude-depth projection while 16019 was in the eastern basin, 

 and (d) longitude-depth projection after 16019 became entrained in Ring S9.

Figure 12. Time series of particle 16019 latitude (dotted) and temperature at 87°W, 25.5°N, 

 128 m.
22

Another example of particle movement in the deep eastern portion of the GOM is 

demonstrated by particle 17976, which was tracked for the entire 15 years (Figure 13). Particle 

17976 took almost nine years to reach 500 m, during which time the particle made tight spirals in 

the eastern basin. The spirals became wider in the east-west direction from 1000 m to nearly 400 

m in year 33 (Figure 13C) when particle 17976 became entrained in a LC ring in year 33 and 

migrated westward (Figure 13D). Particle 17976 was still near 400-m depth when it reached in 

the northwestern GOM and then took four years to ascend to 100-m depth. Many particles 

eventually rise above 500-m depth in the south-central and southwestern portions of the GOM, 

but the only particles that rose above 200-m depth were entrained in LC rings in the northwestern 

GOM. The upward motion of particles in the northwestern GOM is thought to be associated 

with the spin down of LC rings. The ascent of particles into the surface layer in the northwestern 

GOM is repeated in the following 3 experiments and will be discussed in more detail in sections 

3.2 through 3.4.

Particles that remain in the central and western portions of the deep GOM are observed to 

continually circulate. The 15-year paths of two particles that were initialized in the deep western 

basin are presented in Figure 14. Particle 1007 was initialized at 2800-m depth and primarily 

remained in a region that was over 3000 m deep in the southern portion of the western 

Figure 13. Path of particle 17976: (a) plan view for years 24-38, (b) latitude-depth projection 

 for years 24-38, (c) longitude-depth projection while 17976 was in the eastern basin, 

 (d) longitude-depth projection after 17976 became entrained in Ring S17 in year 33.
23

Figure 14. Paths of two particles in the deep GOM during years 24-38: (a) plan view of particle 

 1007, (b) longitude-depth projection of particle 1007, (c) plan view of particle 4005, 

 and (d) longitude-depth projection of particle 4005.

basin for the entire 15 years (Figure 14 A,B). Particle 4005 was initialized at 2500 m over the 

western Campeche escarpment, but moved over the region that is deeper than 3000 m during the

first part of year 25 and remained at almost exactly a depth of 2800 m for the next 13 years 

(Figure 14 C,D).

The rate at which particles ascend to 2000 m from 2200 m or below is approximately 

0.34% per year from Figure 8. At this rate it would take 294 years for all particles to reach the 

Yucatan Channel sill depth. The rate is slightly less for particles reaching 1000 m. If the 

flushing of these inert particles is representative of the turnover of waters in the deep GOM, then 

a model residence time for deep water is at least 300 years. Figures 11 through 13 demonstrate 

that the loss of particles from the deep is correlated with the frequency of ring separations from 

the LC. The average ring separation period in this model is 6.5 months, which is smaller than 

the average value of 9.5 months reported for 33 observed separation intervals (Sturges and Leben 

2000). The ascent of particles in the model is correlated with the separation of rings from the 

LC, which means the mean model separation interval is controlling the rate of flux of particles 

from the deep. Since the average interval of model ring separations is small, the model residence 

time of deep water is underestimated. If the average separation interval was longer, then the 

calculated rate of ascent would likely be reduced, and the model residence time would increase. 
24

3.2 Experiment 2: Surface Water Tracer Particle Release

The model is initialized with 28,294 particles - one per grid box in the top two layers 

centered at 12.5 m and 37.5 m. The positions of these particles were tracked every 10 days for 

year 24 through 29 (which were repeated from Experiment 1). Approximately 20% of all 

particles were flushed into the North Atlantic and less than 5% were flushed into the Caribbean 

(Figure 15). A low percentage of particles sunk below 50 m (whether the particles started at 12.5 

m or 37.5 m). The majority of particles fluctuate between the top two grid levels.

Figure 15. Percentages of particles found at specific depth ranges during Experiment 2.

Several particles that descended below 50 m were tracked to determine where they were 

sinking. Figures 16 and 17 show examples of two particles that descended ~70 m upon reaching 

the southwestern edge of the west Florida shelf during the latter part of model year 24. The

descent of these particles occurred for all of model year 25 and part or all of year 26. This 

behavior was observed for most particles entering the region bounded by 25°N:27°N and 

85°W:84°W. Some particles were stranded in this region and others moved away from the 

topography and became entrained in the LC. Particle 19829 became entrained in LC ring S6 at 

the end of year 26 and particle 7453 became entrained in LC ring S7 during year 27 and both 

particles migrated westward at a nearly constant depth. The descent of particles in this region 

did not appear to be correlated with fluctuations in the lateral area of the LC and the cause of 

sinking in this region was not determined. Sinking of some particles also occurs along the flanks 

of the entrance to the Southern Straits of Florida. 
25

Figure 16. Path of particle 19829 path during years 24-29 in plan view and cross-sectional view.

Figure 17. Path of particle 7453 path during years 24-29 in plan view and cross-sectional view.

Several particles that were initialized at 12.5 m remained in the upper 50 m during the 

entire 5 years. The starting location for particle 22825 was near 24.5°N and 86.5°W at 12.5 m. It 

crossed the GOM from ~87°W to ~95°W and back two and a half times while remaining above 

40 m. The particle ascended in the western GOM and descended in the eastern GOM, but 

remained at nearly the same depth as it migrated through the back and forth. This particle was 

twice transported into the entrance to the DeSoto Canyon where it completed several 

anticyclonic loops before moving southward and becoming entrained in another LC ring. The 

particle spent the entire fifth year in the northwest corner at between 27 m and 32 m. Particles 

7453 and 19829 also completed several anticyclonic spiral as they ascended in the water column 

in the eddy graveyard.
26

The physical processes associated with the spin down of warm core rings in the GOM 

have been examined by Schmitz and Vastano (1976) and Flierl and Mied (1985). A parametric 

model was used to compute diffusion and entrainment in simulated LC ring. The mean velocity 

field and streamline patterns were computed for a range of eddy coefficients. For the optimum 

case the water enters the ring at 1200 m, is transported upward, and flows outward either above

400m or below 800 m. There is a wedge-like layer between 400 m and 800 m where the velocity 

is directed inward. The 17° isotherm migrates upward by .8 m per day, which is equivalent to a 

net transport into the idealized ring of approximately 1 Sv (Schmitz and Vastano 1976). Flierl 

and Mied (1985) used numerical model to look at vertical radial circulation during the spin down 

of warm-core rings. In the case where the momentum diffusion dominates over buoyancy 

diffusion, there is inflow toward the center of the ring in the deep water (below 600 m), flow 

upward in the center, and outflow in the upper portion of the ring. The process of inflow in the 

lower layer and outflow in the upper layer with upwelling in the center may be applicable to the 

spin down of LC rings in the northwestern GOM. 

The results of Experiment 2 were difficult to analyze, since a small percentage of 

particles remained over deep water. Many particles were either flushed-out of the GOM or 

drifted into shallow shelf water. Only 15% of the particles remaining in the GOM were found 

below 50 m, thereby making it difficult to locate regions of sinking. Another approach to 

locating regions of downward motion in the upper layer would be to repeatedly seed the surface 

layer with particles and track them for shorter duration.

3.3 Experiment 3: Base of Mixed Layer Tracer Particle Release

Experiment 3 was designed to look at the locations and processes where the particles are 

transported from above the mixed layer to below the mixed layer. The mixed layer depth in this 

model was determined from vertical profiles of temperature, salinity, and density at six equally 

spaced locations along 25°N. The profiles represent the average of twelve years (years 24-35) of 

model data for January and July. The pycnocline lies between 130 m and 180 m in summer and 

nearer to130 m in winter, which compares well with the climatological data used to initialize the 

model. This might be expected since the model was initialized with annual mean hydrographic 

data, but the model profiles represent 12-year seasonal means after 23 years of spin-up.

For Experiment 3 the model was seeded with 18814 particles at levels 5 and 6, centered 

at 128.1 m and 181.5 m, respectively. The experiment was run for 3 years of model time (years 

24 through 26 were repeated). By the end of year 26, more than 60% of particles were located 

above 100 m, nearly 30% were located between 100 m and 200 m, and less than 10% were 

located below 200 m (Figure 18). Particles crossed the 200-m isobath in the ring separation 

region, along the ring migration paths, the Campeche Escarpment, the Texas-Louisiana slope, the 

DeSoto Canyon, and the entrance to the Southern Straits of Florida. Tracer particles were 

observed to cross the 250-m isobath in the ring separation region, Southern Straits of Florida and 

the Texas-Louisiana slope.
27

Figure 18. Percentages of particles found at specified depth ranges during Experiment 3.

 

The index numbers of several particles that crossed the 200-m isobath in the ring 

separation region were identified and the paths of these particles were tracked for the entire three 

years (Figure 19). All particles started in a small patch along the northeastern slope of the 

Yucatan Peninsula. Within the first 10 days after initialization, the entire patch of particles 

descended along the slope toward the north and then became entrained in LC ring S1. Except for 

particle 14735 (Figure 19F), the particles were located at approximately 26°N, 92°W at the end 

of the first year. The depth at which westward migration occurred ranged from 180 m to 270 m. 

After the first year the particles paths greatly diverged. The greatest ascent of particles takes 

place in the northwestern GOM as the particles move up over the Texas slope. The particles tend 

to remain at nearly the same depth as they transit the deep water away from the boundaries.

Rapid vertical motion was observed by particles 14727 (Figure 19 E) and 15049 (Figure 

19C) as they encountered the upper slope off of Louisiana between 90°W and 93°W during the 

second half of year 25. Particle 14727 ascended from a depth of 210 m on day 200 to 160 m on 

day 260 at fairly even rate of upward motion. Particle 15049 first descended from 160 m on day 

220 to ~200 m on day 240 and then began to ascend again on day 300 to nearly 140 m on day 

350. The circulation in this region was dominated by the presence of LC ring S3 and a cyclonic 

eddy that formed on the eastern side of the ring (Figure 20). The particles 14727 and 15049 

were adjacent to the vertical wall between levels 5 and 6 with a base of 212.5 m. Particle 14727 

ascended along the northeastern margin of the cyclonic eddy and particle 15049 descended along 

the northeastern margin of ring S3. In Figure 20 there is an area of upwelling near 93°W, 27.7°N 

at the location where particle 14727 was observed to ascend. Two months later on day 300 of 

year 25, ring S3 and the cold-core eddy had migrated westward (Figure 21). Particle 14727 then 

moved eastward at a nearly constant depth, but particle 15049 was now positioned along the 

northern edge of the cyclonic eddy and had begun to ascend. In Figure 21 there is an area of 

upwelling near 90.5°W, 27.8°N on day 300 in the region where particle 15049 was ascending.
28

Figure 19. Plan view and longitude-depth cross-section of particles (a) 14737, (b) 14892, and 

 (c) 15049. The year of observation is indicated.
29

Figure 19. Plan view and longitude-depth projection of particles (d) 14579, (e) 14727, and

 (f) 14735. The year of observation is indicated.
30

Figure 20. Velocity (cm s-1) and temperature (oC) at 181.125 m on model year 25, day 240. 

Figure 21. Velocity (cm s-1) and temperature (oC) at 181.125 m on model year 25, day 300.
31

There are several possible explanations for the vertical motion of these particles. The fact 

that they did cross isopycnals during their ascent and descent suggests that the particles were 

moving within a bottom boundary layer, except that they were against a vertical wall. Fluid was 

being transported toward the wall in the vicinity of particle 14727, and the convergence of the 

flow at the boundary may have generated vertical velocities resulting in vertical transport. As the 

fluid encountered the wall, the fluid may have moved upward rather than downward because of 

the topography below. The opposite occurred for particle 15049, which moved down as fluid 

was transported away from the boundary. It is reasonable for the fluid to be drawn down from 

above because of the topography below. 

Another explanation for the vertical displacements of particles against the wall could be 

that the particle tracing method is not reliable near vertical walls in the presence of strong flows. 

An argument against this is that the particle displacements were consistent and reasonable in 

terms of direction, distance displaced, and rate of movement.

3.4 Experiment 4: Level 11 (1000 m) Particle Release

From previous modeling studies and observations, 1000-m appears to be a transition 

depth from the LC/LC ring dominated upper layer to the vertically homogeneous lower layer. 

Strong mid-depth currents have been reported by SAIC, NOAA, and Texas A&M (Nowlin et al. 

2001). Sporadic strong currents have been observed during offshore exploration by the oil and 

gas industry at slightly shallower depths. Particles released in Experiments 1 through 3 were 

found at 1000 m near the boundaries or in the eastern GOM. This experiment was designed to 

provide information about the vertical and lateral advection of particles near 1000-m depth. The 

simulation of model years 24 through 26 was repeated with 7594 particles initialized at the 

center of level 11 (1000 m). Over the three-year period, more than 70% of all particles which 

were initially in the east region within level 11 moved above 850 m or below 1150 m. Less than 

6% descended below level 11. The remaining ~64% were found within level 10 (between 550 m 

and 850 m) at the end of three years. Approximately 20% of all particles that originated in the 

central region and 10% of all particles that originated in the west region ascended to level 10. A 

negligible number of particles from the west region descended below level 11 and less than 3 % 

of particles from the central region descended below level 11.

As in the previous three experiments, individual particles were tracked to determine 

where particles were moving vertically in the water column and why. The greatest vertical 

motion was found in the ring separation region between 85°W and 87°W at 25°N. Significant 

vertical motion was also observed in the northwest GOM and in the Southern Straits of Florida.

Lateral motion of particles released at 1000 m was quite interesting. Particle paths in the 

Campeche Bay had different characteristics depending on whether they were over water depths 

greater than 3000 m or if they were over the slope. Particles observed over the deep, relatively 

flat bottom, moved in large cyclonic gyres (200 to 250 km diameters) with tight cyclonic spirals 

imbedded in their paths (Figure 22). Over the slope, the particle paths became elongated ellipses 

primarily in the southwest to northeast direction (Figure 23). Particle 4100 made repeated 

cyclonic loops as it migrated along the Campeche Escarpment from the southern Campeche Bay 

to the northern tip of Campeche Bank over a three year period. Particle 3100 oscillated back and 

forth over the 3-year period and was positioned southwest of its starting point after three years.
32

Figure 22. Particles (a) 1500 and (b) 2500 in the Campeche Bay during model years 24-26. 

 These particles remained at depths between 925 m and 1010 m.

An examination of particle paths over the northern slope of the GOM was also made. A 

line of particles from 93.95°W to 86.45°W at 26.95°N with .5 degree spacing was tracked for 3 

years. Particles west of 93°W moved immediately south and west. Two of these particles 

remained in the northwest GOM, but one particle moved into the Campeche Bay and completed 

several cyclonic loops similar to the paths of particles described. Particles from 89.5°W to 

92.5°W moved to the northeast into the DeSoto Canyon. Particles east of 89.5°W moved 

southward into the ring separation region of which 4 particles ascended to above 750 m. These 

four particles made the largest vertical excursion of all the particles.
33

Figure 23. Particles 1100 and 3100 in the Campeche Bay during model years 24-26. These

 particles were initialized at 1000 m.

3.5 Experiment 5: Eulerian Framework Study at 2200 m

During the first four experiments the inert particles were tracked as they moved freely 

about the domain. This method of tracking particles was not suitable for examining the changes 

in the circulation at fixed locations over time. This experiment was designed to observe the 

range of particle displacements over time by repeatedly seeding the same locations with particles 

at a fixed interval in time. 116 locations were chosen directly over the slope at a depth of 2200 

m in water of depths less than or equal to 2800 m. Particles were released at the beginning of 

each month at each of the 116 locations and tracked every 10 days for 30 days over a two-year 

period. 

The particle motion is primarily along slope around the entire basin (Figure 24). 

Topographic rectification is apparent along the 2200-m isobath from 87°W to 95°W along the 

northern slope and again from 88°W to 95°W along the southern slope. The maximum lateral 

displacements observed along the western slope over the 30-day sequences are nearly 2 degrees 

of latitude (~200 km), which is equivalent to sustained speeds of nearly 8 cm s-1. The maximum 

straight-line lateral displacements over a 30-day period in the north central GOM and the south 

central GOM are approximately 100 km and 150 km, respectively. The maximum rate of 

displacement by particles in these regions was 4 to 6 cm s-1 over 30 days.
34

Figure 24. 30-day particle displacements for each month in model year 25.

The method of repeatedly initializing the particles at a given location every 30 days was 

very useful for examining the vertical displacements of particles near the 2000-m isobath. 

Along-slope 30-day particle displacements versus depth are plotted for each of three regions in 

Figure 25. The vertical displacements appear large compared to the lateral displacements, but 

the range on the vertical axis is only 800 m, compared to more than a range of 120 km on the xaxis. The largest vertical displacements are in the entrance to the South Florida Straits near 

84°W and just below the sill of the Yucatan Channel (Figure 25A). The vertical displacements 

along the western slope between 22°N and 25°N are generally less than ~200m. In the latitude 

band between 25°N and 28°N, there is a cluster of larger than average vertical displacement 

vectors near 92W where the Sigsbee Escarpment protrudes southward (Figure 25B) and again at 

95°W. The average 30-day vertical displacement vectors along the northern slope are less than 

~100m. Along the western slope (west of 94.5°W) (Figure 25C), the average vertical 

displacement is the smallest of the three regions (except for the northwest corner). The particle 

trajectories along the western slope south of 25°N exhibit a greater cross-slope component in the 

x-y projection. Particle displacements may be affected in this region by LC rings and deep eddies 

are colliding with the western slope.
35

Figure 25. Vertical displacements of particles during model years 24 and 25 of Experiment 5:

 (a) all particles between 22oN and 25oN, (b) all particles between 25oN and 28oN, 

 and (c) all particles west of 94.5oW.

 a)

 b)

 c)
36

Particle displacements are oriented along slope and appear to oscillate back and forth. 

The time dependence of these oscillations was examined by plotting the 30-day along-slope 

component versus time for several particles in different locations (Figure 26). There is a strong 

correlation between the times each current switched direction and the shedding of a LC ring. In 

the far eastern GOM at 84.35W, 23.45N the current switched from east to west immediately 

following the separation of rings S1 through S4 (Figure 26A). At the base of the Mississippi Fan 

(-87.95W, 26.25N) there is a shift from westward currents to eastward current coinciding with 

the separation of rings S1 through S3 (Fig 26B). There is some variability in the timing of these 

events due to the variability of the ring size and proximity of the rings to the bathymetry. Over 

the Sigsbee Escarpment at 91.15W, 25.85N the currents flow toward the west at the time of ring 

separation and switch to the east between shedding events (Fig. 26C).

 Figure 26A. 30-day longitude displacement vectors for particle 112 at 

 84.35oW, 23.45oN.
37

 Figure 26B. 30-day longitude displacement vectors for particle 77 at 

 87.95oW, 26.25oN.

 Figure 26C. 30-day longitude displacement vectors for particle 54 at 

91.15oW, 25.85oN
38

CHAPTER 4

MEAN CIRCULATION

4.1 Deep Mean Hydrographic and Velocity Fields

One of the main objectives of this project has been to look at ventilation of the deep 

water. The area beneath the LC was identified as an important region for the upward flux of 

particles from the deep waters. Downward flux appears to occurring primarily along the 

boundaries. Water mass distribution in the deep was examined by plotting the 12-year average 

hydrographic and flow fields at various levels (Figure 27). In the near-surface level the LC 

extends northwestward into the central GOM transporting relatively warm, saline water (Figure 

27A). Upwelling at the Catoche Tongue of the Yucatan Peninsula greatly influences the 

hydrography of the entire Campeche Bank. The eddy graveyard is apparent in the mean as an 

anticyclone in the northwestern corner of the GOM. A weak cyclone is observed in the western 

half of the Campeche Bay

At 1000 m depth the most noticeable features in the temperature field is the upwelling of 

relatively cool, fresh water around the eastern side of the Yucatan Peninsula (Figure 27B) which 

extends westward from the tip of the Yucatan Peninsula. An area of relatively warm water is 

observed in the northwest GOM from 1000 m to 1600 m (Figure 27B). There is a depression of 

the isotherms in this region due to the anticyclonic circulation of the decaying LC rings. This 

region is also an active area for vertical motion of tracer particles as evidenced in Experiments 1 

through 4.

A band of relatively cool water is observed along the slope off the southwest coast of 

Florida at every level down to 3100 m. From 1600 m to 1900 m cool water appears to enter 

through the Yucatan Channel along the northwestern tip of Cuba. The relatively cool water is 

then advected northward along the eastern side of the mean cyclonic circulation. An anticyclone 

is positioned just to the northwest of the mean cyclonic flow. This mean cyclone-anticyclone 

pair is highly coherent in the vertical from 1600 m to 3100 m and is contained by the 3000-m 

contour. Relatively cool water also appears to be advected southward by the eastern side of the 

mean anticyclone and is possibly being upwelled by the interaction of the current with the slope. 

The coldest water is observed on the eastern side of the juncture of the cyclone-anticyclone and 

is advected toward the southwest between the centers of the cyclone-anticyclone pair.

From 2200 m through 2800 m mean cyclonic flow is observed along the 3000-m contour 

in the western and central basins to the west of 86°W. This mean cyclonic flow is concentrated 

at or very near the boundary at 3100 m and 3400 m. The temperature gradients in the deep are 

extremely small (.017°C at 3100 m and .005°C at 3400 m), but the temperature patterns are 

repeated from 2500 m to the bottom. The isotherms tilt up to the north in the central Gulf, which 

is consistent with the cyclonic circulation around the basin. These modeling observations are 

consistent with the theory that North Atlantic Deep Water overflows the Yucatan Sill on the east 

side, flows counterclockwise around the perimeter of the GOM, and returns to the Caribbean 

through the west side of the Yucatan Channel (Maul et al. 1985).
39

Figure 27A. 12-year average model temperature (oC) and velocity vectors (cm s-1) at 12.5 m.

Figure 27B. 12-year average model temperature (oC) and velocity vectors (cm s-1) at 1000 m.
40

Figure 27C. 12-year average model temperature ((T-4)*100oC) and velocity vectors (cm s-1)

 at 2200 m.

Figure 27D. 12-year average model temperature ((T-4)*100oC) and velocity vectors (cm s-1)

 at 3100 m.
41

The mean cyclonic circulation around the deep basin below 2000m is in good agreement 

with University of Colorado Princeton Ocean Model (CUPOM) output for years 1993 through 

1999 (Nowlin et al. 2001). Another similarity between these models is the anticyclonic current 

flowing along the 1000 m contour off the Mexico slope at 24N to the Louisiana slope near 90W. 

An important feature of both models is the deep mean cyclonic gyre centered at approximately 

24.5N and 85W, which has a companion anticyclonic gyre positioned just northwest of the 

cyclone. There are differences in the mean surface circulation between this model and CUPOM. 

The mean position of the LC is similar, although the LC extends farther west in the this model 

resulting in a mean westward current in the center of the GOM and an eastward current south of 

Louisiana between 26N and 28N. The CUPOM model circulation features an anticyclone 

centered over the axis of the DeSoto Canyon at 88W, 29N whereas this model produces mean 

flow up the axis of the DeSoto Canyon. Other differences include a mean anticyclonic gyre in 

the CUPOM model centered at ~23.5N over the Mexican Slope, a mean cyclonic gyre in the 

south-western Campeche Bay in this model, and a larger anticyclonic gyre in the ‘eddy 

graveyard’ in this model, that is much weaker in the CUPOM model. Many of the differences in 

the surface features may be due to the surface fluxes of heat, salt, and momentum. The CUPOM 

model is a data assimilation model that was used to hindcast currents in the GOM from 

observations. This project used climatological mean fields to force the model in an effort to 

reproduce the LC system, as well as the prevailing seasonal circulation features.

4.2 Kinetic Energy Distribution

By initializing the model with particles at 2200 m and recording their 30-day 

displacements, particle motion was observed to be primarily along the slope. This was especially 

true in the central and western basin of the GOM. The deep mean flow fields described in the 

preceding section suggest that there is cyclonic flow around the perimeter of the deep central and 

western portions of the GOM. Significant advection of particles could result from energetic 

events and wave phenomena. Recent efforts to model deep water in the GOM, including several 

projects funded by Minerals Management Service, primarily focus on energy levels associated 

with Topographic Rossby Waves (TRW) and jets. From the viewpoint of operational end 

environmental problems in the deep water, advection resulting from energetic events and wave 

phenomena is one of the primary issues. Therefore, in addition to examining particle advection, 

the kinetic energy levels in the deep are examined for evidence of wave phenomena.

The velocity at each grid point in the lower levels was saved every three days for three 

years during Experiment 5. The eddy kinetic energy averaged over three years was vertically 

coherent from 2500 m through 3100 m, with slight bottom intensification in the regions of 

highest eddy kinetic energy. The bands of eddy kinetic energy are within the LC ring formation 

region, offshore of the DeSoto Canyon, and along the slope in the central and western portions 

(Figure 28).
42

Figure 28. The 3-year average eddy kinetic energy at a) 2500 m and b) 3400 m.
43

The velocity data was low pass filtered with a cutoff of 180 days to look at the kinetic 

energy associated with the low frequency events, such as eddy shedding. The average kinetic 

energy computed using the low-pass filtered data is highest in the ring separation region and has 

smaller maxima along a band from 23°N to 25°N along the western slope (Figure 29). The 

original velocity data was also band pass filtered between 40 days and 10 days and used to 

compute the kinetic energy averaged over the three-year period. The band-passed data has 

several localized maxima (Figure 30). The maxima near 88°W, 92°W, and 95°W may be 

associated with generation regions of TRW=s. 

Values for the mean kinetic energy were compared for the bottom three grid levels at 

seven one-degree intervals of longitude along 25.5oN. Bottom intensification of the mean kinetic 

energy computed from the high-pass filtered velocity fields was observed at all seven locations. 

The kinetic energy computed using the low-pass filtered velocity fields was analyzed for the 

bottom levels at the same geographic locations. The kinetic energy exhibited a slight increase 

toward the bottom for the low-pass filtered velocities at only 95oW, 92oW, 91oW and 89oW, but 

not at 94oW, 93oW, and 90oW. The bottom intensification of the high-pass filtered velocity field 

along the northern slope is consistent with the propagation of TRW’s.

 Figure 29. Low-pass filtered (180 day cut-off) kinetic energy at 2800 m 

averaged over 3 years.
44

 Figure 30. Band-pass filtered (40 to 10 days) kinetic energy at 2800 m 

 averaged over 3 years.
45

CHAPTER 5

DISCUSSION AND SUMMARY

5.1 Particle Paths in the Deep Water

The model was seeded below 2000 m with 19,105 inert particles that drift freely within 

the model domain in response to the velocity field. By tracking individual particles, it can be 

seen that the upward motion of particles from below 2000 m to above 1000 m occurs almost 

exclusively in the ring separation region in the eastern GOM. Individual particles are observed 

to spiral upward with each separation of a new ring from the LC. An anticyclone-cyclone eddy 

pair develops in the deep eastern basin each time the LC reaches its northernmost extent and 

sheds a ring. The tracer particles are advected away from the Campeche Bank into the deep 

water of the eastern basin by the northward currents on the western side of the leading 

anticyclone. A southward current associated with the western side of the trailing cyclonic eddy 

moves the particles back toward the Campeche Bank. The particles remain at nearly the same 

depth as they are advected away from the slope, but they move slightly upward in the water 

column as they are pushed back toward the slope. With each separation of a LC ring, the 

particles experience a net upward motion.

The deep eddy pairs, or modons, that are generated in the deep eastern basin propagate 

westward. There is a constriction in the bottom topography between 86.5oW and 88.5o that 

forms an east-west channel below 3000 m. Since the deep barotropic motions ‘feel’ the bottom, 

the eddy pairs become compressed in the north-south direction and squeeze through the channel. 

The eddy pairs reform on the western side of the channel and continue to propagate westward. 

Individual deep eddies can sometimes be tracked all the way to the western slope, and other 

times they become indistinguishable from the background eddy field. Although the particles in 

the deep are continually moved about in response to the deep eddy field, the velocities are weak 

compared to the upper layer eddies (<20 cm/s) and the deep eddies only transport the tracer 

particles short distances. During a 15-year simulation in which the waters below 2200 m were 

seeded with nearly 20,000 particles, fewer than 10% of particles were transported across 88oW in 

either direction. The advective properties of deep eddies may change as they squeeze through 

the deep channel, which would limit the exchange of fluid from the eastern and central basins. 17 

Profile Autonomous Lagrangian Circulation Explorer (PALACE) floats were deployed in the 

northern GOM that were designed to drift at approximately 900 m. The floats were designed to 

remain at a nearly constant depth for 6.23 days and then come to the surface for 11.5 hours to 

transmit their geographic position to polar orbiting satellites. Weatherly (personal 

communication 2000) analyzed the trajectories of these floats and noted that there was limited 

exchange of the floats across 89oW at 900 m.

There are complex bottom topographic features over the slope that range in magnitude 

from carbonate mounds and salt domes (tens of meters) to submarine canyons (several 

kilometers). The presence of rough bottom topography implies possible topographically-induced 

current instability and enhanced vertical mixing (e.g., Polzin et al. 1997). To the west of the 

Mississippi Delta in water depths of approximately 3000 m a field of sedimentary furrows have 
46

been observed by Bryant at Texas A&M. These long, narrow channels which are oriented 

parallel to the escarpment and cover ~100 km may be attributed to strong near bottom flows.

During a recent current meter deployment near a depth of 2000 m the northern GOM 

occasional strong flows of 85 cm s-1 were measured (Hamilton and Lugo-Fernandez 2001). 

These current measurements were recorded only 10 m from the bottom while the LC was 

shedding an eddy. Hamilton and Lugo-Fernandez (2001) conclude that the LC and LC frontal 

eddies are forcing the deep layer in such a way as to generate TRW’s that propagate westward 

with periods of 10-14 days. This study also provided evidence that near bottom currents of 40-

50 cm⋅s

-1

 are occurring at a regular rate as wave trains continually pass over the site. A numerical 

study of TRW propagation in the northern GOM by Oey and Hamilton (2000) indicated that 

there is a concentration of kinetic energy in the 20 to 100 day band along the 3000-m isobath 

extending westward from under the LC. 

The numerical model was seeded with particles close to the bathymetry at 2200 m. 

Particle positions were saved every 10 days for 30 days over a 2-year period with the particles at 

the same location at the beginning of every month. The particle motion near the slope exhibited 

topographic rectification and individual particles were transported as far as 50 km in 30 days. 

The direction of particle motion along the northern slope is highly correlated with the proximity 

of the LC and the passage of LC eddies. The vertical excursion of the particles over each 30-day 

interval was generally less than 20 m, but on one occasion did exceed 100 m. Since the sampling 

interval is10 days the particle tracers could not be used to detect TRW activity in the 10-14 day 

range.

Daily snapshots of model velocity fields were saved at all levels from 2200 m to 3700 m 

during a three-year simulation in order to compute kinetic energy in the deep water. There is 

relatively high concentration of eddy kinetic energy in bands along the slope below 2200 m in 

the model output. Slight bottom intensification of the kinetic energy in these bands in consistent 

with TRW propagation. The analyses of the model currents, hydrographic fields, and tracer 

particles strongly indicate that the deep currents are coupled with overlying upper layer features 

in the eastern GOM.

5.2 Particle Paths in the Upper Layer

Many particles eventually rise above 500-m depth in the south-central and southwestern 

portions of the GOM, but the only particles that rise above 200-m depth are entrained in LC rings 

in the northwestern GOM. The upward motion of particles in the northwestern GOM is thought 

to be associated with the shoaling of isotherms during the spin down of LC rings. The ascent of 

particles into the surface layer in the northwestern GOM was examined in more detail by 

initializing the center of each grid box in the top two grid levels with 28,294 particles and 

tracking their positions for 5 years.

The ultimate goal of Experiment 2 was to see if particles that were initially near the 

surface (above 50 m) would descend into the lower layer below 1000 m. Less than 20% of all 

particles were observed below 50 m at any time. The majority of particles fluctuated between the 

top two grid levels. Several particles that descended below 50 m were tracked to determine 

where they were sinking and what processes could be responsible. Particles that descended 

below 100 m were near the LC or LC rings. The few exceptions were in the southern Campeche 

Bay, the edge of the west Florida shelf, the northwestern coast of Cuba, and the Yucatan 
47

Channel. Of the 28,294 particles, only 4 descended below 150 m during the 5-year simulation. 

One of these particles was located on the outer west Florida shelf near 84°W between 25°N and 

27°N and the other three were on the northwest tip of Cuba. All particles that moved onto the 

outer west Florida shelf between 25°N and 27°N were observed to slowly descend from above 

20 m to nearly 80 m over periods of up to two years. This slow descent takes place over several 

ring separation cycles and is not related to a single energetic event. Particles that entered the 

eddy graveyard typically moved upward in the water column as they completed several 

anticyclonic spirals, although many became entrained in the current that extends eastward along 

the Louisiana shelf break.

Segregation between the eastern and western basins of the GOM was observed in the 

behavior of 12,542 surface drifters that were released from 1955-1987 (Lugo-Fernández et al. 

2001). Drifters that were recovered along the coast in the eastern GOM originated in the eastern 

GOM, but drifters that were recovered along the coast in the western GOM may have originated 

in either the eastern or western GOM. Lugo-Fernández et al. (2001) attribute this distribution of 

landings to prevailing easterly winds. The results from the second model experiment featuring 

the initialization of particle tracers in the upper 50 m of the water column are in good agreement 

with these observations. The following similarities are noted between the model particle tracers 

and the surface drifters: a general movement of particles from east to west; westward movement 

of particles along the outer shelf off of Louisiana during winter; and avoidance by particles of the 

west Florida Shelf. Eastward transport of particles in the upper layer during the first 60 days of 

the model simulation occurs near the coast of the northern GOM and along the northern edge of 

the LC. The transport of model tracer particles from the eastern basin to the central and western 

basins occurs primarily due to westward migration of eddies. At the end of 60 days, none of the 

model tracer particles that originated west of 92°W had moved to the east of 91°W, while 

particles initially to the east of 88°W were transported into the central GOM within a LC ring or 

were advected westward by the upwelling plume over the Campeche Bank.

Experiment 3 featured the initialization of particles at depths of 128 m and 181 m and 

provided interesting results on the behavior of particles over the outer shelf and upper slope. 

Vertical motion over steep topography was observed in response to strong flows. A patch of 

particles was observed to descend from ~181m to ~240 m during the first 10 days of model year 

24 at approximately 87°W, 23°N when the LC was flowing directly over this location with 

current speeds greater than 1 m/s. The vertical motion of tracer particles over the upper 

Louisiana slope coincided with the passage of a cold-core eddy and a LC ring that produced 

strong currents up against the slope. 

5.3 Mixing and Ventilation of the Deep Water

The deep waters of the GOM are well oxygenated (> 5.4 ml/l) with the highest nearbottom values measured in the eastern GOM and the lowest values in the far western GOM. 

(Berbian and Cantillo 1999). The residence time of deep water GOM is estimated to be only 100 

years from both simple volume flux calculations and analysis of barium concentrations in the 

deep sea (Buerkert 1997). Since deep water below the sill depth is completely isolated from 

outside, the only mechanism to ventilate the deep water must be via vertical mixing. The results 

from Experiment 2 indicate that there is very little downward mixing of the surface waters. Less 
48

than 10% of all particles initially at a depth of 12.5 m descended below 50 m and only 4 particles 

out of more than 28,000 that were initially above 50 m descended below 150 m during a 6 year 

simulation (Figure 15). The results of Experiment 2 were influenced by the fact that many 

particles were flushed out of the GOM or became stranded near the coast. 

The final experiment was designed to observe the particle tracer behavior near 2000 m 

over the slope and has provided the best indication of how oxygen-rich water is reaching the 

deep GOM. Upper North Atlantic Deep Water (UNADW) is an oxygen-rich water mass (>6. 

ml/l) that flows into the Cayman Basin of the Caribbean Sea over the Windward Passage (sill 

depth 1688 m) (Wust, 1964). UNADW then mixes with Caribbean water and flows into the 

Yucatan Basin. Maul et al. (1985) consider the possibility that UNADW flows into the GOM on 

the eastern side of the Yucatan Channel to explain the difference between T-S properties at sill 

depth in the Florida Straits and Yucatan Channel. Since the water in the Caribbean below the sill 

depth of the Yucatan Channel is cooler than water at the same depths in the GOM, it is natural to 

expect that the water flowing over the Yucatan sill would sink below sill depths. The largest 

vertical particle displacements occurred in the eastern GOM in Experiment 5. The greatest 

descent was observed to occur in over the steep topography north of the Yucatan Channel and 

west of the Southern Straits of Florida. 

From the 12-year mean temperature and salinity fields, it appears that relatively cold, 

salty water flowing over the sill of the Yucatan Channel, hugs the eastern side of the channel and 

then flows northward along the eastern side of the basin. The cold water continues to follow the 

bathymetry and flow northward and then westward at progressively greater depths. Relatively 

warm water appears to upwell over the northern Campeche Bank at the same depths. The Loop 

Current flows over the sill at a depth of 1400 m into water depths greater than 3000m. The 

upwelling may be caused by the Loop Current entraining water just below sill depth on the 

northern side of the sill as the Loop Current moves over deeper water. A generally east-west 

temperature gradient is observed in the model mean fields from 2500 m through 3100 m. A 

barotropic, cyclonic current that follows the 3000-m isobath is present in the 12-year mean 

velocity fields. A conceptual model for the ventilation of the deep water has developed from this 

study that features downwelling of relatively cold, salty water in the far eastern basin, the deep 

mean cyclonic current, and upwelling of older, relatively warm water onto the Campeche Bank 

beneath the western limb of the LC. The dissolved oxygen that is present in the deep water of 

the GOM is likely derived from the relatively cold, highly oxygenated mixture of UNADW and 

Caribbean water overflowing the Yucatan sill. This conceptual model is nearly identical to the 

explanation by Maul et al. (1985) as to why T-S properties at the sill of the Florida Straits and 

the sill of the Yucatan Channel are different. It would seem at first that there should be a net loss 

of heat from the deep layer if cold water was added in the far eastern basin and warmer water 

was being extracted over the northern Campeche Bank. The loss of heat from the deep waters in 

the eastern basin may be balanced by the transport of heat to the western GOM by LC rings
49

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