OCS Study 

MMS 2005-031

 

Climatology of Ocean Features 

in the Gulf of Mexico 

Final Report 

U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 
OCS Study 

MMS 2005-031 

Climatology of Ocean Features 

in the Gulf of Mexico 

Final Report 

Author 

Fred M. Vukovich 

Prepared under MMS Contract 

1435-01-03-PO-74302 

by 

Science Applications International Corporation 

615 Oberlin Rd., Suite 100 

Raleigh, North Carolina 27587 

Published by

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region New Orleans

June 2005 
DISCLAIMER 

This report was prepared under contract between the Mineral Management Service 

(MMS) and Science Applications International Corporation (SAIC). This report has been 

technically reviewed by MMS and approved for publication. Approval does not signify 

that the contents necessarily reflect the views and polices of MMS, nor does mention of 

trade names or commercial products constitute endorsement or recommendation for use. 

It is, however, exempt from review and compliance with the MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of this report may be obtained from Public Information Office (Mail Stop 

5034) at the following address: 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Office (MS 5034) 

1201 Elmwood Park Blvd. 

New Orleans, LA 70123-2394 

Telephone Number: 1-504-736-2519 or 

 1-800-200-GULF 

CITATION 

Suggested citation: 

Vukovich, F.M., 2005. Climatology of Ocean Features in the Gulf of Mexico: Final 

Report. U.S. Department of the Interior, Minerals Management Service, Gulf of 

Mexico OCS Region, New Orleans, LA. OCS Study MMS 2005-031 58 pp. 

ABOUT THE COVER 

The cover picture shows a series of NOAA/AVHR SST images depicting warm core 

rings that separated from the Loop Current in the Gulf of Mexico in 1997, 1998, and 

1999. 

iii
 ABSTRACT 

The objective of this project is to develop a climatology of certain ocean features in the 

Gulf of Mexico (GOM) using data sets that span long time periods (i.e., as long as 32 

years in some cases) to create the statistics. Twelve (12) separate statistics were created, 

some of which described characteristics of the Loop Current, while others were involved 

with warm core eddies that separate from the Loop Current and cold core eddies. The 

data resources used to create this climatology included analyzed in-situ data from ships of 

opportunity, from field programs in the GOM, and from the projects managed by the 

various oil and gas companies. However, the principal data used for this study were 

satellite remote sensing data, which included sea-surface temperature data from 

radiometers aboard TIROS-M, HCMM, SEASAT, and the numerous NOAA polar 

orbiting satellites; ocean color data from the CZCS, which was aboard the Nimbus polar 

orbiting satellite, SeaWiFS, and MODIS; and altimeter data from the TOPEX/Poseiden 

and the ERS instruments. These data were used to develop ocean front analyses in the 

GOM, which provided the “characteristic” position of the fronts for a given month. It is 

expected that these statistics will be useful in planning field programs, to marine 

biologists, to oceanographers, as background information for environmental impact 

statements (EISs), in evaluating and directing improvements for models that calculate the 

ocean dynamics in the GOM, and for oil and gas operations in the GOM. 

ACKNOWLEDGMENTS 

The author wishes to thank Dr. Carole Current, the MMS Contracting Officer’s Technical 

Representative, and Dr. Alexis Lugo-Fernandez for their continuing support of this 

project. I would also like to thank the people, too numerous to cite, who contributed 

information and data that helped create the continuous datasets from which many of the 

statistics cited in this report were derived. 

v
TABLE OF CONTENTS 

Page 

ABSTRACT..................................................................................................................... v

ACKNOWLEDGMENTS ............................................................................................... v 

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

LIST OF TABLES...........................................................................................................xi 

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

1.1 Objective and Data............................................................................................... 1 

1.2 Frequency of Loop Current Water in the EGOM................................................ 2 

1.3 Frequency of Loop Current Water on the West Florida Shelf............................. 2 

1.4 Frequency of the Position of WCR Centers in the GOM .................................... 5 

1.5 Summary of Findings........................................................................................... 7 

2.0 INTRODUCTION ..................................................................................................... 9 

3.0 ANALYSIS OF FRONTS IN THE GOM.................................................................11 

4.0 THE LOOP CURRENT AND LOOP CURRENT WATER.....................................13 

4.1 Frequency of Loop Current Water in the EGOM................................................13 

4.2 Orientation of the Loop Current ..........................................................................14 

4.3 Frequency of Isolated Warm Water in the EGOM ..............................................17 

4.4 Frequency of Loop Current Water on the West Florida Shelf.............................21 

5.0 WARM CORE RINGS..............................................................................................25 

5.1 Frequency of Ring Separation .............................................................................25 

5.2 Ring Path..............................................................................................................31 

5.3 WCR Speed in the WGOM..................................................................................36 

5.4 Decay of Ring Size ..............................................................................................36 

5.5 Frequency of WCR Water in the WGOM ...........................................................40 

5.6 Frequency of the Position of WCR Centers in the GOM.........................................43 

6.0 COLD CORE RINGS................................................................................................47 

6.1 Frequency of the Position of CCR Centers in the GOM. ....................................47 

7.0 SUMMARY OF FINDINGS.....................................................................................53 

8.0 REFERENCES ..........................................................................................................55 

APPENDIX A-ACRONYM LIST...................................................................................59 

vii
LIST OF FIGURES 

Page 

Figure 1.1 Spatial frequency (%) of the water mass associated with the LC in 

the EGOM based on data for the period 1976-2003......................................3 

Figure 1.2 Spatial frequency (%) of LC water on the West Florida Shelf based 

on data for the period 1976-2003...................................................................4 

Figure 1.3 Spatial frequency (%) for the location of WCR centers using a 27-

year (1977-2003) database.............................................................................6 

Figure 3.1 Example of an analysis of fronts in the GOM................................................11 

Figure 4.1 Spatial frequency (%) of the water mass associated with the LC in 

the EGOM based on data for the period 1976-2003......................................14 

Figure 4.2 NOAA/AVHRR SST composite image for 31 January 2002. .......................15 

Figure 4.3 Depiction of method used to determine the Loop Current orientation 

angles. ............................................................................................................16 

Figure 4.4 Frequency distribution for the Loop Current orientation angles....................17 

Figure 4.5 NOAA/AVJRR SST image for 16 February 1999 depicting warm 

water pools in the EGOM ..............................................................................18 

Figure 4.6 Spatial frequency (%) of isolated warm pools in the EGOM based on 

data for the period 1976-2003........................................................................19 

Figure 4.7 SeaWiFS ocean color image for 18 January 2000 with TOPEX 

altimetry data superimposed (left figure); NOAA/AVHRR SST 

image for 23 January 2000 (right figure).......................................................20 

Figure 4.8 NOAA/AVHRR SST image for 15 November 1999. ....................................21 

Figure 4.9 Spatial frequency (%) of LC water on the West Florida Shelf based 

on data for the period 1976-2003...................................................................22 

Figure 4.10 Surface currents on the West Florida Shelf from surface drifter data 

for (A) March 1996, (B) August 1996, and (C) November 1996..................23 

Figure 4.11 Monthly mean sea-surface velocity vectors on the west Florida 

Shelf (28 °N, 84 °W) from surface drifter data for 1996...............................24 

Figure 5.1 Time series of SSH analyses from TOPEX/Poseiden showing the 

apparent development of a WRC immediately west and slightly 

south of the LC...............................................................................................26 

Figure 5.2 NOAA/AVHRR SST image for 15 March 2002 shows two WCRs 

that separated from the LC in March 2002. ...................................................27 

Figure 5.3 Measurement procedure for the Loop Current's northern boundary. .............28 

ix
LIST OF FIGURES (continued) 

Figure 5.4 LC’s northern boundary displacements in the period 1977-2003. .................29 

Figure 5.5 NOAA/AVHRR SST images for the period February-March 1998 

depicting a ring separation event. ..................................................................29 

Figure 5.6 Histogram of the LC’s eddy-shedding periods for the period 1972-

2003................................................................................................................30 

Figure 5.7 Histogram of the LC eddy separation as a function of month in a year 

based on data for the period 1972-2003.........................................................31 

Figure 5.8 Variance-preserved spectrum of the LC’s northern boundary 

variations based on data for the period 1977-2003........................................32 

Figure 5.9 Paths of WCRs for the period 1976-2003. .....................................................34 

Figure 5.10 Frequency at which WCRs follow each of the three prescribed ring 

paths (i.e., the Northern Path, the Central Path, and the Southern 

Path) based data for the period 1976-2003 ....................................................35 

Figure 5.11 Frequency distribution for the speed of WCRs based on data for the 

period 1972-2003...........................................................................................37 

Figure 5.12 Path of a warm ring in the summer of 1983. ................................................37 

Figure 5.13 Decay of the diameter of WCRs at the surface over time based on 

data for the period 1976-2003........................................................................39 

Figure 5.14 NOAA/AVHRR SST image for 23 April 2001 showing a WCR that 

just separated from the LC.............................................................................40 

Figure 5.15 The spatial frequency (%) of surface warm water associated with 

major WCRs that separate from the Loop Current in the WGOM................42 

Figure 5.16 NOAA/AVHRR SST image in the northwestern corner of the 

WGOM for 28 January 1986. ........................................................................43 

Figure 5.17 Analysis of the sea-surface height in the GOM from satellite 

altimetry data for 15 April 2003. ...................................................................44 

Figure 5.18 Spatial frequency (%) for the location of WCR centers using a 27-

year (1977-2003) database.............................................................................45 

Figure 6.1 TOPEX/ERS sea-surface height image for 1 October 2003. .........................48 

Figure 6.2 Analysis of the spatial frequency (%) for the location of CCR centers 

using a 12-year (1992-2003) database. ..........................................................49 

Figure 6.3 Interaction of a CCR on the northwestern boundary of a WCR/LC in 

mid-September 2004......................................................................................50 

x
LIST OF TABLES

Page 

Table 5.1 Year-to-Year Change in the Period Having the Most Significant 

Variance from the Spectrum and in the Average Eddy-Shedding 

Period from the Histogram Data Set Over the Last Decade ..........................32

xi
1. EXECUTIVE SUMMARY 

1.1 Objective and Data 

The objective of this project is to develop a climatology of certain ocean features in 

the Gulf of Mexico (GOM) using data sets that span long time periods (i.e., as long as 32 

years in some cases) to create the statistics. The climatology focused on three principal 

ocean features: the Loop Current (LC); warm core rings (WCRs); and cold core rings 

(CCRs). Twelve (12) separate statistics were created. Four of these twelve statistics 

dealt with aspects of the LC; that is, the spatial frequency with which the LC is found in 

the eastern GOM (EGOM), the frequency of the orientation angle of the LC, the spatial 

frequency with which isolated pockets of warm LC water are found in the EGOM, and 

the spatial frequency with which isolated pockets of warm LC water are found on the 

West Florida Shelf. A major portion of the effort focused on WCRs. Seven separate 

statistics were created for WCRs; that is, the dominant frequency of separation of WCRs

from the LC, which was examined using two separate statistics, the frequency with which 

WCRs move along certain paths as they move through the western GOM (WGOM) to the 

western wall, WCRs’ speeds as they traverse across the WGOM, the mean decay of the 

WCR size as they move through the WGOM, the spatial frequency with which WCR 

water is found in the WGOM, and the spatial frequency with which the centers of WCRs

water are found in the GOM. Only one statistic could be created about CCRs. CCRs are

short-lived phenomena compared to the lifetime of WCR. They are much more difficult

to detect in satellite remote sensing data other than altimetry data. Their motion is very 

often associated with the LC or with WCRs. The statistic that was created for CCRs is 

the spatial frequency with which the centers of CCRs are found in the GOM. This report 

describes the procedures used to create the statistics and relates the statistics to known 

processes in the GOM. 

The data resources used to create this climatology included analyzed in-situ data from 

ships of opportunity, from MMS field programs in the GOM, and from GOM projects 

managed by the various oil and gas companies. The bulk of the information was, 

however, obtained from satellite remote sensing data. These data included sea-surface 

temperature (SST) data from radiometers aboard TIROS-M, HCMM, SEASAT, GOES,

and the numerous NOAA polar orbiting satellites; ocean color data from the CZCS, 

which was aboard the Nimbus polar orbiting satellite, and from the SeaWiFS system; and 

altimeter data from the TOPEX/Poseiden and the ERS systems. These data were used to 

develop monthly ocean front analyses in the GOM from which many of the statistics 

were derived. These analyses were created using an integral of available satellite clearsky images (i.e., satellite SST, ocean color, and altimetry imagery) and of any available 

information and/or analyzed in-situ data obtained in a particular month. The LC front 

was the approximate average front for the month unless a WCR separated from the LC 

during that month. In that case, it is the position of front after separation had occurred. 

WCR fronts were the approximate average front for the month. As a result, these

analyses provided the “characteristic” position of the fronts in the GOM for a given 

month. 

The development of the front analyses was based, for the most part, on satellite remote 

sensing data. In the periods 1976 through 1978 and 1986 through 1991, only SST data 

1
were available to create the ocean front analysis in the GOM. Most of the SST data were 

obtained from the NOAA/AVHRR, though significant use was also made of GOES SST

and SST data from NASA’s Heat Capacity Mapping Mission (HCMM). As a result of 

using only SST data in the periods mentioned above, the frontal analyses could only be 

determined for as little as 5 months and as many as 8 months in a year, depending on 

when the mixed layer developed in the summer and was removed in the autumn. 

However, in those periods when satellite SST data were not useful, significant use was 

made of “ship-of opportunity” data to fill data gaps in climatological data sets for certain 

ocean features (e.g., position of the northern boundary of the LC, the location of WCRs, 

the month in which separation of a WCR from the LC occurred, etc). In the period 1979 

through 1985, CZCS ocean color data were used to supplement the SST data, which were 

used to detect ocean features in the warm season when SST data were of little use. After 

1991, TOPEX/ERS SSH data were available and the frontal analyses for most features 

could be developed for each month in the year. After 1997, SeaWiFS ocean color data 

were also used to supplement the SST data in the warm season to detect features that 

could not be detected in the SSH data (e.g., intrusions of LC water on the WFS). 

However, only limited amounts of these data were available for analysis because of the 

commercial nature of the SeaWiFS data. 

It is expected that these statistics will be useful in planning field programs, to marine

biologists and oceanographers, as background information for environmental impact

statements (EISs), in evaluating and directing improvements for models that calculate the 

ocean dynamics, and for oil and gas exploration and exploitation in the GOM. 

The following sections provide examples of the statistics that have been developed. 

1.2. Frequency of Loop Current Water in the EGOM 

The LC frontal boundary (Figure 1.1) seldom reached the 28 ̊ latitude line (i.e., about 5% 

of the time over the 28-year period of the statistic). It reached the 27 ̊ latitude line about 

20% of the time. The LC usually penetrates at least as far north as 27 ̊ N latitude just 

prior to WCR separation. After WCR separation, the LC re-establishes itself further 

south, usually at around 25 ̊ latitude, so that it does not normally occupy the northern part 

of the EGOM (i.e., north of 25  ̊N) for long periods of time. The water mass associated 

with the LC occupied the region south of 24 ̊ N latitude and east of 86 ̊ W longitude the 

greatest amount of time. The analysis suggests that the LC was at times found on the 

Yucatan shelf and the West Florida Shelf. There are cases when LC thermal boundary 

that was found in the satellite SST imagery and in the ocean color imagery were found on 

these shelves. This, of course, does not mean that the high-speed currents associated with 

the LC were found on either shelf. The analysis shows that there is a mean westward tilt 

(i.e., a westward tilt of approximately 30 ̊) of the axis of the LC. 

1.3 Frequency of Loop Current Water on the West Florida Shelf 

The analysis of the spatial frequency of warm LC water on the West Florida shelf (WFS)

shows that intrusions of LC water onto the shelf occurred most often south of 27 ̊ N and 

between the 200-m isobath and the 83 ̊ W longitude line (Figure 1.2); that is, most of 

these intrusions occurred near the shelf break and do not penetrate deep into the shelf 

(i.e., they do not penetrate deep into the shelf as an identifiable features in the satellite

remote sensing data). The inability to detect deep penetration of these features on the 

2
Figure 1.1. Spatial frequency (%) of the water mass associated with the LC in the EGOM based on data for 

the period 1976-2003.

3
Figure 1.2. Spatial frequency (%) of LC water on the West Florida Shelf based on data for the period 1976-

2003.

4
shelf using satellite remote sensing data is presumably due to mixing, which causes shelf 

water to become indistinguishable from LC water. It also appears that the intrusions of 

LC water onto the shelf occurs most often when a CCR, whose circulation causes the 

transport, is located on the southeastern portion of the LC front near the Dry Tortugas. 

The maximum spatial frequency for these events is about 12%, which correspond to a 

frequency of about 1 event every year. 

1.4 Frequency of the Position of WCR Centers in the GOM 

The analysis (Figure 1.3) shows an east-west zone of relatively high spatial frequency for 

the ring centers located in the 25 ̊ N-26 ̊ N latitude belt from 88 ̊ W to 94 ̊ W. Multiple

centers are found in that zone, and the maximum frequency in that zone is about 12%, 

which is a little more than one event every year, on average. One event every year might

be considered rather high considering that major rings do not necessarily separate from 

the LC that often nor is the path of the rings always in this area. There are a couple of 

reasons for this frequency. As shown in this report, during periods when the movement 

of major rings slowed because they were either changing direction or their motion formed 

a loop, the ring, in some case, did not move over a great distance and occupied the same

grid square for more than a one-month period. Another reason for the one event every 

year frequency is the fact that minor rings were included in the analysis. Minor rings 

move slowly, are often quasi-stationary, and dissipate quickly. Under these conditions, 

minor rings can made a serious contribution in the analysis. 

North of the eastern part of the east-west zone of relatively high frequency is a secondary 

center of relatively high spatial frequency for the ring centers (i.e., located at around

27.5 ̊ N and 89.5 ̊ W). The maximum frequency in this center is about 7%, which is 

about one event every 17 months. In this region, minor rings made serious contributions

to the spatial frequency. This center is important because rings located in this region can 

make a marked impact on the circulation on the Texas-Louisiana shelf and the slope in 

that region, which will impact outer continental shelf activities in that region. 

Immediately west of the east-west zone of relatively high frequency is a center of 

relatively low spatial frequency (i.e., located at around 25.5 ̊ N and 94.5 ̊ W). The

minimum frequency in that center is about 3%, which is about one event every 4 years. 

The reason for the minimum in this region is not completely clear. Data provided in this

report indicate that this area is an area where CCRs are often found. The dominance of 

CCRs in this region may inhibit WCRs from invading the region. WCRs are normally

much smaller when they reach the western wall of the WGOM and have about the same

dimensions as CCRs in this area, so that a CCR is just as likely to occupy a position near 

the western wall as a WCR. The data on ring path provided in the report would suggests

that very few major rings move into this region when they traverse the WGOM to the 

western wall. 

North and south of the minimum area discussed in the last paragraph are areas of 

relatively high frequency for ring centers. The area to the north is in the northwestern

corner of the GOM, which region has been called the “graveyard for rings.” Rings have 

been observed to reside in this area for long periods of time; and at times, those rings 

have become revitalized for a period of time due to interactions with other WCRs 

(Vukovich and Waddell, 1991) and/or with CCRs. The maximum frequency in that area 

is about 10%, which is about one event every year. 

5
Figure 1.3. Spatial frequency (%) for the location of WCR centers using a 27-year (1977-2003) database.

6
The zone of relatively high frequency in the area to the south of the minimum area is

located near the western boundary of the GOM and the maximum frequency in that area

is about 12%, which is a little greater than one event every year. The fact that there is a 

persistence of WCR centers in this area supports the notion that these rings are major 

contributors to the transport in the WGOM and the maintenance of an anticyclonic cell 

along the western boundary of the GOM (Brooks, 1984; Nowlin and McLellan, 1967; 

Elliott 1982). 

1.5 Summary of Findings 

The following is an outline of the major findings from this study: 

Loop Current 

The LC front reached 28 ̊ N latitude in the EGOM only about 5% of the time and 27 ̊ N 

about 20% of the time. About 80% of the LC’s orientation angles are between 0 ̊ (a

north-south orientation) and 30 ̊ (a north-northwest to south-southeast orientation), which 

represent a stable orientation for the LC; that is, ring separation is not expected anytime

soon. Less than 20% of the LC’s orientation angles are greater than 30 ̊, which represent 

an unstable mode for the LC and ring separation is usually expected soon. 

Isolated warm pools of LC water were found throughout the region north of 26 ̊ N in the 

EGOM. The highest frequency for the warm pools in the EGOM was about 14% and it 

was found near 27 ̊ N and 85 ̊ W, just west of the shelf break for the WFS. This center of

high frequency was created, for the most part, as a result of many cases in which there 

was transport of warm LC water by the circulation associated with CCRs located on the 

boundary of the LC. 

Intrusions of LC water onto the WFS are, for the most part, due to transport associated 

with a CCR located on the eastern boundary of the LC. Most of these intrusions are 

found south of 27 ̊ N and between the shelf break and 83 ̊ W. The maximum frequency is 

about 12%, which corresponds to about one event per year. 

Warm Core Rings 

The frequency distribution for the period for separation of major rings from the LC had a 

bi-model distribution with modes at 6 and 11 months and a range of 5 to 19 months. The 

average period was 11 months and the standard deviation was ± 4 months. The average 

period was identical to one of the modes of the frequency distribution, suggesting that 11 

months might be the most characteristic period for eddy shedding. The average period did 

not change substantially over the 10-year period 1994-2003, remaining at about 11 

months. A high frequency of ring separation was found in March, and no rings were 

observed to separate from the LC in December. 

The period with the most significant variance in the variance-preserved spectrum of the

LC northern boundary variations was 12 months, suggesting that 12 months might be the 

most characteristic period for eddy shedding based on these data. The most significant 

variance showed a one-month variation over the last ten years (1994-2003), presumably a 

result of the year-to-year variability in the eddy-shedding period. 

In terms of the path that the WCRs take through the WGOM, the Central Path had the 

highest frequency (62%). The Northern Path had the next highest frequency (24%), and 

the least likely trajectory for the WCRs was the Southern Path (18%). The average speed

7
of the WCRs through the WGOM was 4.4 km/day with a standard deviation of ± 2.9 

km/day. The frequency distribution of WCR speeds had a bi-model distribution with 

modes in the increments of 4.0 to 4.9 km/day and 1.0 to 1.9 km/day. 

WCRs decayed, on average, to about 55% of their initial size in about 8 months. There 

was a period of about two months, starting at the fourth month after separation of the 

ring, when the decay rate decreased significantly. 

The maximum frequency of WCR water in the WGOM occurred near 25 ̊N and 90 ̊ W 

and was 24%. There was a zone of relatively high frequency of WCR water in the 

WGOM that stretched along a west-southwest--east-northeast line from 26 ̊ N and 90 ̊ W 

to 24.75 ̊ N and 95 ̊ W where the frequency varied from 24% to 16%, which appeared to 

be associated with the Central Path of the major rings through the WGOM. The 

frequency contours for WCR water ridged into the northwest corner of the WGOM where 

major WCRs are often observed. 

An east-west zone of relatively high frequency of WCRs was found in the WGOM in the 

25-26 ̊ N latitude belt from 88 ̊ W to 94 ̊ W in which the maximum frequency was about 

12%, corresponding to little more than one event per year. A secondary region of 

relatively high frequency of WCRs was found at around 27.4 ̊ N and 89.6 ̊ W for which 

minor rings played a significant role. Another region of relatively high frequency of 

WCR was found in the northwest corner of the WGOM, where, as previously indicated, 

major rings are often observed. Another region of relatively high frequency of WCRs was

found near the western wall between 22 ̊ N and 25 ̊ N, in which the maximum frequency 

was about 12%, corresponding to little more than one event per year. This region 

supports the notion that WCRs may be major contributors to transport in the WGOM and 

the maintenance of an anticyclonic cell along the GOM’s western boundary (Brooks, 

1984; Nowlin and McLellan, 1967; Elliott 1982). In the WGOM, WCRs play an 

important role in the heat and salt balance (Elliot, 1982). These data indicate that WCRs

have their most significant effect on the heat and salt balance north of 24 ̊ N. 

Cold Core Rings 

In the EGOM, there was a northwest-southeast oriented zone of relatively high frequency 

for CCRs, having two centers of maximum. The northwestern center was located at 

around 27.5 ̊ N and 88.5 ̊ W, which is in an area where CCRs have been previously found

to intensify and/or develop (Vukovich, 1988a), and the maximum frequency in this area 

was about 30% (the highest frequency noted for any kind of ring—warm or cold), which 

corresponds to about 3-4 events per year. The southeastern center was located at around 

25.5 ̊ N and 85.5 ̊ W, which is in the Dry Tortugas region where CCRs have been 

previously noted (Maul and Herman, 1985 and Vukovich 1988a and b), and the 

maximum frequency in this area was about 17%, which corresponds to about 2 events per 

year. In the WGOM, four region of relatively high frequency for CCRs were found. The 

maximum frequency in these regions varied from 16-20%, corresponding to about 1-2 

events per year. 

This study showed that in the EGOM, CCRs, which are very persistent and/or numerous 

in that region, play an important role in redistributing heat and salt. They accounted for a 

large portion of the development of isolated pools LC water in the EGOM and for all 

intrusions of LC water onto the WFS through transport associated with their circulation. 

8
2. INTRODUCTION 

In the late 1980s, a climatology (Vukovich and Hamilton, 1990) of a limited number of 

oceanographic features in the Gulf of Mexico (GOM) was created that was based on 

satellite remote sensing and in-situ data. Other aspects of the climatology of features in 

the GOM have been subsequently reported in the literature over the years (Vukovich, 

1995; Vukovich; 1988a; Herring et al., 1997; Vukovich et al., 1978 and 1979b; Vukovich 

et al., 1979a; Sturges, 1992; Sturges, 1995; Sturges and Leben, 2000; Jacobs and Leben, 

1990). Among the features included in those studies were the frequency of Loop Current 

(LC) water in the eastern GOM, LC eddy separation periods, cold core rings (CCRs) on 

the LC boundary, and warm core ring (WCR) water in the GOM. Up to 28 years of data 

were used in at least one of those studies. Most, however, used datasets that had much 

smaller periods (i.e., about 10-15 years, on average). The data set that is available to 

develop a climatology of oceanographic features in the GOM now span up to 32 years,

which would lend greater statistical significance to the climatology. Furthermore, it is 

possible to develop additional statistics that were previously unattainable because the

required data were not available. It is the objective of this project to develop the most upto-date and statistical significance climatology possible of certain ocean features in the 

GOM. 

The statistics on which this climatology is based on, are as follows: 

1. Spatial frequency of Loop Current water in the Eastern GOM (EGOM). [Database 

period: 1976-2003]; 

2. Spatial frequency of warm water associated with major WCRs in the Western

GOM (WGOM). [Database period: 1977-2003] 

3. Spatial frequency of centers of WCRs in the WGOM (based on the location of the

centers of both major and minor WCRs). [Database period: 1977-2003] 

4. Frequency of the path of major WCRs in the WGOM (based on three potential 

ring-path subdivisions—the northern path, central path, and southern path). 

[Database period: 1976-2003] 

5. Frequency of the speed of major WCRs in the Western GOM [Database period: 

1972-2003] 

6. Mean decay of the size of the major WCRs (Ring Dissipation) [Database period: 

1976-2003] 

7. Frequency of major eddy shedding periods from the LC. [Database period: 1972-

2003] 

8. Periods of oscillation of the LC’s Northern Boundary. [Database period: 1977-

2003] 

9. Frequency of the orientation of the LC. [Database period: 1976-2003] 

9
10. Spatial frequency of LC water in the EGOM not directly associated with the LC 

or with major WCRs (This water mass is, for the most part, associated with minor 

WCRs and with the advection of warm LC water by cold core rings [CCRs]). 

[Database period: 1976-2003] 

11. Spatial frequency of intrusions of LC water onto the West Florida shelf (WFS). 

[Database period: 1976-2203] 

12. Spatial frequency of centers of CCRs in the GOM. [Database period: 1992-2003] 

The data resources used to create this climatology included analyzed in-situ data from 

ships of opportunity and from various MMS field programs in the GOM; information

from the GOM programs managed by the various oil and gas companies; sea-surface 

temperature (SST) data from radiometers aboard TIROS-M, HCCM, SEASAT, GOES, 

and the numerous NOAA polar orbiting satellites; ocean color data from the CZCS, 

which was aboard the NIMBUS polar orbiting satellite, SeaWiFS, and MODIS; and 

altimeter data from the TOPEX/Poseiden and the ERS instruments. The application of 

the data from these sources to develop each statistic is discussed in the report. 

It is hoped that the information that is derived from this integrated climatology may be 

useful in planning field programs, to marine biologists and oceanographers, as 

background information for environmental impact statements (EISs), and for oil and gas

exploration and exploitation in the GOM. It has also been shown that the information 

from a climatology of ocean can useful in evaluating and directing improvements for 

models that calculate the ocean dynamics in the GOM (Herring et al., 1997). In the

following sections of this report, a description of the steps that were taken to create the 

various statistics and a description of the features of each of the derived statistics are 

presented.

10
3. ANALYSIS OF FRONTS IN THE GOM 

The principal data set used to derive many of the statistics that are presented in this

report, are monthly analyses of the fronts in the GOM. These analyses provide the 

“characteristic” position of the fronts in the GOM for a given month. They were created 

using an integral of available satellite clear-sky images (i.e., satellite SST, ocean color, 

and altimetry imagery) and of any available information on ocean features and/or 

analyzed in-situ data for the month in question in the GOM. The LC front is the 

approximate average front for the month unless a WCR separated from the LC during 

that month. In that case, it is the position of front after separation has occurred. WCR 

fronts are the approximate average front for the month. If isolated pools of warm LC 

water not directly associated with the LC or with major WCRs are found in the EGOM in 

a given month, then it is represented in the frontal analysis by its most widespread effect

in the EGOM for that month that is found in the available data resources. If an intrusion 

of warm LC water onto the WFS is found in the EGOM in a given month, then it is also 

represented by the most widespread effect for that month on that shelf that is found in the 

available data resources. These frontal analyses have been created for the period 1976-

2003. Figure 3.1 is an example of a frontal analysis used to derive statistics. 

Figure 3.1. Example of an analysis of fronts in the GOM.

The solid blue line is the LC front and the dashed lines branching off of the LC are warm

water features in the EGOM and on the WFS, the only source of the warm water being, 

of course, the LC. A more detailed discussion of the nature of these features will be 

given later in this report. The dotted lines represent the average position of the 

boundaries of WCRs in the WGOM. 

11
The development of these front analyses is based, for the most part, on satellite remote

sensing data (i.e., SST, ocean color, and sea-surface height [SSH] data from altimetry 

data). In the periods 1976 through 1978 and 1986 through 1991, only SST data were 

available to create the ocean front analysis in the GOM. Most of the SST data were 

obtained from the NOAA/AVHRR, though significant use was also made of GOES SST

and SST data from NASA’s Heat Capacity Mapping Mission (HCMM). As a result of 

using only SST data in the periods mentioned above, the frontal analyses could only be 

determined for as little as 5 months and as many as 8 months in a year, depending on 

when the mixed layer developed in the spring/summer and was removed in the autumn. 

However, in the periods when SST data were not useful, significant use was made of 

ship-of opportunity data to fill data gaps that existed in climatological data sets for certain

ocean features in the GOM (e.g., position of the northern boundary of the LC, the 

location of WCRs centers, the month/year in which separation of a WCR from the LC

occurred, etc). In the period 1979 through 1985, CZCS ocean color data were used to 

supplement the SST data, which were used to detect ocean features in the warm season 

when SST data were of little use. After 1991, TOPEX/ERS SSH data were available and 

the frontal analyses for most features could be developed for all twelve months in the 

year. After 1997, SeaWiFS and MODIS ocean color data were also used to supplement 

the SST data in the warm season to detect features that could not be detected in the SSH 

data (e.g., intrusions of LC water on the WFS). 

12
4. THE LOOP CURRENT AND LOOP CURRENT WATER

4.1 Frequency of Loop Current Water in the EGOM 

4.1.1. Introduction 

The LC enters the EGOM through the Yucatan Strait and exits through the Straits of

Florida (Maul et al., 1985). In the EGOM, the LC penetrates northward until instabilities 

form in the current and a WCR separates from the LC. As a result of this process, the 

amount of time that the water mass associated with the LC resides in any one location in 

the EGOM varies from one location to the next. The statistic presented in this section is

the spatial frequency of water directly associated with the LC and provides information 

on the percent of time that the LC water mass resides at locations in the EGOM. This

statistics can also be used to provide a crude estimate of the spatial frequency of the 

location of high speed current associated with the LC front in the EGOM. The statistic is 

based on a 28-year (i.e., 1976-2003) database, and was derived using, for the most part, 

satellite remote sensing data (i.e., IR SST, ocean color, and altimetry data). 

4.1.2 Procedure 

The frequency of LC water in the EGOM was determined using the LC frontal boundary 

analyses described in Section 3. A 0.5 ̊ latitude by 0.5 ̊ longitude grid was constructed

and overlaid onto the frontal analyses. If the water mass associated with the LC occupied

more than 50% of any 0.5 ̊ latitude by 0.5 ̊ longitude grid square in any given month, 

then it was assumed that the LC water mass was present in that grid square at that time. 

It should be noted that since this statistic was derived from monthly frontal analyses that 

depended primarily on satellite remote sensing data, there were periods (i.e., the mid 

1970s and the late 1980s) when only SST data were available to create the LC frontal 

boundary analyses, and as a result, the LC front could only be determined for periods of 5 

to 8 months in particular years, depending on when the mixed layer developed in the 

spring/summer and was removed in the autumn. When ocean color and/or altimetry data 

were available, the LC front was determined for all months in the year. 

4.1.3 Analysis 

Figure 4.1 presents the spatial frequency of the water mass associated with the LC in the 

EGOM. The LC frontal boundary seldom reached the 28 ̊ latitude line (i.e., about 5% of 

the time over the 28-year period). It reached the 27 ̊ latitude line about 20% of the time. 

The LC usually penetrates about as far north as 27 ̊ N latitude just prior to WCR 

separation. After WCR separation, the LC re-establishes itself further south, usually at 

around 25 ̊ latitude, so that it does not normally occupy the northern part of the EGOM

(i.e., north of 25  ̊N) for long periods of time. The water mass associated with the LC 

occupied the region south of 24 ̊ N latitude and east of 86 ̊ W longitude the greatest 

amount of time. The analysis suggests that the LC was at times found on the Yucatan 

shelf and the West Florida Shelf. There are cases when LC thermal boundary that was 

determined using satellite SST imagery and ocean color imagery, were found at these 

locations. This, of course, does not mean that the high-speed currents associated with the 

LC were located on either shelf. The analysis shows that there is a mean westward tilt 

(i.e., a westward tilt of approximately 30 ̊) of the axis of the frequency analysis. The 

variation of the orientation (i.e., the westward tilt) of the LC is associated with the

separation of WCRs from the LC and will be discussed in detail in the next subsection. 

13
Figure 4.1. Spatial frequency (%) of the water mass associated with the LC in the EGOM based on

data for the period 1976-2003 

4.2 Orientation of the Loop Current 

4.2.1 Introduction 

The LC orientation is defined as the angle made by the intersection of a line drawn 

parallel to the eastern and western boundaries of the LC through the frontal axis and a 

longitude line. When a tilt of the axis of the LC occurs, it is always a westward tilt. The

orientation of the LC changes in association with cycle of the LC; that is, the northward 

penetration of the LC into the EGOM, the eventual separation of a WCR from the LC, 

and the re-establishment of the LC after eddy separation. The westward tilt of the LC is 

greatest at the time a WCR is about to separate. The orientation, or tilt, angle of the LC, 

is usually about 30 ̊- 40 ̊, on average, when a ring separates (i.e. the LC is oriented 

northwest-southeast). However, much larger orientation angles (i.e., orientation angles 

the order of 60 ̊) and tremendous westward extension of the LC have been observed in 

the last decade (Figure 4.2). When large tilt angles are achieved, the high-speed currents

associated with the LC can influence central GOM and parts of the WGOM as Figure 4.2 

14
indicates. After the ring separates, the LC re-establishes itself around 25 ̊ N, in most

cases, and, at that time, is oriented north-south (i.e., the orientation angle is 0 ̊). As the 

LC penetrates again into the EGOM, a westward tilt of the LC begins to emerge and the 

LC’s orientation angle increases as processes associated with the LC lead once again to

the separation of a WCR. 

Figure 4.2. NOAA/AVHRR SST composite image for 31 January 2002. The LC orientation angle is on the 

order of 60 ̊ and the LC appears to extend as far west as 93 ̊ W in this case. 

4.2.2 Procedure 

The LC orientation was determined by drawing a straight line parallel to the eastern and 

western boundaries of the LC (Figure 4.3) and measuring the angle between that line and 

a longitude line (i.e., a line directed due north). These lines were drawn subjectively so 

that the angles determined are approximations. If that line was parallel to the longitude

line, the orientation of the LC is north-south and the angle is zero degrees. If the line was

parallel to a latitude line, the orientation of the LC is west-east and the angle is ninety

degrees. The west-east LC orientation has never occurred in the more than 30 years that

satellite data has been available to observe the LC behavior. As indicated previously, the

maximum orientation angle observed to date was about 60 ̊, which occurred in the last 

decade. 

The LC orientation angle was determined for each month using the monthly frontal 

analyses. The data were used to determine standard statistics (i.e., the average, the mode,

the standard deviation for the, the maximum, and the minimum orientation angle) and a 

15
frequency distribution for the orientation angle. The frequency distribution was 

established by binning the data in 5 ̊ increments (i.e., the binning took the form of 0 ̊- 4 ̊, 

5 ̊- 9 ̊, 10 ̊- 14 ̊, etc.). The database for these statistics covered the period 1976-2003. 

Figure 4.3. Depiction of method used to determine the Loop Current orientation angles. 

As in the case of the frequency of LC water in the EGOM, this statistic was derived using

monthly frontal analyses that depended primarily on satellite remote sensing data. As

previously indicated, there were periods when only IR SST data were available to create

the LC frontal boundary analyses, and as a result, the LC front could only be determined

for 5 to 8 months in particular years, depending on when the mixed layer developed in 

the spring/summer and was removed in the autumn. When ocean color and/or altimetry 

data was available, the LC front was determined for all months in the year.

4.2.3 Analysis 

Figure 4.4 provides the frequency distribution for the LC orientation angle. The figure 

indicates that about 80% of the angles lie between 0 ̊ and 30 ̊ (i.e., the LC orientation is 

between north-south and north-north west--south-southeast). Orientation angles less 

than 30 ̊ are normally associated with a relatively stable LC (i.e., an LC not about to 

16
shed a WCR anytime in the near future). Orientation angles greater than 30 ̊ are found 

about 20% of the time. Normally when the LC orientation angles are this large, ring 

separation is soon to follow. The primary mode in the histogram is in the 0 ̊- 4 ̊ band. 

Secondary modes are noted in the bands 10 ̊- 14 ̊ and 20 ̊- 24 ̊. These bands are 

orientation angles for a stable LC. The LC, at times, persists in these orientations for 

extended periods prior to separation process. Over the period 1976-2003, the average 

orientation angle was about 17 ̊ with a standard deviation of about 14 ̊. The single most 

observed orientation angle (i.e., the mode of the data set) was 0 ̊--a north-south 

orientation of the LC, which is, of course, the minimum angle observed. The maximum 

angle observed over the period was 63 ̊. An angle greater than 55 ̊ only occurred twice 

over the 28-year period, and angles between 50 ̊- 54 ̊ were also observed only twice. 

0

10

20

30

40

50

Percent

0 10 20 30 40 50 60 70 80 90

Loop Current Oriention Angle (degrees)

Figure 4.4 Frequency distribution for the Loop Current orientation angles. This analysis is

based on data from the period 1976-2003. 

4.3 Frequency of Isolated Warm Water in the EGOM 

4.3.1 Introduction 

Warm pools of water that become isolated from the LC, are often found in the EGOM. 

Though these warm water pools are isolated from the LC, the LC is the obvious source of 

these features. They can be established in EGOM by minor, short-lived rings that 

separate from the LC and dissipate in the EGOM, by the advection of LC water by with 

the circulation associated with CCRs that are found on the northern boundary of the LC

(Vukovich, 1988a; Wang et al., 2003), or by dragging surface water from the LC 

northward through the action of a strong wind stress. These pools are an indication of 

the transport of LC water mass and of any foreign mass (i.e., oil spilled, dinoflagellates, 

etc.) residing in the LC, from the LC into the common water area of the EGOM. Figure 

4.5 is a NOAA/AVHRR SST image showing various types of warm pools that can be 

17
found in the EGOM. The warm pool north and somewhat west of the LC may be the 

result of a small WCR separating from the LC or it may be due to northward advection of 

LC water by the CCR that is located at the northwestern boundary of the LC. The warm 

water pool that is located near the eastern boundary of the LC, and which is partially on 

the WFS, is due to transport of LC water by the CCR that is found along the LC’s eastern 

boundary. The analysis that was created and described in this section provides specific 

information on the spatial frequency of these warm pools and in so doing, gives an 

indication of the frequency of processes that create the pools.

Figure 4.5. NOAA/AVJRR SST image for 16 February 1999 depicting warm

water pools in the EGOM.

4.3.2 Procedure 

To produce the analysis of the spatial distribution of the frequency of the isolated warm 

pools water in the EGOM, a 0.5 ̊ latitude by 0.5 ̊ longitude grid, identical to the one used 

to obtain the spatial frequency of LC water, was used in the EGOM. The frequency was 

determined for each grid square by overlaying the grid on the characteristic monthly

ocean frontal analyses. For each grid square, the number of times that an isolated warm 

water pool encompassed that grid-point, was determined over the database period. As in 

18
the case of the spatial frequency of the LC, if the water mass associated with a warm pool 

occupied more than 50% of a grid square in any given month, then it was assumed that 

the water mass was present in that grid square at that time. It should be noted that these 

features could only be detected using satellite ocean color and SST data, and therefore,

suffer from the lack of data in the warm seasons when only SST data were available. 

When ocean color data were available, these features could be determined for all months 

in the year. This analysis used data for a 28-year period (1976-2003). 

4.3.3 Analysis 

Figure 4.6 provides the spatial frequency of the isolated warm water pools in the EGOM. 

Pools of LC water were found throughout the region north of 25 ̊ N in the EGOM over 

the period. The highest frequency, at which those pools were found, was approximately 

14 percent and the center of high frequency was located immediately west of the shelf 

break of the WFS at about 27.25  ̊N and 85.25  ̊W. This high frequency feature was

created, for the most part, as a result of many cases in which there was advection of

warm LC water by the circulation associated with CCRs, which were located on the 

northeastern boundary of the LC (See the example in Figure 4.7). The 14 percent 

maximum frequency corresponds to a frequency of about 1 event every 10 months or 

about 7 events in 6 years. 

Figure 4.6. Spatial frequency (%) of isolated warm pools in the EGOM based on data for the period

1976-2003. 

19
 

LC 

Water 

Cold 

Core 

Ring 

Loop 

Current 

Figure 4.7. SeaWiFS ocean color image for 18 January 2000 with TOPEX altimetry data superimposed

(left figure); NOAA/AVHRR SST image for 23 January 2000. Both data sets show the

northward transport of LC water by a CCR in the northeast part of the LC (right figure). 

There is also an interesting feature in the analysis that is located at around 29 ̊ N and

87.25  ̊W. This feature is described in the 4 percent isoline where it extends northward 

toward the DeSoto Canyon area. That feature was principally a result of minor, shortlived rings, which moved northward into the DeSoto Canyon area and then dissipated. 

Warm water advection associated with CCR also played a role in creating this feature, 

but this process played a minor role. In the spring 1977, significant amounts of surface 

water from the Loop Current were transported northward into the DeSoto Canyon area 

solely by the effect of a strong wind stress, which also contributed to this frequency 

feature. The highest frequency in this region that suggests an extension into DeSoto 

Canyon was about 5 percent. A 5-percent frequency corresponds to about 1 event every 

2.5 years. 

It should be noted that the association of a frequency value with an event, which is

directly related to a process, in this case and cases discussed later in this report, does not

necessarily refer to separate events. This is particularly true in those cases when a 

frequency value is associated with more than one event per year. An event may persist 

for more than a month in a given location, creating a frequency that is related to the 

event’s persistence rather than to multiple occurrences of similar events in that case. 

20
4.4 Frequency of Loop Current Water on the West Florida Shelf 

4.4.1 Introduction

The cyclonic circulation associated with CCRs on the eastern boundary of the LC 

occasionally cause a transport of warm LC water onto the WFS (See the example in 

Figure 4.8). In time, the LC water mass mixes with the shelf water, so that the LC water

mass is no longer distinguishable from shelf water using satellite SST or ocean color 

data. As in the previous case, the transport of LC water onto the shelf also means that 

any foreign mass (i.e., oil spilled, dinoflagellates, etc.) residing in the LC will also be 

transported onto the shelf. This analysis provides specific information on the spatial 

frequency of warm LC water that was transported onto the WFS and in so doing, 

provides information on the frequency of processes that cause this kind of transport. 

Figure 4.8. NOAA/AVHRR SST image for 15 November 1999. 

4.4.2 Procedure 

The procedure to obtain the spatial frequency of LC water on the WFS is similar to that

for the spatial frequency of the LC in the EGOM and the spatial frequency of isolated 

warm water pools in the EGOM. A 0.5 ̊ latitude by 0.5 ̊ longitude grid was constructed 

21
and overlaid onto the frontal analyses, which indicated the presence of LC water on the 

WFS. For each grid square, the number of times that LC water was found in that grid 

square was determined over the database period. As in previous cases, if the warm water

mass on the shelf occupied more than 50% of a grid square in any given month, then it 

was assumed that the water mass was present in that grid square at that time. It should be

noted that this feature could only be detected using satellite ocean color and SST data.

Therefore, this analysis suffered from the applicability of SST data in the warm season in 

periods when only SST data were available. The database used for this analysis covered 

the period 1976-2003. 

4.4.3 Analysis 

The analysis of the spatial frequency of warm LC water on the WFS shows that 

intrusions of LC water onto the shelf occurred most often south of 27 ̊ N and between the 

(i.e., the 200-m isobath) and the 83 ̊ W longitude line (Figure 4.9); that is, most of these 

intrusions occurred near the shelf break and do not penetrate deep into the shelf (i.e., they 

do not penetrate deep into the shelf as an identifiable features in the satellite remote 

sensing data). The apparent lack of identifiable deep penetration of these features on the 

shelf is presumably due to mixing. It also appears that the intrusions of LC water onto 

the shelf occurs most often when a CCR, whose circulation causes the transport, is 

located on the southeastern portion of the LC front near the Dry Tortugas. The

maximum spatial frequency for these events is about 12%, which correspond to a

frequency of about 1 event every year, in this case. 

Figure 4.9. Spatial frequency (%) of LC water on the West Florida shelf based on data

for the period 1976-2003.

22
It should be noted that the circulation on the WFS can possibly augment or deter the 

effects of the intrusions of LC water onto the shelf. A program sponsored by the MMS

examined the circulation on the WFS in 1996. The shelf was seeded with hundreds of 

satellite-tracked surface drifters during that year. The drifter data was used to determine 

the characteristic circulation on the shelf by integrating all drifter locations for the a given 

month in that year and using those data to determine the speed and direction of the flow

at the surface on the shelf for that month. Figure 4.10 shows the surface flow on the 

WFS for the months of March, August, and November 1996. Super-imposed on the 

figure are satellite SST data (i.e., color coded) and SSH contours from satellite altimetry 

data. The analyses in Figure 4.10 show flow to the south-southeast on the WFS in March 

and November 1996 and flow to north-northwest in August 1996. 

A B

C

Figure 4.10. Surface currents on the West Florida Shelf from surface drifter data for (A) March 1996, (B) 

August 1996, and (C) November 1996. Super-imposed are satellite SST and SSH data.

Figure 4.11 provides the mean monthly current speed and direction at 28 ̊ N and 84 ̊ W 

that were derived from the drifter data. The data in that figure shows that southerly 

currents, which would prevent the widespread northward transport of LC water and any 

mass in the intruded water mass on the shelf, were found on the shelf in the fall and 

winter. Northerly currents, which would augment widespread northward transport of LC 

23
water on the shelf, were noted in the spring and summer. Data were not available to 

determine whether the 1996 data represent the characteristic cycle of the surface flow on 

that shelf each year. However, it should be noted that the shelf currents are controlled by 

the wind (He and Weisberg, 2003), and they will vary as the year-to-year wind 

climatology varies. 

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

28 ÞN 

84 ÞW

10 cm/s

Figure 4.11. Monthly mean sea-surface velocity vectors on the WFS (28 °N, 84 °W) from

surface drifter data for 1996. An open square around the black dot means no 

data were available at the location. North is toward the top of the page. 

24
5. WARM CORE RINGS 

5.1 Frequency of Ring Separation 

5.1.1 Introduction 

A considerable number of studies have appeared in the peer-reviewed literature, which 

focused on the separation frequency of large WCRs from the LC. The separation periods 

that have been reported are highly variable: 8-9 months (Vukovich and Maul, 1985); 11 

months (Vukovich, 1988a; Maul and Vukovich, 1993; Vukovich, 1995); 12 months 

(Sturges and Evans, 1983); bimodal periods of 8-9 months and 13-14 months (Sturges, 

1992, 1994); and bimodal periods 6 and 11 months with a minor mode at 9 months 

(Sturges and Leben, 2000). Sturges and Leben (2000) also noted that with considerable

smoothing of the spectrum, the peak distribution was formed around 12-month periods. 

Vukovich (1988a, 1995) and Maul and Vukovich (1993) noted that the 11-month period 

that they cited is an average value and that the eddy separation periods were highly 

variable, ranging from 6-17 months. The eddy-shedding period seldom was the same as 

the average period or any of the other characteristic values cited in the literature.

The studies cited above used data sets with periods as small as 10 years and as large as 26 

years. The variable nature of the results from these studies is, in part, a testimonial to the 

limitations in deriving statistics using data sets that are small when the statistic deals with 

events that occur a very limited number of times in a year. Vukovich (1995) suggested 

that it may be necessary to require data sets that cover a 100-year period in order the 

obtain a statistically stable estimate of the eddy-shedding period. In this section of this 

report, the eddy-shedding period was re-examined using the largest data set currently 

available (i.e., a 32-year data set). The period was actually examined using two 

procedures: a histogram of eddy-shedding periods using a 32-year data set and spectral 

analysis of the LC northern boundary oscillations using a 27-year data set. 

5.1.2 Procedure 

The eddy-shedding period was examined using two procedures. In one procedure, a

histogram of the eddy-separation periods was developed. The eddy-separation periods 

were determined by documenting the month and year of the separation of each major 

ring. A major ring was defined as a large ring (i.e., rings with diameters of about 300 km 

or more at the time that they separated from the LC.), which persisted for more than five

months. The month and year of separation was that time when the major eddy 

completely separated from the LC. In some cases, a WCR separated from the LC only to 

be re-absorbed by the LC. In some cases, a WCR separated from the LC twice only to be 

re-absorbed by the LC. The month and year of separation of the ring from the LC for this 

study was defined as the month and year when complete separation occurred and the ring 

moved into the WGOM. Furthermore, this study only dealt with rings that actually 

separated from the LC. There are situations when large rings develop near the LC 

(Figure 5.1) for no apparent reason. As the figure shows, a large ring developed 

immediately west of the LC and another ring that was centered at about 23.3 N and 93.7 

W on 15 June, intensified in about 25 days. There are indications in the 1 July analysis 

that an energy pulse may have emanated from the LC. In any case, there was obviously 

no separation of a WCR from the LC in this case and the ring that developed was not

considered in this analysis. 

25
 15 June 2004 1 July 2004 

 20 June 2004 5 July 2004 

 25 June 2004 10 July 2004 

 

Figure 5.1. Time series of SSH analyses from TOPEX/Poseiden showing the apparent development

of a WRC immediately west and slightly south of the LC.

26
A 32-year (i.e., 1972-2003) data set was used to determine the eddy-separation periods, 

and in that period, 37 major rings separated from the LC. Under normal circumstances, 

one major WCR separated from the LC each year. In some years, two rings separated, 

but these usually separated in different months as little as 5 months apart. In 2002 

however, two major rings separated from the LC in the same month (Figure 5.2). Two 

rings also separated from the LC in the same month in 2003, but one of these dissipated 

quickly (i.e., in about three months), so that it was not considered to be a major ring. 

Figure 5.2. NOAA/AVHRR SST image for 15 March 2002 shows two WCRs that separated from the LC

in March 2002. The dual separation was the result of the elongated LC noted in Figure 4.2 in 

Section 4. 

For the second approach, a time series of the mean monthly distances between the 

northern most boundary of the LC and the 30 ̊ N latitude line (Figure 5.3) was developed 

using the frontal analyses discussed previously. The average distance for a given year 

was calculated using the monthly averaged values and was subtracted from each 

corresponding monthly averaged value to produce detrended, monthly displacements of 

the northern boundary relative to the mean. A sign convention was developed such that 

positive displacements were to the north of the mean (Figure 5.4). Spectral analysis was 

applied to the time series of detrended, monthly displacements of the northern boundary 

to determine dominant periods. It should be noted that when the LC penetrates into the 

EGOM, the northern boundary generally moves north of its mean annual position. When

a major eddy separates from the LC, the northern boundary moves abruptly south of its 

mean annual position because the eddy is no longer considered as part of the LC (Figure 

27
5.5). As a result, the LC’s northern boundary moves north and south of its mean annual 

position with a frequency, which is, for the most part, associated with eddy shedding. 

Therefore, the dominant periods from the spectral analysis of these displacements will 

provide an indication of the dominant eddy-shedding period. A 27-year (i.e., 1977-2003) 

data set was used to study the LC northern boundary oscillations. 

31

30

29

28

27

26

25

24

23

22

21

20

-90 -89 -88 -87 -86 -85 -84 -83 -82

 Loop Current Boundary

200

200

Figure 5.3. Measurement procedure for the Loop Current's northern boundary.

28
Figure 5.4. LC’s northern boundary displacements in the period 1977-2003. 

 

Figure 5.5. NOAA/AVHRR SST images for the period February-March 1998 depicting a ring separation

event. The image on the left is for 6 February 1998 and that on the right is for 30 march 1998.

The images show the rapid shift of the position of the LC’s northern boundary from about 28 ̊ N 

in early February to about 26 ̊ N in late March.

The spectrum of the LC northern boundary variations was calculated using a spectral 

analysis routine, which was part of the IGOR Pro software package. One Hanning 

smoothing of the spectrum was applied. 

5.1.3 Analysis 

Figure 5.6 provides a histogram of the LC’s eddy-shedding periods for the period 1972-

2003. The frequency distribution is bimodal with primary modes at 6 and 11 months, 

similar to that found by Sturges and Leben (2000). The primary modes have frequencies 

29
of 14%. The range in the data is 5 to 19 months, and the average eddy-shedding period is 

11 months with a standard deviation of ± 4 months. Since the average period and one of 

the primary modes in the histogram are approximately 11 months, this might suggest that 

an 11-month period is a representative period for eddy separation from the LC. However, 

since periods of 6, 9, 10, and 14 months also have significant frequencies, it is more likely

that the eddy-shedding period is highly variable and that no one period can be used to 

represent the LC’s eddy-shedding period. Oey et al. (2003) has suggested that the 

variability of the LC eddy-shedding frequency is a function of three factors: a) the 

steadiness of the transport; b) wind-induced fluctuations through the Greater Antilles

Passage; and c) Caribbean eddies. Their results suggest that any one of these factors can 

induce a shorter or longer eddy-shedding period.

0

5

10

15

20

25

30

35

Percent

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Separation Periods (months)

Figure 5.6. Histogram of the LC’s eddy-shedding periods for the period 1972-2003.

Figure 5.7 provides the histogram for the LC’s eddy shedding as a function of the month 

in the year. The mode of the frequency distribution is at month 3 (March) and the

average is month 6 (June). The standard deviation is ± 3 months. These data indicated 

that about 45% of the major WCRs separated from the LC in the late winter and spring 

period (i.e., March, April, May, and June). It is interesting to note that over the 32-year 

period, no WCRs separated from the LC in month 12 (i.e., December) and the net

frequency for eddy shedding is a minimum in months 11 and 12 (i.e., NovemberDecember). This is the same period when the Straits of Florida transport is normally a

minimum (Maul and Vukovich, 1993).

30
0

5

10

15

20

25

30

35

Percent

1 2 3 4 5 6 7 8 9 10 11 12

Separation Month

Figure 5.7. Histogram of the LC eddy separation as a function of month in a year based on data for 

the period 1972-2003.

Figure 5.8 provides the variance-preserved spectrum of the data in Figure 5.4. A onetime Hanning filter has been applied to the spectrum. The most significant variance has a

period of 12.0 months, which is one month larger than one of the modes in the histogram

(Figure 5.5) and the average of the eddy-shedding periods from the data used in the 

histogram procedure. There are secondary peaks in the spectrum at 9.5 months and 8.1 

months. The histogram (Figure 5.6) has a secondary mode at 9 months. It should be 

noted that the period having the most significant variance in the spectrum varied by one 

month over the last 10 years as each additional year of data was add to the data set of 

LC’s northern boundary displacements data (Table 5.1); that is, it varied from 12.2 

months to 11.1 months and then back to 12 months over the 10-year period, which is most 

probably a consequence of the fact that the eddy-shedding period was highly variable 

over that period. On the other hand, the average eddy-shedding period that was derived 

using the data set that was used to create histograms (Figure 5.6) remained about 11 

months over the past 10 years (Table 5.1) as the eddy-shedding periods that took place 

each year were added to the data set, suggesting that the average eddy-shedding statistic 

may be a stable statistic.

5.2 Ring Path 

5.2.1 Introduction 

When major WCRs separate from the LC, they move into the WGOM. Their direction of 

motion is generally westward until they reach the “western wall” of the GOM (Vukovich 

31
Figure 5.8. Variance-preserved spectrum of the LC’s northern boundary variations 

based on data for the period 1977-2003 

Table 5.1.Year-to-Year Change in the Period Having the Most Significant Variance

from the Spectrum and in the Average Eddy-Shedding Period from the

Histogram Data Set over the Last Decade

Year Period Having the Most

Significant Variance from

Spectrum 

(months) 

Average Eddy-Shedding 

Period from the Histogram 

Data Set 

(months) 

1994 12.2 11.3

1995 12.2 10.8

1996 12.2 10.9

1997 12.1 11.0

1998 12.2 10.8

1999 11.5 11.1

2000 11.5 11.1

2001 11.5 11.3

2002 11.1 11.3

2003 12.0 11.3

32
and Waddell, 1991; Elliott, 1982; Hamilton et al., 1999). During their movement from 

the EGOM through the WGOM, some rings have their centers located north of 26 ̊ N

most of the time. Since these major WCR are large (i.e., having diameters of about 300-

400 km), the circulation associated with these rings under these circumstances interact 

with the Louisiana and Texas shelf and the slope region west of 90 ̊ W. Many rings have 

their centers located between 24 ̊ N and 26 ̊ N when they move through the WGOM. 

These rings travel through the deep Gulf regions (i.e., water depths ≥ 2000 m) for the 

most part, but most of the very large WCRs will affect the slope region west of 90 ̊ W, 

especially when their path is in the northern part of the 24 ̊ N - 26 ̊ N latitude belt. Some

rings move southwestward from the EGOM into the WGOM (i.e., move into the region 

south of 24 ̊ N), then turn northward as they approach the western wall. Sutyrin et al.

(2003) has suggested that the westward motion of rings is governed by the presence or 

absence of a deep circulation associated with the ring and the configuration of the 

northwest corner of the shelf. After reaching the western wall, the WCRs generally have 

no prescribed direction of motion. Some rings move northward. Some move southward, 

while others move eastward. In this section of this report, the frequency at which major

WRCs move along three paths through the WGOM to the western wall is determined

using data for the period 28-year period 1976-2003. 

5.2.2 Procedure 

For each month in the period 1976-2003, the mean positions of the centers of the major 

WCRs were determined. The center of these rings was determined in most cases as the 

geometric center. When altimetry data were available, the center of the ring was defined 

as the location of the largest value for the sea-surface height. Major rings that coalesced 

with other rings and rings that the split, producing two rings, during their motion through 

the WGOM were excluded from this analysis. The mean monthly ring positions were 

plotted (Figure 5.9) in the WGOM. It should be noted that only major rings were

included in the analysis (i.e., minor rings were excluded) and only their path from the 

EGOM to the western wall was considered (i.e., their path after reaching the western wall 

was not included in this analysis). Those rings that spent 75% or more of the time north 

of 26 ̊ N before reaching the western wall were considered to have the Northern Path.

Those that spent 75% or more of the time between 24 ̊ N and 26 ̊ N were considered to

have the Central Path, and those that spent 75% or more of the time south of 24 ̊ were

considered to have the Southern Path. The database used for this study included satellite 

remote sensing data (i.e., IR SST data, ocean color data, and altimetry data), analyzed insitu data from ships of opportunity and from MMS field programs in the GOM, 

information from the GOM programs managed by the various oil and gas companies, and 

satellite-track drifter data. The drifter data was especially useful in the summer periods

when ocean color and/or altimetry data were not available. 

5.2.3 Analysis 

Figure 5.10 provided the frequency analysis for the three ring paths examined. By far, 

the preferred path (i.e., with a frequency of 62%) for the WCRs through the WGOM is the 

Central Path. As indicated previously, the WCRs that travel along the Central Path, 

move through the deep-water regions of the WGOM. However, if the WCR is especially 

large and if it moves through the north portions of the latitude belt that defines the 

Central Path (i.e., 24 ̊ N –26 ̊ N), the circulation associated with these rings will influence

33
the slope region off the Texas-Louisiana coast west of 90 ̊ W. The Northern Path, which 

is the WCR path that will allow the rings to influence the Louisiana and Texas shelf and 

the slope west of 90 ̊ W, has the next highest frequency (i.e., 24%). The least likely path 

for the WCRs is the Southern Path with a frequency of about 14%. It should be noted 

that Hamilton et al. (1999) found no apparent preferred path for the WCRs. They 

examined 10 WCRs between 1985 and 1995. Their findings were based on a 

significantly smaller dataset than used in this study. 

30

29

28

27

26

25

24

23

22

21

20

19

18

-98 -96 -94 -92 -90 -88

200

200

Figure 5.9. Paths of WCRs for the period 1976-2003. 

34
30

29

28

27

26

25

24

23

22

21

20

19

18

-98 -97 -96 -95 -94 -93 -92 -91 -90 -89 -88

Northern Path

(24%)

Central Path

(62%)

Southern Path

(14%)

Figure 5.10. Frequency at which WCRs follow each of the three prescribed ring paths (i.e.,

the Northern Path, the Central Path, and the Southern Path) based data for the 

period 1976-2003

35
5.3 WCR Speed in the WGOM 

5.3.1 Introduction.

Previous studies have indicated that WCRs move through the WGOM at speeds on the 

order of 5 km/day (Cochrane, 1972; Elliott, 1982; Vukovich and Crissman, 1986). This 

statistic has been based on studies that examined small number of rings in most cases, but 

also include a study that examined up to 12 years of data, and involved calculation of the

speeds associated with 13 rings. Presently, there is 32 years (i.e., 1972-2003) with which 

to calculate the speed of WCRs in the WGOM. The ring speed was calculated for 37 

major WCRS that existed in the 32-year period. This database should provide better 

statistic for the ring speed than previously acquire. 

5.3.2 Procedure 

The speed of the rings was estimated using the available 32-year/37-ring database. It was

calculated using successive locations of the center of the ring. As previously indicated, 

the center of the ring was determined in most cases as the geometric center. When 

altimetry data were available, the center of the ring was defined as the location of the 

largest value for the sea-surface height. The west-to-east and south-to-north components

of the ring speed were calculated using the longitudes and latitudes, respectively of 

successive positions of a ring’s center, and the ring speed was estimated using the values 

of the components of the ring speed. 

5.3.3 Analysis 

The average ring speed over the 32-year period was 4.4 km/day with a standard 

deviation of ±2.9 km/day. The median ring speed was 4.1 km/day. The average ring 

speed is slightly smaller than that previously found (i.e., 5 km/day---Cochrane, 1972; 

Elliott, 1982; Vukovich and Crissman, 1986). The frequency distribution (Figure 5.11) 

has a bi-model distribution. The primary mode is found in the interval 4.0-4.9 km/day 

and the mode for the data is 4.1 km/day. A secondary mode occurred at speeds on the 

order of 1 km/day. Ring motion is characterized with periods of “stalls and sprints” 

(Vukovich and Crissman, 1986; Hamilton et al., 1999). Vukovich and Crissman noted 

that the stalls are often associated with periods when the rings make major changes in 

their direction or when their motions is characterized by a tight loop (Figure 5.12). The 

relatively high-speed motion of the rings, the sprints, occurred when the path of the ring 

is characterized by quasi-linear motion. The secondary mode in the frequency 

distribution may be associated with those periods when the path of the rings is 

characterized by major directional changes or the tight loops. 

5.4 Decay of Ring Size

5.4.1 Introduction 

As major WCRs move through the WGOM, they have been observed to dissipate 

(Vukovich and Waddell, 1991; Elliott, 1982; Cochrane 1972). The size of the rings, 

which had diameters the order of 300-400 km when they separated from the LC, 

decrease; have diameters the order of 100-200 km when they reach the western wall; and 

36
0

5

10

15

20

25

Frequency (%)

0 2 4 6 8 10 12 14 16 18 20

Ring Speed (km/day)

Figure 5.11. Frequency distribution for the speed of WCRs based on data for the period 1972-

2003.

31

30

29

28

27

26

25

24

23

-95 -92.5 -90 -87.5 -85

200

200

Loop in WCR Path

Direction Change

Figure 5.12. Path of a warm ring in the summer of 1983. 

37
an energy transfer often to takes place between the WCRs and CCRs that were found in 

close proximity to the WCR (Elliott, 1979; Merrill and Morrison, 1981; Merrill and 

Vazquez, 1983; Brooks, 1984; Hurlburt and Thompson, 1982; Wallcraft, 1986; Smith 

and O’Brien, 1983; Smith, 1986). Previous studies of the decay of the ring size have

used databases as small as 12 years (Vukovich and Crissman, 1986) and as large as 22 

years (Herring et al., 1997). For this study, a 28-year (i.e., 1976-2003) database was used 

to determine the mean rate of decay of the WCRS as they travel through the WGOM. 

5.4.2 Procedure 

The average decay of the size of WCRs was determined using data for a 28-year period 

1976-2003. The ring dissipation was estimated in terms of the month-to-month change of 

the diameter of a circle whose area was identical to that determined for a major ring that 

separated from the LC. The area of the ring was calculated for each month using the 

portrayal of the ring in the monthly frontal analysis. That area was, in many cases, 

determined as the area of the ellipse that best fit the ring in the analysis, which was then

used to estimate the diameter of a circle whose area was identical to that of the ring. 

The diameters of the rings were determined for each month during the period when the 

ring traveled from the EGOM to the western wall in the WGOM. This was done

primarily because for certain portions of the period 1976-2003, only satellite SST data 

were available to track the ring. When the ring became small (i.e., diameters of about 

100-200 km), the ring very often could not be detected using the SST. When ocean color 

and/or altimetry data were available, the rings could be tracked for considerably longer 

periods. However, for consistency in the period over which the ring diameter was 

determined for each ring, only data in the period when the ring traveled from the EGOM 

to the western wall was considered in these calculations. In the 28-year period, 32 rings 

separated from the LC. However, complete data sets on the evolution of the diameter of 

the rings could only be obtained for 24 rings. The 8 rings for which complete data sets 

could not be obtained, were rings that separated from the LC in periods when only 

satellite SST data were available and detection of the ring size was not possible even with 

best efforts to enhance the satellite data during image processing. 

The time registration in this case was as follows. For the month when the ring separated 

from the LC, the month was set to zero. For the following months, month = 1, 2, 3, 

....etc. By normalizing time relative to the separation month, the average decay of the 

size of WCRs could be calculated with ease. The average decay of the diameter of 

WCRs was also normalized relative to the average diameter at the time of separation (i.e.,

at month = 0, the average normalized ring diameter equaled 1.0). 

5.4.3 Analysis 

Figure 5.13 shows the variation of the average, normalized diameter of WCRs over an 8-

month period. The average diameters were normalized by the average diameter of the 

ring at the time of separation (i.e., at month = 0, and in this case, the average diameter 

was 330 km). Data for an eight-month period is only provided in the average because, 

though data were available for longer periods in some cases, the 8-month period provided 

a near-uniform number of data points to calculate the average for each month. After the 

8th month, the number of data points available to calculate an average decreased 

markedably. Also shown in the figure is a best-fit Gaussian curve for the data: that is,

38
y = ao – a1 exp[-(a2 x-a3)

2

];

where ao, a1, a2, and a3 are model coefficients determined by least squares, x is months 

(i.e., x = 0, 1, 2,....12), and y is the normalized ring diameter. The best-fit curve had an r

= 0.97 and average error of about 9 km relative to the observations. 

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Normalized Ring Dia

meter

0 1 2 3 4 5 6 7 8 9 10 11 12

Month

 Average Observed, Normalized Ring Diameter

 Best-Fit Curve

Figure 5.13. Decay of the diameter of WCRs at the surface over time based on data for the period 1976-

2003. 

According to the observations, the average ring diameter decayed to approximately 55% 

of it initial size at separation in about 8 months or about 240 days. At that time, the rings 

that were used in the calculations were located near or at the western wall in the WGOM.

There is a 2-month period starting at month = 3 and ending at month = 5 when the ring 

decay was very weak. After that period, the ring decay was about equivalent or greater

than that found in the first three months. It is not clear why the ring decay became

weaker in the period month = 3 to month = 5, but this feature, which was associated with

momentary ring intensification, appeared to take place for many of the rings used to 

create this statistic. 

39
According to the best-fit curve, the ring diameter decayed at a rate of about 17 

km/month, on average. The actual decay rate was larger than the average indicated by 

the best-fit curve in the first few months (i.e., the average decay rate in the first 4 months 

of about 18 km/month) and the last few months (i.e., the average decay rate in the last 

three month was about 30 km/month). The maximum decay rate in the data was found 

between month 6 and 7 and was 40 km/day. The best-fit curve did not capture the period 

of weak decay of the ring noted in the observations in the period month = 3 to month = 5, 

and indicated that the e-folding took place in approximately 12 months. 

5.5 Frequency of WCR Water in the WGOM 

5.5.1 Introduction 

When WCRs move through the WGOM, the warm water that defined the rings affected a 

large surface area in the WGOM. The area affected by a single ring is generally largest 

when the ring enters the WGOM immediately after it has separated from the LC (Figure 

5.14), and it is smallest when the ring reaches the western wall of the WGOM, because 

the size of the ring has decayed to about 55%, on average, of its initial size by the time it 

has reached that wall (See Section 5.4.3). Some rings expand in the north-south direction 

when they encounter the western wall (Vukovich and Waddell, 1991). However, the net 

effect of many rings moving through the WGOM is to have the ring water affect almost 

the entire northern half of the WGOM (Vukovich and Hamilton, 1990). Some parts of

the WGOM will be considerably more affected by ring water than other parts because

there are paths that the WCRs will follow when they move through the WGOM more 

often than others (See Section 5.2). These rings play an important role in the heat and 

salt budget in the WGOM (Elliott, 1982). Therefore, an analysis of the frequency of ring 

water in the WGOM will define areas in the WGOM most affected by ring water and, 

therefore, where the rings most often directly affect the heat and salt budget in the 

WGOM. 

Figure 5.14. NOAA/AVHRR SST image for 23 April 2001 showing a WCR that just 

separated from the LC. 

40
In this section, a new analysis of the frequency of surface ring water in the WGOM is 

presented using a 28-year (i.e., 1976-2003) database. Previous studies of the frequency 

of ring water in the WGOM used a 9-year (1976-1984) database (Vukovich and 

Hamilton, 1990). Changes in the spatial analysis of the frequency of ring water in the 

WGOM are expected because the new database covers a period more than 3 times larger. 

5.5.2 Procedure 

The frequency of surface ring water in the WGOM was created using a 28-year (i.e., 

1976-2003) database. As in other cases of this kind, a 0.5 ̊ latitude by 0.5 ̊ longitude grid 

was constructed and overlaid onto the frontal analyses. If the water mass associated with

a WCR occupied more than 50% of any 0.5 ̊ latitude by 0.5 ̊ longitude grid square in any

given month, then it was assumed that the WCR water mass was present in that grid

square at that time. This statistic was derived using monthly frontal analyses that 

depended primarily on satellite remote sensing data. In the mid 1970s and the late 1980s, 

only IR SST data were available to create the frontal boundary analyses, and as a result, 

the WCR fronts could only be determined for about 5 to 8 months in particular years, 

depending on when the mixed layer developed and was removed. When ocean color and 

altimetry data were available, WCR fronts could be determined for each month in the 

year. 

5.5.3 Analysis 

Figure 5.15 provides the analysis of the spatial frequency of surface water in the WGOM

associated with major WCRs that separate from the LC. The magnitude of the 

frequencies is lower than that for LC water (Figure 4.1) because of the intermittent and 

non-stationary nature of warm rings. The maximum frequency (i.e., ~24%) is found near 

26 ̊ N and 90 ̊ W in the eastern portion of the WGOM where the WCRs first enter into the

WGOM. There is a zone of high frequency of ring water that stretches along a line 

oriented west-southwest–east-northeast from about 26 ̊ N and 90 ̊ W to about 24.75 ̊ N 

and 95 ̊ W. The spatial frequency varied along that line from 24% to 16%. This line 

appears to be parallel to the Central Path of the major WCRs (See Section 5.2), the path 

that the major rings most often take when they travel through the WGOM. The data 

indicate that there is a low probability of finding water associated with major rings in the 

southern portions of the WGOM (i.e., the Bay of Campeche). This effect is due in part 

because rings are no longer included in the frontal analysis when the diameter of the rings

become on the order of 150 km. The diameter of major rings normally decays to sizes ≤

150 km soon after the ring reaches the western wall in the WGOM. Rings whose 

diameters are usually about 150 km or smaller, are the kinds of rings normally found in 

the Bay of Campeche. 

The frequency contours ridge into the northwestern corner of the WGOM. This is

because major WCRs are often observed in this region. The region has been called the

“graveyard for rings.” Rings have been observed to reside in this area for long periods 

of time, and, at times, become revitalized for short periods of time. It is also possible that 

an anticyclonic circulation may be generated in that region when the warm water is 

transported into the northwestern corner of the WGOM by the effect of a ring colliding

with the western wall south of the northwest corner. As a result of a ring colliding with

the wall south of the northwestern corner, they expand to the north and south, sending 

ring water into the northwestern corner of the WGOM and producing a weak anticyclonic 

41
Figure 5.15. The spatial frequency (%) of surface warm water associated with major WCRs that

separate from the Loop Current in the WGOM. This analysis is based on a 28-year

(1976-2003) database. 

circulation in that region (Figure 5.16). Drifter data have clearly shown that in a 

January 1986 case that the circulation in the northwestern corner was anticyclonic 

(Vukovich and Waddell, 1991). Sometimes warm water is also transported into the 

northwestern corner by circulation of a CCR that was located along the western wall 

just south of the northwestern corner. 

42
Anticyclonic 

Flow Region

Figure 5.16 NOAA/AVHRR SST image in the northwestern corner of the WGOM for 28 January

1986. Anticyclonic flow was detected in the northwest corner of the GOM (See

Vukovich and Waddell, 1991).

5.6 Frequency of the Position of WCR Centers in the GOM 

5.6.1 Introduction 

Major WCRs separate from the LC about once every 11 months, on average, though the 

actual frequency is highly variable. These rings move into the WGOM where they 

dissipate. However, minor WCRs (i.e., WCRs that are smaller and/or that do not have 

deep vertical structure) also separate from the LC. Some of these rings move into the 

WGOM, while others stay in the NEGOM. The minor rings dissipate quickly (i.e., in 

about 2 to 5 months), but play a role in the heat and salt balance and on the circulation in 

the GOM. Warm rings are believed to be a major contributor to the transport in the 

WGOM and the maintenance of an anticyclonic cell along the western boundary of the

GOM (Brooks, 1984; Nowlin and McLellan, 1967; Elliott 1982). The anticyclonic 

circulation associated with WCRs has a profound effect on the surface trajectories in the 

WGOM (Hamilton et al., 1999; Lewis and Kirwan, 1985). Obviously, information on the 

persistence of WCRs in the GOM is vital to our understanding of the circulation in the 

GOM. In this section, an analysis of the spatial frequency of the location of the centers 

of WCRs is presented. This analysis was created using a 27-year (i.e., 1977-2003)

database. 

5.6.2 Procedure 

The analysis of the spatial frequency of the location of the centers of WCRs was created 

using a 27-year (i.e., 1977-2003) database. For the period 1977-1991, the location of the 

centers of WCRs was derived from the frontal analyses discussed in Section 3.0. The 

frontal analyses for that period were based, for the most part, on satellite SST data and 

CZCS ocean color data when they were available. Minor rings were included in the 

43
frontal analyses for this period when they could be detected and for as long as they were 

detectable. In the mid 1970s and the late 1980s, only SST data were available to create 

the frontal boundary analyses, and as a result, the WCRs could only be determined for 

about 5 to 8 months in particular years, depending on when the mixed layer developed 

and was removed. Furthermore, minor rings could only be detected during this period in 

the first month or two after separation from the LC. For the period 1992-2003, the 

database used for this analysis also included of satellite altimetry data. When satellite

altimetry data was included in this analysis, data were available for this analysis each 

month in the year. The altimetry data provided daily analyses of all rings (i.e., either 

WCRs or CCRs) in the GOM (Figure 5.17). 

Figure 5.17. Analysis of the sea-surface height in the GOM from satellite altimetry data for 15 April 

2003. The red/yellow areas with solid lines define areas with large values of sea-surface 

height and the blue areas with dashed lines are areas with low values of sea-surface

height. 

A 1 ̊ latitude by 1 ̊ longitude grid was constructed and overlaid onto the frontal analyses 

or the satellite altimetry analyses. If a WCR center occupied a 1 ̊ latitude by 1 ̊ longitude 

grid square, then it was assumed that the ring center was present in that grid square for 

that month. The center of the ring was defined as the location of the largest value for the

sea-surface height in the altimetry data or the geometric center in the frontal analysis

created using SST and/or ocean color data. When the largest-valued closed contour that 

defined the center of the ring in the analysis of the altimetry data encompassed a broad

area, the geometric center of the closed contour was used as the center of the ring.

NOTA BENE: this analysis took into account major and minor rings, including major 

rings for periods after they reached the western wall. Minor rings were detected at most 

three (3) months after they separated from the LC using SST and/or ocean color data. 

Major rings could be detected at most four months after they reached the western wall 

44
using those data. Major and Minor rings could be detected until they dissipated using 

altimeter data. 

5.6.3 Analysis 

The magnitude of the spatial frequencies of the location of the centers of WCRs (Figure 

5.18) is smaller than that for ring water (Figure 5.15). The surface water associated with 

a single ring sweeps out an area as the ring moves through the WGOM, which will affect 

many grid squares; whereas the center of a WCR is a single point in space that will affect 

a single grid square at any point in time and only a small number of grid squares as the 

ring moves westward.

Figure 5.18. Spatial frequency (%) for the location of WCR centers using a 27-year (1977-2003) database.

The analysis shows an east-west zone of high spatial frequency for the ring centers 

located in the 25 ̊ N-26 ̊ N latitude belt from 88 ̊ W to 94 ̊ W. Multiple centers are found 

in that zone, and the maximum frequency in that zone is about 12%, which is a little more 

45
than one event every year, on average. One event every year might be considered rather 

high considering that major rings do not necessarily separate from the LC that often (See 

Section 5.1) nor is the path of the rings always in this area (See Section 5.2). There are a 

couple of reasons for this frequency. In periods when the movement of major rings 

slowed because they were either changing direction or their motion formed a loop (See 

Section 5.3), the ring, in some case, did not move over a great distance and occupied the 

same grid square for more than a one-month period. Another reason for the one event 

every year is the fact that minor rings were included in the analysis. Minor rings move 

slowly, are often quasi-stationary, and dissipate quickly. Under these conditions, minor 

rings can make a serious contribution in the eastern part of the analysis. 

North of the eastern part of the east-west zone of high frequency is a secondary center of 

relatively high spatial frequency for the ring centers (i.e., located at around 27.5 ̊ N and 

89.5 ̊ W). The maximum frequency in this center is about 7%, which is about one event

every 17 months. In this region, both major and minor rings made serious contributions 

to the spatial frequency. This center is important because rings located in this region can 

make a marked impact on the circulation on the Louisiana shelf and the slope in that 

region.

Immediately west of the east-west zone of high frequency is a center of low spatial 

frequency (i.e., located at around 25.5 ̊ N and 94.5 ̊ W). The minimum frequency in that 

center is about 3%, which is about one event every 4 years. The reason for the minimum 

in this region is not completely clear. In the next section (Section 6.1), it will be shown 

that this area is an area where CCRs are often found. The dominance of CCRs in this 

region may inhibit WCRs from invading the region. WCRs are normally much smaller 

when they reach the western wall of the WGOM and have about the same dimensions as 

CCRs in this area, so that a CCR is just as likely to occupy a position near the western 

wall as a WCR. The data on ring paths in Section 5.2 would suggests that very few major 

rings move into this region when they move across the WGOM to the western wall. 

North and south of the low frequency area discussed in the last paragraph are areas of 

relatively high frequency for ring centers. The area to the north is in the northwestern

corner of the GOM, which region, as previously discussed, has been called the graveyard 

for rings. Rings have been observed to reside in this area for long periods of time; and at 

times, those rings have become revitalized for a period of time due to interactions with

other WCRs (Vukovich and Waddell, 1991) and/or CCRs. The maximum frequency in 

that area is about 10%, which is about one event every year. 

The zone of relatively high frequency in the area to the south of the low frequency area is

located near the western boundary of the GOM and the maximum frequency in that area 

is about 12%, which is a little greater than one event every 12 months. This feature was 

created by a large number of major rings that reached the western wall, that were 

reduced considerably in size at that time, and that moved southward after reaching the 

western wall. Considerably more rings moved southward than northward after they 

reached the western wall. The fact that there is a persistence of WCR centers in this area 

supports the notion that these rings may be major contributors to the transport in the 

WGOM and the maintenance of an anticyclonic cell along the western boundary of the 

GOM (Brooks, 1984; Nowlin and McLellan, 1967; Elliott 1982). 

46
6. COLD CORE RINGS 

6.1 Frequency of the Position of CCR Centers in the GOM 

6.1.1 Introduction 

CCRs are an integral part of the dynamics in the GOM. They are generally small in size 

(i.e., diameters on the order of 100 km). They have been observed in the GOM along the 

boundary of the LC (Vukovich et al., 1979a and b; O’Connor, 1981; Vukovich and Maul, 

1985; Vukovich, 1986, Vukovich, 1988a), off the Dry Tortugas (Maul and Herman, 

1985, Vukovich, 1988b), and on the boundary of WCRs (Elliott, 1979; Merrill and 

Morrison, 1981, Merrill and Vazquez, 1983, Brooks, 1984; Kelley and Brooks, 1986; 

Kelley et al., 1986; Kelley et al., 1987; Vukovich and Waddell, 1991). Their circulation 

can influence the water mass characteristics on the West Florida Shelf (See Figure 4.8 in 

Section 4) and in the northeastern GOM (See Figures 4.5 and 4.7 in Section 4). CCRs

have been observed to develop or intensify along the northwestern portions of the LC 

boundary when the LC penetrates deep into the EGOM (i.e., when the northern boundary 

of the LC is found as far north as 27 ̊ N) [Vukovich, 1988a]. They move along the LC 

boundary in the direction of the flow from the Yucatan Strait to the Straits of Florida. 

They have been observed to be associated with the separation of a WCR from the LC 

(Figure 6.1) [Vukovich et al., 1979a; Vukovich, 1988a]. The production of CCRs on the 

boundary of WCRs is considered part of the dissipation process associated with WCRs

(Cushman-Roisin, 1987; Smith 1986). They move along the boundary of the WCRs in a 

clockwise manner associated with the anticyclonic flow in the WCRs. CCRs are highly 

mobile. However, if these rings were persistent in regions of the GOM, the circulation

associated with these rings can potentially influence local water characteristics and local 

surface trajectories. In this section, an analysis of the spatial frequency of the location of 

the centers of CCRs is presented, which will provide information on the persistence of 

these rings in certain areas in the GOM. This analysis was created using a 12-year (i.e., 

1992-2003) database. 

6.1.2 Procedure 

The procedure for creating the analysis of the spatial frequency of the location of the

centers of CCRs was identical to that used to create the analysis of the spatial frequency 

of the location of the centers of WCRs. The analysis was created using a 12-year (i.e., 

1992-2003) database. The database used for this analysis was entirely made up of 

satellite altimetry data. The altimetry data provided daily analyses of all rings (i.e., either 

WCRs or CCRs) in the GOM (See Figure 5.17 in Section 5). A 1 ̊ latitude by 1 ̊ longitude 

grid was constructed and overlaid satellite altimetry analyses. If the center of a CCR 

occupied a 1 ̊ latitude by 1 ̊ longitude grid square in a given month, then it was assumed 

that the ring center was present in that grid square for that month. The center of the ring 

was defined as the location of the smallest value for the sea-surface height. When the 

smallest-valued closed contour that defined the center of the ring in the altimetry analysis 

encompassed a broad area, the geometric center of the closed contour was used as the 

center of the ring. Since this analysis was dependent on only satellite altimetry data, data 

were available for this analysis each month in the year. 

47
Figure 6.1. TOPEX/ERS sea-surface height image for 1 October 2003. 

CCRs are found off the Campeche Bank to the southwest of 

the WCR and off the West Florida Shelf to the southeast of the 

WCR. This configuration of rings is a common occurrence 

with WCR separation. 

6.1.3 Analysis 

Figure 6.2 provides the analysis of the spatial frequency of the location of the centers of 

CCRs using the 12-year (i.e., 1992-2003) satellite altimetry database. In the EGOM (i.e., 

the region east of 90 ̊ W in the GOM), the analysis shows a northwest-southeast oriented 

zone of relatively high frequency of CCR centers, having two centers of high frequency. 

The center in the northwestern part of that zone (i.e., the center located at about 27.5 ̊ N 

and 88.5 ̊ W) has a maximum frequency of about 30%, which corresponds to about 3-4 

events per year. This center of maximum frequency is found in an area of the EGOM

where CCRs have been noted to either develop or intensify on the boundary of the LC 

(Vukovich,1988a). When CCRs develop or intensify on the boundary of the LC, they 

have been observed to become quasi-stationary for a period of time (e.g., one-four 

months), before the move downstream, so that a frequency of 3-4 events per year does

not necessarily mean 3-4 individual CCRs, but that a CCR may have persisted for 3-4

months in a year. If a WCR separates after the CCR develops or intensifies on the 

boundary of the LC in this area, then the CCR may also contribute to this center of 

maximum frequency a number of times also as it moves along the boundary of the WCR 

while the WCR moves westward into the WGOM. It should also be noted that because 

48
of the high frequency with which these CCRs are found in this region, they often interact 

with the shelf off the Louisiana and Alabama coast, transporting shelf water into the deep 

Gulf (Figure 6.3). 

Figure 6.2. Analysis of the spatial frequency (%) for the location of CCR centers using a 12-year (1992-

2003) database. 

49
A B

 

CCR 

Transport of 

Shelf Water 

into the Deep

GOM 

WCR 

Figure 6.3. Interaction of a CCR on the northwestern boundary of a WCR/LC in mid-September 2004. (A)

MODIS ocean color image and (B) SSH from the JASON Altimeter. 

The center in the southeastern portion of that zone (i.e., the center located at about 25.5 ̊

N and 85.5 ̊ W) has a maximum frequency of about 17%, which corresponds to about 2 

events per year. This center is found in an area where CCRs have been observed off the 

Dry Tortugas (Maul and Herman, 1985, Vukovich, 1988b) and where they have been 

observed to be associated with the separation of a WCR from the LC (Vukovich et al., 

1979a; Vukovich, 1988a). These CCR have been known to persist in that location for 1-2 

months at a time, so that, as in the previous case, a frequency of about 2 events per year 

does not necessarily mean 2 separate CCRs, but that a single CCR may have persisted in 

that area for 2 months in a year.

In the WGOM, the analysis shows four major centers of relatively high frequency of CCR 

centers: one at 24.5K N and 91.5 ̊ W; one at 26.5 ̊ N and 93.5 ̊ W; one at 25.5 ̊ N, 94.5 ̊

N; and one at 22.5 ̊ N and 94.5 ̊ W. The center located at 24.5K N and 91.5 ̊ W has a

maximum frequency of about 18.4 %, that at 26.5 ̊ N and 93.5 ̊ W has a maximum 

frequency of about 16.3%, that at 25.5 ̊ N and 94.5 ̊ W has a maximum frequency of 

about 17%, and that at 22.5 ̊ N and 94.5 ̊ W has a maximum frequency of about 20.6%. 

In each case, the maximum frequency corresponds to about 1-2 events per year. These 

centers are produced by CCRs associated with a number of process events that take place 

in the WGOM. These CCRs are most often associated with major WCRs that separate 

from the LC, and they are found on the boundary of the WCRs (Elliott, 1979; Merrill and 

Morrison, 1981; Merrill and Vazquez, 1983; Brooks, 1984; Kelley and Brooks, 1986; 

Kelley et al., 1986; Kelley et al., 1987; and Vukovich and Waddell, 1991). The major 

WCRs that move through the WGOM, generally enter the WGOM north of the center of 

maximum frequency for the CCRs located at 24.5K N and 91.5 ̊ W in about 80% of the 

cases, which suggests that the CCRs that contribute to that center are most probably 

found on the southern boundary of those WCRs. The CCRs that contribute to the centers

of maximum frequency that are located near the western wall of the WGOM (i.e., those 

located at 26.5 ̊ N, 93.5 ̊ W; 25.5 ̊ N, 94.5 ̊ N; and 22.5 ̊ N, 94.5 ̊ W) are associated with

the CCRS that are observed with a major WCR when it reaches the western wall. A ring

50
pair (i.e., a WCR and a CCR, which are about the same size) is often observed together at 

the western wall and the ring pair has been linked to the dissipation of the WCR

(Cushman-Roisin, 1987 and Smith 1986). Sometimes, the CCR in the pair is found to the 

north of the WCR, and sometimes it is found to the south. It must be made clear, 

however, that CCRs associated with WCRs are major contributors to the development of 

these three centers of relatively high frequency of CCR centers, but they are not the only 

contributors. CCRs that are not directly associated with WCRs have been observed in 

these regions and have contributed to the development of these centers. In most cases, 

these CCRs are unique, non-recurring events. 

51
7.0 SUMMARY OF FINDINGS 

The following is an outline of the major findings from this study: 

Loop Current 

The LC front reached 28 ̊ N latitude in the EGOM only about 5% of the time and 27 ̊ N 

about 20% of the time. About 80% of the LC’s orientation angles are between 0 ̊ (a

north-south orientation) and 30 ̊ (a north-northwest to south-southeast orientation), which 

represent a stable orientation for the LC; that is, ring separation is not expected anytime

soon. Less than 20% of the LC’s orientation angles are greater than 30 ̊, which represent 

an unstable mode for the LC and ring separation is usually expected soon. 

Isolated warm pools of LC water were found throughout the region north of 26 ̊ N in the 

EGOM. The highest frequency for the warm pools in the EGOM was about 14% and it 

was found near 27 ̊ N and 85 ̊ W, just west of the shelf break for the WFS. This center of

high frequency was created, for the most part, as a result of many cases in which there 

was transport of warm LC water by the circulation associated with CCRs located on the 

boundary of the LC. 

Intrusions of LC water onto the WFS are, for the most part, due to transport associated 

with a CCR located on the eastern boundary of the LC. Most of these intrusions are 

found south of 27 ̊ N and between the shelf break and 83 ̊ W. The maximum frequency is 

about 12%, which corresponds to about one event per year. 

Warm Core Rings 

The frequency distribution for the period for separation of major rings from the LC had a 

bi-model distribution with modes at 6 and 11 months and a range of 5 to 19 months. The 

average period was 11 months and the standard deviation was ± 4 months. The average 

period was identical to one of the modes of the frequency distribution, suggesting that 11 

months might be the most characteristic period for eddy shedding. The average period did 

not change substantially over the 10-year period 1994-2003, remaining at about 11 

months. A high frequency of ring separation was found in March, and no rings were 

observed to separate from the LC in December. 

The period with the most significant variance in the variance-preserved spectrum of the

LC northern boundary variations was 12 months, suggesting that 12 months might be the 

most characteristic period for eddy shedding based on these data. The most significant 

variance showed a one-month variation over the last ten years (1994-2003), presumably a 

result of the year-to-year variability in the eddy-shedding period. 

In terms of the path that the WCRs take through the WGOM, the Central Path had the 

highest frequency (62%). The Northern Path had the next highest frequency (24%), and 

the least likely trajectory for the WCRs was the Southern Path (18%). The average speed

of the WCRs through the WGOM was 4.4 km/day with a standard deviation of ± 2.9 

km/day. The frequency distribution of WCR speeds had a bi-model distribution with 

modes in the increments of 4.0 to 4.9 km/day and 1.0 to 1.9 km/day. 

WCRs decayed, on average, to about 55% of their initial size in about 8 months. There 

was a period of about two months, starting at the fourth month after separation of the 

ring, when the decay rate decreased significantly. 

53
The maximum frequency of WCR water in the WGOM occurred near 25 ̊N and 90 ̊ W 

and was 24%. There was a zone of relatively high frequency of WCR water in the 

WGOM that stretched along a west-southwest--east-northeast line from 26 ̊ N and 90 ̊ W 

to 24.75 ̊ N and 95 ̊ W where the frequency varied from 24% to 16%, which appeared to 

be associated with the Central Path of the major rings through the WGOM. The 

frequency contours for WCR water ridged into the northwest corner of the WGOM where 

major WCRs are often observed. 

An east-west zone of relatively high frequency of WCRs was found in the WGOM in the 

25-26 ̊ N latitude belt from 88 ̊ W to 94 ̊ W in which the maximum frequency was about 

12%, corresponding to little more than one event per year. A secondary region of 

relatively high frequency of WCRs was found at around 27.4 ̊ N and 89.6 ̊ W for which 

minor rings played a significant role. Another region of relatively high frequency of 

WCR was found in the northwest corner of the WGOM, where, as previously indicated, 

major rings are often observed. Another region of relatively high frequency of WCRs was

found near the western wall between 22 ̊ N and 25 ̊ N, in which the maximum frequency 

was about 12%, corresponding to little more than one event per year. This region 

supports the notion that WCRs may be major contributors to transport in the WGOM and 

the maintenance of an anticyclonic cell along the GOM’s western boundary (Brooks, 

1984; Nowlin and McLellan, 1967; Elliott 1982). In the WGOM, WCRs play an 

important role in the heat and salt balance (Elliot, 1982). These data indicate that WCRs

have their most significant effect on the heat and salt balance north of 24 ̊ N. 

Cold Core Rings 

In the EGOM, there was a northwest-southeast oriented zone of relatively high frequency 

for CCRs, having two centers of maximum. The northwestern center was located at 

around 27.5 ̊ N and 88.5 ̊ W, which is in an area where CCRs have been previously found

to intensify and/or develop (Vukovich, 1988a), and the maximum frequency in this area 

was about 30% (the highest frequency noted for any kind of ring—warm or cold), which 

corresponds to about 3-4 events per year. The southeastern center was located at around 

25.5 ̊ N and 85.5 ̊ W, which is in the Dry Tortugas region where CCRs have been 

previously noted (Maul and Herman, 1985 and Vukovich 1988a and b), and the 

maximum frequency in this area was about 17%, which corresponds to about 2 events per 

year. In the WGOM, four region of relatively high frequency for CCRs were found. The 

maximum frequency in these regions varied from 16-20%, corresponding to about 1-2 

events per year. 

This study showed that in the EGOM, CCRs, which are very persistent and/or numerous 

in that area, play an important role in redistributing heat and salt. They accounted for a 

large portion of the development of isolated pools LC water in the EGOM and for all 

intrusions of LC water onto the WFS through the transport associated with their 

circulation. 

54
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58
Appendix A-ACRONYM LIST

AVHRR Advanced Very High Resolution Radiometer 

CCR Cold Core Ring 

CZCS Coastal Zone Color Scanner 

EGOM Eastern Gulf of Mexico 

EIS Environmental Impact Statements 

ERS European Remote Sensing 

GOES Geostationary Operational Environmental Satellite 

GOM Gulf of Mexico 

HCMM Heat Capacity Mapping Mission 

IR Infrared 

LC Loop Current 

NASA National Aeronautics and Space Administration

NEGOM Northeastern Gulf of Mexico 

NOAA National Oceanic and Atmosphere Administration 

SEASAT Sea Satellite 

SeaWiFS Sea-Viewing Wide Field-of-view Sensor 

SSH Sea-Surface Height 

SST Sea-Surface Temperature 

TIROS Television Infrared Observation Satellite 

TOPEX Topography Experiment 

WCR Warm Core Ring 

WFS West Florida Shelf 

WGOM Western Gulf of Mexico 

59
The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has 

responsibility for most of our nationally owned public lands and natural resources. This 

includes fostering sound use of our land and water resources; protecting our fish, wildlife, 

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