OCS Study 

MMS 2003-048 

 

Deepwater Observations in the 

Northern Gulf of Mexico from 

In-Situ Current Meters and PIES 

Volume I: Executive Summary

I1

I2

I3

500 m

1000 m

2000 m

2500 m

1500 m

Sigsbee Escarpment

Sigsbee Escarpment

Green Knoll

U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 
OCS Study 

MMS 2003-048 

Deepwater Observations in the 

Northern Gulf of Mexico from 

In-Situ Current Meters and PIES 

Volume I: Executive Summary 

Authors 

Peter Hamilton 

James J. Singer 

Evans Waddell 

Science Applications International Corporation 

Kathleen Donohue 

University of Rhode Island 

Prepared under MMS Contracts 

1435-01-00-CT-31132 

and 

1435-01-96-CT-30825 

by 

Science Applications International Corporation 

615 Oberlin Rd., Suite 100 

Raleigh, North Carolina 27605 

Published by 

U.S. Department of the Interior

Minerals Management Service 

Gulf of Mexico OCS Region 

New Orleans

August 2003 
DISCLAIMER 

This report was prepared under contract between the Minerals Management Service (MMS) and 

Science Applications International Corporation. This report has been technically reviewed by 

the MMS, and it has been approved for publication. Approval does not signify that the contents 

necessarily reflect the views and policies of the 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 the report may be obtained from the Public Information Office (Mail Stop 5034) 

at the following address: 

 U.S. Department of the Interior 

 Minerals Management Service 

 Gulf of Mexico OCS Region 

 Public Information Office (MS 5034) 

 1201 Elmwood Park Boulevard 

 New Orleans, Louisiana 70123-2394 

 Telephone Number: 1-800-200-GULF or 

 504-736-2519 

CITATION

Suggested citation: 

Hamilton, P., J.J. Singer, E. Waddell, and K. Donohue. 2003. Deepwater Observations in the 

Northern Gulf of Mexico from In-Situ Current Meters and PIES. Final Report. Volume I: 

Executive Summary. U.S. Dept. of the Interior, Minerals Management Service, Gulf of 

Mexico OCS Region, New Orleans, LA. OCS Study MMS 2003-048. 14 pp. 

ABOUT THE COVER 

The map shows the Sigsbee Escarpment south of New Orleans that was the primary focus of 

measurements from instruments on the indicated moorings. Moorings I1-4 and J1 were normal 

tautline moorings. Moorings K1-K3 were Inverted Echo Sounders with Pressure (PIES). The 

picture is of a PIES on the back deck of the mooring deployment vessel. The “orb” contains all 

the instrumentation associated with operations and recovery of the instrument. The stand is used 

in regions where strong bottom currents are expected, such as the present study area.

ii
ACKNOWLEDGMENT 

The insight of Dr. Willis Pequegnat was verified with this project. One of the authors 

remembers well listening to Willis many years ago describe how his biological sampling gear 

lowered near the present study area could not reach the bottom due to very strong bottom 

currents. 

Both the MMS and BP provided support to this measurement program. This cooperative funding 

was essential to acquisition of the resulting horizontal and vertical coverage as well as the 

measurement duration. Dr. Alexis Lugo-Fernandez, the MMS COTR, and Mr. David Driver, the 

BP representative, worked cooperatively to define a program that accomplished both industry 

and government objectives. 

iii 
TABLE OF CONTENTS 

 Page

LIST OF FIGURES .............................................................................................................v 

1.0 INTRODUCTION .....................................................................................................1 

2.0 MEASUREMENTS...................................................................................................1 

3.0 RESULTS ..................................................................................................................5 

4.0 REFERENCES ........................................................................................................14

iv
LIST OF FIGURES 

Figure Page

1. Detailed bathymetric map of the Sigsbee Escarpment in the region 

south of the Mississippi delta.. ......................................................................2 

2. Time lines of data return for the current meter moorings. ............................3 

3. PIES on the back deck of the deployment vessel in a cradle used in 

regions of expected strong bottom currents. .................................................4 

4. The first year of 40-HLP velocity vectors from below 1000 m at I1, 

I2, 13 and J1. .................................................................................................6 

5. 40-HLP temperatures and current velocities at indicated depths. .................7 

6. Amplitudes of inertial (26 hour) current oscillations at the indicated 

depths of mooring I1. ..................................................................................10 

7. Comparison of temperature time series measured by the I1 mooring 

(red) and temperatures estimated at the same set of depths by the 3 

nearby PIES (blue) using the GEM interpretation of the tau 

 (τ=RT travel time) measurements. ..............................................................11 

8. Comparison of salinity time series measured by the I1 mooring (red) 

and salinity estimated at 450 dbar by the 3 nearby PIES (blue) using 

the GEM interpretation of the tau (τ) measurements. .................................12 

9. Comparison of velocity measured by the I1 mooring (right) and 

velocity estimated at the same set of depths by the 3 nearby PIES 

(left) using the GEM interpretation of the tau (τ) measurements 

and referencing geotrophic shears to the deep current meter on the 

I1 mooring. ..................................................................................................13 

v 
EXECUTIVE SUMMARY 

1.0 Introduction 

As an extension of the MMS-Funded DeSoto Canyon Eddy Intrusion Study (Hamilton et al., 

2000) additional current/temperature measurements were made at the base of the Sigsbee 

Escarpment in water depths of approximately 2,000m. Deepwater and near-bottom current 

measurements in this specific area, supported by BP, indicated that periodic higher speed events 

occurred that could be of considerable importance to expected oil/gas exploration and 

development in this deepwater area. In order to have representative and pertinent knowledge of 

conditions in these development areas, the MMS funded a series of fairly site-specific, deepwater 

current measurements. In conjunction with MMS support, BP provided support to expand the 

depth and areal coverage of these measurements in what was essentially a cooperatively funded 

government-industry measurement program. 

The specifics of the various components of measurement program and a characterization of 

many of the key attributes of the observations are presented in this report. 

2.0 Measurements 

The specific types and locations of measurements described in this report varied somewhat over 

the two-year field program. During most of the study, MMS and BP jointly funded field and 

measurement activities. Although the specific supported measurements varied, the continuity 

and areal extent of measurements would not have been possible without this joint governmentindustry support. As shown in Figure 1, Moorings I1, I2 and I3 were deployed over the entire 

two-year interval. Moorings J1 and I4 were deployed during the first year and the last six 

months respectively. Moorings K1, K2 and K3 were Inverted Echo Sounders with Pressure 

(PIES) and were deployed during the final six months in a triangle around the only full-depth 

mooring (I1). 

Moorings J1, I2, I3 and I4 were near bottom or “short” moorings in that they each supported 

three current meters at 10 m, 100 m and 400 m above the local bottom. This emphasis on nearbottom conditions reflects the focus on documenting and understanding better the higher speed 

current that were previously measured at a series of BP-funded moorings. Mooring I1 was a full 

depth mooring with a series of ADCPs, fixed-point current meters, conductivity/temperature 

instruments, and temperature sensors. This was to provide relatively complete documentation of 

conditions at depths ranging from 10 m above the bottom to 72 m below the water surface in a 

water depth of 2100 m. A timeline of resulting observations for the various moorings and 

instruments in this study are shown in Figure 2. Note, velocity time series resulting from ADCPs 

are for selected representative depths. Field work occurred on cruises conducted every six 

months. 

A PIES is a bottom-mounted instrument (Figure 3) that transmits an acoustic signal and 

measures the vertical acoustic travel time (VATT) for the sound signal to travel to the ocean 

varied ocean locations, University of Rhode Island scientists have developed and implemented 

data processing methods and algorithms (Gravest Empircal Mode – GEM) which uses extensive 

1 
I1

I2

I3

500 m

1000 m

2000 m

2500 m

1500 m

K1

K2 K3

Figure 1. Detailed bathymetric map of the Sigsbee Escarpment in the region south of the 

Mississippi delta. The positions of the MMS moorings (I1, I2 and I3, black circles) 

and PIES (K1, K2 and K3, red squares), and the BP moorings (J1 and I4, green 

circles) are shown. 

J1 I4

Sigsbee Escarpment

Sigsbee Escarpment

Green Knoll

2
ADCP

ADCP

ADCP

ADCP

ADCP

ADCP

Figure 2.Time lines of data return for the current meter moorings. Solid and dashed lines 

represent velocity and scaler (e.g. Temperature and Salinity), respectively. On 

Mooring I1, a 150 kHz, up-directed ADCP failed near the end of the first six 

months and was replaced by various in-situ current meters for the remainder of 

the mooring deployments.

C/T/P

C/T/P

C/T

T

T

T

T

T

S4

C/T

GO/MKII

T

S4

S4

RCM

RCM

RCM

RCM

RCM

GO/MKII

GO/MKII

RCM

RCM

GO/MKII

RCM

RCM

GO/MKII

RCM

RCM

GO/MKII

RCM

RCM

Instr. Failure - Not Deployed

Not Deployed

Not Deployed

Not Deployed

Not Deployed

Not Deployed

72m

90m

95m

100m

150m

175m

200m

240m

250m

300m

400m

410m

450m

452m

525m

600m

602m

650m

225m

800m

1000m

1200m

1400m

1600m

1800m

1989m

1979m

1600m

1800m

1989m

1775m

1975m

2164m

972m

1172m

1361m

1557m

1757m

3
Figure 3. PIES on the back deck of the deployment vessel in a cradle used in regions of expected 

strong bottom currents. Prior to deployment, the ropes to the weights (chain links) 

were removed, as were the rubber "stoppers" around the equatorial rib. This 

configuration orients vertically on a sloping bottom since the weights tend to orient the 

instrument toward the vertical. The transducer is at the top of the orb, and the release, 

radio and light are at the bottom. When the acoustic release is activated, the unit 

separates from the chain links and the orb rises to the surface with the transducer 

pointing downward. This causes the light and radio to be at the water surface.

Transducer

Assembly

4
basin and regional hydrographic measurements to estimate the depths of various isotherms, 

isopycnals, and isobars (e.g. Watts et al., 2001). Essentially, it produces a time series of profiles 

of these variables. In turn, knowledge of the density field at two locations allows estimation of a 

geostropic velocity profile. Having pressure information at three known and proximate 

locations, allows estimation of a profile of the relative velocity vector (i.e. a velocity shear 

profile). Using an appropriate reference velocity measurement, a shear profile can be converted 

to an absolute velocity profile. In conjunction with bottom-pressure measurements, the relative 

contribution of the barotropic and baroclinic components of the total geostrophic pressure field 

can be estimated. Using historical C/T/D profiles to develop an appropriate GEM and three 

PIES in a triangle about Mooring I1 provided information needed to estimate absolute velocity 

profiles. Because I1 was a full depth mooring, additional measured current, temperature and 

salinity time series were available against which the PIES-based velocities were be compared. 

3.0 Results 

The two years of data, from the three main moorings in the region of the Sigsbee Escarpment on 

the lower slope south of the Mississippi Delta, have shown unusually strong deep current flows, 

in particular near the base of the Escarpment (Figure 4). The bottom intensified, nearly depthindependent motions are typical of topographic Rossby waves (TRW) observed in this and other 

regions of the deep Gulf. However, energy levels at these sites are exceptional and the dominant 

periods of the motions are approximately 10 to 14 days which is short compared to other regions 

of the eastern and western Gulf (Hamilton, 1990). Maximum currents at the I2 mooring were of 

order 85 to 95 cm/s for an event early in the record (Figure 4). This energy seems to spill over 

the escarpment to the J1 site at the top of the Escarpment on some occasions when very-high 

energy events occurred at I2. Otherwise, the velocities at J1 were much less energetic than at the 

moorings south of the Escarpment. Similarly, bottom velocities at another site (I4) just west of 

I2, on the Escarpment, were also much weaker than measured at the base of the escarpment and 

in deeper water depths. 

The records show a number of distinct wave trains passing through the site (Figure 5). The first, 

between September 1999 and January 2000, showed the highest current speeds and at the 

beginning of the record there were indications that upper-layer disturbances, caused by cyclonic 

frontal eddies on the LC front, were coherent with the lower layer flows. In this first period, 

upper-layer currents were vigorous due to the presence of the LC and later the presence of Eddy 

J. The second period began in April 2000 and lasted to the end of August 2000. These TRW’s 

were also energetic though with slightly longer characteristic periods, stronger bottom trapping, 

and larger westward amplification between I1 and I2, than observed in the first period. The 

upper-layer currents were quiescent during this second interval as Eddy J had moved off into the 

western Gulf. Thus, there was no evident connection with the initiation of the April TRW’s with 

5 
I3

I1

I2

J1

N

N

N

N

Velocity (cm/s) Velocity (cm/s) Velocity (cm/s) Velocity (cm/s)

Figure 4. The first year of 40-HLP velocity vectors from below 1000 m at I1, I2, I3 and J1. 

The direction of the y (V-component) axis with respect to North for each mooring is 

indicated.

6

simultaneous fluctuations of the upper-layer currents. A third interval of energetic TRW’s which 

began in February 2001 and was associated with the shedding of Eddy M and associated 

cyclonic circulations. In all, five wave trains, with differing periods and wavelengths were 

identified in the bottom records. Ray-tracing suggests that (1) the west side of the LC is the most 

likely source region for these waves and (2) it is difficult for such short period waves to penetrate 

into the western Gulf basin. This is an accord with earlier measurements (Hamilton 1990) that 

showed 20- to 30-day TRW’s dominating in the central and western Gulf. If the mechanism for 

coupling of surface propagating eddies or meanders with deep TRW motions that has been put 

forward for TRW generation by the Gulf Stream in the North Atlantic (Pickart, 1995) applies in 

this portion of the Gulf of Mexico, then small cyclonic frontal eddies on the LC and LCE fronts 

could be candidates for generating deep, short-period TRW’s in the eastern Gulf. 

It is apparent from these measurements and the previous one-year of current data obtained by BP 

at a site close to I1 (Hamilton, 1998), that TRW activity is fairly continuous at the base of the 

Escarpment in this region(Figure 4). The distribution of energy across the array was quite 

inhomogeneous, and varied with the different observed wave trains. The rotation of the principal 

axes of the motions from partly across-isobath at I3 to along-isobath at I1 and I2 suggests that 

the relatively steep slope of the Escarpment may be influencing the propagation of the TRW’s, 

possibly by reflecting the energy back into deep water. The weak currents at J1 and I4 suggest 

that the Escarpment in this region was an effective barrier to TRW motions propagating into 

shallower water. 

Upper-layer currents were dominated by the passage of the two major LC anticyclones and 

associated processes (Figure 5). The periphery of these eddies seem to have produced the most 

energetic temperature and velocity fluctuations that were often associated with rapidly 

translating cold cyclones. The passage of Eddy M into the western Gulf, between February and 

June 2001, allowed partial analysis of radial and azimuthal flows from mooring I1. The path and 

characteristics of Eddy M were analyzed using drifter trajectories acquired from Horizon Marine, 

Inc. These data in conjunction with satellite altimetry showed this eddy moved further north 

than the previous Eddy J, and the center of the Eddy M passed within 60 km of the I1 mooring, 

so that a substantial portion of the eddy interior was measured. Eddy M was probably forced 

northwards and towards the slope by a large cyclone on its western side that formed while the 

eddy was still attached to the LC. This cyclone eventually moved up onto the slope and blocked 

further westward passage of Eddy M, which moved rapidly southwards during May 2001, and 

then resumed a more commonly seen southwestward path into the western deep Gulf. The 

azimuthal velocity and temperature profiles behaved roughly as expected for an elliptical eddy in 

solid body rotation with a period of 8 to 10 days. However, there were significant anomalies that 

were associated with either attachment to the Loop Current or interactions with the cyclone and 

slope. The passage of the eddy allowed estimation of the distances from the center to the 

maximum velocity, along the semi-major axis, and the width of the cyclonic shear zone outside 

the maximum velocity position. These were 150 and 50 km, respectively. The narrow cyclonic 

shear zone implies a large positive vorticity anomaly and thus a generation region for non-linear 

instabilities. 

8 
Analysis of upper-layer inertial currents show that they were present and varied in amplitude 

with time and depth. In the January – February, 2001 interval, before the arrival of Eddy M, 

when the upper-layer velocities were dominated by vigorous interacting cyclones, strong inertial 

currents were generated in the surface layer (Figure 6). These were more vigorous than in the 

fall and winter of 2000, when background currents were small and wind forcing was stronger. It 

was speculated that the interacting eddies were partially responsible for the increased amplitudes 

of the inertial oscillations. The January – February inertial waves did not propagate to deeper 

depths until Eddy M arrived. In Eddy M, the region just above the main thermocline had greater 

inertial energy than the surface indicating the propagating inertial internal waves were being 

trapped by the negative vorticity anomaly in the center of the eddy. Below 800 to 1000 m, 

inertial oscillations had negligible amplitudes. 

Three bottom-mounted PIES were deployed in a triangle around mooring I1 to test the viability 

of using the GEM methodology to generate low-frequency temperature, salinity and geostrophic 

velocity depth profiles. Deployment of an array of PIES and bottom current meter moorings is 

much more cost effective for large-scale measurement programs in deep water, than deploying 

arrays of full-depth moorings like I1. A preliminary GEM was constructed from historical CTD 

data for the Gulf and applied to the bottom pressure and travel time records from the three PIES 

deployed in the last 6-months of the study. The results derived profiles of temperature, salinity 

and geostrophic currents were compared with appropriate direct measurements of these 

quantities from mooring I1(Figures 7, 8 and 9). The time series comparisons were good for all 

depths with high statistical significance and confidence. The last six months of the deployment 

included part of the passage of Eddy M. This tests point to the confident use of PIES in future 

deep Gulf physical oceanographic measurement programs. 

9 
40-HLP 

Wind Speed at

BURL1

C Eddy J C Eddy M Cyclone(s)

Figure 6. Amplitudes of inertial (26 hour) current oscillations at the indicated depths of 

mooring I1. Top panel shows the 40-HLP wind speed at the BURL1 C-MAN 

Station. The vertical axes show the amplitudes (in cm/s) of inertial current 

oscillation. The dashed vertical lines help identify intervals when LC eddies and 

their peripheral cyclones (C) dominated the upper layer at Mooring I1. 

Quiescent

cm/s m/s

10
15

20

25

I1 MOORING PIES

150 m ; rms = 0.62,1.28,0.68 °C

temperature [°C]

5

10

15

20

300 m ; rms = 0.59,1.11,0.75 °C

temperature [°C]

5

10

15

20

400 m ; rms = 0.59,0.93,0.59 °C

temperature [°C]

6

8

10

12

14

525 m ; rms = 0.52,0.48,0.42 °C

temperature [°C]

4

6

8

10

Feb Mar Apr May Jun Jul Aug

2001

650 m ; rms = 0.52,0.23,0.41 °C

temperature [°C]

 Figure 7. Comparison of temperature time series measured by the I1 mooring (red) and 

temperatures estimated at the same set of depths by the 3 nearby PIES (blue) using the 

GEM interpretation of the tau (τ = RT travel time) measurements.

11
34.5

35

35.5

36

Feb Mar Apr May Jun Jul Aug

2001

I1 MOORING PIES

450 m ; rms = 0.08,0.06,0.08 

salinity (psu)

Figure 8. Comparison of salinity time series measured by the I1 mooring (red) and salinity 

estimated at 450 dbar by the 3 nearby PIES (blue) using the GEM interpretation of the 

tau (τ) measurements.

12
 50

0

50

PIES I1 Mooring Depth=122 m

RMS = 11.9,11.2 cm s-1

 20

0

20

40

300 m

RMS = 10.3,11.0 cm s-1

 20

0

20

40

410 m

RMS = 12.2,10.0 cm s-1

 20

0

20

1400 m

RMS = 8.7, 6.5 cm s-1

 20

0

20

1800 m

RMS = 3.3, 2.6 cm s-1

 20

0

20

Feb Mar Apr May Jun

2001

1979 m

Feb Mar Apr May Jun Jul

2001

velocity (cm s-1) velocity (cm s-1) velocity (cm s-1) velocity (cm s-1) velocity (cm s-1) velocity (cm s-1)

Figure 9. Comparison of velocity measured by the I1 mooring (right) and velocity estimated at 

the same set of depths by the 3 nearby PIES (left) using the GEM interpretation of the 

tau (τ) measurements and referencing geostrophic shears to the deep current meter on 

the I1 mooring.

13
4.0 References 

Hamilton P., Berger, T.J., J.H. Churchill, R.R. Leben, T.N. Lee, J.J. Singer, W. Sturges, and E. 

Waddell. 2000. DeSoto Canyon eddy intrusion study; Final report. Volume II: Technical 

report. U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS 

Region, New Orleans, LA. OCS Study MMS 2000-080. 269 pp. 

Hamilton, P. 1998. Analysis of currents at the BP Atwater Valley Block 618 exploration site. 

SAIC Final Report, submitted to BP Exploration Inc. 

Hamilton, P. 1990. Deep currents in the Gulf of Mexico. J. Phys. Oceanogr. 20:1087-1104. 

Pickart, R.S. 1995. Gulf Stream-generated topographic Rossby waves. J. Phys. Oceanogr. 

25:574:-584. 

Watts, D.R., C. Sun, and S. Rintoul. 2001. Gravest empirical modes determined from 

hydrographic observations in the subantarctic front. J.Phys. Oceanog. 31: 2186-2209. 

15 
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