OCS Study 

MMS 2001-093 

Velocity and Transport Characteristics 

of the Louisiana-Texas Coastal Current 

during 1994 

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Longitude 

US. Department of the Interior 

Minerals 7management Service 

Gulf of Mexico OCS Region 
OCS Study 

MMS 2001-093 

Velocity and Transport Characteristics 

of the Louisiana-Texas Coastal Current 

during 1994 

Authors 

Stephen P . Murray 

Ewa Jarosz 

Edward T. Weeks III 

Prepared under MMS Contract 

14-35-001-30724 

by 

Coastal Studies Institute 

Howe-Russell Geoscience Complex 

Louisiana State University 

Baton Rouge, Louisiana 70801 

Published by 

U .S. Department of the Interior 

Minerals Management Service New Orleans 

Gulf of Mexico OCS Region November 2001 

DISCLAIMER 

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

Coastal Studies Institute, Louisiana State University . This report has been technically reviewed 

by the MMS and approved for publication. Approval does not signify that the contents 

necessarily reflect the views or policies of the Service, nor does mention of trade names or 

commercial products constitute endorsement or recommendation for use. It is, however, exempt 

from review and compliance with MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of this report may be obtained from the Public Information Office (MS 5034) at the 

following address: 

U.S . Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Office (MS 5034) 

1201 Elmwood Park Boulevard 

New Orleans, Louisiana 70123-2394 

Telephone Number: (504) 736-2519 or 

1-800-200 GULF 

CITATION 

Suggested citation : 

Murray, S .P., E. Jarosz and E.T. Weeks, III. 2001 . Velocity and Transport Characteristics of the 

Louisiana-Texas Coastal Current during 1994. OCS Study MMS 2001-093 . U.S . Dept. of the 

Interior, Minerals Management Service, Gulf of Mexico Region, New Orleans, LA, 89 pp. 

iii 

TABLE OF CONTENTS 

LIST OF FIGURES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii 

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

IN'I'RODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

PART I: A REPEAT SECTION STUDY WITH CONTIGUOUS CURRENT 

METER MOORINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

A. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

B. The Study Area and Data Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

C. Results of Data Analyses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

1 .Repeat Section Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

1 .1 .Meteorological Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

1 .2 .Hydrography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

1 .3 .Vertical Distribution of the Horizontal Velocity 

Components. . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

1 .4 .Volume Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

2. Tidal Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

3 . Near Inertial Motion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

3 .1 . Complex Demodulation of Observed Currents . . : . . . . . . . . . . . . . . . . 25 

3 .2. Relation of Near Inertial Oscillations to Wind Forcing . . . . . . 30 

4 . Low Frequency Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 

4.1 . Monthly Flow Field from Current Meters. . . . . . . . . . . . . . . . . . . . . . . . . . . 32 

4.2 . Spatial Correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 

4.3 . Spectral Analysis of the Low Frequency Flow . . . . . . . . . . . . . . . . . . . 39 

4.4 . Possible Driving Forces of Low Frequency Currents - 

Multiple and Partial Coherence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 

D. Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 

PART n: TRANSPORT OF THE LTCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 

A. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 

B. Characteristics of Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 

C. Results of Data Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 

1 . Structure of Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 

2. Cross-Sectional Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 

3 . Decorrelation Time Scales across the Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 

4. Spatial Correlation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 

5. Multiple Coherence Analysis from Current Meter Data . . . . . . . . . . . . . . . . . . . . 76 

6. Transport and Wind Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 

7. A Wind-Driven Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 

8 . Salinity Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 

D. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 

v 

LIST OF FIGURES 

Figure I-1 . Map of the study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Figure I-2. Daily Mississippi River discharge at Tarbert Landing in summer 1994 . . . . . . . 7 

Figure I-3 . Histogram of wind speed vs. number of observations, 06/01-08/31/1994 . . . . 7 

Figure I-4. Histogram of wind direction vs . number of observations, 

06/01-08/31/1994 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Figure I-5 . The time series of the wind velocities measured at MB 19 and Grand Isle, 

06/23-07/04/1994 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

Figure I-6 . Vertical distribution of salinity along (a) S 1, (b) T2, (c) S2, 

and (d) TO sections during Pass I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

Figure I-7. Vertical distribution of salinity along S 1 during (a) Pass II and (b) 

Pass In. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

Figure I-8 . Vertical distribution of the alongshore velocity current component 

along S 1 and S2 during (a) Pass I, (b) Pass II, and (c) Pass III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

Figure I-9 . Vertical distribution of the cross-shore velocity current component 

along S 1 during Pass I. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

Figure I-10 . Amplitudes of the K, and O, tidal currents obtained from the 15-day 

overlapping subsets for the upper current meter at mooring M2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

Figure I-11 . The 3-40 hour band pass east-west and north-south velocity components 

of currents at the upper meter of mooring M2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

Figure I-12 . Time series of the east-west and north-south components of the 20-28 h 

band pass residual currents from upper meters at (a) Ml,(b) M2, (c) M3 

and (d) M4 mooring sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

Figure I-13 . Time series of the east-west and north-south components of the 20-28 h 

band pass residual currents from lower meters at (a) M1, (b) M2 and (c) M4 

mooring sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 

Figure I-14. Time series of the east-west component of the 20-28 h band pass 

residual currents from the upper and lower meters at M2 mooring site . . . . . . . . . . . . . . . . . 29 

Figure I-15 . Magnitudes (a) and phases (b) of the demodulated currents 

recorded by upper meters at M1, M2 and M3 mooring sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 

Figure I-16. East-west (a) and north-south (b) velocity components of the 

modeled and observed inertial oscillations at M2 location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 

vii 
Figure I-17. Monthly average current vectors during (a) July 1994, 

(b) August 1994 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 

Figure I-18. Correlation coefficients of east-west and north-south current velocity 

components as a function of cross-shelf mooring separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 

Figure I-19. Correlation coefficients of east-west and north-south current velocity 

components as a function of along-shelf mooring separation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 

Figure I-20. Spectra of the east-west and north-south velocity current component 

for the upper meter at M4 mooring site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 

Figure I-21. Multiple coherence and partial coherences between the east-west 

velocity component of the upper meters with the east-west pseudo wind 

stress, north-south pseudo wind stress, along-shelf pressure gradient 

and buoyancy forcing at (a) M1, (b) M2, (c) M3, and (d) M4 mooring 

locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 

Figure I-22. Multiple coherence and partial coherences between the northsouth velocity component of the upper meters with the east-west pseudo wind 

stress, north-south pseudo wind stress, along-shelf pressure gradient 

and buoyancy forcing (b) at (a) M1, (b) M2, (c) M3, and (d) M4 mooring 

locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 

Figure n-1. Regional map containing study transect of LTCC south 

of Cameron, LA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 

Figure n-2. Detailed map of bathymetry along mooring line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 

Figure II-3 . Details of instrumentation along mooring line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 

Figure n-4. Data log of current meters and SeaCat sensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 

Figure II-5 . Example of hourly current vectors from the meters on the three levels 

of mooring C during winter downcoast flow November 10-30, 1996 . . . . . . . . . . . . . . . . . . . . 51 

Figure II-6 . Example of hourly current vectors from the meters on the 3 levels 

of mooring C during summer upcoast from July 10-30, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 

Figure u-7 . Example of hourly current vectors from Freshwater Bayou 

for the winter period November 10-30, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 

Figure II-8 . Example of wind data from Freshwater Bayou and B42035 for the 

summer period July 10-30, 1996. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 

Figure II-9 . Energy spectra of the winds observed at Freshwater Bayou 

(a) in the winter season and (b) in the summer season . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 

viii 
Figure II-10. Energy spectra of the winds observed at B42035 

(a) in the winter season and (b) in the summer season . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 

Figure u-11 . The Atchafalaya River discharge at Simmsport 1995-97 illustrates 

the seasonal cycle of buoyant fresh water into the MACP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 

Figure u-12. Example of pressure gauge data converted to water level from (a) the 

Oyster Bayou site and (b) Freeport site November 10-30, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 

Figure II-13 . Example of pressure gauge data converted to water level from (a) the 

Oyster Bayou site and (b) Freeport site July 10-30, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 

Figure n-14. A difference between demeaned water levels at Oyster Bayou and 

Freeport for (a) November 10-30, 1996 and (b) July 10-30, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . 58 

Figure II-15. Energy spectra for the EW and NS current components of the top 

meter at mooring C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 

Figure II-16 . Coherence squared for the alongshore current component between 

the upper and mid depth meters at mooring C for four months of winter 

1996-1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 

Figure n-17 . Coherence squared for the alongshore current component between 

the mid-depth and lower meters at mooring C for four months of winter 

1996-1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 

Figure II-18. Energy spectra for both EW and NS components of the top meter 

at mooring C from June 1, 1996 through August 31, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 

Figure II-19. Coherence squared for the alongshore current component between 

the upper and mid depth meters at mooring C for three months of 

summer 1996. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 

Figure II-20. Coherence squared for the alongshore current component between 

the mid-depth and lower meters at mooring C for three months of 

summer 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 

Figure II-21 . Temporal evolution of the vertical structure of the currents at mooring 

C, September 1996 through January 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- . . . . 63 

Figure II-22. Cross-sectional distribution of subtidal alongshore currents, 

October 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 

Figure u-23 . Cross-sectional distribution of subtidal alongshore currents, 

July 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 

Figure II-24.Transport in the LTCC during the winter downcoast regime, 

September 1996 through January 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 

ix 
Figure II-25 . Transport in the LTCC during the summer upcoast regime of 1996 . . . . . . . .66 

Figure II-26. Cross-sectional distribution of alongcoast speeds on October 6, 1996, 

illustrating a downcoast intensification during the winter regime . . . . . . . . . . . . . . . . . . . . . . . . . . 67 

Figure u-27 . Cross-sectional distribution of alongcoast speed on November 26, 

1996. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 

Figure II-28 . The strongest upcoast intensification of the alongshore current field 

occurring on July 23, 1996 is notable for its moderate speeds barely in excess 

of 20 cm/s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 

Figure II-29. A strong downcoast anomaly driven by persistent easterly winds 

lasted for over 10 days with peak on August 23, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 

Figure II-30. Autocorrelation diagrams of 40 hour low passed data from the upper 

level instruments at moorings B and F for the winter observation interval . . . . . . . . . . . . 70 

Figure II-31 . Decorrelation time scales t* for the alongshore components of 

the upper level current meters for both the winter and summer observation 

intervals plotted as a function of distance offshore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- . . . . . . . 71 

Figure II-32. Autocorrelation diagrams of 40 hour low passed data from the upper 

level instruments at moorings C and G for the summer observation interval. . . . . . . . . 71 

Figure II-33. Cross-correlation function of the alongshore and cross-shore 

components of currents measured at the upper meters of mooring B and F 

for the downcoast flow regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 

Figure a-34. Cross-correlation separation distance diagram of the alongshore 

current components for the downcoast flow regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 

Figure II-35 . Cross-correlation separation distance diagram of the cross-shore 

current components for the downcoast flow regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 

Figure n-36. Cross-correlation function of the alongshore and cross-shore current 

components between the upper meters at mooring A and E for the 

upcoast flow regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 

Figure u-37 . Cross-correlation separation distance diagram of the alongshore 

current components for the upcoast flow regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 

Figure II-38. Cross-correlation separation distance diagram of the cross-shore 

current components for the upcoast flow regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 

Figure n-39 . Multiple and partial coherence between the transport observed 

during the downcoast flow regime and (a) alongshore wind stress ; 

(b) cross-shore wind stress ; (c) alongshore sea level slope; (d) Atchafalaya 

River discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 

x 
Figure II-40. Multiple and partial coherence between the transport observed 

during the upcoast flow regime and (a) alongshore wind stress ; 

(b) cross-shore wind stress ; (c) alongshore sea level slope ; 

(d) Atchafalaya River discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 

Figure n-41 . The alongshore wind stress plotted against the transport normalized 

by the cross-sectional area for the downcoast flow regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 

Figure n-42. The alongshore wind stress plotted against the transport normalized 

by the cross-sectional area for the upcoast flow regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 

Figure II-43. The alongshore 6 .5m current observed at mooring F during the 

downcoast flow regime compared to the predictions obtained from 

equation II-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 

Figure II-44. The alongshore 5 .5m current observed at mooring G during the 

upcoast flow regime compared to the predictions obtained from equation n-3 . . . . . 82 

Figure II-45. A comparison of salinity from the outer edge and mid-region of 

the coastal current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 

Figure II-46. A composite of the segments of salinity recorded from mooring F 

over a two-year period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 

xi 

LIST OF TABLES 

Table I-1 .Section volume fluxes and net box flux in m'/s, section area in m2 . . . . . . . . . . . . . . . 15 

Table I-2 .Tidal constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

Table I-3 . Semi-major axis, semi-minor axis, semi-major axis direction and 

sense of rotation of tidal currents for NLZ, S29 N21 K,, O, and Q, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

Table I-4.Semi-major axis, semi-minor axis, semi-major axis direction and 

sense of rotation of tidal currents for NLz, SZ, N2, K,, O, and Q, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

Table I-S .Inertial periods of the mooring sites on the Louisiana-Texas shelf 

south of Isles Dernieres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

Table I-6.Mean, standard deviation and root mean square error for M1, M2, M3, 

M4, LA 14, LA 15 and LA 16 mooring sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 

Table I-7.Correlation coefficients of the east-west and north-south velocity 

components as a function of cross-shelf separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 

Table I-8 .Correlation coeffcients of the east-west and north-south velocity 

components as a function of along-shelf separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 

Table I-9.Magnitude of complex correlation coefficients of the upper layer low 

frequency currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37 

Table n-1 .Mooring names, location, total depth, and instrument configurations . . . . . . . . . . 49 

Table u- 2.Decorrelation time of alongshore and cross-shore current components 

for the summer and winter flow regimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

xiii 

INTRODUCTION 

Observations from the five cruises conducted by the Mississippi River Plume 

Hydrography study portion of the Louisiana-Texas Physical Oceanography Program 

(LATEX B) (Murray et al. 1997) have provided an unprecedented visualization of the 

coastal plume that emanates from the combined discharge of the Atchafalaya and 

Mississippi Rivers into the Gulf of Mexico in central Louisiana. Conductivitytemperature-depth (CTD) casts, underway acoustic Doppler current profilers (ADCP), 

and thermosalinography on these cruises documented the significant temporal and spatial 

variability along the length of the Mississippi-Atchafalaya coastal plume (MACP). In 

addition, the ADCP data, in particular, showed a distinct coherent velocity structure 

associated with the MACP. Thus, we also use the term Louisiana -Texas Coastal Current 

(LTCC) to identify the alongshore velocity intensification that extends from these river 

mouths along the coast westward and downcoast into south Texas . We will also use the 

conventional terminology downcoast and upcoast. Downcoast refers to the direction of 

propagation of a coastal Kelvin wave that must travel with the coast to its right in the 

northern hemisphere (Csanady, 1982). Downcoast movement is thus westward on the 

Louisiana inner shelf, and southwestward and southward on the Texas coast. Upcoast 

movement correspondingly is northward, northeastward, and eastward moving from 

south Texas into Louisiana. 

In addition to documenting the spatial and short-term variability exhibited by the 

LTCC, Murray et al . (1997) demonstrated a strong but asymmetrical seasonal cycle in the 

transport of the LTCC consisting of a nine-month-long downcoast (winter) regime and a 

three-month-long upcoast (summer) regime. While seriously aliased by shifting wind 

patterns and long cruise length scales, the LATEX B data show that from about midAugust (through the fall, winter, and spring) to mid-May dominant easterly wind 

components drive a coastal current downcoast to the west and south. This current 

transports about 96 x 103 m3/s of coastal plume waters of which 29% (28 x 103 m3/s) is 

fresh water. In a normal year, based on Mississippi River discharge and regional winds, 

the summer flow (e .g . July 1994) in the reversed coastal current reaches 48 x103 m3/s, of 

which 22°Io is fresh water. 

Dynamical analysis (Murray et al ., 1997) demonstrates that the downcoast 

current regime is controlled by local wind forcing and that buoyancy forcing appears 

inconsequential. The summer upcoast flow regime off Louisiana and east Texas is 

clearly not controlled by wind, and the partial coherence analysis suggests that largescale, alongcoast pressure gradients are probably an important driving mechanism. 

While extremely enlightening, these results on the transport of the LTCC are, 

however, subject to a large error band caused by the spatial and temporal aliasing 

inherent in underway cruise data. Thus, the aim of the present study as a follow-on and 

enhancement of the LATEX B study is to conduct high resolution studies of the LTCC 

and MACP to further quantify the transport and its temporal variability. 
The program has two elements, each addressing specific questions arising from 

the LATEX B analysis. The first is a repeat section study in which the same research 

ship used in LATEX B analysis transited a control "box" 3 times in about 3 days, each 

transit taking 20 to 25 hours . The box captured day-to-day variability in the velocity and 

hydrographic structure across the LTCC just east of the Atchafalaya River mouth. 

Underway ADCP and a novel underway profiling CTD were used to assess the 

robustness of our previous single pass estimates of transport and hydrographic structure. 

Additionally, current meter moorings were deployed on 3 sides of the box that allowed 

coherence studies over distance of tens of kilometers and time scales of 40 to 50 days. 

The second and largest element of the program is a mooring current meter array 

on a line roughly across the LTCC just east of Cameron, Louisiana. The objective was to 

obtain a long-term time series of transport in the LTCC from in situ current meter 

observations . Great difficulty was encountered in this objective because of instrument 

malfunctions and numerous disruptions to the moorings from oil field activity . 

Nonetheless, data were sufficient for significant new results to emerge. 

2 
PART I 

A REPEAT SECTION STUDY 

WITH CONTIGUOUS CURRENT METER MOORINGS 

A. Introduction 

The Mississippi River is the major source of fresh water, sediment, nutrients, and 

pollutants for the northern shelf of the Gulf of Mexico. It drains over one third of the 

continental watershed of the United States and has an annual average discharge rate of 

about 19,000 m3/s (Wiseman et al ., 1997). The outflow usually peaks in spring and is at a 

minimum in late summer. The river enters the Gulf of Mexico through the Mississippi 

River birdfoot delta and the Atchafalaya River, which is a distributary of the Mississippi 

River diverted from the main channel at Old River north of Baton Rouge. The thirty 

percent of the total discharge is delivered to the Louisiana shelf by the Atchafalaya River. 

The remaining seventy percent flows through the Mississippi River birdfoot delta. 

However, estimates suggest that only about fifty percent of the fresh water discharged by 

the birdfoot delta is carried to the western Louisiana shelf (Dinnel and Wiseman, 1986) . 

Because this large input of fresh water acts to create a buoyant surface layer, one 

may expect that regional wind forcing would be a great importance. Winds over this part 

of the Gulf of Mexico show seasonal fluctuations (Gutierrez de Velasco and Winant, 

1996). During fall, winter and spring the winds are generally easterly. The dominant 

synoptic-scale disturbances in this part of the year are cold fronts, propagating mainly 

from northwest to southeast and bringing strong winds from the northern quadrant. 

Characteristic time scales of these disturbances in winter are 3 to 10 days (DiMago et al ., 

1976). The summer months are characterized by weak southerly and southeasterly winds 

associated with the dominance of the Bermuda high-pressure system (Grout et al., 1984). 

During this season, a sea-breeze system exists, and squalls, tropical storms and hurricanes 

aperiodically may impact the region (Wiseman et al., 1997) . 

A portion of the Mississippi River outflow, which flows westward, joins with the 

Atchafalaya River discharge, mixes with the shelf waters and ultimately forms a coastal 

current (Wiseman and Kelly, 1994; Murray et al ., 1997). This coastal flow has been 

called various names in the literature : for instance, it was referred to as the Louisiana 

Coastal Current by Wiseman and Kelly (1994) and the Texas current by Vastano et al . 

(1995) . The latest studies (Murray et al ., 1997) show that despite strong spatial and 

temporal variability this current can be generally traced west of the Mississippi River 

delta along the entire Louisiana and Texas coasts . Therefore, it seems reasonable to refer 

to it as the Louisiana-Texas Coastal Current (LTCC). 

During much of the year the Louisiana-Texas Coastal Current flows westward 

along the shore from Louisiana to Texas. Such a simple scenario may suggest that the 

flow is in general agreement with geostrophic theory and is driven mainly by the pressure 

gradient . The theory states that the river outflow is deflected to the right upon entering 

3 
the sea in the northern hemisphere (the Gulf of Mexico) producing a buoyant coastal 

current trapped against the coastline. Such behavior of the flow has been observed, for 

instance, in the Delaware coastal current (Munchow and Garvine, 1993) and also has 

been successfully modeled numerically (Chao, 1988). 

Observations and analyses do not, however, support such a simple behavior of the 

Louisiana-Texas Coastal Current. Smith (1975, 1978) reports seasonal variability of the 

current on the central Texas shelf with a strong south-southwesterly flow in winter and 

weak north-northeasterly or south-southwesterly flow in summer. He also implies that 

many features of the nearshore current pattern can be explained as a response to surface 

wind stress . Crout et al . (1984) describe such variability of the current on the western 

Louisiana inner shelf. They conclude that the summer current is generally disorganized 

and probably driven by non-local forcing. In contrast to the summer flow, the winter 

alongshore current is better organized and is well correlated with alongshore wind stress . 

Cochrane and Kelly (1986) report that the alongshore wind stress is the major driving 

force of the coastal current on the Louisiana-Texas shelf in a region west of 92.SW. 

During much of the year, the current flows downcoast (in the sense of Kelvin wave phase 

propagation), and it is highly correlated with the downcoast wind stress component . In 

late summer there is reversal of the flow direction and the upcoast flow regime (in the 

opposite sense of Kelvin wave phase propagation) prevails. This flow is coherent with the 

upcoast wind stress component. Li et al. (1997) also report such a bimodal annual pattern 

of the nearshore circulation on the northern shelf of the Gulf of Mexico and its 

dependence on the alongshore wind stress . 

Murray et al . (1997) confirm the downcoast and upcoast flow regimes of the 

Louisiana-Texas coastal current. They verify that during the fall-winter-spring downcoast 

regime, current fluctuations on the Louisiana-Texas shelf are controlled largely by the 

alongshore wind stress and secondarily off central Texas by the alongshore surface slope. 

During the summer upcoast flow regime, the current fluctuations in southern Texas are 

significantly associated with the alongshore surface slope and wind stress components. 

However, variability of the current in central Texas and central Louisiana is highly 

coherent with the sea level gradients. Murray et al .'s (1997) scaling analysis of the 

alongshore momentum balance also shows that the wind stress, basal stress and the 

barotropic pressure gradient are dominant forcing mechanisms for the downcoast flow. 

For the upcoast regime, the importance of wind stress decreases, and the barotropic 

pressure gradient and Coriolis forces are dominant terms of the alongshore momentum 

balance. 

In addition to the seasonal fluctuations of the coastal current, there are two 

energetic higher frequency motions on the Louisiana-Texas shelf: near inertial and tidal, 

which are occasionally important. The near inertial oscillations are difficult to determine 

because of the proximity of the local inertial period to the diurnal tidal period . Despite 

these difficulties, Daddio et al . (1978) and Chen et al . (1996) describe the presence of 

such oscillations with amplitudes as large as 40 cm/s on this shelf. They also report that 

these oscillations are generated by shifting winds that are usually associated with the 

passage of atmospheric fronts . Recent studies (LATEX A report, 1997) confirm that the 

4 
near inertial oscillations are generated by wind in non-summer months on the shelf. 

However, they also imply that in summer months the large amplitude oscillations are 

thermally induced. 

Tidal currents are rather small and diurnal on the Louisiana-Texas shelf. 

According to data from the Texas-Louisiana Shelf Circulation and Transport Study 

(LATEX A report, 1997; DiMarco and Reid, 1998), the Kl tidal component ranges from 

3% to 75°Io of the total tidal variance and is, therefore, the largest tidal current component 

on the shelf. 01, which ranges from 6 to 48% of the total variance, is the second largest 

tidal component. Among semidiurnal components, MZ is the dominant one. Together they 

account for 76% of the total near-surface tidal energy . These tidal currents generally 

rotate clockwise and their semi-major and semi-minor axes do not exceed 6 cm/s, except 

in a region south of Atchafalaya Bay where the semi-major axis is 9.58 cm/s for M2, 8 .49 

and 8.22 cm/s for 01 and Kl, respectively . 

Our understanding of the motions of various frequencies on the Louisiana-Texas 

shelf has improved greatly with recent studies. However, knowledge of the behavior of 

these motions in different locations and seasons remains decidedly lacking. This report 

focuses on the description of summer flow regime on the portion of the Louisiana-Texas 

shelf located south of Isle Dernieres, just east of the injection of the Atchafalaya River 

discharge into the eastward flowing Louisiana-Texas coastal current. This description 

will be based on a data set from the Louisiana Coastal Current Summer Flow Regime 

project (LOCCOSUM) that was designed to provide a more detailed characterization of 

the variability of currents and hydrography in the summer season . Thus, the objectives of 

this study are: (1) to describe day-to-day changes in horizontal velocity components and 

hydrography (salinity) along four sections across and along the coastal current; (2) to 

estimate the average tidal currents in this region ; (3) to determine whether weak to 

moderate summer winds can induce near inertial oscillations in the upper layer of the 

water column; and (4) to describe the variability of the low frequency flow and determine 

what mechanisms drive the low frequency current on this portion of the Louisiana-Texas 

shelf. 

B. The Study Area and Data Sources 

The area examined in this study is located on the inner shelf of Louisiana south of 

Isles Derniere between 90° 27'W and 91° 15'W longitude and 28' 30'N and 29' 1 .2'N 

latitude (Figure I-1) . This region of the shelf is rather shallow with a depth range from 

about 6.5 m near the coast to above 40 m in its southern part. The shape of the coastline 

and isobaths is quite complicated in the region and its vicinity . They do not run simply 

from east to west but bulge seaward as a result of subsidence and reworking of 

abandoned Mississippi River deltas . Generally, they trend west-northwest to eastsoutheast in the western part and northeast to southwest in the eastern part . A change in 

the isobath direction appears approximately in the middle of the study area. 

5 
30.0 

29 .5 

N 

7 

J 

29.0 

28 .5 

" 1~ r 

.s 

OLA16 

S1 

-92.0 -91 .5 -91 .0 -90.5 -90.0 -89.5 

Longitude 

Figure I-1 . Map of the study area (depth contours are in meters; M1-M4 are the current 

meter moorings deployed for this study ; LA 14-LA 16 are moorings from the 

LATEX A project; S l, S2, T0, and T2 are the ship transit lines; MB 19 

identifies a m eteorological buoy from the LATEX A project ; locations of a 

CIVIAN meteorological and water level stations at Grand Isle and a water level 

gauge at Atchafalaya Bay are indicated by diamonds) . 

Hydrography and circulation of this particular part of the shelf are highly 

impacted by fresh water, which is introduced to the system by the Mississippi and 

Atchafalaya Rivers . During the summer of 1994 both rivers had relatively low outflows . 

Except for the very beginning of June their combined discharge measured at Tarbert 

Landing did not exceed 12,000 m3/s and had a minimum on August 22 (Figure I-2) . 

In addition to buoyancy forcing represented here by the river outflow, wind is also 

an important mechanism that influences both hydrography and circulation in this region . 

A three month-wind record (June, July, August) from a meteorological buoy (MB 19, see 

Figure I-1 for location) shows that the winds during the summer of 1994 were typical 

with speeds lying generally between 3 and 6 m/s (Figure I-3) and blowing mainly from 

southwestern, southern and southeastern directions (Figure I-4) . 

LOCCOSUM data originate from two sources: a four-day cruise to collect 

current, salinity and temperature data, and four moorings deployed around the periphery 

of the study area. The cruise was conducted from June 30 to July 03 1994, and this part of 

the project was named "repeat section study" (RSS). The ship track of the RN Pelican 

along with annotated sections of RSS is shown in Figure I-1 . The sections marked S1, T2, 

S2, and TO were profiled underway with two instruments: a 1200kHz acoustic doppler 

current profiler (ADCP) and a conductivity temperature density profiler (CTD) . 

6 
zooao 

18000 

16000 

14000 

Annul average discharge 

12000 

m 10000 ar 

R 

5000 

N 

6000 

4000 

2000 0 L 

06101/94 07101/94 07/31/94 08/30/94 

Figure I-2 . Daily Mississippi River discharge at Tarbert Landing in summer 1994. The 

thin horizontal line indicates the annul averaged discharge of 14,000 m3/s. 

520 

480 

440 

400 

p 360 

c 

320 

A 

ZN 280 

H 

O 240 

200 `ro 

a 

E 160 

120 

80 

° o F .31 X3 .4] (4 .5] ('S .sl X6 .77 (7 .8] (8 .91 (9 .10] (10,11] 

Spaced (m/s) 

Figure I-3 . Histogram of wind speed (1 m/s intervals) vs . number of observations, 06/O1-

08/31/1994. 

The shallowness of the study area allowed using the ADCP instrument in the 

bottom-tracking mode. Horizontal velocity components obtained from the ADCP had a 

one-meter vertical bin resolution and a two-minute sampling interval . After applying 

correction coefficients and removing ambiguous readings, the velocity data were 

obtained for a depth range from 3.5 m to 15 .5 m in 1-meter increments . However, if the 

total depth (H) was less than 15.5 m the depth range was lower, and it extended from 3 .5 

m to about 0.8H meters with the same vertical increment. The CTD instrument mounted 

in a side-scan sonar housing profiled at a one-second sampling interval allowing the 

excellent horizontal and vertical resolution of salinity, temperature and density data. 

7 
225 

210 

195 

180 

165 

150 

c 

0 135 

.~ 120 

105 

a 

0 90 

75 

a 60 

45 

Z 

30 

15 

n o W o n o W o h o in o W o W o W o W o n o W o 

H7 V 10 P Ol O N M H 10 OD O1 N V h P OD O M ~ 10 

O h O h O ~ N N N N c 't e`7 1'1 M M M 

`-' v v ~ ~ O N O L(f O H O 1(f O N O N O h O N O N 

0~7 O N C'f ~l1 t0 00 OI N a N P CD O e'9 q 

N N N N N N 1y M M M 

Direction (°) 

Figure I-4. Histogram of wind direction (15° intervals) vs. number of observations, 

06/01-08/31/1994 . 

The circuit around the box was repeated three times with each pass starting at the 

northeastern corner of the S1 section, which was located at 29° 0.96'N and k=90° 30'W. 

The first pass took 25.17 hours to be completed. The second and the third passes took 

20.43 and 20.67 hours, respectively . 

On the third and fourth days of the cruise four moorings were deployed (M1, M2, 

M3, M4) at locations shown in Figure I-1 . Each mooring had the same instrument 

configuration and consisted of two Endeco current meters and a SeaCat. The current 

meters were deployed at depths of 5 .46 m and 10 m. The SeaCat was placed 6 .86 m 

below the sea surface . The current meters collected speed and direction of currents with a 

five-minute time interval . The SeaCat measurements of temperature and salinity were 

recorded with a ten-minute time interval. Except for the lower current meter at M3, all 

other instruments worked properly and nearly two-months of continuous data were 

obtained for further analyses. 

In addition to the measurements of this project, current meter data (LA16, LA15, 

LA 14) and wind data (MB 19) collected for the Texas-Louisiana Shelf Circulation and 

Transport Study (LATEX A project) were also analyzed (see Figure I-1 for locations) . 

The LA 16 and LA 14 meters were located 9 m below the sea surface . The LA 15 meter 

was located at 10 meters . The meteorological buoy (MB 19) contained an anemometer, 

which was placed 3 .6 m above the sea level and measured wind speeds and directions . 

Additionally, wind data from the CMAN station at Grand Isle, water levels from gauges 

at Atchafalaya Bay near Eugene Island and at Grand Isle and discharge of the 

Atchafalaya River at Simmesport were also used in the analyses . 

8 
C. Results of Data Analyses 

1. Repeat Section Study 

1.1 . Meteorological Conditions 

Winds measured at station MB 19 and Grand Isle (located about 220 km apart) 

before and during the repeat section study (RSS) are shown in Figure I-5. The cruise was 

preceded by a week of moderate southwesterly winds, which should set up an eastward 

current in the study area. According to MB 19, Pass I started (17:30GMT, June 30 1994) 

~-

' ~ P a s 8I 

,HUOy ,19 

~ P ass II~ 

' P a saIII' 

a 17 d Is]e 

10 ./a 

2 3 2 4 2 5 2 6 2 7 2 8 2 9 3 0 0 1 0 2 0 3 0 d 0 5 

June 1 9 9 4 Judy 1 9 9 4 

Figure I-5 . The time series of the wind velocities measured at MB 19 and Grand Isle, 

06/23-07/04/1994 (the day number marks the beginning of the day). 

with very light southerly winds, which accelerated to 3 m/s and then blew steadily from 

the south for 6 hours. During the remaining part of the cruise they were generally weak to 

moderate (0 .5-5 m/s) and rotated clockwise, except for the winds measured on July 2 

1994, which were easterly for most of the day . 

1.2. Hydrography 

During the entire RSS, a well-developed stratification with three distinct salinity 

layers was observed all around the box. Examples of this salinity structure are shown in 

Figure I-6 . The upper part of the water column was occupied by low salinity waters (-22 

9 
psu) . This freshwater layer was underlain by a well-defined halocline, which was usually 

4 m thick and located from 6 to 10 m below the sea surface, except for T0 . Below the 

base of the halocline layer there was a relatively homogeneous deep layer filled with Gulf 

waters (34-36 psu). Thickness of these layers varied from one place to another. The weak 

and moderate winds present during the cruise were not able to break down this 

stratification, and the three-layer salinity pattern was maintained along all sections 

through Pass II and Pass III. 

Superimposed on this general three-layer structure were some notable deviations 

in the salinity distribution . During Pass I on line S1, we observed a lens of low salinity 

waters detached from the shoreline and the halocline sloped markedly up to the coast, 

both classic signals of upwelling conditions . The measurements from the next two passes 

(Figure I-7) showed a shoreward movement of this lens, for example, the 20 psu isohaline 

moved shoreward about 30 km from Pass I to Pass III . Such behavior of the lens and 

downward movement of the halocline at the inshore end of this section may suggest the 

relaxation of the upwelling episode along S 1 . The wind records from MB 19 and Grand 

Isle prior to the RSS cruise indicate that the winds were indeed upwelling favorable. 

During the cruise, the winds weakened and blew from various directions. Therefore, the 

measurements probably recorded the end of the upwelling event when the system began 

returning to equilibrium . 

The end of this upwelling episode was not observed along the S2 cross-section 

located about 74 km west of S1 . That may have resulted from the different configuration 

of the isobaths and coastline in this region . The winds, which caused the upwelling along 

S 1, apparently were not able to set up such conditions at S2 because they were 

approximately perpendicular to the coast and the isobaths, rather than more parallel as 

favored by upwelling theory. 

Another interesting feature in the salinity distribution was observed along the 

shallowest inshore section of the RSS, T0. In its upper few meters, very fresh water with 

salinity of 14 psu was observed during all three passes . This distinct lens of the light 

waters was 2-3 m thick and had an alongshore length scale of 10 to 20 km. It was 

generally located in the western part of the section and may have represented the 

eastward flowing Atchafalaya River discharge and/or the outflow from channels and bays 

situated northward of T0 . 

In contrast to the salinity distribution, vertical temperature structure did not show 

much variability. All sections contained warm waters, in which temperature varied from 

30°C in the very upper part of the water column to 21°C in the deepest parts of the study 

area. Density distribution followed closely the vertical salinity structure at all sections as 

expected in coastal waters highly influenced by large fresh water input. 

10 
0 

-2 -

-4 

-s 

-s 

-10 

-12 

y -14 -

a 

~+ -1 6 

-18 

-20 

-22 -

-24 -

-26-

-284, 

0 

0 

-2 

-6 

-8 

-10 

-12 

r -14 

r+ 

-16 

-18 

-20 

-22 

-24 -

-26 

-28 

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 

Distance (km) 

Figure I-6 . Vertical distribution of salinity (psu) along (a) S1, (b) T2, (c) S2, and (d) TO 

sections during Pass I. 

5 10 15 20 25 30 35 40 45 

Distance (km) 
0 

-2 

-4 

-6 

-8 

-10 

-12 

r -14 

a+ 

-16 

-18 

-20 

-22 

-24 

-26 

-is 

C 

52 

0 5 10 15 20 25 30 35 

Distance (km) 

0 

-4 

E 

s .. 

a 

-6 

-8 

-10 

-2 

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 

Distance (km) 

Figure I-6. Vertical distribution of salinity (psu) along (a) S 1, (b) T2, (c) S2, and (d) TO 

sections during Pass I (continued). 

12 
0 

2 

-10 

-12 

-14 

-16 

-18 

-20 

-22 

-24 

-26 

-28 

0 

0 

2 

-10 

£ -12 

t -14 r.+ 

Q 

-16 

-18 

-20 

-22 

-24 

-is 

-is 

5 10 15 20 25 30 35 40 45 

Distance (km) 

b 

S1 

0 5 10 15 20 25 30 35 40 45 

Distance (km) 

Figure I-7. Vertical distribution of salinity (psu) along S1 during (a) Pass II and (b) Pass 

III. 

13 
1.3 . Vertical Distribution of the Horizontal Current Velocity Components 

Vertical distributions of horizontal current components around the box showed 

that the alongshore component of the flow at each section was usually more energetic 

than the cross-shore component through all three passes . Day-to-day changes in the 

current field were more distinct in the shallowest pans of the study area (total depth less 

than 10 m) where the flow was generally driven by the wind and responded quickly to 

any variations in wind speed and direction . Offshore of this shallow region, in general, 

we observed a well-developed upcoast flow (in the opposite sense of Kelvin wave phase 

propagation) in the upper part of the water column throughout the entire RSS. 

Examples of the observed changes in the alongshore component through Pass I to 

Pass III along two cross-sections (S 1 and S2) normal to the coast are shown in Figure I-8 

(a-c). During the first pass, the strong upcoast flow was observed at both sections (Figure 

I-8a) . These upcoast currents were characterized by a subsurface maximum, which was 

more energetic at S1 . Such a subsurface intensification of the flow may result from a 

cross-shore baroclinic pressure gradient. Observed sloping density contours imply that 

such a cross-shore baroclinic pressure gradient was present during Pass I . The difference 

in the subsurface speeds of the flow at S 1 and S2 may again originate from a difference 

in the cross-shore baroclinic pressure gradient, which depends on the cross-shore density 

gradient. The density gradient at S 1 was larger than that measured at S2. Therefore, if the 

intensification is baroclinic in origin, the speeds should be higher at S 1 . At S2, a surface 

intensification of the flow was also observed and its origin is difficult to explain. It may 

have come from the cross-shore barotropic pressure gradient or it may have been wind 

driven . 

Subsequent to Pass I, the upcoast flow generally decelerated and a deeper 

countercurrent emerged during the next two passes (Figure I-8b and I-8c). However, both 

deceleration and countercurrent were more distinct along the S 1 section . Such 

deterioration of the upcoast flow was also present along the southernmost section, T2 (not 

shown). This upcoast flow, which was observed throughout the entire RSS, was probably 

set up by the southwesterly winds blowing prior to the cruise and may have represented a 

well-developed phase of the LTCC in its summer flow regime. 

The weak narrow downcoast current flowing below the energetic upcoast current 

was first observed only at S 1 (Pass I) . During the second pass, this countercurrent became 

stronger and broader (Figure I-8b). Figure I-8c shows that a weak downcoast flow was 

even present at S2 when the section was sampled for the third time. The countercurrent, 

especially that at S1, may originate from the upwelling episode observed along this 

section since such a current is a persistent feature observed in upwelling regions (Smith, 

1968). According to Yoshida's two-layer model (1967), a uniform wind field implies a 

condition of no net alongshore transport requiring a compensating countercurrent under 

the surface layer. 

As has been stated previously, the signature of the end of the upwelling episode 

was observed along the S 1 line in both salinity and alongshore flow structure . The crossshore flow field was also consistent with a circulation pattern observed in upwelling 

14 
regions . During Pass I, the vertical structure of the cross-shore velocity component still 

showed very well developed upwelling circulation (0-20 km) with an onshore flow in the 

lower part and an offshore flow in the upper part of the water column (Figure I-9) . 

1.4 . Volume Transport 

The sections sampled during the repeat section study were designed to enclose a 

volume; therefore, it was of interest to determine the volume flux for each section and to 

investigate the volume divergence for the box . Unfortunately, the horizontal velocity 

components were available only to a depth of 15 .5 m. Therefore, the volume transport 

was determined by integrating the velocity speed normal to each section from the sea 

surface down to 15 .5 m if the total depth was higher or to the total depth in the shallow 

part of the study area. Two assumptions were required in order to estimate the net box 

transport. The first stated that there was no exchange of volume through the sea surface 

that bounded the box from the top (evaporation minus precipitation is zero). Another said 

that there was also no volume exchange through the bottom of the box. The latter was 

made because of the lack of the vertical velocity data and might be false . Results are 

listed in Table 1 . 

Table I-1 

Section volume fluxes and net box flux in m3/s, section area in m2. 

Section Pass I Pass u Pass III Area 

S 1 120,830 27,545 -10,583 717,000 

T2 -56,381 -25,361 -29,214 1184,000 

S2 121,611 84,338 48,835 459,000 

TO -49,894 -11,714 2,539 564,000 

Box -5,706 43,146 27,665 

The positive and negative flux of S 1 and S2 are considered to be eastward and 

westward, respectively . T2 has positive northward and negative southward transport. The 

coordinate system was rotated 7 .7° counterclockwise from the north in order to estimate 

the velocity component perpendicular to T0, and positive and negative values of the flux 

are directed toward 7° and 187 .7°, respectively . The box net transport is estimated by 

assuming that the waters flowing into the box have positive volume flux and the waters 

flowing out of the box have the negative volume flux . 

The results show that all section volume fluxes decreased from Pass I to Pass II . 

The most significant decrease appeared for the S 1 and TO sections and resulted from 

drastic changes in the current flow field. During the third pass, the transport even had an 

opposite sign as a result of further modifications of the current flow in these two sections. 

The smallest section, if area is considered, S2, had the largest and most unidirectional 

volume flux that was eastward during all three passes . However, this eastward transport 

decreased on account of decelerating eastward flow from one pass to another. Similar to 

S2, the flux of T2 was unidirectional (southward), which diminished from Pass I to Pass 

15 
a 

0 

-2 

-4 

-6 

-8 

-10 

-12 

s -14 .F+ 

0- -16 

-18 

-20 

-22 

-24 

-26 

-28 

Distance (km) 

Figure I-8. Vertical distribution of the alongshore velocity current component (cm/s) 

along S 1 and S2 during (a) Pass I, (b) Pass II, and (c) Pass III (positive values 

are upcoast; negative values are downcoast) . 

16 

0 5 10 15 20 25 30 35 40 45 
b 

0 

-2 

-4 

-6 

-8 

-10 

v 

-12 

-14 

C'- -16 

-18 

-20 

-22 

-24 

-26 

-28 

Distance (km) 

Figure I-8. Vertical distribution of the alongshore velocity current component (cm/s) 

along S 1 and S2 during (a) Pass I, (b) Pass II, and (c) Pass III (positive values 

are upcoast; negative values are downcoast) (continued) . 

17 

0 6 10 16 20 25 30 35 40 45 
C 

0 

_2 

-4 

-6 

-8 

-10 

-12 

r -14 

m -16 

-18 

-20 

-22 

-24 

-26 

-28 

Distance (km) 

Figure I-8. Vertical distribution of the alongshore velocity current component (cm/s) 

along S 1 and S2 during (a) Pass I, (b) Pass II, and (c) Pass III (positive values 

are upcoast; negative values are downcoast) (continued) . 

18 

0 5 10 15 20 25 30 35 40 45 
E 

s 

a 

d 0 

-2 

-10 

-12 

-14 

-16 

-18 

-20 

-22 

-24 

-26 

-28 

\ , 1

/ 01 

Is 30 f ~o \ 

5 

_~ o 

IV 

O 

20 

ii . . : . +i- . 

h 

10 15 20 25 30 35 40 45 

Distance (km) 

Figure I-9 . Vertical distribution of the cross-shore velocity current component (cm/s) 

along S 1 during Pass I (positive values are onshore; negative values are 

offshore). 

II and then increased for the last pass. However, this increase (about 4000 m3/s) might be 

considered insignificant because of the ADCP data resolution (-2 cm/s) . 

The results also show that the net box flux is not equal to zero for all passes . Since 

an error of the volume flux estimations resulting from instrument accuracy is 7240 m3/s, 

the net flux of the first pass might be considered approximately zero. The next two are 

significantly higher, and it might suggest that there was exchange of the volume through 

the lower boundary of the box. Such negative vertical flux may have occurred since the 

end of the upwelling episode was observed during the RSS, at least in the eastern part of 

the study area. Lack of information about vertical velocity distribution prevents 

determining whether such downward flow of waters through the lower boundary of the 

box really existed. However, the available data allow at least estimating the average 

vertical velocity that is required to compensate the box net transport. Its value calculated 

by division of the net flux by the area of the box bottom is O(10-3) cm/s. Vertical velocity 

may be also approximated by division of the displacement of the 32 psu isohaline by time 

of the first or second pass . The obtained results give the same amplitude order of the 

vertical velocity as that estimated from the net box transport. These estimates seem to be 

reasonable and may explain why the net box fluxes of the last two passes are not equal to 

zero. Finally, periodic motion, which was not removed from the raw data, may be 

responsible for different than zero net box flux . 

19 
2. Tidal Motion 

Since little is known about tidal forcing in the study area, time series obtained 

from seven current meters deployed for this project were examined to evaluate average 

tidal currents . It should be noted that the currents estimated in this chapter represent the 

average tidal currents during the period July 3 through August 29 1994, and they are not 

corrected for the nodal variation of the moon's declination . 

Based on the Rayleigh criterion, three diurnal tidal periods (O,, K, and Q) and 

three semi-diurnal tidal periods (S, N1, and N) were analyzed (Pugh, 1987) . The phase 

and amplitude were estimated from 3- to 40-hour band pass filtered data of the horizontal 

velocity components. The names, symbols and periods of all analyzed tidal constituents 

are given in Table I-2 . 

Table I-2 

Tidal constituents . 

Symbol Name Period (solar hours) 

Semi-diurnal constituents 

MZ principal lunar 12 .42 

NZ larger lunar elliptic 12.66 

SZ principal solar 12.00 

Diurnal constituents 

KI luni-solar diurnal 23 .93 

O, principal lunar diurnal 25.82 

Q, large lunar elliptic 26.87 

The tidal constituents were determined using least squares harmonic analysis 

(Dronkers, 1964) that treats the observed tides as the sum of a finite number of 

harmonics. The method seeks to fit the following model to the data 

M 

ll(t) _ !gyp + I [Ak COS(Wkt) + Bk S1I1(CVkt)] ~I -

k=1 

where Ao, Ak and Bk are coefficients to be found, t is the time and the wkare known 

frequencies representing a tidal constituent. u denotes a horizontal velocity component. 

This model, when least squares error constraint is imposed, gives for each velocity 

component a system of 2M+1 linear equations that can be easily solved . 

Table I-3 summarizes the results of the harmonic analysis performed for seven 

meters from four mooring places M1, M2, M3 and M4 (see Figure I-1 for locations) . The 

table contains information about semi-major and semi-minor axes, direction of the semimajor (positive anticlockwise from the east) and rotation sense. These parameters were 

20 
constructed from the phase (referenced to 22:00 LJTC, July 3, 1994) and amplitudes of 

the east-west (u) and north-south (v) components of each tidal current constituent 

(Mooers, 1973 ; Pugh, 1987) . 

The results show that the M2 tide dominates among semi-diurnal constituents . 

Values of the major and minor axes are in fairly good agreement with the results obtained 

for current meters of the LATEX A project (LATEX A report, 1997 ; DiMarco and Reid, 

1998). At the easternmost mooring (M1) values do not exceed 2 and 0 .5 cm/s for the 

major and minor axes, respectively . Farther to the west, the M2 tidal currents increase 

with the maximum at the mooring M4. That westward increase of the length of the major 

axis was also observed in data from the LATEX A program (LATEX A report, 1997 ; 

DiMarco and Reid, 1998). The S2 and N2 tidal currents are weaker than those described 

above. Similar to the M2 current, their maximum major axis was found for the lower 

meter of the M4 location . The direction of the major axis and sense of rotation generally 

vary from one place to another. 

According to the data, the largest tidal currents on this portion of the shelf are 

diurnal and the largest constituents are K, and O, . They all rotate clockwise. However, 

their major and minor axes are significantly different from those evaluated from the data 

of LATEX A project (LATEX A report, 1997 ; DiMarco and Reid, 1998). The first 

possible reason for this discrepancy may be the shorter time span of the records that were 

used in the analysis. The longer the data record, the better are the estimates of the 

deternunistic tidal currents, and the less other tidal frequencies contribute to the final 

results (Chen et al., 1996). Another reason, closely associated with the previous one, is 

the presence of strong near inertial oscillations. The latitude of the moorings on the shelf 

defines the range of inertial period of 24.81 hours at the northernmost mooring and 25 .11 

hours at the southernmost mooring. This range of inertial periods is slightly different 

from the periods of the diurnal tides . Therefore, if near inertial oscillations are present in 

the data, they may contaminate the estimates of the diurnal constituents obtained from 

harmonic analysis. 

To show that both short record and near inertial oscillations may cause 

discrepancies, the data were divided into 15-day overlapping segments. The first subset 

began on July 3, 1994 and lasted to July 17, 1994 . The second segment contained data 

from July 4, 1994 to July 18, 1994 and so on. Forty-two subsets were obtained for 

determining amplitudes of the tidal currents . Results of the analyses at all current meters 

show identical behavior of the diurnal amplitudes to those for the upper current meter at 

mooring M2 (Figure I-10). Figure I-11 presents the 3-40 hour band pass of the horizontal 

velocity components for the same instrument . The conclusion that may be drawn by 

comparing these two figures is that whenever the amplitudes of both horizontal velocity 

components are large and included in the subsets, the estimates of the diurnal amplitudes 

are extremely high. 

Finally, in order to obtain a more adequate approximation of the barotropic tidal 

current on this part of the shelf, a different approach was applied to the data. The velocity 

components measured by the upper and lower current meters at moorings M1, M2 and 

21 
Table I-3 

Semi-major axis, semi-minor axis, semi-major axis direction and sense of rotation (CW 

=clockwise. CCW=anticlockwise) of tidal currents for M. S N K O, and Q, . 

Mooring 

Number 

Depth of an 

Instrument 

(m) 

Semi-major 

Axis (cm/s) 

Semi-minor 

Axis (cm/s) 

Major Axis 

Angle (~ 

Rotation 

Sense 

M, 

M1 5.46 1 .13 0.04 -47.24 CCW 

10 1 .77 0.31 76.05 CCW 

M2 5.46 2.39 1.07 -17.41 CW 

10 2.83 1.10 -58.93 CW 

M3 5.46 2 .32 0.79 26.34 CCW 

M4 5.46 3 .69 0.71 27 .86 CW 

10 5 .53 1 .22 34.26 CW 

S, 

M1 5 .46 0.32 0.08 14.52 CCW 

10 0.62 0.02 -4 .42 CW 

M2 5 .46 0.85 0.23 -54.02 CW 

10 1 .52 0.55 7.60 CW 

M3 5.46 0.41 0.24 6.00 CW 

M4 5.46 1.02 0.11 -8 .20 CCW 

10 2.23 1.42 27 .38 CW 

NZ 

M1 5.46 1 .12 0.61 -13 .32 CW 

10 0.82 0.07 -12.46 CW 

M2 5 .46 1 .51 1 .18 -57.86 CW 

10 1 .33 0.27 48.51 CW 

M3 5 .46 1 .23 0.47 60.41 CW 

M4 5 .46 1 .28 0.10 -36.91 CCW 

10 3.36 0.07 39.53 CCW 

K, 

M1 5.46 3.91 1 .54 -16.54 CW 

10 3.75 1 .07 16 .13 CW 

M2 5.46 11 .93 9.99 55 .41 CW 

10 8.28 5 .35 20.02 CW 

M3 5 .46 6.32 4.59 68 .85 CW 

M4 5 .46 3.28 1 .50 -53.86 CW 

10 7.99 2.62 -3 .18 CW 

22 
Table I-3 

Semi-major axis, semi-minor axis, semi-major axis direction and sense of rotation (CW 

=clockwise, CCW=anticlockwise) of tidal currents for NLL, S2, NZ, K,, O, and Q, 

(continued). 

O, 

M1 5.46 4.58 2.03 5 .46 CW 

10 6.87 5.2 21 .36 CW 

M2 5.46 7 .82 5 .11 62 .33 CW 

10 6.40 4.86 23 .37 CW 

M3 5 .46 4.36 2.57 82.09 CW 

M4 5 .46 2.96 1 .58 35.26 CW 

10 4.12 0.19 2.51 CW 

Q, 

M1 5.46 2.45 1.88 2 .35 CW 

10 3 .49 2 .02 0.00 CW 

M2 5 .46 1 .96 0.83 2228 CW 

10 1 .01 0.12 -9 .92 CW 

M3 5 .46 4.22 3 .76 44.54 CW 

M4 5 .46 1 .03 0.08 -7 .90 CW 

10 3.03 0.75 -33 .83 CW 

is 

is 

24 

22 

20 

18 

16 

v .. 

d 14 

v 

12 

E 10 

Q 

8 

6 

4 

2 

0 

K, :A (--O-), q,l--[3-) : 01 :A,(-W), A,("&') 

07/03 07/07 07111 07/15 07/19 07/23 07/27 07/31 08/04 08108 08/12 

1994 

Figure I-10. Amplitudes of the Kl and 01 tidal currents obtained from the 15-day 

overlapping subsets for the upper current meter at mooring M2 (time 

displayed on the horizontal axis should be considered as the first day of each 

subset) . 

23 
100 

80 

60 

40 

20 

V 

d 

V 

7 

a -20 

E 

a -40 

-60 

-80 

-100 

07/01/94 08/01/94 

1994 

09/01/94 

Figure I-11 . The 3-40 hour band pass east-west (u) and north-south (v) velocity 

components of currents at the upper meter of mooring M2. 

M4 were averaged, and then the average data were subjected to harmonic analysis (S.P. 

Murray and W.J . Wiseman Jr., personal communication, 1997) . M3 mooring was 

excluded from the calculations because of the lack of data from the lower meter. Table I4 summarizes the results . The estimation error of the principal axis size (Godin, 1972) 

varies from one location to another: it is 0.49 cm/s for mooring M1 and 0 .66 cm/s and 

0.65 cm/s for M2 and M4, respectively . Values of the semi-diurnal tidal major and minor 

axes are generally comparable to those determined earlier for each instrument separately. 

The MZ tidal currents dominate among the semi-diurnal constituents, and they become 

stronger in the western portion of the study area as observed earlier. The M2, S2 and NZ 

currents rotate clockwise, except for the MZ and S2 constituents at mooring M2 that have 

a counterclockwise rotation. 

The Kl and 01 currents are again the largest in the diurnal frequency band. The 

extremely high values of their axis lengths have decreased significantly, and now they are 

similar to those found from the data of the LATEX A project (LATEX A report, 1997; 

DiMarco and Reid,1098) . The 01 currents dominate at moorings M1 and M2. The major 

and minor axes are 5 .12 cm/s and 3 .07 cm/s for M1 and 5.56 cm/s and 3 .47 cm/s for M2. 

The K, current is the largest at the M4 mooring. Its major and minor axes are 5 .31 cm/s 

and 1 .84 cm/s, respectively. The sense of rotation of the diurnal currents is clockwise, 

except for the 01 constituent at the M4 mooring location that rotates counterclockwise . 

To find the net tidal forcing present in the study area during the LOCCOSUM 

project, the average tidal currents previously estimated for each constituent separately 

24 
were combined and a root mean squared (RMS) amplitude was estimated for the eastwest and north-south components at the M1, M2, and M4 mooring sites. Results again 

confirmed that the tidal currents in the study area were weak. Values of the east-west 

RMS amplitudes varied a little from one location to another, and they were 3 .95 cm/s at 

M1, 4.03 cm/s at M2, and 2.55 at M4. The north-south RMS amplitude was equal to 2.4 

cm/s at M1, 2 .5 cm/s and 3.86 cm/s at the M2 and M4 mooring sites, respectively . 

Table I-4 

Semi-major axis, semi-minor axis, semi-major axis direction and sense of rotation 

(CW-clockwise, CCW=anticlockwise) of tidal currents for M2, S2, NZ, K1, O, and Q1 . 

Mooring 

Number 

Semi-major 

Axis (cm/s) 

Semi-minor 

Axis (cm/s) 

Major Axis 

Angle (°) 

Rotation 

Sense 

M1 123 0.35 61.85 CCW 

M2 2.09 0.73 -23.84 CW 

M4 3 .60 0.62 32.71 CW 

SZ 

M1 0.4 0.06 -24.75 CCW 

M2 1 .02 0.31 -14 .02 CW 

M4 1 .06 0.36 -39 .43 CW 

NZ 

M1 0.88 0.31 -19 .88 CW 

M2 127 0.78 -21 .43 CW 

M4 2.15 0.01 43 .37 CW 

K~ 

M1 3.42 0.82 -1 .55 CW 

M2 3 .05 1 .41 -11 .71 CW 

M4 5 .31 1 .84 7 .74 CW 

O, 

M1 5 .12 3.07 -70.23 CW 

M2 5.56 3.47 -5 .18 CW 

M4 2.48 0.18 -17 .99 CCW 

M1 2.78 1.82 -2 .87 CW 

M2 0.91 0.11 28 .62 CW 

M4 1 .90 0.29 -29.05 CW 

3. Near Inertial Motion 

3.1 . Complex Demodulation of Observed Currents 

The existence of another high frequency motion mode, near inertial oscillations, 

on the Louisiana-Texas shelf is well known and has been investigated by several 

researchers (Daddio et al ., 1978 ; Chen et al ., 1996) . Unlike the tidal currents that are 

25 
predictable, the near inertial oscillations are non-deterministic signals . Despite their 

randomness, they are usually strong enough to be isolated from the tidal motion and 

background noise, especially when the inertial period is significantly different from the 

major tidal constituent periods and long data records of the current velocity components 

are available. 

The inertial periods at the mooring sites of the study area are listed in Table I-5 . 

They range from 24.81 hours to 25 .11 hours. As has been mentioned in the previous 

chapter, these periods are slightly different from the periods of the diurnal tides (K1 and 

01) that generate the strongest tidal currents on the shelf. This closeness of the periods 

may complicate analysis and separation of these different motions in the study area. 

However, knowing that the barotropic diurnal tidal currents are rather small and the near 

inertial oscillations are usually characterized by large amplitudes (Daddio et al ., 1978 ; 

Chen et al ., 1996), these oscillations still can be isolated from the tidal flow . 

During the fifty eight-day observation interval, several days of large amplitude 

oscillatory motion were recorded by current meters. The east-west (u) and north-south (v) 

velocity components of the upper and lower meters at four mooring sites (see Figure I-1 

for locations) are displayed in Figures I-12(a-d) and I-13(a-d), respectively. To exclude 

tidal influence, at least partially, the earlier estimated average barotropic tidal currents 

were subtracted from the raw velocity measurements. Then the residual components were 

band passed (20-28 hours) in order to decrease the impact of the higher or lower 

frequency motions on the amplitudes of the near inertial oscillations . 

Table I-5 

Inertial periods of the mooring sites on the Louisiana-Texas shelf south of Isles 

Dernieres. 

Mooring Number Inertial Period 

(hours) 

M1 24.81 

M2 25 .11 

M3 25.01 

M4 24.89 

According to the data of the current velocity components from the upper meters 

(Figure I-12), the most pronounced event was present approximately from July 5 to July 

12, 1994. The near inertial oscillations appeared almost simultaneously in three of four 

upper meters (M1, M2 and M3) of the LOCCOSUM project. Another major episode 

occurred generally on July 20 and lasted until August 4, 1994 and was recorded at the 

same upper instruments. However, the precise beginning and end of this event differed 

from one mooring location to another. There were two other episodes after August 4, 

1994 but the probable near inertial oscillations of the smaller amplitude were generally 

observed at very different times only at M1 and M2 moorings. The last event occurred at 

the end of the deployment starting approximately at the same time at M2, M3 and M4 

and was only partly recorded. The oscillations reported by the lower instruments (Figure 

I-13) display different behavior at different sites. Based on salinity measurements and the 

26 
50 

30 

,o d 

C 

2 -t0 

a 

-30 

-50 

u(-),v( °--°) a 

vy i 6.4 PV A. W. 

Mooring M1 

50 

30 

v CI ,o _,o 

Q -30 

-50 

50 

30 

v 10 

d 

7 

10 

a 

-30 

-50 

50 

u(-).wl --°) 

30 d 

10 

m 

-10 

. E -30 Mooring M4 

Q 

-50 

07/04/94 07/14 07/24 08/03 08/13 08/23 

Figure I-12. Time series of the east-west (u) and north-south (v) components of the 20-28 

h band pass residual currents from upper meters at (a) M1, (b) M2, (c) M3, 

and (d) M4 mooring sites. 

repeat section study data, these meters were situated just below the halocline and may be 

considered as representative of the lower layer. In the case of a two-layer system the near 

inertial oscillations in the lower layer are not generated by the downward flux of the 

inertial energy from the upper layer, but they appear as a result of long internal near 

inertial waves propagating offshore . In such a case, these upper and lower layer 

oscillations are out of phase (Millot and Crepon, 1981). Our data suggest that this may be 

a reasonable explanation for observations the M1 and M2 moorings. Figure I-14 shows 

u(-),v( . . . . . . ~ 

b 

Mooring M2 

W-),w( . . . . . ~ 

C 

Mooring M3 

27 
that during the first episode the east-west component of the upper and lower meters at M2 

are clearly out of phase (about 180°). Identical behavior was observed for the north-south 

components at M2 and both components at the M1 site for the first and second events . 

Additionally, amplitudes are smaller in the lower meters . 

The results from the M4 mooring located on the western end of the study area are 

totally different from Ml and M2. The amplitudes of both velocity components observed 

at the lower meter are generally higher than those in the upper layer. Lack of amplitudes 

in the upper layer suggests that there was no inertial motion at this location . The larger 

amplitudes of the current components in the lower layer are difficult to explain. They 

might have been increased by internal tides or by propagating offshore long internal 

waves generated by the inertial oscillations observed in the eastern part of the study area. 

so 

30 

t ,o 

01 

7 

10 

a 

E 

-30 

-50 

~ a w-i,vi . . . . . . 

AAA 

Mooing M1 

50 

 30 

'YI 

t U 10 

L 

10 

a 

E 

a -30 

-50 

30 

10 

b ~(-).v~ . . . . .~ 

" ~ . 

. . : " 

. . Mooring M2 

50 

41 

3 -10 

a 

Q -30 

07/04/94 07/14 07/24 08/03 08/13 08/23 -50 

u(-),v( .... . . ~ 

C 

Mooring M4 

Figure I-13 . Time series of the east-west (u) and north-south (v) components of the 20-28 

h band pass residual currents from lower meters at (a) M1, (b) M2, and (c) 

M4 mooring sites. 

28 
To verify the existence of the near inertial oscillations at three moorings (M1, M2 

and M3) complex demodulation was performed (Bloomfield, 1976) . The complex 

demodulation may be regarded as a local version of harmonic analysis . It transforms a 

signal at frequency co, which may be concealed by large contributions from other 

frequencies, to a demodulated signal having much reduced frequency. Similar to 

harmonic analysis, it describes the signal in terms of its magnitude and phase . It is local 

in the sense that these parameters are determined only by the data in the neighborhood of 

a particular time, t, rather than for the entire record . In this study, the time t is chosen to 

be equal to two inertial periods. Therefore, the magnitude and phase are computed as a 

function of only those currents observed over the time t ± 1 inertial period and are 

independent of all other observed currents velocities . 

Results obtained from the method described above are shown in Figure I-15(a-b). 

The demodulated signals from the upper meters at moorings M2 and M3 clearly show 

significant magnitudes associated with the first two episodes described earlier (Figure I15a). Furthermore, these peaks are generally accompanied by a nearly constant phase as 

expected for inertial oscillations (Figure I-15b) . Small magnitude episodes observed at all 

instruments after August 4, 1994 are very close to the amplitudes of the tidal currents and 

are probably tidal in origin . Since the inertial and tidal periods are so close, tidal currents 

unfortunately also are preserved in the demodulated signals with little change in 

magnitude (Daddio et al ., 1977). 

V 

m 

7 

z 

a 

a 

50 

40 

30 

20 

10 ., 

~ . 

Mooring M2 : upper( -) end lower( --- ) meters 

-10 

-20 

-30 

-40 

-50 

4 5 6 7 8 9 10 11 12 13 14 

July 1994 

Figure I-14. Time series of the east-west (u) component of the 20-28 h band pass residual 

currents from the upper and lower meters at M2 mooring site . 

The magnitudes (not shown) of the demodulated currents for the lower meter at 

M2 are 24 cm/s for the first event and 17 cm/s for the second event. The phase varies 

very slowly and generally might be considered constant . The magnitude (not shown) of 

the signal at the lower instrument at the M1 site does not exceed 6 cm/s for either episode 

29 
and might be generated by the tidal currents . However, the constant phase associated with 

these periods of time may suggest that they are inertial in origin . 

In summary, the time series analyses and complex demodulation results confirm 

that near inertial oscillations were present on the inner shelf. The two largest and most 

clearly resolved oscillations were observed in early July and between July 20 and August 

4, 1994. The magnitude of these oscillations decreased with depth (for example, the 

upper and lower meters at M2 site) and onshore (the upper meters at M2, M3 and M1 

moorings). Such a downward and onshore decrease of the oscillation amplitudes was also 

reported by Chen et al . (1996) . Finally, the oscillations in both layers were clearly out of 

phase . Such behavior suggests that the lower layer oscillations were generated by long 

near inertial internal waves, which are generated in the transient phase of the geostrophic 

adjustment at the shore in a two layer system and propagate offshore after the near 

inertial oscillations have been initiated at the surface layer (Millot and Crepon, 1981). 

35 

30 

25 

v 20 

v 

15 

c 

10 

5 

0 

a_ 

/ - \`' \ . 

/ .. 

M1(-),M2(---),M3( """ ' " ') 

., : 

/ - 

250 

150 

so 

:. 

N 

a -50 

-,50 

-250 

b 

I 

07/05/94 07/15 07/25 08/04 08/14 08/24 

Figure I-15 . Magnitudes (a) and phases (b) of the demodulated currents recorded by 

upper meters at M1, M2 and M3 mooring sites. 

3.2 . Relation of Near Inertial Oscillations to Wind Forcing 

Inspection of time series and spectrum of the wind measured at Buoy 19 and 

Grand Isle and surface intensification of the near inertial oscillations may imply that the 

recorded near inertial motion in the upper layer is generated by wind. According to the 

30 
wind measurements from the buoy, the first episode was associated with the wind speed 

increase (from 2 m/s to 7 m/s) recorded on July 4, 1994. The same data showed that the 

second event was initiated by clockwise rotating wind and later maintained by winds 

shifting from southwesterly to northeasterly. 

To test the hypothesis that the observed near inertial oscillations in the upper layer 

were generated by wind, a model developed by Pollard and Millard (1970) was utilized . 

This model assumes that the currents in a homogeneous mixed layer are generated by 

wind stress, which is instantaneously and uniformly distributed through this layer. The 

model currents are computed using the vertically averaged, linearized equations of 

motion : 

au _ fv = ~X - cu (1-2) 

at ph 

av 

+ fu = 

iY 

at 

- cv (I-3) 

P 

where i = (iX,iy ) is the wind stress, p is the water density, h is the mixed layer depth, f 

is a Coriolis parameter, u and v are the east-west and north-south velocity components of 

the model currents, and c-1 is the e-folding decay time that models the dispersion effect 

by introducing a decay factor of the form exp(-ct) . The wind stresses are computed from 

i = cDAaIvIv (I-4) 

where v is the hourly wind velocity ; the density of the air is taken as pa =1 .22* 10-3 

g/cm3, and cD=1 .2*10-3. There is a wide range of values for CD and for summer winds 

observed along the Louisiana-Texas coast, this value of CD seems to be appropriate as 

suggested by Hsu (1988), and it was also used for modeling of the inertial oscillations by 

Kundu (1976b). Assuming zero initial conditions (I-2) and (I-3) equations are integrated 

forward in time using a backward scheme for decay terms, a trapezoid scheme for the 

oscillatory terms and a time step of one hour, yielding time series for the east-west and 

north-south components. 

When the wind from Buoy 19 was used as input with the decay time of 1 .6 days 

the results were in fairly good agreement with the currents observed in the upper layer at 

the M2 and M3 mooring sites . The model also gave a good simulation of the currents 

observed at the Ml site in early July, when the wind from Grand Isle was used. Figure I16(a-b) shows the modeled and observed velocity components for the M2 location. It can 

be seen clearly that the wind was able to initiate the near inertial oscillations found in the 

upper layer. The time of appearance of the modeled inertial currents is well correlated 

with the time when these currents were actually present. However, there is an apparent 

phase difference between modeled and observed currents . This time lag is, first of all, 

introduced by the numerical scheme used for the oscillatory terms and is equal to 4.2 

hours for the inertial period of the M2 site and the one-hour time step. It may also result 

from the fact that the input wind, which was used in the model, was not measured at the 

same location or in the close vicinity of the mooring (the distance between the mooring 

31 
and buoy is about 120 km). There is also the third possibility that long near inertial 

internal waves may have contributed to the phase difference. The amplitudes generated 

by wind are also comparable with those of the observed oscillations, except for a 

discrepancy that was found after July 29, 1994. That may result from the last two reasons 

that are used to explain the phase difference. 

40 

30 

20 ,a 

r 

4) 0 

-10 

E .20 

- 30 

-40 

-a u observed ( -) 

u modeled ( ----) 

. T-YJ1JT1.'HTV" 

4 6 8 10 12 14 16 18 20 22 24 26 28 30 1 3 5 7 

July 1994 August 1994 

40 

30 

20 

10 

0 

-10 

-20 

Q -30 

-40 

-b 

-r-~~ , _ . . . . . . . . . _ 

v observed ( -) 

v modeled ( ----) 

4 6 8 10 12 14 16 18 20 22 24 26 28 30 1 3 5 7 

July 1994 August 1994 

Figure I-16 . East-west (a) and north-south (b) velocity components of the modeled and 

observed inertial oscillations at M2 location (east and north are positive ; west 

and south are negative). 

4. Low Frequency Motion 

4.1 . Monthly Flow Field from Current Meters 

It has been shown that the low frequency flow on the seasonal time scale on the 

Louisiana-Texas inner shelf has two different flow regimes : upcoast flow usually from 

June to August and downcoast flow during the rest of the year. According to the monthly 

average of the LATEX A current measurements, the upcoast flow regime was observed 

only in June and July 1994 and then in August 1994, a weak downcoast flow began 

(Murray et al ., 1997). The low frequency currents obtained from the current meter data 

from this project seem to confirm their different behavior in July and August 1994. 

32 
The monthly average of the low frequency current east-west (u) and north-south 

(v) components are given in Table I-6 and displayed in Figure I-17(a-b). Additionally, the 

table and figures include information about the mean flow computed for the LATEX A 

instruments (see Figure I-1 for locations) . In addition to the monthly average, the table 

contains standard deviations and root mean square errors for each component determined 

separately from the hourly data for the July and August subsets . The error of the monthly 

average was calculated as (Kundu and Allen, 1976): 

E = 6(T / 2L)-i 12 (I-5) 

where F_ is the error of the mean estimates, 6 the standard deviation of the signal, T the 

length of the time series, and L is the integral scale defined as half of the area under the 

autocorrelation function . 

Table I-6 

Mean, standard deviation and root mean square error for M1, M2, M3, M4, LA 14, 

LA15, and LA16 mooring sites. 

Mooring 

Number 

Instrument 

Depth (m) 

umean 

(cm/s) 

a 

(cm/s) 

E 

(cm/s) 

v~~ 

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

Jul 3 - Jul 31 1994 

M 1 5.46 5 .29 9.8 4.07 2.15 5.93 1 .12 

10 0.04 5.49 2.39 -1 .79 3.44 0.68 

M2 5.46 2.33 13 .25 5 .14 -0 .44 9.16 2.69 

10 3 .83 9.97 4.87 -0 .82 8.34 3 .24 

M3 5.46 22.57 11 .36 5 .61 -0.61 6.42 1 .98 

M4 5.46 16.73 9.17 2 .12 -9 .34 7.52 1 .54 

10 6.79 11.08 4.42 -4.03 5.24 1 .61 

LA14 9 5 .05 8.24 4.07 -0 .39 5.80 1 .49 

LA15 10 14.08 9.29 2.64 2.33 8.09 2.48 

LA16 9 -3 .90 4.68 0.89 -1 .96 2.77 0.50 

Au ust 1 - August 29 1994 

M1 5.46 -2 .59 9.21 3 .64 -0.84 4.67 0.87 

10 -2 .96 10 .81 4.02 -4 .27 6.03 1 .81 

M2 5.46 -3 .84 11 .57 6.41 4.11 11 .00 3.52 

10 -3 .52 12 .03 6.13 -0 .82 6.17 2.07 

M3 5 .46 -3 .19 15 .03 6.54 0.73 9.93 4.95 

M4 5.46 -3 .34 10 .63 4.77 -0 .98 6.41 1 .95 

10 -3 .43 12 .93 4.99 -1 .91 7.13 1 .15 

LA14 9 -0 .82 9.43 4.63 -1 .31 9.67 3.19 

LA15 10 -1 .39 10.66 3 .75 -0 .48 7.25 1 .95 

LA16 ~ 9 -4 .51 8 .74 2.79 -1 .78 3.91 127 

33 
Latitude 

30.0 

29.8 

29.6 

29.4 

29.2 

29.0 

28.8 

28.6 

28.4 

-92.0 -91 .8 -91 .6 -91.4 -91 .2 -91 .0 -90.8 -90.6 -90.4 -90.2 -90.0 -89.8 -89.6 -89.4 

Longitude 

Latitude 

30.0 

29.8 

29.6 

29.4 

29.2 

29.0 

28.8 

zs.s 

28.4 

-92.0 -01.8 -91 .6 -91 .4 -91.2 -91 .0 -90.8 -90.8 -90.4 -90.2 -90.0 -89.8 -89.6 -89.4 

Longitude 

Figure I-17 . Monthly average current vectors during (a) July 1994; (b) August 1994 

(thick arrows - upper meters, thin arrows - lower meters). 

The July averages of the east-west components show eastward flow for all upper 

and lower level instruments, except at the LA16 site (Figure I-17a) . The mean flow 

speeds generally decrease with depth, except at M2; however, neither mean is statistically 

different from 0 at M2 mooring . Therefore, it is rather difficult to say whether such a 

decrease of the mean flow speeds is present or not. The ume. of the lower level 

I I I I I 

, I~ I] Y 

b -- 

.. -, 

ri 

r 

1 

w 

1 ~-- 

34 
instrument at M1 is also much lower than its error and is not statistically meaningful. All 

moorings with a well-defined eastward flow are also characterized by much smaller 

means of the north-south components . Table I-6 shows that only half of the estimates of 

the mean of the north-south component are statistically significant. 

The monthly averages of the east-west component for the August data set are 

totally different. All instruments have rather weak westward flow. Contrary to the July 

means, August means are smaller than the errors and should be rather considered nonsignificant, except for that at mooring LA16. The means of the north-south component 

are southward, except for that of the upper meter at M2, and most are also statistically 

insignificant. In summary, the August flow field (Figure I-17b) seems to be less 

organized than that observed for the July subset . 

A common property for both subsets is large standard deviations. They are 

generally much greater than the estimated monthly averages, with an exception at the 

locations where the strong eastward flow is dominant. Such large deviations indicate the 

presence of a strong fluctuation field during July and August 1994 . The standard 

deviations generally decrease with depth for the first subset and in the onshore direction 

for both subsets . Another interesting feature, which emerges from this analysis, is the 

decorrelation time of the components . The root mean square error depends on this time. 

Examination of the error values show that they are always larger for the east-west 

component than those of the north-south component. Therefore, it may be deduced that 

the decorrelation time of the u component is larger than that of the v component. It ranges 

from 2 to 3 days and from 5 to 10 days for the north-south and east-west component, 

respectively, if calculated for the entire data record. 

In summary, the July estimates show rather a good example of the summer 

upcoast flow that seemed to be confined by the 30-meter isobath . Farther offshore the 

eastward flow was still present, however, the currents were much weaker. This horizontal 

decrease of the flow speed suggests the existence of a different low frequency motion 

farther out on the shelf. In August, the mean flow was not well defined and probably 

represented a transition period between the upcoast and downcoast regimes of the 

Louisiana-Texas coastal current. 

4.2 . Spatial Correlations 

To determine how the currents in the study area are spatially related, the crosscorrelation coefficients of the east-west (u) and north-south (v) components as a function 

of the distance of separation were calculated for current meters at depths of 9 and 10 m. 

For the purpose of this analysis, it was assumed that the low frequency currents for 

instruments measured one meter apart in the vertical, were similar. Two different spatial 

correlation scales were determined: cross-shelf and along-shelf. Both scales were 

computed from the 40-hour band pass data for a period of time between July 3 and 

August 29, 1994. 

35 
The cross-shelf spatial scale of the east-west and north-south components was 

obtained for the meters at LA16, M1, LA15 and LA14 mooring sites . The standard error 

of the correlation coefficients was 0.3 for the east-west current component and 0.2 for the 

north-south component. The results are displayed in Table I-7 and Figure I-18 . The table 

contains the 0 lag and maximum correlation coefficients for each pair of the instruments 

and the time lag when this maximum occurred. The results of this analysis show that all 

correlation coefficients are much larger than the error, but it can also be seen that they are 

rather small and do not exceed 0 .6 . The cross-shelf correlation scales approximated by 

the 0-lag coefficients for both components decrease with increasing separations of the 

mooring sites. The correlation of the north-south component, however, seems to decline 

faster than that of the east-west component (Figure I-18) . Thus, the cross-shelf correlation 

scale of the east-west component is longer than that of the north-south component. 

The along-shelf correlation scale of the velocity components was estimated using 

current records from M1, M2, M4, LA14, LA15 and LA16 mooring sites for the same 

period of time as the previously computed cross-shelf spatial scale. Unfortunately, the 

number of current mooring pairs is even smaller than that used for the cross-shelf spatial 

correlation scale, so the results are less reliable (Table I-8, Figure I-19). However, they 

give some insight on what the along-shelf scale might be. It can be seen that the alongshelf correlation of the east-west component is the highest for the separations less than 30 

km and it stays fairly large even for large mooring separations. This may suggest that the 

scale of the east-west component is much higher in the along-shelf direction than that of 

the cross-shore one. The same scale of the north-south component is negative and small 

for all mooring pairs and distances . Such behavior may imply that similar to the crossshelf spatial scale, the along-shelf scale of this component is very small and probably 

does not exceed a few kilometers. 

Table I-7 

Correlation coefficients of the east-west (u) and north-south (v) velocity components as a 

function of cross-shelf separation. 

Mooring Distance 0 Lag u Maximum La 0 Lag v Maximum La 

Sites (km) Correlation U Hours Correlation v Hours 

Correlation Correlation 

M1 11 .5 0.48 0.59 -18 0.56 0.56 0 

LA 16 

M 1 26.6 0.50 0.56 1S 0.26 0.32 12 

LA 15 

M1 50 .0 0.20 0.46 140 -0 .05 -0 .28 170 

LA 14 

LA16 37.1 0.13 0.43 94 0.20 -0.33 157 

LA 15 

LA16 60.5 0.18 0.33 98 0.16 -0 .28 200 

LA 14 

LA 14 23 .5 0.42 0.53 -85 0.34 034 0 

LA 15 

36 
Table I-8 

Correlation coefficients of the east-west (u) and north-south (v) velocity components as a 

function of along-shelf separation . 

Mooring Distance 0 Lag u Maximum La 0 Lag v Maximum La 

Sites (km) Correlation u Hours Correlation v Hours 

Correlation Correlation 

M4 82.0 0.26 0.44 37 -0.15 -0 .38 -28 

LA 16 

M4 74.0 035 0.51 62 -0 .27 -0 .46 25 

M1 

M2 24.0 0.76 0.76 0 -0 .14 0.20 140 

LA 14 

M2 27.0 0.64 0.64 0 -0 .09 0.30 190 

LA 15 

Table I-9 

Magnitude of complex correlation coefficients of the upper la yer low fre uenc currents . 

Mooring 

Sites 

Total Depth 

(m) 

Mooring Separation 

(km) 

Magnitude 

M4:M3 14.9 :23.0 28.0 0.94 

M4:M2 14.9 :36 .0 60.0 0.28 

M4:M1 14.9 :19.0 74.0 0 .56 

M3 :M2 23 .0 :36.0 32.0 0.34 

M3 :M1 23.0 :19.0 60.0 0.53 

M2:M1 36.0:19.0 47.0 0.22 

The July monthly averages of the currents show that the strong para-isobathic 

flow in the upper layer is generally shoreward of the 30-meter isobath . To determined 

whether this visual observation is valid or not the complex coefficient (Kundu, 1976a) 

was computed for the upper meter records at M1, M2, M3 and M4 mooring sites . This 

quantity is a complex number whose magnitude (<1) gives the overall measure of 

correlation between two current records . The coefficient was calculated for the July 

subset of the data only. Table I-9 shows the results from this analysis . 

The magnitudes of the complex correlation coefficients are consistent with the 

previously stated hypothesis about probable presence of the different flow regimes 

separated by the 30-meter isobath over the inshore pan of the study area and farther out 

on the shelf in July 1994. All mooring pairs (M1, M3, and M4) deployed at depth less 

then 30 meters are characterized by large magnitudes. It is also valid even for the large 

separation of the mooring sites. However, if the current record from the M2 location, 

where the total depth is 36 m, is included in the analysis, the value of the correlation 

coefficient decreases significantly regardless of the distance between the current meters . 

37 
1 

0.9 

0.8 

0.7 

0.6 

r 

0.5 

0.4 

d 

0.3 

.

0 0.2 

d 0.1 

0 

-0 .1 

° -0 .2 

-0 .3 

-0 .4 

-05 

U( " ) . w( + ) 

f 

t 

- - - - - - - - - - - - - - - - - - - - - - 

0 10 20 30 40 50 60 70 80 

Cross-shelf separation [km] 

Figure I-18 . Correlation coefficients of east-west (u) and north-south (v) current velocity 

components as a function of cross-shelf mooring separation. 

1 

0.9 

0.8 

0.7 

0.6 

c 

d 0.5 v 

d 0 .4 

0 

0 .3 c 0 

'~ 0 .2 

0.1 

0 

v 0 

o -0 .1 

-0 .2 

-0 .3 

-0 .4 

-0 5 

u( 0 ), v( + ) 

------------------------------

+ + 

0 10 20 30 40 50 60 70 80 90 100 110 

Along-shelf separation [km] 

Figure I-19 . Correlation coefficients of east-west (u) and north-south (v) current velocity 

components as a function of along-shelf mooring separation. 

38 
4.3 . Spectral Analysis of the Low Frequency Flow 

The energy spectra of both current components at all moorings did not show any 

significant peaks in the low frequency band. The energy generally increased with 

decreasing frequencies . A representative example of the averaged spectrum is displayed 

in Figure I-20. The raw spectra of the east-west (u) and north-south (v) components were 

computed for the upper meter at the M3 mooring for the entire available record . Then 

they were averaged over eight frequencies yielding to 16 degrees of freedom. The only 

significant peaks present in the spectrum of both components are associated with the 

inertial-diurnal and semi-diurnal bands. At subtidal frequencies (_< 0 .6 cpd) the energy 

increases almost linearly with decreasing frequency. Another feature, which is also 

common for all moorings, is that the energy computed for the east-west component is 

always higher than that of the north-south component for the low frequency band. 

Assuming a linear relation between current components, the coherence squared 

was estimated. For 16 degrees of freedom, the coherence greater than 0.35 was 

significant at the 95% confidence level. 

Results obtained for the east-west current component showed that the currents 

recorded in the upper layer at M1, M3, and M4 were coherent (coherence squared >0.5) 

for the frequencies less than 0.3 cpd. Surprisingly, the coherence squared was totally 

insignificant between M4-M2, M3-M2 and M1-M2 mooring pairs (coherence 

squared<0 .35). Despite the proximity of the M2, M3, and M4 mooring sites, the eastwest component did not show any coherence in any frequency band between these station 

pairs . 

io" 

103 

101 

io' 

E 

Y 

'oo 

N 

70'1 

10-2 

10-* 

10"2 10'1 100 101 101 

Cycle par day (cpd) 

Figure I-20 . Spectra of the east-west (u) and north-south (v) velocity current 

component for the upper meter at M4 mooring site . 

39 
The coherence squared of the upper layer north-south component showed that 

there was no significant dependence between the majority of mooring pairs at any 

frequency band, except for the M4-M2 and M3-M2 pairs. However, in both cases, the 

currents measured at one mooring could explain less than 50°Io of the variance of those at 

the second mooring for the very narrow frequency band centered at 0.5 cpd. 

The coherence squared of the north-south component computed for the lower 

meters was not significant for any pair. The same quantity of the east-west components 

was significant for all pairs, but not for the entire low frequency band. The currents at M4 

and M1 mooring sites were coherent at frequencies <0.3 cpd with the maximum 

coherence (0 .67) at 0.2 cpd. The coherence squared between M1 and M2 was only 

significant for very low frequency (<0.22) and does not exceed 0.5 . The last possible pair 

M4-M2 exhibited coherence at frequencies less than 0.5 cpd band and, on average, the 

coherence squared was equal to 0.45. 

In the vertical, the best values of the coherence squared (>0.6) of the east-west 

component were found at the M4 mooring where the currents were highly coherent at all 

low frequencies (<0.58 cpd) . At M2, the same component recorded at the upper and 

lower meters was coherent for the frequencies less than 0 .3 cpd, but the coherence 

squared did not exceed 0 .6. Similar results were obtained for the M1 mooring site where 

the currents were correlated for the same band. The values of the coherence squared for 

the north-south component were poorer than those of the east-west velocity component. 

At the M1 location, the upper currents could explain only about 50% of the variance of 

the lower currents at very low frequencies (<0 .18 cpd). Similar results were observed at 

the M4 site ; however, the frequency band was broader (0.1-0 .3 cpd) . At the M2 mooring, 

there was no correlation between the upper and lower current components throughout the 

entire low frequency band. 

In summary, the coherence squared analysis gave similar results to those 

presented earlier in this chapter (spatial correlations). The east-west components 

consistently exhibited higher coherence than the north-south velocity components. The 

coherence squared of the north-south component was generally insignificant for the 

majority of the mooring pair combinations. The coherence of the east-west component 

was usually greater for the moorings with a short separation distance or when the 

moorings were located in waters with a total depth of less than 30 m. 

4.4. Possible Driving Forces of Low Frequency Currents - Multiple and Partial 

Coherence 

To determine which driving mechanisms control currents in the study area, 

multiple and partial coherences were estimated for both velocity components . Similar to 

the analysis described by Murray et al . (1997), the simple linear model with four inputs 

and one output was designed. The inputs were the possible driving forces and the output 

was one of the velocity components. The following forcing functions were chosen: (a) 

east-west pseudo wind stress ; (b) north-south pseudo wind stress ; (c) along-shelf 

barotropic pressure gradient approximated by water level differences ; and, (d) buoyancy 

40 
forcing proxied by river discharge as in Munchow and Garvine (1993) . The pseudo wind 

stress defined as 

tP = MV (1-6) 

where 'v is the wind velocity vector is directly proportional to wind stress (compare with 

equation I-4) and has the advantage of being independent of the choice of the drag 

coefficient. For the following statistical analyses, results are independent of whichever 

parameterization is used. The pseudo wind stresses were computed from the wind 

velocity components measured at Buoy 19. The water level differences were found for 

gauges at Grand Isle and Atchafalaya Bay near Eugene Island . The buoyancy forcing was 

represented by Atchfalaya River discharge. 

Figure I-21 displays results obtained for the east-west components of the upper 

meters at all LOCCSUM mooring locations . The multiple coherence is statistically 

significant for all considered cases. It means that the four forcing variables together were 

important driving mechanisms of the east-west current component observed in July and 

August 1994. Percentage of the component variance explained by these driving forces 

depends on the frequency and mooring location, but it is at least 50% at M1, 45% at M2, 

45% and 60% at M3 and M4, respectively. 

Looking at the partial coherences, the east-west pseudo wind stress (Figure I-21a) 

was a dominant forcing mechanism for the currents at the M1 mooring (frequencies 

<0.25 cpd). Surprisingly, at M2, this wind stress components was unimportant for the 

entire low frequency band, and the buoyancy forcing seemed to be significant at the 

frequency band centered around 0.22 cpd (Figure I-21b) . At M3 (Figure 1-21c) and M4 

(Figure I-21d), the east-west wind stress dominated for the frequencies less than 0.4 cpd. 

The north-south wind stress influenced current fluctuations in a narrow frequency band 

centered around 0.22 cpd in both locations . Finally, at M4, the alongshore pressure 

gradient and buoyancy forcing became important and statistically significant for the 

different frequencies (pressure gradient <0.4 cpd; buoyancy forcing <0.25 cpd). 

Results (not shown) of the multiple coherence analysis of the east-west 

component for the lower instruments at the M1, M2 and M4 mooring sites gave similar 

results. For all these cases, the multiple coherence was statistically significant. However, 

how much variance of the component was explained by four inputs depended on the 

location. The highest values of multiple coherence were found at the M2 and M4 sites 

where, on average, the driving mechanisms accounted for at least 70°l0 of the current 

variance . Such high values were only present at very low frequencies at M1 . The rest of 

the band was characterized by the multiple coherence less than 0.6 . From an investigation 

of the partial coherences, the north-south wind stress seemed to be dominant at M1 for 

the frequencies less than 0 .25 cpd. Surprisingly, at the M2 mooring, buoyancy forcing 

played an important role as a forcing mechanism in the frequency range from 0.01 to 0 .4 

cpd. At the M4 site, the east-west wind stress appeared to be statistically important for 

the very low frequencies (<0.15 cpd). 

41 
1 .0 

04 

0.3 

a.7 

0.8 

as 

o.~ 

0.3 

0.7 

0.1 

0.0 

a 

w 

e 

1 

i.a 

e 

0.9 

0 .1 

0 .7 

o.e 

c4 

o.s 

o.~ 

a. : 

. . 

o . i 

o.a 

0.00 0 .05 0.10 OAS 0.20 0.23 0.30 0 .35 0 .40 0.45 0.50 0.55 000 

Cycle par day (cpd) 

6 

/ ,11` . V 

~. . \ .~ 1 

a 

- - - l 

i 

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.30 0.55 0.e0 

Cycle 

P. 

day (Cpd) 

Figure I-21 . Multiple coherence (mc) and partial coherences between the east-west 

velocity component of the upper meters with the east-west pseudo wind 

stress (tu), north-south pseudo wind stress (tv), along-shelf pressure 

gradient (dp) and buoyancy forcing (b) at (a) M1, (b) M2, (c) M3, and (d) 

M4 mooring locations (the lower straight solid line is the 95% significance 

level) . 

Multiple and partial coherences of the upper meter north-south current velocity 

components computed for all LOCCSUM locations are shown in Figure I-22. The 

multiple coherence was only statistically significant for the entire low frequency band at 

the M2 and M4 mooring sites . The driving forces could explain at least 60% of the 

component fluctuations at these locations. At M1 and M3, the multiple coherence dropped 

below the 95°Io confidence level for some frequencies (Figure I-22a,c). This means that 

the four forces were unimportant driving mechanisms of the north-south current 

42 
components in July and August 1994 for these frequencies at M1 and M3. Looking at the 

forces separately, the east-west wind stress seemed dominant at all mooring sites, but its 

influence varied with the location and frequency band. The most visible impact was 

observed at the M2 and M4. 

For the lower meters, the highest values of multiple coherence (not shown) of the 

north-south current component were observed at the M1 and M4 mooring sites . At least 

55% of the total variance of the current fluctuations was explained by the forcing 

variables at the first mooring where the dominant forcing seemed to be the east-west 

wind stress . At the M4 site, at least 60% of the current variance was accounted for by the 

forces . The partial coherences showed that different mechanisms were important at 

different frequencies, but the highest values are found for the north-south wind stress in 

the frequency band 0.2-0 .4 cpd. At M2, the multiple coherence was statistically 

significant for all frequencies, except for a small range from 0.34 to 0.38 cpd. Both wind 

stresses components, especially the north-south one, separately seemed to have some 

influence on the current fluctuations at 0 .22 cpd. 

In summary, the multiple coherence analysis shows that four model variables 

together were important driving mechanisms for the low frequency flow in the study area 

in July and August 1994. The percentage of the total variance explained by them depends 

on flow component, frequency band considered and mooring location . The multiple 

coherence of the east-west component is statistically significant for all frequencies at all 

mooring locations . On average, at least 60% of the current fluctuations is accounted for 

by the driving forces for the majority of the analyzed cases. The partial coherences show 

that the east-west component is generally highly coherent with the pseudo wind stresses, 

especially with the east-west stress at the moorings located in waters with a total depth of 

less than 30 m. However, the influence of this force is different for different locations and 

frequencies . Surprisingly, farther offshore (M2 site), the east-west component is well 

correlated with the buoyancy forcing for some frequencies . For the north-south 

component, four driving mechanisms explain between 50 to 80% of the total variance for 

the frequencies less than 0.3 cpd. The principle forcing mechanisms for this component 

are also the wind stresses . However, the significance of these forces depends again on the 

frequency and location considered . 

D. Summary and Conclusions 

The purpose of this report was to describe the summer flow regime observed on 

the portion of the Louisiana-Texas shelf located south of Isle Dernieres, just east of the 

injection of the Atchafalaya River waters into the eastward flowing Louisiana-Texas 

coastal current. A description of this regime was based on a data set from the 

LOCCOSiJM project, which was particularly designed to examine variability of the 

current field and hydrography. The data obtained from CTD and ADCP instruments were 

used to describe daily changes in salinity and the flow field along four sections. Next, 

high frequency motions, e.g . barotropic tidal currents and near inertial oscillations, were 

43 
1 .0 

o.s 

0.9 

0.7 

p 0 .8 

0 .5 

Y 

o

.a 

0 .3 

0 .2 

0 .1 

0 .0 

O.i 

o .e 

o.~ 

o.e 

~a.s 

o.~ 

8 

0.3 

0.2 

0.1 

o . o 

a 

me 

\ to 

r 

rv 

. .\e ~ 

- : 

b 

. . . . 

d 

-- 

/ ~ \\ 

.\ 

0.00 0.05 0.10 DAs 0.20 0. 

'S 

0.30 0 .35 0.40 0 .45 0.50 0 .55 0 .60 0.00 0.05 0.,0 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.e0 

CV 'l . par day (cva) 

Cycle 

per day (cpd) 

Figure I-22. Multiple coherence (mc) and partial coherences between the north-south 

velocity component of the upper meters with the east-west pseudo wind stress 

(tu), north-south pseudo wind stress (tv), along-shelf pressure gradient (dp) 

and buoyancy forcing (b) at (a) M1, (b) M2, (c) M3, and (d) M4 mooring 

locations (the lower straight solid line is the 95°Iosignificance level) . 

examined in the study area. Finally, the low frequency circulation and its possible driving 

forces were described. 

Day to day changes in the hydrography (salinity) and the flow field in the study 

area were examined around a box, which was sampled three times. Each pass took 

roughly a day. This study shows that salinity distribution was characterized by a threelayer structure : (1) the upper layer filled with low salinity waters (-22 psu); (2) the 

halocline layer; (3) the lower layer with high salinity waters (-34 psu). This structure was 

observed along all sections and this pattern was maintained through all three passes . A 

few modifications of this basic salinity distribution, e .g . downward movement of the 

44 
halocline and onshore movement of the low salinity water lens (-20 psu) along the S 1 

section, were associated with the end of an upwelling episode observed along this line . 

The flow field exhibited more changes than the salinity distribution from one pass 

to another . These modifications were more distinct in the shallowest part of the study 

area (total depth less than 10 m) where the flow was generally driven by wind and 

responded quickly to any variations in wind speed and direction . Offshore of this shallow 

region, in general, the upcoast flow, which decelerated with time, was present in the 

upper part of the water column. This flow was probably set up by southwesterly winds 

blowing prior to the cruise and may have represented a well-developed phase of the 

Louisiana-Texas coastal current in its summer flow regime. The southwesterly winds 

ceased just before the repeat section study (RSS) began, and the modifications of the 

upcoast flow observed during the RSS seemed to be associated with the changing wind 

field and salinity (density) structure (e.g . relaxation after the upwelling episode). 

Aliasing, originating from the presence of high frequency motion, such as near inertial 

and tidal oscillations, may have been also responsible for these apparent changes. 

However, current records from LA14 and LA15 mooring locations did not show any near 

inertial motion before and during the RSS in the study area, at least in its eastern part, and 

tidal analysis show that the barotropic tidal currents are small over this part of the shelf. 

Therefore, the observed modifications of the flow were associated with the wind and 

salinity changes . 

In order to examine current fluctuations at longer tidal scales in the same region, 

four moorings were deployed along three sides of the box. Data from these moorings 

indicate that the average barotropic tidal currents are small in the study area. Among 

eight computed constituents (M2, N2, S2, K1, 01 and Q1), the diurnal tidal currents (Kl 

and 01) are dominant. These diurnal tidal currents generally rotate clockwise. Values of 

their major and minor axes vary from one place to another. The K, and 01 major axes do 

not exceed 5.6 cm/s for all mooring locations. The minor axis values of the K, and 01 

constituents are even smaller than those of their major axes, and the difference between 

them is approximately 2 cm/s at all examined mooring sites. The net tidal forcing, which 

originates from the eight estimated constituents, is also small and similar at all moorings : 

RMS values of both (east-west and north-south) tidal velocity components are smaller 

than 4.5 cm/s . 

Another high frequency motion, near inertial oscillations, was observed at three of 

four mooring places (M1, M2 and M3). Since the moorings were located fairly close to 

each other, the lack of these oscillations at M4 suggests that the spatial correlation scale 

of this motion might be small . Results from the model proposed by Pollard and Millard 

(1970) are consistent with the oscillations observed in the upper layer being initiated by 

moderate winds present over the shelf in July and August 1994. Comparison of the eastwest and north-south components of the near inertial currents showed that both 

components measured by the meters in the upper and lower layers were clearly out of 

phase (- 180°) . Such behavior suggested that the near inertial oscillations in the lower 

layer were created by long internal near inertial waves . These waves are generated at the 

shore in a two-layer system, which was present in July and August 1994 in the study area, 

and propagate offshore after the near inertial oscillations have been initiated in the upper 

45 
layer (Millot and Crepon, 1981) . Magnitudes of the observed oscillations decreased with 

depth and in the onshore direction. The maximum magnitudes were found at the 

southernmost upper meter (M2 mooring), and they were approximately 30 cm/s and 20 

cm/s for the first and second episodes, respectively . 

The low frequency flow recorded at four mooring sites exhibited several 

interesting features . The behavior of this flow was different in July and August 1994 . In 

July, the upcoast flow dominated, and in August, the low frequency currents were less 

organized and probably represented a transient period between the upcoast and 

downcoast flow regimes of the Louisiana-Texas coastal current. The spatial correlation 

scale of this flow depended on the current component and was anisotropic. This scale 

seemed to be larger for the alongshore direction than that for the cross-shore direction, 

and it was much longer for the alongshore current component than that of the cross-shore 

current component. The small number of the moorings did not allow estimating the exact 

length of the spatial correlation scales ; however, the character of the alongshore flow 

seemed to change significantly at the 30-meter isobath in the cross-shore direction over 

this part of the shelf. Finally, fluctuations of this flow were fairly well explained by four 

forcing mechanisms : alongshore and cross-shore wind stresses, alongshore pressure 

gradient, and buoyancy forcing, among which the alongshore wind stress was a dominant 

driving force. Its impact on the currents decreased, as expected, with increasing depth . 

Murray et al. (1997) report that in summer, the primary forcing of the Louisiana-Texas 

coastal current on the Louisiana shelf is the alongshore pressure gradient. This conclusion 

was based on data from the summer of 1992. In this year, the LTCC flowed upcoast in 

June, July and August. In 1994, such a flow was observed in July. Lack of data from June 

1994 and the different flow pattern in August 1994 probably account for different results . 

A three-day repeat section study and sixty days of current meter records have 

provided more detailed look at the summer hydrography and current characteristics. 

However, several questions still need to be answered. For example, what causes the 

counter current observed during the RSS on this portion of the shelf, and whether this 

current is a permanent or transient feature. Despite the earlier studies, it is still not clear 

what forcing mechanisms drive the coastal current in the summer season, and how far 

this current extends in both vertical and horizontal directions . Therefore, further studies 

and data are required to improve our knowledge about this current since this low 

frequency flow is a major agent responsible for distribution of pollutants, nutrients, and 

sediment in the region . 

46 
PART II 

TRANSPORT OF THE LTCC 

A. Introduction 

Initial estimates of transport in the Louisiana-Texas coastal current (LTCC) were 

made in the LATEX B study (Murray et al ., 1997) by averaging underway ADCPderived transport data from shore-normal lines . Given the available data, this approach 

yielded a broad temporal and spatial average extending over five to eight days and 

hundreds of kilometers . 

The approach here was to remove spatial and temporal aliasing on the transport 

values by constructing and maintaining a transport-resolving array of current meters 

across the LTCC at a suitable location. Figure II-1 is a location map of the LouisianaTexas shelf showing the locations off Cameron, Louisiana, of (a) the transport resolving 

array ; (b) pressure and water level gauges; and, (c) anemometer locations used in the 

analysis of the transport data. The Cameron location for the transport array was selected 

based on two considerations: (a) first, to be far enough downstream of the Atchafalaya 

River mouth to escape its influence and the influence of the large Tiger and Trinity shoals 

on the flow field; and (b) for any mooring in the water column on the inner Louisiana 

shelf to survive oil industry traffic and fishing pressure, it must be located near an 

existing oil field structure to deter trawling and general boat traffic . Availability of 

nearby platforms or stand pipes thus also played a key role in the mooring location . 

Despite our caution and coordination with oil structure owners, we lost several moorings 

and pressure gauges during the study when structures were removed with our mooring 

lines and gauges attached. 

30 .00 

29 .50 

m 29 .00 

o 

4 

28 .50 

28.00 

~+m!ron LOUISIANA At afalaya M(asl pi 

aver Rlv 

TEXAS if 

Freshwater Bayou 

B42035 Mooring Line 

Oyster Bayou`u 

Freeport 

9- Pro" : gouge 

A -wind tation 

-98.00 -95.50 -95.00 -94.50 .84.00 -93.50 -93.00 -92.50 -92.00 -91.50 -91.00 -90.50 -90.00 .89.50 -89.00 

Longitude 

Figure II-1 . Regional map containing study transect (mooring line) of LTCC south of 

Cameron, LA. Data from our shallow pressure gauges at Oyster Bayou and 

Freeport, and wind data from our station at Freshwater Bayou and the US 

buoy off Galveston (B42035) are used in analysis . 

47 
Figure u-2 shows the location of seven moorings on the cross-shore monitoring 

line labeled A through G from offshore to onshore . This line is seven km east of the 

sampling line SS (along 93° 15' W) that was repeated on all five coastal plume cruises of 

the LATEX B program (Murray et al ., 1997) . Table II-1 gives the location and water 

depth at each mooring along with instrument depth and serial number of the current 

meters, all Endeco 174 impeller models. Note the mooring line extends in a cross-shore 

direction over 65 km, from about the 22-m isobath inshore to the 10-m isobath over a 

fairly uniformly sloping bottom. 

30.00 

29.90 

29 .80 

29 .70 

29.60 

N 

29.50 

29 .40 

29 .30 

29 .20 

29.10 

-94 .00 -93 .90 -93.80 -93 .70 -93.60 -93 .50 -93 .40 -93.30 -93.20 -93 .10 -93 .00 

1CALCA IEU 

SABINE L E p " LAKE 

Cameron 

V O 

~0 ~1 

o ~ F 

s 

OE 

I 1` p J 

c 

a 

"B 

u 

so 

Longitude 

Figure u-2. Detailed map of bathymetry along mooring line (depths in meters). Survival 

demands mooring lines be located near oil well structures to deter trawler 

damage. 

The mooring design is shown in Figure II-3 (see also Table II-1 for details) . The 

shallowest mooring G carried only one current meter, while moorings F, E, D, and B 

carried two ; the mid-section mooring C and outermost mooring A carried three. 

Additionally, four SeaCats recording salinity and temperature were deployed across the 

array, one each at the five-to-six-meter level on moorings A, F, and D, and one deep 

SeaCat at the 12-m level on mooring D. The SeaCats were intended to determine the 

48 
Table II-1 

Mooring names, location, total depth, and instrument configurations . 

Mooring Mooring 

Name 

Longitude Latitude Total 

De th [m] 

Instrument Instrument 

De th [m) 

A Annabella 93.16277W 29.08033N 23.1 SeaCat 5.8 

A Annabella 93.16277W 29.08033N 23 .1 Current meter 7.8 

A Annabeila 93.16277W 29.08033N 23 .1 Current meter 13 .3 

B Brigitte 93.18233W 29.19233N 19 .5 Current meter 5.5 

B Brigitte 93.18233W 29.19233N 19 .5 Current meter 10.0 

C Cathy 93.24219W 29.3289N 17 .7 Current meter 6.4 

C Cathy 93.24219W 29.3289N 17 .7 Current meter 11 .0 

C Cathy 93 .24219W 29 .3289N 17 .7 Current meter 15.0 

D Debra 93.243W 29.39167N 15 .0 SeaCat 4.5 

D Debra 93.243W 29.39167N 15 .0 Current meter 5.5 

D Debra 93.243W 29.39167N 15 .0 SeaCat 12.5 

E Eudora 9320417W 29.53033N 13 .1 Current meter 5.7 

E Eudora 93 .20417W 29 .53033N 13 .1 Current meter 10.0 

F Fiona 93.16433W 29.59967N 12 .0 SeaCat 5.0 

F Fiona 93 .16433W 29 .59967N 12 .0 Current meter 6 .5 

F Fiona 93 .16433W 29 .59967N 12 .0 Current meter 10.0 

G Greta 93 .16084W 29.66033N 10 .0 ~ Current meter J 5 .5 

L 

r 

a 

N 

13 

Mooring A Mooring B Mooring C Mooting D Mooring E Mooring F Mooring O 

'Annabslla " 'Brigitte' 'Cathy' 'Debra' 'Eudora' 'Flona' 'GreU' 

- ~ . ~.- 

.~.- sc, 

Y ~ ~ 

cu . CY 

D 

U .D U 

d 

U .D 

U 

.~.- ~ 

cY 

r .a.- c~~ 

U U,D 

" 70 

c

.f 

ur 

q.b- CYZ10.- CYO IOI~- CY7 1lM- -~, 

-11 p UID 

o ..- .y 

D u~. 

_1 ; 

n..,- cY~ u.~. 

uu .- -3 I,n~a1 

-18 U,D \\\\ 

-20 

n.~ .- 

`y' 

CM " curnnt m~pr 

_ZT \ SC-SUCH 

75 70 65 60 55 SO 45 en a s en vc in .a 

Distance offshore (km) 

Figure II-3 . Details of instrumentation along the mooring line (D indicates instruments 

whose data were analyzed for the downcoast regime and U indicates 

instruments whose data were analyzed for the upcoast regime). 

49 
reduced salinity characteristic of coastal plume waters through the duration of the 

development. 

A data inventory is presented in Figure II-4 for the 15 current meters and the 4 

SeaCats . Logistical difficulty from equipment malfunctions and severe weather limited 

the data return in 1995 . Mooring operations were resumed using improved equipment in 

May 1996, with greatly improved data returns. Figure II-4 shows that the period of good 

data coverage extended from the beginning of May 1996 to the end of January 1997. 

This conveniently allowed for the study of the two distinct flow regimes : the upcoast 

summer flow regime of 1996 and the downcoast winter flow regime of 1996-97. The 

labels U and D adjacent to each current meter in Figure II-3 indicate which instruments 

were used in the summer and winter regime studies, respectively . 

MG-top 

MF bot 

MF_top 

ME bot 

M E_top 

MD bot 

M D_top 

MC bot 

MC mid 

MC_top 

MB bot 

MB_top 

MA bot 

MA mid 

MA_top 

MF_sc(5m) 

MD_sc(12m) 

MD_sc(6m) 

MA_sc(6m) 

09/01/95 01/01/96 05/01/96 09/01/96 01/01/97 05/01/97 

Figure II-4 . Data log of current meters and SeaCat sensors (where, for instance, MA 

means mooring A, top (mid/bot) means a top (middle bottom) current meter 

instrument, and sc(Sm) means a SeaCat instrument at depth 5 meters) . 

B. Characteristics of Data 

The data fundamental to this study are the velocity vectors obtained from the 

moored current meters . The Endeco 174 was set for nominal 10 minute samples. The 

standard stored data is hourly vector averages, which are displayed in Figure II-5 as an 

example for the period November 10-30, 1996, from the three levels of mooring C. 

Moderate to strong events are commonly coherent through the water column as in 

November 22 and November 27 . A strong westward current pulse as expected for this 

time of year occurs November 15-17. Diurnal oscillations, either tidal or inertial, are 

evident throughout the record (e.g ., November 24-25) . The temporal variability over a 

50 
several hour time scale as a result of tidal, inertial and atmospheric forcing can be clearly 

significant. 

The example of summer regime current data in Figure u-6 (July 10-30, 1996) 

shows similar strong daily variability. Eastward events can be seen July 13-15 and July 

20-24. This visual inspection of the hourly unfiltered data suggests that the eastward 

summer regime is present but highly intermittent in nature . Thus, we will use 40-hour 

low pass data to remove the ubiquitous tidal and inertial oscillations from our transport 

computations . 

In Figures II-7 and II-8 we show the corresponding wind records from the 

Galveston Buoy 42035 (only for the summer period) and the anemometer at Freshwater 

Bayou. These two stations are separated by a distance of about 200 km. The Freshwater 

Bayou station is 78 km and the Galveston Buoy is 119 km away from a mid-point of the 

Cameron transect. 

The November winds (Figure II-7) during the downcoast flow regime are 

generally strong with speeds frequently 10 m/s or more and variable in their direction . 

The first part of the record (11-17 of November) shows a period of persistent easterly 

winds with speeds up to 12-13 m/s . Beginning on the 20 of November, the winds are 

characterized by speeds between 3 and 12 m/s and a clockwise rotation. Note that there is 

high visual coherence between the currents at mooring C (Figure II-5) and the winds 

observed in November. 

;Top ;insUpmerit 

~Mid i e i ' ime .t 

S 

~~T?` /I ~r 

Bottprtf~iqstru~rient ; 

mrs 

~ ~ 

N 

zoC 1 

10 11 12 13 14 15 16 17 18 19 20 27 22 23 24 25 26 27 28 29 3u 

November 1996 

Figur u-5 . Example of hourly current vectors from the meters on three levels of mooring 

C during winter downcoast flow from November 10-30, 1996 . 

51 
-~-- r i I I I ! ! 

Top; instrpmapt 

r 

Middle iqstruipent ; ~\ 

Bottom i~struineni 

-T- V 

7 20cr" ; 

10 11 12 13 14 15 76 77 78 19 20 27 22 23 24 25 26 27 28 29 30 37 

July 1996 

Figure n-6. Example of hourly current vectors from the meters on three levels of mooring 

C during summer upcoast flow from July 10-30, 1996. 

Xrestiwater Bayou 

1 -,r 

mTs 1 

70 11 72 73 14 75 16 17 18 79 20 21 22 23 24 25 26 27 28 29 30 01 

November 1996 

Figure n-7. Example of hourly wind vectors from Freshwater Bayou for the winter 

period November 10-30, 1996. 

52 
~res6watpf Bayou 

Oil 

ilouy 4205 

;10 m1Y I ; 

10 71 12 73 74 15 16 17 78 19 20 21 22 23 24 25 26 27 28 29 30 31 

July 7996 

Figure u-8. Example of wind data from Freshwater Bayou and B42035 for the summer 

period July 10-30, 1996 . 

During July 1996, winds are 5-7 m/s and blow quite steadily onshore along the 

Louisiana coastline as expected. However, there is surprisingly little visual coherence 

between the highly variable currents at mooring C (Figure u-8) and the winds (Figure II8). 

Spectral analysis (Mooers, 1973) of the observed winds at Freshwater Bayou and 

Buoy 42035 shows (Figure n-9a and Figure II-l0a) that in the winter season, a major part 

of their energy is associated with the low frequencies (<0.6 cpd) for both clockwise and 

anticlockwise components . Both components are characterized by a small peak present in 

the frequency band between 0.1 and 0.5 cpd. The winds in both stations are in phase and 

are highly coherent with the coherence squared (not shown) varying from 0 .6 to 0.9 and 

from 0 .7 to 0.95 for the anticlockwise and clockwise spectral components, respectively . 

In summer, the low frequency part of the spectrum is rather `red' but again contains 

majority of the wind energy (Figure II-9b and Figure II-10b) . The coherence (not shown) 

between the winds at Galveston Buoy and Freshwater Bayou is smaller than that 

observed in the winter season, and becomes even statistically not different from zero for 

the anticlockwise component at frequencies higher than 0.4 cpd. Similar to the winter 

period, the summer phase between the winds at these two stations is almost zero, except 

for frequencies higher than 0.4 cpd. 

In addition to the wind driving, we may expect influence of the Atchafalaya River 

discharge on the low frequency currents of the LTCC . Figure u-11 shows the 

Atchafalaya River discharge at Simmsport 1995-1997. Note that the time periods 

53 
goo 

50 

10 

5 

v 

a 

v 

0 .5 

m 

v 0.1 

31 

y 0 .05 

0 .01 

0 .05 

0 001 

a 

- - , \ 

V ~I 

j es %cl 

I 

0.01 0.05 0 .1 0 .5 1 5 1 0 

Cycle per day (cpd) 

100 

50 

10 

5 

al f 

'a 0 .5 

v 0 .1 

y 0 .05 

0.01 

0.005 

0.001 

50 100 

b 

+95%CI ~1~f 

I 

0 .01 0 .05 0 .1 0.5 1 5 10 50 100 

Cycle per day (cpd) 

Figure II-9 . Energy spectra (counterclockwise - solid line, clockwise - dash-dot line) of 

the winds observed at Freshwater Bayou (a) in the winter season and (b) in 

the summer season . 

54 
goo 

50 

10 

5 

v 

V 

0.5 N 

E 

m 

;, 0 .1 

0 .05 

0 .01 

0 .005 

0 001 

a 

1 

V 

1 i 

95%CI 

I f 

. 0 .01 0 .05 0 .1 0 .5 1 5 10 

Cycle per day (cpd) 

50 100 

100 

50 

10 

5 

v 

a 

H 0 .5 Na 

a 

0 .1 

01 

y 0 .05 

0.01 

0 .005 

0 001 

b 

I 

}II I 

+D6%CI ~` ~V 

I `i fd 

0 .01 0 .05 0 .1 0 .5 1 5 10 50 100 

Cycle per day (cpd) 

Figure II-10. Energy spectra (counterclockwise - solid line, clockwise - dash-dot line) of 

the winds observed at B42035 (a) in the winter season and (b) in the summer 

season . 

55 
selected for detailed analysis are (a) the summer upcoast regime of June 1 through 

August 30, 1996, which begins just before the spring 1996 Atchafalaya discharge 

maximum of 12,000 m3/s and continues through the rapid drop to the minimum discharge 

of 3,000 m3/s; and (b) the winter downcoast regime of September 20, 1996 through 

January 17, 1997, which occurs over a modest rise from 3,OOOm3/s to 8,000 m3/s. 

Maximum freshwater conditions on the shelf are probably sampled by the summer 

observations, while the winter observations are samples from a low freshwater storage 

situation on the shelf. 

zo 

ie 

1s 

is 

iz 

N 

A 

E 10 

C1 

t0 

L 

V 

b g . 

C 

Figure II-11 . The Atchafalaya River discharge at Simmsport 1995-97 illustrates the 

seasonal cycle of buoyant fresh water into MACP. 

Subsequent data analysis also involves use of pressure gauge data (to calculate 

alongshore pressure gradients as a sea level slope) . Examples of data for the winter 

observation period from the two stations, Freeport, Texas, and Oyster Bayou, Louisiana, 

which bracket the transport monitoring, are shown in Figure II-12. Clearly, there is a 

high degree of visual coherence between the two sites at both tidal and sub-tidal 

frequencies. Note that the water level set-ups and set-downs (Figure n-12) reach 40 to SO 

cm on November 17 and 26, respectively, at both sites and that these two events seem to 

be related to the observed winds (Figure II-7) . In summer (Figure II-13) these low 

frequency water elevation changes are only 10 to 20 cm in amplitude reflecting reduced 

wind energy levels . 

It is, of course, the water level difference between sites representing the 

barotropic pressure gradient along the coast that would be a potential driving force for the 

coastal current. In Figure II-14a, b the demeaned 40-hour low passed water level 

difference between sites (Oyster Bayou minus Freeport) is shown for the same two 

periods illustrated previously . Both July (Figure n-14b) and November (Figure u-14a) 

differences show some fluctuations of different amplitudes . However, in July, there 

seems to be present a small but positive trend of the water level difference of the order of 

56 
80 

70 

80 

50 

- 40 . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . ~ . ~ . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----------

. 

... pp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .~.i. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . 

10 . . . . . . . . . . . . . . . .. . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . 

> 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

d .gyp . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..

.~

.~. . . . ~. . . . . . .. . . . . . . . . . . . . . . . . . . . . 

J 

. . . . . . . 

-20 . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . ." ._'"' . . . . . . . . . . . . . . . . . . . . . . . . . . . . v -30 ~

. . 

~ . . . . .U . . . . . . . . . . . . . . . . .U. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -60 . . . . . . . .u 

m . ._ . . . . . . . . ._ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . ._ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . -70 . 

: :: :

. . . .

: : :

. . . . .

: : : :

. .

: : : :

. . . .

: : :

. . . .

: : : :

. . . . . .

:: :

. . . . . .

: : : :

. . . . . . : . 3 -so Oyster Bayou . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

-eo 

11/70198 11/12/98 11/14/98 11118/98 11/18/98 17/20/98 11/22198 11/27/98 11/28/98 17/28/98 71/30/98 

80 

70 

80 

50 

r= 40 . . . . . . . .. ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . .

.. 

.. 2 

0 

. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . \. t. . . ~ . . . . . 

. . 

- 10 . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ."'V""'. . .V. . ." . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .~ . . 

> 0 . . .. . . . . . .. . . . . .. . . . . . . .. . . . U. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .~ . ~. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . 

d -1 . .. . . . . . . . . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .lU . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

J -200 . .. . . . . . .{. .{ . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.~

. . . . . . 

d -30 . . . . . . .,.} . . . . . .~. . .. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

-40 . . . . . . . ..~V~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . 

~50 . . . . . . . . . . . . . ._ ._ . . . . . ._ . . . . . . . . . . . . . . . . . .__ . . . .__._ . . . . . . . ._ . . . . . . . . .__. . . . . . . . . . . . . . . . . . ._ . . . . . . . . . . . . . . . 

.ea Freeport . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . 

. . . 

.e o 

71/10/98 11/12/08 11114/98 11/18/98 11/18/96 11/20/98 11/22/98 11/21/98 11/28/98 11/28/98 11/30/98 

Figure II-12. Example of pressure gauge data converted to water level from (a) the Oyster 

Bayou site and (b) Freeport site November 10-30, 1996. Note the hourly 

traces showing tidal activity are over-plotted with the 40-hour low passed 

data. 

80 

50 40 . . b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 

v pp . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . 

y p . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .1 .} . . . . . . . . . . . .. . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . .u~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . -10 

-20 . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -- 30 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .U . . . .~ . .~ . . . o : : : :v : : : : : :u: : : : : : :v : :: : : : ~ : : : : : ::~ : . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . W , 

3 -

.eSoO 

. . . FreePort . . .. . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 

. . . . . .. . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . . . .. . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 

.70 

07/10/98 07/12/98 07/14/98 07/18/98 07/18/98 07/20/98 07/22/98 07/24/98 07/48/98 07/28/98 07/30/98 08/01/98 

Figure II-13 . Example of pressure gauge data converted to water level from (a) the Oyster 

Bayou site and (b) Freeport site July 10-30, 1996. Note the hourly traces 

showing tidal activity are over-plotted with the 40-hour low passed data. 

57 
vo 

v 35 

30 

V 25 

20 

y 15 

10 

O 5 

d 0 

-5 d J -10 

y -15 

A -20 

11/10/98 11/12/88 11/14/96 11118/98 11/18/98 71/20/98 71/22/98 11/24/98 11/28/98 71/28/98 11/30198 

E e 

U 

d g . ._ . . . . . . . .

. . . . . . . .. . . . . . . . . . . 

. . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . ..--------

. 

U 

C 5 . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

d 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . I. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . J p . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . 

. . . . . . 

/ 

d 

m -2 

07/10/98 07/12/98 07/11/96 07/18/98 07/18/96 07/20/98 07/22/96 07/24/98 07/28/98 07/28/98 07/30/96 08/01/96 

Figure u-14. A difference between demeaned water levels at Oyster Bayou and Freeport 

(OB-FP) for (a) November 10-30,1996 and (b) July 10-30, 1996 . Note the 

difference in scale of the vertical axes . 

1-2 cm. The major fluctuation (- 6 cm) occurred on July 15 and it appears to be wind 

related. Note an increase in wind strength at both wind stations (Figure II-8). In 

November, however, this water level difference is negative for the major part of the 

record with values reaching up to -11 cm. Distinct positive anomalies are present on 

November 25 and 30. Inspection of the wind record (Figure II-7) shows that they both are 

associated with the wind field created by the atmospheric front passages. Note also that 

for both July and November, persistent trends of the water level difference oppose 

currents observed in the study area at the same time. 

C. Results of Data Analyses 

1 . Structure of Currents 

Before presenting the transport computations, it is instructive to examine spectral 

details of the currents at the central mooring C. In Figure II-15, the energy spectra 

(Bendat and Piersol, 1986) of the north-south (cross-shore) and east-west (alongshore) 

components of the current at the top instrument on mooring C for a seven month period 

(downcoast regime, 09/19/96-04/16/97) show strong peaks at both the diurnal and semidiurnal frequencies. The semi-diurnal peak represents the important tidal constituent 1VI.1 . 

The diurnal peak, however, contains both the major diurnal tidal constituents, K, and 0,, 

and the inertial energy, as the diurnal tidal and inertial periods at this latitude are only 

separated by about 30 minutes. Both spectra for the very low frequencies (<0.1 cpd) are 

almost constant and in a case of the alongshore component this part contains majority of 

energy . The frequency band 0.1 to 0.5 cpd (2-to 10-day periods) includes small peak in 

the energy spectra for both current components. This band is the so-called weather band, 

58 
which results from wind driving in the migrating atmospheric pressure systems that are 

common features in the northern Gulf of Mexico most of the year. Note, especially, that 

at the diurnal and semi-diurnal frequencies, the cross-shore (N-S) energy peaks are higher 

than the alongshore (E-W) peaks, but the energy in the weather band (2 to 10 days) is 

highly polarized in the alongshore direction . 

1000 

soo 

goo 

so 

a 

V 

" 

!A 10 

s 

W 

u 

d 

N 

0 .5 

0 .1 

0 .05 

001 

l 

l 

95%CI ~ 

V 

0 .01 0 .05 0 .1 0 .5 1 5 10 50 100 

Cycle per day (cpd) 

Figure II-15 . Energy spectra for the EW (solid line) and NS (dash-dot line) current 

components of the top meter at mooring C (September 19, 1996-April 

16,1997) . 

The current spectra of the middle instrument (not shown) are generally similar to 

those obtained for the top instrument at mooring C in the winter period . The spectra of 

the currents recorded by the bottom meter, however, differs from these described above 

and the major differences are as followed: (1) energy level at the inertial-diurnal 

frequency band drops off drastically for both current components; (2) such a extreme 

drop of energy is also present at the semidiurnal frequency for the alongshore component ; 

(3) energy associated with subtidal motion is about two times smaller than that observed 

at the top and middle instruments. The bottom instrument was placed just about 3m 

above the bottom, therefore, this decrease of the energy level at different frequencies is 

probably associated with bottom friction . 

During the downcoast flow regime, coherency of alongshore currents between the 

three depth levels on mooring C is strong . Figure II-16 shows a broad band of significant 

coherence from 0 .1 to 0.5 cpd (2 to 10 days) between the upper and mid-depth meters . 

The coherence decreases in strength from 10-to 2-day periods, but the phase lag (not 

59 
0.9 

0.8 

0.7 

a 

CI 

0 .6 

c 

d 0 .5 

u 

c 

ID 

0 .4 

0 

U 

0 .3 

0 .2 

0 .1 

0 

96% Significance L*vel 

0 0 .05 0.1 0 .15 0 .2 0 .25 0 .3 0 .35 0 .4 0 .45 0 .5 

Cycle per day (cpd) 

Figure n-16. Coherence squared for the alongshore current component between the upper 

and mid-depth meters at mooring C for four months of winter 1996-1997. 

shown) is uniform and is not statistically different from zero across the band. The 

coherence between the middle and bottom meters at mooring C, shown in Figure II-17, is 

even stronger across the entire subtidal band between 0.1 and 0.2 cpd, again, with about a 

zero phase lag. 

These diagrams show that for periods of 2.5 to 10 days, the alongshore currents in 

the water column respond barotropically between the top and bottom current meters . The 

lower water column, however, responds barotropically all the way down to periods of 

one-day duration, apparently reflecting the more transient influence of wind forcing on 

the upper water column . 

The spectra of the subsurface currents observed at mooring C in the summer 

(Figure n-18) have the same major peaks associated with semidiurnal and diurnal tidal, 

and inertial motions. The low frequency part of these spectra (<_ O .Scpd) is characterized 

by almost linear increase of energy with decreasing frequencies for both alongshore and 

cross-shore components. Resembling the winter season, the subtidal energy is polarized 

in the alongshore direction, however, this polarization only appears for motion with 

periods larger than 4 days . The pattern is closely replicated at the middle instrument level 

(data not shown). Similar to the downcoast regime, energy level of the currents recorded 

by the bottom instrument, which was deployed -3 m above the bottom, is smaller for the 

same frequency bands than those observed for the currents of the subsurface and middle 

instruments. This energy drop off is probably again associated with the bottom friction . 

60 
1 

0 .9 

0 .8 

0.7 

v 

77 

N 0.6 

7 

Q 

tq 

d 0 .5 

C1 

C 

d 

d 0 .4 r 

0 

U 

0 .3 

0 .2 95% Significance Level 

__ 

0 .1 

p i , - 

0 0 .05 0 .1 0 .15 0 .2 0 .25 0 .3 0 .35 0 .4 0 .45 0 .5 

Cycle per day (cpd) 

Figure II-17. Coherence squared for the alongshore current component between the middepth and lower meters at mooring C for four months of winter 1996-1997 . 

1000 

soo 

ioo 

50 

v 

(a7 

H'° 10 

s 

a 

u 

a 

N 

0 .5 

o ., 

0 .05 

001 

1 

.l/ 1 I 

I 

' 1~`'I 1 

96%CI 

i 

0 .01 0.05 0 .1 0 .5 1 5 10 50 100 

Cycle per day (cpd) 

Figure n-18 . Energy spectra for the EW (solid line) and NS (dash-dot line) current 

components of the top meter at mooring C from June 1, 1996 through August 

31,1996 . 

61 
i 

0.9 

0 .8 

0 .7 

v 

0 .6 m 

c 

m 0.5 

V 

C 

d 

d 0. 

t 

O 

U 

0 .3 

0 0 .05 0 .1 0 .15 0 .2 0 .25 0 .3 0 .35 0 .4 0 .45 0 .5 

Cycle per day (cpd) 

0 .2 

0 .1 

0 

96% Significance L. 11 

- - - - - - - 

- 

- - - - - 

Figure u-19. Coherence squared for the alongshore current component between the upper 

and mid-depth meters at mooring C for three months of summer 1996. 

o.s 

0.8 

0.7 

o.s 

Q 

y 

m 0 .5 

V 

C 

d 

m 0. L 

O 

U 

0 .3 - - - - - - - ~ - 95% Significe-e Level 

0 .2 

0 .1 

0 

0 0 .05 0 .1 0 .15 0 .2 0 .25 0 .3 0 .35 0 .4 0 .45 0.5 

Cycle per day (cpd) 

Figure II-20 . Coherence squared for the alongshore current component between the middepth and lower meters at mooring C for three months of summer 1996. 

62 
e 

a 

G1 

O 

W1 "-J 1 

"60 -10 -30 -20 -10 0 10 20 cm/s 

Figure II-21.Temporal evolution of the vertical structure of the alongshore current 

component at mooring C (horizontal lines indicate depths of the instruments), 

September 1996 through January 1997. Note the characteristic vertically 

coherent upcoast and downcoast pulsations; speeds are in cm/s. 

Coherency of the alongshore currents between the three depth levels on mooring 

C in summer is not as strong as it is for the winter period . Significant coherence in the 

subtital frequency band is only observed between the upper and mid-depth meters for 

frequencies less than 0.3 cpd and centered on 0.38 cpd (Figure II-19) . The phase lag (not 

shown) is uniform and is not statistically different from zero across the subtidal band. 

The coherence between the mid-depth and bottom meters is statistically significant only 

for subtidal motion with frequencies less than 0.3 cpd (Figure II-20) with the zero phase 

lag. 

2. Cross-Sectional Transport 

In order to calculate the transport in the LTCC through the section off Cameron, 

we first produce a time series of the 40-hour low passed current component at each 

current meter. A gridded data set is then constructed at the cross-section at each hour 

using 1-m depth by 2-km cross-shore mesh. To rigorously define the offshore extent of 

the coastal current is obviously difficult because of its inherent temporal variability as 

illustrated in Figures II-5 and II-6 . 

Figure II-21 shows that the subtidal behavior of the water column at mooring C 

near the middle of the section is largely characterized by vertically coherent upcoast and 

downcoast fluctuations lasting several days to a week. Alongshore upcoast (downcoast) 

63 

Oct - 96 Nov - 96 Dec - 96 Jan - 97 
speeds are positive (negative) . Accordingly averaged over a month of positive and 

negative pulses, mean speeds are low but consistent with the expected seasonal cycle. 

Figure II-22, for example, shows the average speeds across the section for October 1996, 

well into the downcoast flow season. Mean speeds are 5 to 10 cm/s and all downcoast, as 

expected for this time of the year. 

The mean alongshore speed distribution for July 1996 (Figure II-23) is not only 

less energetic, (3 to 5 cm/s) with the exception at the seaward edge, but directed upcoast 

(positive) as expected for the summer flow season. The limited salinity data recovered 

from the moorings suggest the presence of coastal plume water across the section even as 

far out as mooring A, except for late August 1996. Thus, we make an operational 

definition of the coastal current transport by integrating across the section between 

moorings A and G and recognizing that this is an estimate of the lower band. It does, 

however, have the advantage of being consistent with the ship-mounted ADCP estimates 

of Murray et al . (1997) . 

L 

a 

D 

0 

_5 O 

-1O 

p 

b\ 

_p 

15 20 25 30 35 40 45 50 55 60 65 70 75 80 

Distance (km) 

Figure II-22. Cross-sectional distribution of subtidal alongshore currents (monthly means 

in cm/s), October 1996. Small open circles indicate the current meter 

location; negative values indicate the downcoast flow (the flow to the west in 

this location). 

The alongshore transport through the section during the winter downcoast regime 

of 1996-97 is shown in Figure II-24. Note the persistent downcoast transport of 100 to 

150 x 103 m3/s with a record length mean of - 60,774 .2 m3/s (with a mean squared error 

of 18,553 m3/s). Significant bursts of elevated downcoast transport occur on October 6, 

November 17, and January 8 and 13 . We will examine the spatial characteristics of 

several of these bursts subsequently . 

64 
L 

d 

O 

v 

O5 

O 

10 

s 

0 

A 

15 20 25 30 35 40 45 50 55 60 65 70 75 80 

Distance (km) 

Figure II-23. Cross-sectional distribution of subtidal alongshore currents (monthly means 

in cm/s), July 1996. Small open circles indicate the current meter location; 

positive values indicate the upcoast flow (the flow to the east in this 

location). 

4*10-

3*10r 

2-1 0' 

" Or 'IA I I A A- M A 11 k IV\ d A 11 

V V 

';E -1

.1 ; 

:~ '2*105 

0 M 

in _3*105 

-4-106 

4*106 

-7*106 

4*106 . . . . . . . . . 09120/96 09/30 IOMO 10120 10130 11109 11N8 71128 12/08 12N9 12/29 01/08/97 01/18 

Time 

Figure II-24. Transport in the LTCC during the winter downcoast regime, September 

1996 through January 1997. Note the average downcoast transport 

of -60,774.2 m3/s and the extreme temporal variability (positive values 

indicate the upcoast transport, negative values indicate the downcoast 

transport) . 

65 
3'105 

2'105 

1 .105 

U 

M 

E 

o -1 .105 

a 

a 

;°. -2.105 

-3'105 

-4`105 

-5'10s r"' 

06/01/96 06/11 06/21 07/01 07/11 07/21 07/31 08/10 08/20 08/30 

Time 

Figure II-25. Transport in the LTCC during the summer upcoast regime of 1996 . The 

average transport for this time period is 19,798 .9 m3/s (positive values 

indicate the upcoast transport, negative values indicate the downcoast 

transport). 

The corresponding transport during the preceding summer upcoast regime is 

shown in Figure n-25 . As expected, regional forcing conditions in the summer of 1996 

are such that the coastal current reverses direction and decidedly shifts upcoast by June 9 

and remains upcoast for most of the summer. Note, however, the extreme temporal 

variability in the flow . Fluctuations in transport, which range from slightly below 0 

(downcoast flow) to maximums of 150 to 200 x 103 m3/s (upcoast flow), occur on roughly 

a 5-day time scale. A record length mean for this period is 19,799 m3/s with a mean 

squared error of 41,296 m3/s . 

The most energetic of these oscillations during the winter period appears to be 

closely related to direct wind driving. The event between October 2 and 11 is a prime 

example to investigate. Ten days of northeasterly winds exert alongcoast and downcoast 

wind stresses to produce the cross-sectional speed distribution shown in Figure n-26 . 

Speeds are in excess of 30 cm/s across the entire section with two zones of 

intensification. A high-speed jet in excess of 60 cm/s occupies the outer edge of the 

section. Its seaward extent, unfortunately, is unresolved by the mooring array. A second 

jet, nearly as strong, occupies the inshore end of the section. Previous sections taken 

during LATEX B cruises suggest that intense vertical mixing, combined with zones of 

intensified horizontal density gradients (fronts), are associated with these jets in the 

coastal current. The cessation of the southwestward wind stresses driving the October 6 

intensification of the downcoast flow event is followed by an upcoast flow event, 

probably driven by the pressure gradient setup to balance and oppose the wind stress . 

66 
a 

D 

\\ I I I I 

-5- 

_1 0- 

N bl b Y 70 F~ 

-15 3 I 

-20- 

15 20 25 30 35 40 45 50 55 60 65 70 75 80 

Distance (km) 

Figure II-26. Cross-sectional distribution of alongcoast speeds on October 6, 1996, 

illustrating a downcoast intensification during the winter regime. Note highly 

energetic jets in excess of 50 cm/s at the inshore and offshore ends of the 

section (positive values indicate the upcoast flow, negative values indicate 

the downcoast flow) . 

18 

E 

r 

a 

D 

= \ 19 

14 \ 'e 

12 

e 

s 

15 20 25 30 35 40 45 50 55 60 65 70 75 80 

Distance (km) 

Figure II-27. Cross-sectional distribution of alongcoast speed on November 26, 1996. An 

example of an intensified upcoast reversal during the winter downcoast 

regime (positive values indicate the upcoast flow, negative values indicate the 

downcoast flow). 

67 
wb 

-5 5 O 

,5 

10- 

14 

12 

m 

1 O 

o 

N 

L 

a -

CS 

40 45 50 55 60 65 

Distance (km) 

Figure II-28 .The strongest upcoast intensification of the alongshore current field 

occurring on July 23, 1996 is notable for its moderate speeds barely in excess 

of 20 cm/s (positive values indicate the upcoast flow, negative values indicate 

the downcoast flow). 

0 

b1 

.y 

C 

L 

a 

D 

15 20 25 30 35 40 45 50 55 60 65 70 75 80 

Distance (km) 

Figure II-29. A strong downcoast anomaly driven by persistent easterly winds lasted for 

over 10 days with peak on August 23, 1996. Speeds across the section were 

20-40 cm/s (positive values indicate the upcoast flow, negative values 

indicate the downcoast flow). 

68 
Intense upcoast events occur intermittently during the winter downcoast regime. Figure 

II-27 shows an example of the upcoast flow with speeds of 10-20 cm/s across the section 

that occurred on November 26, 1996 . A zone of low speeds near the bottom 15 to 30 km 

offshore perhaps suggests baroclinic effects (a lens of higher density water) . 

The time series of transport during the summer upcoast regime (Figure II-25) is 

similarly characterized by intense fluctuations of several days to week time scales . The 

strongest upcoast intensification-centered on July 23 (Figure II-28)-is notable for its 

rather modest current speeds of 20 cm/s. Wind driven upcoast currents are upwelling 

favorable, leading to offshore transport in the surface layer and consequent (a) 

suppression of cross-shore density gradients and (b) reduction of baroclinic alongshore 

velocities . 

A period of anomalously strong easterly winds in late August flipped the upcoast 

regime (Figure II-29) into a downcoast state for over 10 days . Speeds of 20 to 40 cm/s 

extend across the section . This downwelling favorable flow typically intensifies density 

gradients (Murray and Young, 1985) driving a strong baroclinic current component, 

which concentrates and intensifies the coastal current. 

3. Decorrelation Time Scales across the Array 

We begin this investigation into temporal and spatial coherence characteristics by 

interpreting the autoconelation function (Bendat and Piersol, 1986) of the 40-hour low 

passed data from the upper level instruments at an inshore mooring F and at an offshore 

mooring B (Figure n-30). The area under the autocorrelation curve integrated out to the 

first zero crossing is referred to as an integral time scale and is often used as an estimate 

of the decorrelation time scale. Alternatively, the first zero crossing or its interpolation if 

noise prevents a zero crossing, is also often used as an estimate for the decorrelation time. 

The decor-relation times for all instruments are listed in Table II-2 . 

It is immediately clear that the cross-shore (v) components at both moorings have 

a short decorrelation time close to 35 hours. In contrast, the alongshore (u) component at 

the offshore mooring B, 65 km from the shoreline, has a decorrelation time of 75 hours 

while the decor-relation time of the same component at the inner mooring F, which is only 

20 km from the coast, is reduced to 60 hours. Thus, we see very little coherence in the 

cross-shore motion at subtidal frequencies but longer decor-relation times of 2.5-3 days in 

the alongshore components may suggest that this flow is better organized. Larger eddy 

length scales are, of course, expected farther offshore and this is consistent with the 

observed alongshore decorrelation time scales . The other upper level instruments at 

moorings E and C, which operated during the winter interval showed similar behavior in 

their autocorrelation function, i.e . minimal decorrelation times in the cross-shore 

components and 2-3 days in the alongshore components. 

The decorrelation time scales for the four upper level instruments that operated 

during the winter period are plotted as a function of a distance offshore for each mooring 

69 
Table II- 2 

Decorrelation time of alongshore and cross-shore current components for the summer 

(Summer u and Summer v) and winter (Winter -u and Winter v) flow regimes. 

Decorrelation time is given in hours : values without brackets - first zero crossing, values 

in brackets - autocorrelation function reaches its standard error value that is 0 .021 for 

both components. 

Mooring/Instrument Summer -u Summery Winter -u Winter -v Distance 

Offshore (km) 

MA to 402(286) 494(489) 76.92 

MA mid 397(288) 308(304) 76.92 

NM top 76(74) 134(32) 64.45 

MB bot 71(68) 63(61) 64.45 

MC to 163(158) 125(82) 66(64) 34(24) 49.54 

MC mid 221(208) 60(58) 60(39) 49(47) 49.54 

MC bot 182(175) 64(50) 61(59) 54(53) 49 .54 

NID_to 86(81 ) 112(91) 42 .61 

ME_to 92(90) 88(85) 55(53) 32(30 27 .06 

NIF to 60(58 30(23 19 .78 

MF bot 46(44) 63(61) 19.78 

MG-top 86(84) 68(64) 13 .42 

os 

09 

0 .7 

Ob 

OS 

c 

OA 

03 

0 

U 

O 02 

7 

0 1 

0 

-0 .1 

-0 .2 

-03 

-0 " 

alonpshore components at mo'orlny B (solid Ilns) end F (dash line) 

-11 _ _ _ - _cross-shor. components at mooring B (dotted Iln@) end F (duh-dot line) _ 

- - - - - - --- - - - - - 

11I1~ - ;- ' - - - - ~ - - - 

- - - ' - - - - - -' - - - - - f - - - - 

- - I 1 -'- - ' - ' ' ' ' - - - - - ` - - - - - -f- - - - - - ' - - ' ' - -

- - -'I- \\- 1 - - - ' - - - - - - - - - - - - - - - - - - - - - - - - - - - -

- - - - - - - 

- 

- - - - - 

--- 

- - - - - 

- 

- - - `,, ,-' - , \-\ - ,- - - j - ~ - -' ~ - j - - ~ - - ~ - ' ~ - , ' - j 

--- 

-------i--- j-~ i --~---- ;------ i------

0 50 100 150 300 250 300 

Time lay In hours 

Figure n-30. Autocorrelation diagrams of 40 hour low passed data from the upper level 

instruments at moorings B and F for the winter observation interval. 

70 
N 

0 

r 

m 

E 

c 

"N0 

m 

O u 

0 

300 

280 

260 

240 

220 

200 

180 

160 

140 

120 

100 

ao 

so 

ao 

20 

0 

----- " 

---~-----~-----------

"

---- r 

------_----~-----------------~ 

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 

Distance offshore (km) 

Figure II-31 . Decorrelation time scales t* for the alongshore components of the upper 

level current meters for both the winter (squares and dash line) and summer 

observation (circles and solid line) intervals plotted as a function of distance 

offshore . Note greatly increased values of t* during the summer regime. 

o .s 

0 .8 

0 .7 

0 .6 

0 .5 

0 

-0 .1 

-0 .2 

-0 .3 

"0 .4 

0 50 100 150 200 250 300 

Time lag in hours 

Figure II-32. Autocorrelation diagrams of 40 hour low passed data from the upper level 

instruments at moorings C and G for the summer observation interval. Crossshore decorrelation time scales are negligible while alongshore decorrelation 

time scales are several days . 

c 

0 

0 .4 

m 

0 .3 

0 .2 

Q 

0 .1 

- - - - - - 

- - - - - - - - - - 

- -- 

1` ~ alonpahore c~mponenti at m¢orinp C (solid line) and G (dash line) 

cross-shore components at mooring C (dotted line) and G (dash-dot line) 

_ _ _ f _ _ _ _ _ _ _ _ I _ 

_ _ _ _ _ _ _ ' _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

__ 

1 

_ _ _ _ _ _ _ _ f _ _ _ _ _ _ _ _ ' _ _ _ _ _ _ _ _ f _ _ _ _ _ _ _ _ I _ _ _ _ _ _ _ __ 

r 

\-~--------'i\ -------1 -------- ; ------% -i -\ -------

--------- -------~ 

- --- - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - -- 

_ 

---------~-~,J"~--I -------- - ----1`~---~-----~ -----` 

L 

% 

`~-~------ ------ -- 

71 
in Figure II-31 . While ranging between 55-75 hours the limited data are consistent with 

an increase of the decorrelation time with distance offshore . 

The rate of increase with distance offshore of the decorrelation time scales is 

notably different during the summer regime, however. Figure II-31 shows that the field of 

points associated with the summer regime does not only overlap with the winter points 

but that the decorrelation time scales increase four times faster in summer than in winter . 

Autocorrelation diagrams for the summer regime show distinctly larger time 

scales than that of the winter for the subtidal fluctuations in the alongshore (u) 

component. Figure u-32 shows that at the C mooring (50 km offshore) the u 

decorrelation time is 100-120 hours, while at the G mooring (14 km offshore) it is about a 

half of that value. Similar to the downcoast flow, the cross-shore (v) components at both 

moorings show much shorter decorelation times than those of the alongshore 

components. 

Mooring A is 27 km further offshore than mooring C and shows significantly 

different characteristics as illustrated in Table 2. Decorrelation times appear to be over 

300 hours in the u component and over 400 hours in the cross-shore (v) component. 

Mooring A in summer appears to be largely decoupled from the damping influence of the 

coast. Physically, this observation is consistent with our understanding of the summer 

reversal consisting of the relict plume waters spreading out offshore under the influence 

of Coriolis forcing as it works its way upcoast . The much greater offshore extent of the 

surface layer (dominated by plume waters) presumably allows for greater length and, 

hence, greater time scales of the subtidal fluctuations. 

4. Spatial Correlation 

The cross-correlations between the upper meters at moorings B and F (a 

separation distance of 45 km) for both the alongshore and cross-shore current 

components for the winter observation period are shown in Figure II-33 . Maximum 

correlation at essentially zero time lag is 0 .83 for the alongshore component and reduces 

to 0 .3 for the cross-shore component. 95% significance levels (Sciremammano, 1979) of 

O lag correlation coefficients of the alongshore and cross-shore components for the winter 

season are 0.31 and 0.21, respectively. Results are basically similar for all other available 

mooring pairs except for those immediately adjacent to each other. For these contiguous 

pairs the cross-shore components also become significantly correlated. 

The results of the cross-correlation analysis are summarized on correlationseparation distance diagrams (Figures II-34 and Figure II-35) . Figure II-34 shows that for 

all six mooring pairs the correlation coefficient is in the 0.8-0 .9 range for the alongshore 

flow component in the downcoast regime. Such high correlation coefficients suggest that 

the cross-shore spatial length scales for the alongshore velocities is very much greater 

than 50 km. As expected the cross-shore spatial length scales of the cross-shore speeds 

are quite different. Figure II-30 shows a rapid decrease of the correlation coefficients 

with an increasing mooring separation implying that the spatial length scale of this 

72 
1 

0.9 

0.8 

0.7 

0.6 

C 0.5 

;0 0.4 

y 0.3 

0 0.2 

y 0.1 

o 

V -0 .1 

-0 .2 

-0 .3 

-0 .4 

-0 .6 

-06 

---------------- 

---------------- --------------- -------------------------------------------------------------------- 

---------------- -------------- 

---- -------------- ---------------- 

---------------- ------------- --- -- ------------- ---------------- -------------------------------- 

---------------- ------------ --- - -- ------------- ---------------- -------------------------------- 

------------------------------------------- -------------------------- ----- ----- ---- ------------------------------

~ .t 

-------- ---- --- ---------- 

I ` ;~~ 1 ~ ! - _r 1 

- ---- ------- I % 

----------------- ---------------------------------- ---------------- 

crosscorrelailon ofthe alongshOre current components- solid line 

-------------------------------~---------------- r----------------%----------------~----------------

crosscorrel~tion ofthe cross-shore current compdnents- dash line 

X00 -200 -100 0 100 200 300 

Time lag in hours 

Figure II-33. Cross-correlation functions of the alongshore (solid line) and cross-shore 

(dash line) components of currents measured at the upper meters of moorings 

B and F for the downcoast flow regime . 

1 .0 

0 .9 

0.8 

0.7 

0 0.6 

d 0.5 

0 

0.4 

A 

0 .3 

0.2 

0.1 

00 

( .) ( . 

(CE) (CF) ~ (BF) 

0 10 20 30 40 50 60 70 80 

Separation distance (km) 

Figure II-34 . Cross-correlation separation distance diagram of the alongshore current 

components for the downcoast flow regime (letters in brackets indicate 

mooring pairs) . 

73 
1 .0 

0.9 

0 .8 

0 .7 

0 0 .6 

d 0.5 

t 

0 

0.4 

m 

° 0.3 

0.2 

0 .1 

00 

(EF) 

(B E) 

(B F) 

(CE) 

(CF) 

0 10 20 30 40 50 60 70 80 

Separation distance (km) 

Figure II-35 . Cross-correlation separation distance diagram of the cross-shore current 

components for the downcoast flow regime (letters in brackets indicate 

mooring pairs). 

1 

0.9 

0.8 

0.7 

0.6 

C 0.5 

0.4 

0.3 

0 0.2 

H 0.1 

L 0 

U -0 .1 

-02 

-0 .3 

-0 .d 

-------------------------------------------------- 

- ----------------------------------------------------------------------------------- 

----------------- ----------------- ---------------- 

-------------------------------------------- ----------------- ----------------- ---------------- ----------------- 

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

- - - - - - - - - - - - - - - - 

i--`-'% ------- --------------- 

_ ; _--- ----- --- - ------------- ;~i~ ~ ------- - ---- - --r 

~ ,may r 

~- 

% 

v - 

%: OP 

, / .. '.. %i 

crosscorretation of the alongshore current components - solid line 

---------------- ------------- crosscorretation of the cross-Shore current comppnents - dash line 

300 -200 -100 0 100 200 300 

Time lag in hours 

Figure II-36. Cross-correlation functions of the alongshore (solid line) and cross-shore 

(dash line) components of currents measured at the upper meters of moorings 

A and E for the upcoast flow regime. 

74 
component is of about 25 km. Higher values of 0.3-0 .35 at a separation distance of 35-45 

km are considered spurious at this point. 

In the summer observation interval, the behavior of the cross-correlation 

coefficients of the alongshore currents is strikingly different with the cross-shore 

correlation of the alongshore flow much reduced. Figure II-36 illustrates this point by 

showing that the maximum correlation coefficient between the alongshore components at 

mooring A and E separated by 50 km reaches only 0 .4 while in the winter interval it 

approached 0.8. The cross-shore speeds between these two sites in summer (Figure II36) are hardly correlated with the very small cross-correlation coefficient of 0.12 ( 95°Io 

significance levels of O lag correlation coefficients of the alongshore and cross-shore 

components for the summer season are 0.33 and 0.26, respectively) . 

c 0 

`o C1 

rn to 

0 

(DC) 

(G D) 

(GE) " (GA) 

( :C) (EA) 

(ED) 

(EC) 

(AC) 

1 .0 

o .s 

0.8 

0.7 

0 .6 

0 .5 

0.4 

0 .3 

0.2 

0 .1 

0.0 

10 20 30 40 50 60 70 

Separation distance (km) 

80 

Figure II-37. Cross-correlation separation distance diagram of the alongshore current 

components for the upcoast flow regime (letters in brackets indicate mooring 

pairs) . 

The summer regime cross-correlation can also be summed up on a correlationseparation distance diagram as in Figure II-37. In contrast to an almost constant value of 

0.8 overall separation distances seen during the downcoast regime (Figure u-34) the 

summer alongshore flow shows a steep decrease down to the correlation coefficient of 

about 0.3 with only a 20-30 km separation distance . Over the 30 km to 65 km separation 

distances the correlation coefficient jumps back to a fairly steady values of 0.4-0 .5 . Such 

behavior of the correlation coefficients suggests that they may be divided into two 

clusters . The first cluster of points (those with 0 to 30km separation distance) has the 

expected decay curve, but the second contains points (separation distance over 30km) 

that are statistically different from zero and may represent another process that we fail to 

understand at this point. 

75 
In Figure II-38 we present the correlation-separation distance diagram for the 

cross-shore summer regime flow for all mooring pairs. This figure with the large 

population of moorings points indicates a well-behaved trend for the upcoast regime 

cross-shore flow with a spatial length scale of about 35 km. It is difficult to contrast this 

summer spatial correlation with that of the winter regime (Figure II-35) due to the small 

number of the winter points (only six mooring pairs) but no significant difference is 

readily apparent except for the two high correlation points (-0.3) at 35 km and 45 km 

separation present in the winter but not in the summer. 

c 0 

N v 

O 

v 

rn 

m 

0 

1 .0 

o .s 

o .s 

0.7 

0.6 

0.5 

0 .4 

0 .3 

0 .2 

0.1 

0.0 

(DC) 

(ED) 

t 

(EC) (GA) 

(G E) 

) (G C) (EA) 

(AC) 

" (DA) 

10 20 30 40 50 60 70 80 

Separation distance (km) 

Figure n-38 . Cross-correlation separation distance diagram of the cross-shore current 

components for the upcoast flow regime (letters in brackets indicate mooring 

pairs) . 

The question arises then why the spatial length scales of the alongshore flow for 

the upcoast (summer) and downcoast (winter) regimes are so distinctly different. One 

possible explanation is that the large spatial length scale of the downcoast regime flow 

could be the result of the coherence imposed on the surface layer by the larger scale 

energetic wind forcing of the winter regime as opposed to the less organized summer 

regime. These tendencies are also clearly evidenced in SCULP drifter tracks presented in 

Murray et al (1997) . 

5. Multiple Coherence Analysis from Current Meter Data 

To seek further insight into dynamical relationships controlling the transport 

observed along the transect, we studied multiple and partial coherence (Bendat and 

Piersol, 1986) between the transport and four forcing functions : (a) alongshore wind 

stress ; (b) cross-shore wind stress ; (c) alongshore pressure gradient (a sea level slope) ; 

and (d) buoyancy forcing proxied by river discharge as in Munchow and Garvine (1993) . 

Wind stress was computed from the Galveston Buoy 42035 for the upcoast flow regime 

76 
and from the anemometer at Fresh Water Bayou for the downcoast regime. The sea level 

slope was calculated from subsurface pressure gauges located at Oyster Bayou and 

Freeport for both seasons, and buoyancy forcing is represented by Atchafalaya River 

discharge. 

Figures II-39 and II-40 show multiple and partial coherence over 0.05 to 0.5 cpd 

frequency band (2- to 20-day period band) between the transport and possible driving 

mechanisms. For the winter observation period (Figure II-39), 60 to 95% of the total 

transport fluctuations (shown by the multiple coherence curve) are explained by the four 

forcing variables considered . Looking at individual partial coherences, the alongshore 

wind stress clearly accounts for majority of variance of the transport through the crosssection. It explains at least about 50% of its variance throughout the 0.05 to 0.5 cpd 

frequency band. There is also an indication that the cross-shore wind stress might be 

considered as a significant forcing for the transport fluctuations with periods between 3 

and 5 days. Partial coherence of the alongshore sea level slope and river discharge with 

the winter transport is rarely statistically significant, and, therefore, their importance as 

possible sources of subtidal winter transport fluctuations is extremely small, if any . 

1 

0.9 

0 .8 

0.7 

0.6 

0.5 

c 

01 

t 0.4 

O 

U 

0.3 

0.2 

0.1 

0 

95°/aSi niiicance Level 

0 0 .1 0.2 0 .3 0.4 0.5 0.6 

Cycle per day(cpd) 

Figure n-39. Multiple (thick line) and partial coherence (symbol lines) between the 

transport observed during the downcoast flow regime and (a) alongshore 

wind stress (solid line with asterisks) ; (b) cross-shore wind stress (dash-dot 

line) ; (c) alongshore sea level slope (dashed line); (d) Atchafalaya River 

discharge (dotted line); the lower straight solid line is the 95% significance 

level. 

77 
In the summer flow regime, 45 to 90% of the transport fluctuations (Figure n-40) 

is accounted for by four possible driving forces over the entire frequency band examined. 

During this regime, the transport fluctuations are not only well coherent with the 

alongshore wind stress but also with the alongshore surface slope at various frequencies, 

except for a narrow band centered around 0.28 cpd where they both become statistically 

insignificant . In addition, the cross-shore wind stress might be considered as an important 

forcing particularly for the transport fluctuations with periods between 5 and 10 days but 

its importance is rather secondary in comparison to the alongshore wind stress and sea 

level slope. Partial coherence between the transport and buoyancy forcing represented by 

the Atchafalaya discharge only appears statistically significant for the frequencies lower 

than 0.18 cpd and the percentage of variance explained by this possible driving force 

reaches about 45% at 0 .1 cpd. 

Additionally, for the summer period the multiple and partial coherence analyses 

(not shown) were performed with the wind stress computed from the wind data collected 

by the anemometer at Fresh Water Bayou. The multiple coherence results are similar to 

those obtained when the wind stress is computed from the wind data form Galveston 

Buoy. However, the partial coherence results are quite different, particularly those for the 

0 .9 

0.8 

0.7 

0.6 

d OS 

c 

G! 

L 0.4 

0 

U 

0 .3 

0.2 

0.1 

0 

---/ -- ~ 

\\ ~ 

95%S' fticance Level 

i 

. ' \ ' . 

0 0.1 0.2 0 .3 0.4 0.5 0.6 

Cycle per day(cpd) 

Figure n-40. Multiple (thick line) and partial coherence (symbol lines) between the 

transport observed during the upcoast flow regime and (a) alongshore wind 

stress (solid line with asterisks) ; (b) cross-shore wind stress (dash-dot line); 

(c) alongshore sea level slope (dashed line) ; (d) Atchafalaya River discharge 

(dotted line); the lower straight solid line is the 95°Io significance level. 

78 
sea level slope (smaller percentage of the transport variance is accounted for by this 

variable). At this point, we do not understand why these results are so different. 

In summary, partial coherence analysis shows that the alongshore wind stress and 

sea level slope appear to be the dominant driving mechanisms of the upcoast flow regime 

transport fluctuations and these two variables explain alone most of the transport variance 

for the frequencies between 0.1 and 0.5 cpd except for the band centered at 0 .28 cpd. In 

the downcoast flow regime, the alongshore wind stress, which alone explains at least 

50% of the transport variance, seems to be a dominant forcing of the transport 

fluctuations in the frequency band between 0.1 and 0.5 cpd. 

6. Transport and Wind Stress 

Murray et al . (1997) showed that the alongshore momentum balance might be 

reduced to the simple linear relationship between the transport and alongshore wind stress 

assuming that other momentum balance terms are negligible . This relationship was 

described by the following equation: 

T 1 

'~sX A p-r 

(II-1) 

where T is the transport, A is the cross-sectional area, p is the water density, r is the 

linear friction coefficient, and ix, is the average alongshore wind stress . 

Our partial coherence analysis shows that the alongshore wind stress, especially in 

the downcoast flow regime, is a dominant driving force of the LTCC transport, therefore, 

(1) was applied to the data to learn whether there is a simple linear relationship between 

the observed transport and wind stress . 

In Figure II-41 and u-42, the transport area is plotted against the alongshore 

component of the wind stress for the winter and summer periods, respectively . The 

alongshore wind stress component was computed from the wind data measured at 

Freshwater Bayou using approach proposed by Large and Pond (1981). 

For both flow regimes, the relation of the alongshore wind stress to transport is 

reasonably linear (correlation coefficient is 0.74 and 0 .69 for the winter and summer 

periods, respectively), which supports the linear frictional balance expressed by (1) . The 

summer and winter slopes of the best fit line indicates a linear friction coefficient of 0.05 

cm/s that is in a range (0.1 to 0.01 cm/s) of that deduced from water level studies by 

Chuang and Wiseman (1983) . 

79 
V.3 

0.2 

0.1 

0.0 

?N 

C 

-0 .1 

~i -0 .2 0 

a 

N 

R -0 .3 

H 

-0 .4 

-0 .5 

-06 

-0 .30 -0 .25 -0 .20 -0 .15 -0 .10 -0 .05 0.00 0.05 0.10 0 .15 0 .20 

Alongshore wind stress [N/A] 

Figure n-41 . The alongshore wind stress plotted against the transport (T) normalized by 

the cross-sectional area (A) for the downcoast flow regime. 

0.25 

0.20 

T/A = 0.02025 + 2.3630 ' Wind stress (correlation coefficient=0.69) 

0 .15 ~~ . 

0 .10 f 

0.05 

0 .00 

-0 .05 

Goes 

$, -0 .10 

a 

c 

i -0 .15 

-0 .20 

-0 .25 

-0 .30 

-0 .35 

-0 .12 -0 .10 -0 .08 -0 .06 -0 .04 -0 .02 0 .00 0 .02 0.04 0 .06 0.08 

Alongshore wind stress [N/ M2] 

Figure n-42. The alongshore wind stress plotted against the transport (T) normalized by 

the cross-sectional area (A) for the upcoast flow regime. 

80 
A regression intercept value for the downcoast flow regime is negative and equal 

to 0.0146, which, although potentially related to other driving forces such as buoyancy 

or/and alongshore sea level slopes, is not significantly different from zero (p-value 

=0.05) . This may be interpreted that other forces are relatively unimportant compared to 

the wind stress in the alongshore momentum balance for the winter period . The summer 

intercept (0.02025, p-value =0.001), however, can not be statistically considered as equal 

to zero. Therefore, the sectional transport observed during the upcoast flow regime may 

be driven not only by the alongshore component of the local wind but also by buoyancy 

or/and alongshore barotropic gradient . 

7. A Wind-Driven Model 

To continue in more detail examinations of the importance of the local wind stress 

as a major controlling forcing mechanism of the current variability within the LTCC, a 

wind-driven model, which was used to study variability of the currents within the 

Amazon River plume (Lentz, 1995), was employed. The model assumes that the input of 

energy from the wind into the surface layer plume is balanced by linearly approximated 

bottom friction and the temporal acceleration of the alongshore flow . This alongshore 

momentum balance is expressed as followed: 

au + T ' U - 'r sx 

at h p-h 

(1 1- 2) 

where u is the alongshore wind driven current, p is the reference water density, r is the 

linear friction coefficient, h is a plum thickness and ixs is the alongshore wind stress . 

Integrating (2) in time and solving for u yields 

c 

u(t) = f USX exp(-r ~ (t - t' ) / h)dt'+u(t = 0) ~ exp(-r ~ t / h). (11-3) 

oP'h 

The alongshore wind stress component is computed from the wind data measured at 

Freshwater Bayou using approach proposed by Large and Pond (1981) . The linear 

friction coefficient is chosen to yield the best agreement with the observations . 

Taking the averaged plume thickness (h) equal to 12 m and density of 1021 kg/m3, 

the alongshore current component was calculated from equation 3 for the downcoast flow 

regime period and then compared with the observed subsurface currents at three different 

mooring locations (B, C and F). Figure II-43 shows the model estimates and observations 

(40-hour low passed data) from a subsurface instrument at mooring F. Comparison of the 

wind-driven alongshore velocities based on (3) and observed 6.5m velocities at mooring 

F shows quite reasonable agreement. Maximum zero lag correlation between the 

observations and estimates based on equation II-3 with r=0.0003 m/s is 0.81 and it is 

significant at the 95% confidence level. Variability of the estimated wind-driven 

81 
o.s 

o .s 

0.4 

N 0.3 

0.2 

c 

0.1 

0 

0 

0 

-0 .1 

z 

o -0 .2 

d 

d -0 .3 

0 

N -0 .4 

o -0 .5 

Q 

-0 .6 observatlona 

-- model results 

-0 .8 

09/22/96 10/02/96 10/12/96 10/22/96 11101/96 11/11196 11/21196 12/01/96 12/11/96 12/21/96 12/31/96 

Figure n-43 . The alongshore 6.5m current observed at mooring F during the downcoast 

flow regime compared to the prediction obtained from equation II-3 . 

0 .3 

0.2 

0.1 

c 

L 

C 

a 0.0 

E 0 

v 

-0 .1 

0 

-0 .2 

w c 0 

R 

-0 .3 

-0.4 

06/03/96 06113/96 06/23/96 07/03/96 07/13/96 07/23/96 08/02/96 08/12/96 08/22/96 

Figure n-44. The alongshore 5 .5m current observed at mooring G during the upcoast 

flow regime compared to the prediction obtained from equation II-3 . 

82 
alongshore current is very similar to that observed at F, for instance, events observed 

between October 3 and 11, 1996 or October 22 and November 4, 1996 or November 7 

and 19, 1996. However, estimated amplitudes are either too large or small but differences 

between these amplitudes and those observed are never larger than 20 cm/s . Analogous 

conclusions are obtained for the other two moorings (mooring C: correlation 

coefficient=0.76 with r=0.0004 m/s; mooring B: correlation coefficient=0 .76 with 

r=0 .0003 m/s). These results reinforce an earlier conclusion that the alongshore wind 

stress, which explains alone at least 58°Io of the current variance, is a dominant driving 

force of the currents observed within the LTCC in the downcoast flow regime. 

When the wind stress observed in the summer regime is applied to equation II-3 

variability and amplitudes of the calculated wind-driven alongshore velocities are badly 

modeled for almost entire June but they are quite well reproduced for July and August 

1996. The model estimates are again compared with subsurface current observations at 

three different locations (mooring A, C, and G) . The best agreement is found at mooring 

G (Figure II-44) . Maximum zero lag correlation between the observed alongshore 

velocities and estimates with r=0 .00035 m/s, h=10 m, and density of 1010 kg/m' is 0.61 

and it is significant at the 95% confidence level. It is obvious that major fluctuations in 

July and, particularly, in August such as events observed between August 1 and 6 or 

August 20 and 26 are wind driven. However, the June variability of the estimates is 

different from that of the observations with the amplitude differences reaching even 30 

cm/s, which is quite large because of lower speeds usually measured within the LTCC in 

summer. Similar conclusions are found when the estimates are compared with the 

subsurface currents at mooring C and A (correlation coefficients with r=0.0002 m/s are 

0.58 and 0.57, respectively) . These results suggest that the alongshore wind stress, which 

explains alone 37°Io or less of current variance, is an important driving force of the coastal 

current, however, other driving mechanisms such as barotropic and baroclinic pressure 

gradients should be also considered not neglected as possible driving forces of the LTCC 

in the summer flow regime. 

8. Salinity Characteristics 

Results from the SeaCats intended to monitor the cross-shore and vertical salinity 

gradients were quite disappointing, such that only minimal useful analyses could be 

conducted. In Figure II-45 the sub-tidally filtered salinity data at outer mooring A and 

mid-section mooring D are showed. Outer mooring salinity is persistently higher and 

stable probably because the instrument was located in a small salinity gradient zone. 

Fluctuations at the interior mooring D are larger and more energetic which may suggest 

that this instrument was situated in waters with strong salinity gradient . Note also a large 

decrease of salinity to 15 psu between August 23 and 28 that probably resulted from an 

interplay of strong easterly winds and higher Atchafalaya River discharge observed at 

that time. Additionally, note the general increase of salinity at both locations from midJune to late August as the freshwater content of the inner shelf decreases with decreased 

influx of Mississippi-Atchafalaya River water. 

83 
34 

32 

30 

28 

26 

N a 

24 

in 22 

20 

18 

16 

14 

,- 

. " 

v~ ~ V 

. . .' 

06/01/96 06/11 06/21 07/01 07/11 07/21 07/31 08/10 08/20 08/30 

Figure II-45 . A comparison of salinity from the outer edge (mooring A: dash line) and 

mid-region (mooring D: solid line) of the coastal current. 

34 

32 

30 

28 

is 

a 

24 

v~ 22 

20 

18 

16 

14 

09/11/95 11/10/95 01/09/96 03/09/96 05/08/96 07/07/96 09/05/96 11/04/96 01/03/97 

Figure II-46. A composite of the segments of salinity recorded from mooring F over a 

two-year period . 

84 
Figure II-46 shows the longest available salinity record of this project measured at 

mooring F. This plot displays again only sub-tidally filtered data. Time series of salinity, 

which varies between 18 and 32 psu, from this instrument suggests that mooring F was 

constantly in plume waters regardless of a month. Salinity there fluctuates quite rapidly in 

both summer and winter seasons . However, the short summer record suggests that these 

fluctuations are larger in summer (amplitudes of the order of 10 psu) than those measured 

in winter (amplitudes of the order of 5 psu). Additionally, average salinity seems to be 

smaller in summer than that observed in winter . 

D. Conclusions 

In order to determine the transport of the LTCC and its temporal and cross-shore 

variability a mooring line extending 80km offshore was deployed on the cross-shore 

transect near Cameron, LA . In addition to the current meter data, pressure, salinity, wind, 

and Atchafalaya River discharge were also analyzed to better understand the variability 

of the LTCC transport. Based on availability and quality of the data, two periods (one 

during the summer upcoast flow regime and one during the downcoast winter regime) 

were selected for further detailed studies. 

Energy spectra of the near surface currents show expected semidiurnal and 

diurnal peaks associated with the tidal and inertial motions for both summer and winter 

flow regimes . Broad range of energetic motion is also observed in subtidal/weather 

frequency band (periods of 2-10 days) during the winter regime . Summer spectra of the 

near surface currents in the subtidal band may be described as `red', i .e. the energy 

increases with decreasing frequencies . For both analyzed regimes, the energy in this 

frequency band is also clearly polarized in the alongshore direction . In winter, crossspectra show high coherence of the currents through the water column that do suggest 

their barotropic response to the driving forcing. In summer, such high coherence of 

currents in vertical is not observed. 

Transport computations show that in the winter season, the mean transport of 

LTCC is of - 60,774.2 m3/s and 19,799 m3/s in summer. In both seasons the transport is 

characterized by high synoptic scale variability (standard deviations are 131,445 m3/s and 

99,002 m3/s for winter and summer seasons, respectively). 

The data indicate that the decorrelation time scale of the alongshore current 

component in the downcoast flow regime show values of 40-80 hours. In the summer 

observation period, this decorrelation time is much larger and varies from 80 to about 290 

hours. There is also some indication that in summer there is a sharp increase of the 

decorrelation time in the offshore direction. For both observation periods, the 

decorrelation times of the cross-shore component of the LTCC are shorter than those of 

the alongshore except for currents observed at mooring A. These times vary between 20 

to 60 and 50 to 90 hours for the winter and summer regimes, respectively . 

In the winter regime, regardless of the separation distance, crosscorrelation 

coefficients determined from meter pairs vary between 0.8 and 0.9 for the alongshore 

component; therefore, the spatial deconelation length scale is much larger than offshore 

85 
extend of the mooring array (>SOkm). In summer, the crosscorrelation coefficients of the 

alongshore current component present a confusing picture. There appears to be two 

clusters of points : one resembling the behavior of the cross-shore component, and the 

other cluster showing a constant value about 0.5 for large separation distances. During 

the downcoast flow regime, the spatial decorrelation length scale of the cross-shore 

current component appears to be about 40km. In summer, this spatial length scale for the 

same current component is similar to that of the downcoast regime showing a value of 

35-40km. 

Multiple coherence analysis of the transport for the winter regime shows that 60 

to 95% of its variability is explained by four inputs (alongshore pseudo wind stress, 

cross-shore pseudo wind stress, water level difference, and Atchafalaya River discharge). 

Partial coherence clearly shows that the alongshore wind stress is a dominant driving 

mechanisms. In the summer regime, four input variables account for 45 to 90% of 

variability of the transport . In this case, in addition to the alongshore wind stress, the 

alongshore sea level slope seems to be also important driving forcing. 

Our data support the simple frictional balance model and show that. a relation 

between the summer and winter transport fluctuations and the alongshore wind stress is 

reasonably linear. However, for the summer observation period, the fluctuations are not 

terminated when the wind stress is zero. Such behavior suggests that the remaining 

fluctuations are caused by other forcing mechanisms, for instance, barotropic and/or 

baroclinic pressure gradients . 

Results from the model, which assumes that currents are wind driven, confirms 

that the alongshore wind stress is a dominant driving force of the LTCC in the downcoast 

flow regime. In the upcoast flow regime, this wind stress is also an important driving 

force; however, other forcing mechanisms such as barotropic and/or baroclinic pressure 

gradients should not be neglected either. 

Finally, the limited data from the salinity sensors deployed on the current meter 

moorings suggest that the cross-shore array remained in the coastal plume throughout 

both the summer and winter observational intervals . 

86 
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89 

QPQ~MENT Oply~'y The Department of the Interior Mission 

As the Nation's principal conservation agency, the Department of the Interior has responsibility 

a 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, and biological diversity; 

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