OCS Study 

MMS 2000-045 

Coastal Marine Institute 

Dynamic Height and Seawater Transport 

across the Louisiana-Texas Shelf Break 

Final Report 

TEXAS LOUISIANA 

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U.S. Department of the Interior 4 aft Cooperative Agreement 

Minerals management Service 11 r Coastal Marine Institute 

Louisiana State University Gulf of Mexico OCS Region 

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OCS Study 

MMS 2000-045 

Coastal Marine Institute 

Dynamic Height and Seawater Transport 

across the Louisiana-Texas Shelf Break 

Final Report 

Authors 

Carole L. Current 

William J . Wiseman, Jr. 

June 2000 

Prepared under MMS Contract 

14-35-0001-30660-19948 

by 

Coastal Marine Institute 

Louisiana State University 

Baton Rouge, Louisiana 70803 

Published by 

U .S . Department of the Interior Cooperative Agreement 

Minerals Management Service Coastal Marine Institute 

Gulf of Mexico OCS Region Louisiana State University 

DISCLAIMER 

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

the 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 and policies of the Service, nor does mention of trade names or 

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

from review and compliance with MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of the report may be obtained from the Public Information Office (Mail Stop 5034) 

at the following address : 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Office (MS 5034) 

1201 Elmwood Park Boulevard 

New Orleans, Louisiana 70123-2394 

Telephone Number: 1-800-200-GULF or 

504-736-2519 

CITATION 

Suggested citation : 

Current, C. L. and W. J . Wiseman, Jr. 2000. Dynamic height and seawater transport across the 

Louisiana-Texas shelf break. OCS Study MMS 2000-045 . U.S . Dept. of the Interior, 

Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 46 pp. 

ABOUT THE COVER 

The cover art shows the Louisiana-Texas shelf and the locations of ten moorings at the shelf edge 

from which temperature and salinity data used to estimate cross-shelf transport in this study were 

collected . 

iii 

TABLE OF CONTENTS 

CHAPTER PAGE 

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I 

1 .1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

1 .2 Overview of the Present Study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

II . METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

2.1 Data and Data Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

2.2 Dynamic Vertical Structure Functions for Pressure Anomaly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

2.2 .1 Reference Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

2.2 .2 Computation of Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

2.3 Fit of Mooring Data to Dynamic Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

2.4 Estimation of Dynamic Height and Geostrophic Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

III. RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

3.1 Modal Amplitudes Along the Louisiana-Texas Shelf Break . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 

3.2 Dynamic Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 

3.3 Transport Across the Louisiana-Texas Shelf Break . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

IV . DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

V. SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 

LITERATURE CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 

APPENDIX A: VERTICAL PROFILES OF SHELFWIDE TRANSPORT . . . . . . . . . . . . . . . . . . 33 

APPENDIX B: ESTIMATED TRANSPORT BY SHELF REGION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 

v 

LIST OF FIGURES 

Figure Page 

1 The positions of 10 LATEX-A shelf break moorings that provided 

salinity and temperature data from which time dependent pressure 

anomaly profiles, dynamic height, and geostrophic transport were estimated. . . . . . . . . . . . . .2 

2 Typical CTD deployment patterns for LATEX-A hydrographic cruises 

(filled circles) ; above, full shelf; below, half shelf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 

3 Seasonal mean hydrostatic stability profiles for the Louisiana-Texas shelf, 

with examples of seasonally fitted reference columns (thin lines) used for 

vertical dynamic mode computations . (a) fall-winter, (b) spring, and 

(c) summer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 

4 Typical seasonal shelf-wide dynamic modes of pressure anomaly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . l l 

5 First and second baroclinic pressure anomaly mode amplitude coefficients 

B versus seafloor depth on the Louisiana-Texas shelf from LATEX-A 

cruise hydrography and from shelf break mooring data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

6 Monthly shelf-wide modal amplitude means for (a) barotropic, (b) first 

baroclinic, and (c) second baroclinic modes. Upstream (o) and 

downstream (x) modal amplitude means for (d) barotropic, (e) first 

baroclinic, and (f) second baroclinic modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 

7 Two component Fourier fit to seasonal steric water level averages computed 

from LATEX hydrographic cruise data (filled circles) at about 94°W near 

the 200-m isobath on the Louisiana-Texas continental shelf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

8 Mean offshore geostrophic transport per half meter depth across the 

Louisiana-Texas shelf break for (a) downcoast, (b) midshelf, and 

(c) upcoast Louisiana-Texas shelf break regions for time intervals treated 

in this study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21 

9 Seasonal average geostrophic transport per half meter depth in the offshore 

direction across the Louisiana-Texas shelf break. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22 

A 1 Shelfwide transports per half meter depth in the onshore direction across 

the Louisiana-Texas shelf break, 15-day means from April 21 - July 3, 

1992. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35 

A2 Shelfwide transports per half meter depth in the onshore direction across 

the Louisiana-Texas shelf break, 15-day means from September 26 - 

November 19, 1992 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36 

vii 
Figure Page 

A3 Shelfwide transports per half meter depth in the onshore direction across the 

Louisiana-Texas shelf break, 15-day means from January 31 - May 5, 

1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37 

B 1 Estimated offshelf transport per km from April 28 - June 17, 1992, 

as a function of distance north and east along the 200-meter isobath from 

LATEX-A mooring 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41 

B2 Estimated offshelf transport per km from October 3 - November 12, 1992, 

as a function of distance north and east along the 200-meter isobath from 

LATEX-A mooring 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42 

B3 Estimated offshelf transport per km from February 7 - March 9, 1993, 

as a function of distance north and east along the 200-meter isobath from 

LATEX-A mooring 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43 

B4 Estimated offshelf transport per km from March 19 - April 28, 1993, 

as a function of distance north and east along the 200-meter isobath from 

LATEX-A mooring 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44 

BS Estimated offshore geostrophic transport across the Louisiana-Texas 

shelf break by alongshelf region . (a) April 28-June 27, 1992; 

(b) October 3 - November 12, 1992; and (c) February 7 - April 28, 1993 . . . . . . . . . . . . . . . . 45 

viii 
LIST OF TABLES 

Table Page 

1 Seasonal statistics of barotropic, first baroclinic, and second baroclinic mode 

amplitudes for 144 15-day mean pressure profiles on the Louisiana-Texas 

shelf break. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 

2 Alongshelf regional statistics of modal amplitudes for 144 15-day mean 

pressure profiles on the Louisiana-Texas shelf break. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17 

3 Estimated transport (in Sverdrups) off the Louisiana-Texas shelf at the 

200-meter isobath, listed by alongshelf region at specified depth ranges 

and days. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19 

4 Estimated transport (in Sverdrups) off the Louisiana-Texas shelf at the 

200-meter isobath, listed by alongshelf region for the full water column 

and also listed for the entire shelf break at specified depth ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

5 Mean seasonal offshore geostrophic transport at the Louisiana-Texas shelf 

break in Sverdrups (106 m3/s). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26 

6 Monthly average direction of cross-shelf flow at the shelf break (200-meter 

isobath) for the present study and for the Bender study (in parentheses) . . . . . . . . . . . . . . . . . . . .27 

ix 

I. INTRODUCTION 

1 .1 Background 

The transport of seawater onto and off of the Louisiana-Texas continental shelf is relevant to a 

broad variety of CMI and MMS interests in this region, where extensive offshore oil and gas 

production coexists with some of our nation's strongest and most productive fisheries. The 

present study is focused on the understanding and quantification of horizontal transport of 

seawater across the shelf break, as estimated by applying the assumption of geostrophy. 

The Louisiana-Texas continental shelf extends nearly 1,000 km from the Rio Grande river mouth 

to Southwest Pass in eastern Louisiana. Although it is less than 90 km wide in the south, the 

midshelf region near Galveston and Sabine is broad with rough and irregular bottom features 

across more than 200 km width. The total volume of seawater on the shelf is roughly 8 x 1012 m3 

at any given time . The shelf break is located at approximately the 200-m isobath (Figure 1). 

Cochrane and Kelly (1986) describe a seasonal pattern of circulation on the Louisiana-Texas 

shelf in which alongshelf flow on the inner shelf is downcoast (from Louisiana following the 

coastline south and west towards south Texas) except during the summer months, when it 

reverses and upcoast flow prevails. The outer shelf flow is upcoast (from south Texas north and 

east towards Louisiana) throughout the year. Cross-shelf flow is generally offshore in the south 

and onshore in the east, but this too reverses during the summer months . However the results of 

LATEX-A (Texas-Louisiana Shelf Circulation and Transport Processes Study) only partially 

confirmed the Cochrane and Kelly pattern of circulation, and were particularly equivocal on the 

outer Louisiana-Texas shelf. Alongshelf flow on the inner shelf is downcoast except for the 

summer months, and averages more than 0 .25 Sv in the downcoast direction (Nowlin et al . 

1998). It is not clear where this tremendous volume of water originates and how it leaves the 

shelf. The Mississippi-Atchafalaya River system delivers a mass flux of the order of 10% of the 

Louisiana-Texas shelf volume inside the 90-m isobath (Dinnel and Wiseman 1986). Additional 

mass flux to the shelf is delivered by excess rainfall, smaller rivers and eddy-shelf interactions . 

The timing, loci, and removal mechanisms of this water from the shelf in order to maintain a first 

order mass balance is an intriguing and complex oceanographic problem . This variable exchange 

carries with it a variety of dissolved and particulate materials, e.g. salt, nutrients, heat, sediments, 

larvae, and tar balls. 

Bender and Reid determined transports across the Louisiana-Texas shelf break, as reported by 

Nowlin et al . (1998) . Monthly averaged moored current meter velocity data were fitted to EOFs 

(empirical orthogonal functions) from 4 m deep by 10 km long binned ADCP (acoustic doppler 

current profiler) velocity data to estimate box model volume fluxes for each of 18 box sides. 

They concluded that the cross-shelf flux over the 200-m isobath is generally stronger than the 

outer shelf alongshelf fluxes and shows no discernible seasonal pattern. Bender and Reid did not 

focus on geostrophic flow alone, although current meter data along the shelf break contains a 

strong geostrophic flow component. Chen (1995) found that only about 30% of the variance in 

the LATEX-A ADCP data is ageostrophic, and about 70% is geostrophic. Other LATEX-A 

scientists carefully investigated the current meter record data using a variety of time-frequency 

1 
31 

2! 

2 

2 

2 

TEXAS LOUISIANA 

a ~,W New Orlean ~ . 

" Sabine ~ ~4 

10 

Galveston 

0 C CC GULF OF MIXICO 

N O 

r 

-98°W -97°W -96°W -95°W -94°W -93°W -92°W -91°W -90°W -89°W 

Figure 1 . The positions of 10 LATEX-A shelf break moorings that provided salinity and 

temperature data from which time dependent pressure anomaly profiles, dynamic 

height, and geostrophic transport were estimated. 

domain techniques in an attempt to identify free and forced waves and solitons, and seasonality 

of shelf break flow patterns. Results were of mixed success and these investigations are 

continuing. 

The present study is confined to the shelf break region, employing a different methodology than 

that of Bender and Reid to estimate cross-shelf geostrophic transport. The dynamic structure 

functions used in this study are dependent on the long wave assumption as part of the linear 

dynamics used in their computation. Therefore, the use of these structure functions acts as a 

filter, screening out high frequency and nonlinear effects that can produce bias and inaccuracies 

in estimates of geostrophic transport. The use of hydrographic data on which to base our structure 

functions is also conducive to focus on geostrophic flow. The shape of a leveled state reference 

profile based on hydrographic data may be less prone to bias high frequency signal than is a 

reference based on ADCP data. For these reasons any overlap of this and the Bender and Reid 

calculations is complementary, and not a duplication of effort . 

2 
Our study quantifies time dependent, highly vertically resolved geostrophic transport of waters 

from the Gulf of Mexico across the Louisiana-Texas continental shelf break, and provides a first 

order picture of the spatial and temporal variability of shelf break exchange processes. Crossshelf transport is large, and consistent with the findings of Bender and Reid. The annual volume 

of water transported off the shelf across the shelf break is approximately equal to the total 

volume of water on the shelf. 

1 .2 Overview of the Present Study 

The present study begins with the determination of orthogonal patterns of pressure anomaly that 

are characteristic of geostrophic flow, computed from LATEX-A field hydrography and assumed 

dynamics of the system . Shelf break mooring data then is projected onto these patterns of 

variability, which are called dynamic vertical structure functions or dynamic modes (Kundu et al . 

1975 ; Flierl 1978; SAIL 1989; Arango and Reid 1990; Current 1993) . Estimates of time varying 

pressure anomaly profiles and dynamic height are produced by the summation of weighted 

dynamic modes, and estimation of cross-shelf geostrophic transport follows from these profiles 

by invoking geostrophic balance. 

Computation of dynamic vertical structure functions requires a reference column, which is 

determined from hydrography. In order to seasonally adjust the structure functions for more 

efficient representation of the data, these structure functions are re-computed for each period 

during which data was collected, averaged, and fitted to them in the modal analysis using 

reference columns adjusted to that specific time of year. Density profiles from the hydrography 

are used to compute these smoothed, seasonally adjusted leveled state reference profiles . These 

reference columns are used in computing local dynamic vertical structure functions for pressure 

anomaly. The orthogonal patterns of variability are computed from the assumed dynamics of the 

system as well as the hydrographic profiles, and are highly resolved in the vertical. 

The projection of mooring data onto these dynamic modes requires fitting the data at each 

mooring location, for each time step, to the dynamic structure functions in order to determine 

modal amplitudes . Time-dependent pressure anomaly information derived from measurements at 

individual top, middle, and bottom moorings were fitted to three local orthogonal dynamic 

modes by standard techniques to obtain modal amplitudes . High resolution pressure anomaly 

profile estimates were then reconstructed by summing the weighted modes at each mooring 

during each time step. Dynamic height anomalies were estimated using the Montgomery/Csanady 

method (Csanady 1979), and cross-shelf transport was estimated from an assumption of 

geostrophy and pressure anomaly profiles computed relative to a 250 meter reference depth. 

II . METHODS 

2 .1 Data and Data Products 

The recently completed LATEX-A program, sponsored by MMS, collected mooring and 

hydrographic data over the Louisiana-Texas continental shelf during the 32 months from April, 

1992 through December, 1994. The best spatial and temporal coverage of this large data set is 

arguably in the vicinity of the shelf break. Current meter arrays with near surface, mid-depth, and 

near bottom meters, recorded velocity, temperature, and salinity at five minute intervals. These 

arrays were deployed at a spacing of no more than 90 km along the 200-m isobath between 

90.SW and 27.1N. Furthermore, each of the hydrographic cruises collected CTD (conductivitytemperature-depth instrument) profiles along the 200-m isobath at a spacing of approximately 20 

km. 

Spatial correlation scales over the shelf were generally exceeded by separation distances between 

current meter mooring locations (Li et al . 1996) . Despite these current meter separations, Cho et 

al . (1998) found reasonable patterns of shelf-wide monthly mean streamfunction using only near 

surface current meter data and standard analysis techniques . The moorings along the 200-m 

isobath were deployed in an especially closely spaced pattern that is favorable for the reliable 

estimation of dynamic height and cross-shelf transport, as in this study. 

Moorings 4, 5, 6, 7, 8, 9, 10, 11, 48, and 13 deployed for LATEX-A were the source of hourly 

top (- 14 meter), middle (- 100 meter), and bottom (- 190 meter) temperature and salinity time 

series used in this study. These 10 moorings were located along the 200-m isobath at the 

Louisiana-Texas shelf break as depicted in Figure 1 . Data taken from these moorings were 40-

hour low-passed, and density ((Te) time series were computed from temperature and salinity 

values using EOS-80 (Fofonoff 1985) . Three extended time periods with uninterrupted top, 

middle, and bottom 40-hour low-passed values at most moorings were identified and selected as 

the focus of this study in order to maximize the degrees of freedom available for modal 

representation . Summer, fall, winter, and spring measurements were encompassed by this subset 

(April 20 - July 13, 1992, September 26 - November 25, 1992, and January 31 - May 7, 1993) . 

Summer is defined as June through August, fall as September through November, winter as 

December through February, and spring as March through May, for the present study. Prior 

studies often have selected two seasons on this shelf, summer and non-summer. Summer data 

with uninterrupted top, middle, and bottom temperature and salinity series were sparse, however. 

By selecting four seasons, the spring and fall transition periods between winter and summer 

could be treated separately both for dynamic height and transport, and combined non-summer 

transports are discussed in Chapter 5. Mean densities were obtained for 15-day periods with five 

day overlaps, resulting in mean top, middle, and bottom densities at ten day intervals at each 

mooring. A first approximation of these time dependent 6e profiles was made by means of linear 

interpolation between top, middle, and bottom measurements and extension to the surface and to 

the seafloor . This extension was done by linear extrapolation of the trend between the middle 

and the top (or bottom) measurements to the surface (or seafloor). The mean 100 meter hourly 6e 

for all 10 moorings is at approximately mid-depth in the water column for these shelf break 

5 
moorings, and was taken to roughly represent a barotropic mean and was removed from all 6e 

values . The modal amplitudes are found relative to this estimated shelf break mean reference 

density (26.4 kg/m3) in order to provide a realistic estimate of the barotropic vertical mode 

amplitudes of the local pressure anomaly profiles at the shelf break. 

Pressure anomaly profiles were obtained following the method of Montgomery/Csanady 

(Csanady 1979) . Pressure at 250 meters was taken to be 0 .0 decibars with a corresponding 6e of 

28 .1 kg/m3, a mean value estimated by linear extrapolation using the mean ae at the middle and 

bottom measurement depths, approximately 100 meters and 190 meters . The same 250 meter 

estimated ae of 28 .1 kg/m3 was found by linear integration along each cross-shelf line of each 

LATEX-A hydrographic cruise by Current (1996). Pressures were then obtained, with respect to 

250 meters, by integrating upwards through the water column employing the relation 

6~ -F 6j-I ( 

p~ = p~-I g 2 \ZJ-1 - Zl

l 

/ 

The subscripts j and (j-1) refer to the value at the present and next deepest level, respectively, at 

intervals of 0 .5 meters in the 200-m water column, and a is the ae value adjusted as described. 

Seasonal reference profiles and the dynamic vertical structure functions based on them were 

determined independently from CTD data collected on full shelf and half shelf 

hydrographic cruises (Figure 2). These hydrographic profiles were available digitally with 

measurements separated by only 0.5 meters depth, which allowed the computation of high 

resolution dynamic structure functions. Stability profiles were computed from the density 

profiles using the relation 

p az 

2 .2 Dynamic Vertical Structure Functions for Pressure Anomaly 

Hydrographic data were used to compute vertical dynamic structure functions that are based on 

seasonal shelf-wide reference columns of pressure anomaly and additional information provided 

by the assumed dynamics of the system. These structure functions are obtained by application of 

linear dynamics to a reference density profile, and have the advantage (in comparison with purely 

empirical structure functions such as EOF's) of screening out the effects of high frequency and 

nonlinear phenomena. Dynamic structure functions of this kind are particularly suited for use in 

the estimation of geostrophic transport. 

Modal amplitudes of pressure anomaly were estimated by fitting the modes to pressure anomalies 

computed from mooring data. The resulting modal amplitudes then provide estimated pressure 

anomaly profiles at the mooring location to within 0.5 m of any depth, and were integrated from 

6 
30° N 

29° N 

28° N 

27° N 

26° N 

30° N 

29° N 

28° N 

27° N 

26° N 

TEXAS LOUISIANA O 1 

Houston 6 W Orlean ~ . 

10 i 

~i 20 ~' f . . 

W 

2 

v  

. 

s ~93, . 48 

%s ~~ 1000 

°o °0 2~~~ GULF OF MDCICO 

N O 

-98° W -97° W -96° W -95° W -94° W -93° W -92° W -91° W -90° W -89° W 

TEXAS LOUISIANA D 

i 

Houston New Orlea A 

10 

20 

_. 50 48 

~ y 9 :ea~po,=1: l "- 

13 

~ts 

" -~ 1000 

d " -- 0`5 

., 

. . 

a4 

C) C, 

°n N o 2000 GULF OF MDCICO 

-98° W -97° W -96° W -95° W -94° W -93° W -92° W -911W -90° W -89° W 

Figure 2. Typical CTD deployment patterns for LATEX-A hydrographic cruises (filled circles) ; 

above, full shelf; below, half shelf. Vertical dynamic modes were computed using 

seasonally adjusted mean profiles independently obtained from hydrographic data and 

fitted to time series from ten moorings (open circles) . 

7 
surface to seafloor to obtain estimated dynamic height . High resolution estimates of depth 

dependent flow at the shelf break were then obtained by applying the assumption of geostrophic 

balance, and are potentially useful information not previously available to MMS. The focus on 

low frequency geostrophic cross-shelf transport in the absence of effects of tides and wind 

forcing in the weatherband is consistent with the smoothing of mooring data by time averaging 

over a 15 day period with five day overlap. 

2 .2 .1 Reference Profiles 

Seasonal shelf-wide reference columns of hydrostatic stability were fitted to LATEX-A 

hydrographic data, taking NZ to be 

N 2 N 2 + N 2 ~Z _ Z'~ z > 70 in 0 1 (3) 

N 2 t) z <_ 70 in 

where 

N~z = exp(1 .613 * 10-5 z' + 0 .0197z + 5.7220 (4) 

and 

N; _ -103 .9 + 239.5 cos(co t)- 63 .1 sin(co t)+ 64.4 cos(2co t) . (5) 

The second harmonic sine term was also computed but was found to be negligible. The phase 

(co t) equals zero on the first day of August, following Current (1996) . The variable CO is defined 

as the annual frequency, 27c/year . 

Mean stability profiles computed from corresponding hydrographic profiles using (2) are 

depicted in Figure 3 with examples of these seasonally fitted reference columns. Seasonal 

variability was allowed in the upper 70 meters of the water column. This depth was selected by 

subjective examination of residuals remaining from the hydrographic data for each season after 

the initial fit. 

Local dynamic vertical modes of pressure anomaly were calculated with respect to these 

smoothed, seasonally adjusted leveled state reference profiles obtained independently from 

hydrographic cruise data. The use of a seasonal mean stability profile as a reference for 

computing structure functions allows these functions to represent only the variability, or signal, 

superimposed on this mean, and so improves the efficiency of representation of the data. The 

dynamic reference column is smoothed by superimposing the influence of seasonal variability 

(N12) on a basic smooth, exponential stability profile (No ) that does not vary with season. 

Seasonal effects are superimposed at water column depths not exceeding 70 meters . This is done 

by linear interpolation of the initial exponential stability profile above 70 m to Fourier fitted 

mean seasonal stabilities at the surface. Smoothed reference columns constructed by this method 

are depicted in Figure 3 . The smoothing procedure removes irregularities in the mean profile . 

The resulting simplicity of representation improves computational efficiency. 
0 0 0 - 

50 50 50 

E E E 

100 4; 100 

r 

;G 100 CL Q_ 

D D D 

150 150 150 

I b c 

200 . . . 

. 

200 200 

0 500 

2 2 

0 500 0 500 

N (cph ) N2 (cph2) N 2 (cph 2) 

Figure 3 . Seasonal mean hydrostatic stability profiles for the Louisiana-Texas shelf, with 

examples of seasonally fitted reference columns (thin lines) used for vertical dynamic 

mode computations. (a) fall-winter, (b) spring, and (c) summer. 

2.2 .2 Computation of Modes 

The dynamic modes are orthogonal patterns of variability computed from the assumed dynamics 

of the system, providing added information based on physics to improve the resolution of 

estimated profiles obtained from measurements at a given time and location. These dynamic 

modes of pressure anomaly are rigid lid, flat bottom vertical structure functions based on the 

usual Sturm-Liouville boundary value problem arising from the equations of motion, 

a [ 1 aF ] 

+ 1' F = o aZ N- aZ 

1 aF -0 at z=0,h (7) 

Nz 7z 

Here Fn is the non-dimensional mode n structure function for kinematic pressure anomaly, NZ is 

the rest state Brunt-Vaisala stability parameter, and c,, is the modal eigencelerity. The local 

structure functions F are orthonormalized. 

The long wave approximation, which is necessary for the customary separation of variables, 

limits the usefulness of the analysis to the subinertial frequencies, but does not impede analyses 

such as the present study emphasizing low frequency phenomena. 

9 
The boundary value problem defined by (6) and (7) is developed from momentum and continuity 

equations under the hydrostatic approximation (i .e . negligible vertical acceleration) . 

The pressure anomaly modes were found relative to the dynamic reference column of Section 

3 .3, and are depicted in Figure 4 computed relative to typical summer, fall, winter, and spring 

reference columns. Similar local dynamic structure functions were computed for individual 

LATEX hydrographic stations in a pilot study for this project. These structure functions are 

reminiscent of the local vertical structure functions computed throughout the deep Gulf of 

Mexico by Current (1993), although the isopycnic coordinate system employed in that study is 

replaced here by a Cartesian coordinate system. 

2.3 Fit of Mooring Data to Dynamic Modes 

Time-varying amplitudes (B ) of the local dynamic modes were estimated by individually fitting 

time dependent pressure anomaly from each of the 10 shelf break moorings to these orthogonal 

dynamic structure functions using 

' l `Zi-I - ZJ / Bt, = h I (P,-1 Ft, ..- . + P; F',,, (8) . i 2 , 

n = 0,1,2 

for each level j from surface to seafloor . Pressure anomalies were obtained from time dependent 

temperature and salinity measurements by means of relation (1), and linear methods were 

employed to estimate high resolution pressure anomaly (p) profiles and to extend these profiles 

throughout the entire water column. The mooring data analyzed were taken during times when 

measurements were available at all three instrument depths . The degrees of freedom present 

justify the use of no more than three dynamic modes during the periods analyzed. Therefore only 

the barotropic (n = 0), first baroclinic (n = 1), and second baroclinic (n = 2) mode amplitudes 

were estimated. 

High resolution reconstructed profiles of pressure anomaly from sea surface to sea bed were 

obtained by employing the local vertical structure functions (F,,,) and modal amplitudes (B ) at 

each time step for each level j, 

z 

P; _ ~ B,, F',, .. ; 

. .=o 

These time series of reconstructed pressure profiles are the basis for estimation of the time 

evolution of dynamic height and cross-shelf geostrophic transport at the 200-m isobath. Modal 

reconstruction of the data represented 97% of the variance in the data. Contributions of higher 

modes (n = 3, 4, . . ., -) and other possible causes for lack of fit (such as error in the estimation of 

pressure anomaly profiles) are included as residuals. These residuals are especially high during 

the month of June, and discussion of the contribution of higher modes to summer pressure 

anomaly can be found in Chapter 3 . 

10 
0 0 

summer fall 

50 - 50 - 

BC2 

E E BC2 

-=r 100 - 100 - CL (D BT a) BT 

150 - 

B 

3CJ 

C1 

, 1 

BC1 

200 200 

-3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 

0 0 

winter spring 

50 50 

E E 

100 ~ 100 

BC2 BT ~ BT 0 0 

150 150 

BC1 BC1 

200 200 

-3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 

Figure 4. Typical seasonal shelf-wide dynamic modes of pressure anomaly. 
Wind driving is known to influence alongshore currents on the Louisiana-Texas shelf (Cochrane 

and Kelly 1986), and wind driving is substantially associated with phenomena in the 

weatherband (two to ten day period) although longer periods become relatively more important 

during the summer. Because the focus of this study is on geostrophic flow of lower frequency, 

the present analysis was restricted to 15-day averages of density data with five day overlaps . 

Signal in the weatherband frequencies is not resolved by the present method. 

2 .4 Estimation of Dynamic Height and Geostrophic Transport 

Time dependent dynamic heights were directly obtained from the high resolution pressure 

anomaly profiles using equation (1) . Dynamic heights were then adjusted by removal of an 

annual mean estimated by equal weighting of seasonal means. 

The cross-shelf geostrophic flow at the shelf break was estimated by applying the geostrophic 

balance to the high resolution reconstructed pressure anomaly profiles. Then standard linear 

interpolation techniques were used to integrate along the shelf break between moorings given 

their geographic location. Estimated alongshore spatial decorrelation scales of approximately 35 

km were reported by Li et al . (1996) on the outer Louisiana-Texas continental shelf. The mean 

separation distance of the LATEX-A shelf break moorings is 67.7 km, generally exceeding these 

spatial decorrelation scales (Figure 1). Thus, we feel use of the present simple, conventional 

procedure for estimation of geostrophic transport is marginally justified along the LouisianaTexas shelf break. While it will provide estimates of the cross-shelf geostrophic transport, it may 

miss some of the details of the spatial structure of this transport . 

12 
III. RESULTS 

3 .1 Modal Amplitudes Along the Louisiana-Texas Shelf Break 

A comparison of first and second baroclinic mode amplitudes obtained from mooring data with 

those obtained in a pilot study from LATEX-A hydrographic data is shown in Figure 5 . The 

hydrographic data was continuous in vertical resolution and sampled at every half meter in the 

water column, whereas mooring data is more limited in vertical resolution, but the modal 

amplitudes appear to be comparable. A comparison of the barotropic mode amplitudes is not 

illustrated in Figure 5 because the modal analysis was computed relative to different mean 

density in these two studies. The present reference density is a shelf break mean, instead of a 

shelf-wide mean, and this affects the magnitude of barotropic local vertical mode amplitudes at 

the shelf break. Hydrographic cruise data yields a "snapshot" of the temperature and salinity 

profiles at the time of the CTD cast. Although measurements are taken at no more than three 

depths in the water column, the time series available from LATEX-A moorings present sufficient 

degrees of freedom for a three mode analysis to provide some insight into the temporal variability 

of geostrophic transport across the shelf break. 

Goodness of fit was addressed by comparing the adjusted pressure profiles computed from the 

modes and modal amplitudes using (9), to the observations at three levels in the water column at 

0.3 0 .15 

hydrography (Current 1996) ' hydrography (Current 1996) 

0.2 mooring data (present study) 0.10 0 mooring data (present study) 

YCL L 

0.1 . ~ . : }~ O 0.05 

0 . 0 ~ f 0.00 F Y ' 

- 

-0.1 -0.05 

- - - 

. R = - 

-0.2 -0.10 - 

B1 B2 

-0.3 -0.15 

0 50 100 150 200 250 0 50 100 150 200 250 

Seafloor Depth (meters) Seafloor Depth (meters) 

Figure 5. First and second baroclinic pressure anomaly mode amplitude coefficients B versus 

seafloor depth on the Louisiana-Texas shelf from LATEX-A cruise hydrography and 

from shelf break mooring data. 

13 
the same mooring and time . The modal fit accounted for 97% of the variance in the data from all 

10 moorings during the time periods considered in this study. The fit was better in winter than in 

summer, and better at upcoast regions than downcoast. The residual includes contributions of 

higher modes, which were not addressed in this study, to the variance in the data. 

An advantage of the present method of analysis is that the dynamic structure functions are based 

on linear dynamics that depend on the long wave assumption. Therefore they filter the data, 

removing high frequency and nonlinearities in the data that may otherwise obscure fundamental 

seasonal patterns. Loop Current eddies that persist in the outer shelf region are known to 

introduce nonlinearities to the circulation dynamics at the shelf break, and the present 

methodology is one way to remove these effects from a low frequency analysis. Statistics of 

modal amplitudes for pressure anomaly during summer, fall, winter, and spring indicate that the 

absolute value of the first baroclinic mode amplitude tends to be higher during the winter than 

during the summer. The reverse is true for the second baroclinic mode amplitude. In fact, for 

both baroclinic modes the ranges in amplitudes for winter and for summer seasons do not 

intersect, i .e . the observed differences are statistically significant . Seasonal statistics of Bo, B,, 

and BZ modal amplitudes are given in Table 1 including the percent each mode contributes to the 

total variance accounted for by all three modes. The seasonal pattern of mean modal amplitudes 

is illustrated in Figure 6. Throughout the present study, spring is taken to be March, April and 

May, summer is June, July, and August, fall is September, October, and November, and winter is 

December, January, and February. 

Clearly the summertime is characterized by greater contribution of the higher (second) baroclinic 

mode to the modal representation of the pressure anomaly profile. In the summertime, second 

baroclinic mode mean and median amplitudes are larger in absolute value than during other 

seasons. Second baroclinic mode amplitudes were greater in absolute value than 0.09 only during 

the summer month of June, and the highest second baroclinic mode amplitude occurred June 27 

at mooring 5, the southernmost mooring analyzed for that time period . No July, August, or 

September analyses were conducted. The second baroclinic and higher modes appear to be 

required to adequately represent the effects of near surface summer heating. First baroclinic 

mode amplitudes are lowest in summer, and barotropic mode amplitudes are low as well (Table 

1) . 

In the late winter and early spring first baroclinic mode amplitudes were high, and these were 

distinctly higher at upcoast moorings than at those moorings further downcoast (Figure 6). 

Second baroclinic mode amplitudes were low in absolute values shelf-wide during late winter, 

with a small range and standard deviation. The smaller second baroclinic mode amplitudes of 

winter may reflect the absence of strong heating at the sea surface and deepening of the mixed 

layer. Mean and median barotropic mode amplitudes are large in the winter but greatest in the 

springtime and smallest in the fall . 

The spatial distribution of vertical dynamic modes of pressure anomaly along the shelf break was 

examined by compiling basic statistics (Table 2) for the downcoast region (moorings 4, 5, and 

6), the midshelf region (moorings 7, 8, 9, and 10) and the upcoast region (moorings 11, 48, and 

13). The locations of these moorings can be seen in Figure 1 . Despite considerable differences 

14 
Table 1 . Seasonal statistics of barotropic, first baroclinic, and second baroclinic mode 

amplitudes for 144 15-day mean pressure profiles on the Louisiana-Texas shelf 

break. Seasons are summer (June), fall (October, November), winter (February), and 

spring (March, April, May). The notation <Bn 2> indicates the mean square mode n 

amplitude coefficient. 

Mode Statistic 

Barotropic Mean 

Median 

Std Dev 

Maximum 

Minimum 

<Bo > 

variance 

First Baroclinic Mean 

Median 

Std Dev 

Maximum 

Minimum 

<g,2> 

variance 

Second Baroclinic Mean 

Median 

Std Dev 

Maximum 

Minimum 

<132 2> 

variance 

Summer Fall Winter Spring All 

0.69 0.67 0.71 0 .72 0.71 

0.70 0.67 0.71 0 .72 0.71 

0.02 0.01 0.01 0 .01 0.03 

0.72 0.70 0.74 0 .75 0.75 

0.60 0.64 0.69 0.69 0.60 

0.48 0.45 0.51 0.52 0.50 

91 .1 90.8 89 .5 89 .7 90.0 

0.15 0.16 0.20 0.19 0.18 

0.15 0.16 0.20 0.20 0.18 

0.02 0.01 0.01 0.02 0.03 

0.18 0 .18 0.23 0.23 0.23 

0.09 0 .14 0.18 0.16 0.09 

0.02 0 .03 0.04 0.04 0.03 

4.4 5.5 7.2 6.6 6 .1 

-0 .09 -0 .06 -0 .04 -0 .06 -0 .06 

-0 .09 -0 .06 -0 .04 -0 .06 -0 .06 

0.01 0.01 0.00 0.01 0.02 

-0 .07 -0 .04 -0 .04 -0 .04 -0 .04 

-0 .10 -0 .09 -0 .05 -0 .09 -0 .10 

1 0.011 0.00 0.00 0.00 0.00 

1 .61 0. 7 I 0. 4 1 0. 7I 0 8I 

in shelf topography and geometry, the differences in dynamic modal amplitudes between regions 

were not remarkable without separation of seasons. Most notable was the slightly greater 

contribution of the first baroclinic mode to the variance accounted for by the first three modes in 

the upcoast region compared with the midshelf or downcoast regions. This difference occurred 

during February through April (Figure 6) at moorings 11, 13, and 48. However the differences in 

response due to location along the Louisiana-Texas shelf break are minor in comparison with the 

observed seasonal cycles of modal amplitude. 

3 .2 Dynamic Height 

Although coastal sea level depends on the combination of factors including thermally-induced, 

wind-induced, and riverine-induced annual signals, mean sea level at the shelf break in the Gulf 

of Mexico is strongly influenced by the annual steric signal . Pattullo et al . (1955) describe 

seasonal oscillations in sea level. Whitaker (1971) describes the annual shelf break steric signal 

in the Gulf of Mexico and Sturges and Blaha (1976) interpret these findings . Whitaker showed 

15 
0.75 

0.70 

0.65 

0.25 

ozo 

0.15 

0.10 

0.00 

.a BT 

+~+++ 

t4+ 

++ ++ 

Il 7G 

0.70 

d. . . . . . . . . . 

BT 

P~Axee< 

x o6< 

°o 

o 

x 

-0 .05 

-0 .10 

b `BCC 

44++,+++$ 

++ 

++ 

f 

f 

BC2 

+44+ +++ 

t 

f 

t+ 

+,+ 

'g BC1 

o 

rP~ 

0.20 

0.15 

-0 .05 

BC2 

O 

O 

M A M J J A S O N D J F M A M J J A S O I 

Figure 6. Monthly shelf-wide modal amplitude means for (a) barotropic, (b) first baroclinic, and 

(c) second baroclinic modes. Upstream (o) and downstream (x) modal amplitude 

means for (d) barotropic, (e) first baroclinic, and (f) second baroclinic modes. Here the 

upstream region encompasses moorings 11, 48, and 13, and the downstream region 

refers to moorings 4, 5, and 6 . 

16 
Table 2. Alongshore regional statistics of modal amplitudes for the 144 15-day mean pressure 

profiles on the Louisiana-Texas shelf break . Downcoast region is moorings 4, 5, and 6 . 

Midshelf region is moorings 7, 8, 9, and 10. Upcoast region is moorings 11, 48, and 

13 . The notation <B 2> indicates the mean square mode n amplitude coefficient. 

Mode Statistic Downcoast Midshelf 

Barotropic Mean 

First Baroclinic 

Second Baroclinic Mean 

Upcoast Total 

0.70 0.71 0.71 0.71 

0.70 0.71 0.72 0.71 

0.03 0.02 0.02 0.03 

0.74 0.74 0.75 0 .75 

0.60 0.67 0.66 0 .60 

0.49 0.50 0.51 0 .50 

90.4 90 .5 89.3 90.1 

0.18 0 .18 0.20 0 .18 

0.18 0 .17 0.21 0 .18 

0.02 0 .02 0.03 0.03 

0.20 021 023 0.23 

0.09 0.14 0.14 0.09 

0.03 0.03 0 .04 0.03 

5.7 5.6 6.9 6.1 

-0 .06 -0.07 -0 .06 -0.06 

-0 .06 -0 .07 -0 .06 -0 .06 

0 .02 0.02 0.02 0.02 

-0.04 -0.04 -0.04 -0.04 

-0 .10 -0 .10 -0 .10 -0 .10 

.00 0. 

0.8 1 0.9 1 0.7 1 0.8I 

Median 

Std Dev 

Maximum 

Minimum 

<go > 

variance 

Mean 

Median 

Std Dev 

Maximum 

Minimum 

<g,2> 

variance 

Median 

Std Dev 

Maximum 

Minimum 

<g22> 

% variance 

that there is a very large thermally induced range (order 13 cm) in water level off the Texas shelf 

break. In fact, Whitaker found an essentially common annual signal of steric water level 

throughout the northern and western part of the Gulf of Mexico, in particular near the shelf break 

off Cedar Keys, Pensacola, Eugene Island, Galveston, and Tampico . This signal was 

characterized by a single minimum close to the time of the vernal equinox, and a single 

maximum near the autumnal equinox . 

The annual steric water level signal for the Louisiana-Texas shelf break was approximated using 

estimates of the mean steric water level for winter, spring, summer and fall near the shelf break at 

94°W that were obtained from the data provided by a combination of 12 LATEX A and C 

surveys from 1992 - 1994 . A Fourier fit using the annual and semiannual periods gives a 

representation of the annual steric signal very similar to that of Whitaker (1971) but with a 

slightly smaller range. Figure 7 depicts these seasonal means from the hydrographic cruises and 

the Fourier fit to them as presented in Current (1996) . Estimates of the mean steric water level 

across the Louisiana-Texas shelf 200-m isobath for all four seasons were obtained from the 10 

shelf break moorings of Figure 1, and these estimates were adjusted by an annual mean 

17 
0.20 

0.15 

0 0.j0 

0.05 o 

0 .00 -

-0 .05 

10 -0 .10 

-0 .15 

a~ -0 .20 

U) i F M A M J J A S O N D 

Figure 7. Two component Fourier fit to seasonal steric water level averages computed from 

LATEX hydrographic cruise data (filled circles) at about 94°W near the 200-m isobath 

on the Louisiana-Texas continental shelf. Open circles represent seasonal steric water 

level averages computed from LATEX-A shelf break mooring data. The annual mean 

has been removed. 

representing equal weighting of the four seasonal means. These seasonal values from the 10 shelf 

break moorings are also shown in Figure 7. 

The seasonal mean steric water level from the shelf break moorings agrees well with the fitted 

curve computed from the LATEX-A hydrographic data, with the exception of summer water 

level, which is high. This is consistent with the much poorer modal fit of the data in June 1992 

than during other months. Seasonal surface heating and runoff results in more energy being 

moved into the higher modes for adequate modal representation of the data, but due to the 

degrees of freedom available the study was limited to three dynamic modes. The energy 

corresponding to baroclinic modes three and higher is responsible for a higher residual, and some 

aliasing of this energy into lower modes may have occurred as well. Therefore the present 

methodology appears to be better suited to the non-summer months or to hydrographic data with 

its many levels of measurement. 

3 .3 Transport Across the Louisiana-Texas Shelf Break 

Estimated 15-day mean transports for specific depth ranges in the water column are listed by 

alongshore region in Table 3, and for the entire shelf in Table 4. The depth ranges evaluated are 

0-70 meters and 70-200 meters, as well as for the entire 200 meter water column. 

Although geostrophic transport across the Louisiana-Texas shelf break was offshore in the upper 

70 meters of the water column during the spring of 1992, onshore flow was substantial in deeper 

waters during this time particularly in the upcoast region . By late June, transport was essentially 

onshore in direction throughout the water column . In October 1992, the direction of transport 

18 
Table 3. Estimated transport (in Sverdrups) off the Louisiana-Texas shelf at the 200-meter 

isobath, listed by alongshelf region at specified depth ranges . The downcoast region 

extends from LATEX-A mooring 4 to mooring 7 and includes the south Texas shelf. 

The midshelf region extends from mooring 7 to mooring 10, and the upcoast region 

includes the shelf break from mooring 10 to mooring 13 (see Figure 1 for mooring 

locations) . Offshore flow is positive. 

April 28, 1992 

May 8, 1992 

May 18, 1992 

May 28, 1992 

June 7, 1992 

June 17, 1992 

June 27, 1992 

October 3, 1992 

October 13, 1992 

October 23, 1992 

November 2, 1992 

November 12, 1992 

February 7, 1993 

February 17, 1993 

February 27, 1993 

March 9, 1993 

March 19, 1993 

March 29, 1993 

April 8, 1993 

April 18, 1993 

April 28, 1993 

0-70 meters 

Downcoast Midshelf Upcoast 

70-200 meters 

Downcoast Midshelf Upcoast 

0.32 0.24 -0.10 0.19 -0.16 -0.41 

0.09 0.27 0.02 0.12 -0.22 -0.29 

0.18 -0.08 0.13 0.30 -0.30 -0.27 

0.35 -0.06 0.17 0.51 -0.25 -0 .30 

0.24 -0.14 0.26 0.31 -0.28 -0 .25 

0.10 -0.17 0.25 0.17 -0.28 -0 .17 

-0.17 -0.40 0.18 0.04 -0.37 -0 .16 

-0.26 0 .39 -0.30 -0.22 0 .36 0.19 

-0.08 0 .24 -0.31 -0.18 0 .23 0.24 

-0.18 0 .39 -0.34 -0.31 0 .32 0.22 

-0.09 0 .34 -0.13 -0.32 0.33 0.39 

0.05 0 .34 0.03 -0.20 0.26 0.52 

0.22 0 .22 0.34 0 .13 -0 .12 0.21 

0.49 0 .12 0.43 0 .27 -0 .16 0.28 

0.01 0 .42 0.36 -0.01 -0 .01 0.19 

0.46 0 .35 0.19 0 .26 -0 .10 0.16 

0.17 0 .29 0.10 0 .18 -0 .14 0.09 

-0.19 0 .55 -0.04 -0.11 0.04 -0.06 

-0.27 0 .64 -0.23 -0.25 0.09 -0.21 

-0.21 0.54 -0.32 -0.27 0.07 -0.32 

I -0.77 0.90 -0.29 -0.62 0.34 -0.40 

19 
Table 4. Estimated transport (in Sverdrups) off the Louisiana-Texas shelf at the 200-meter 

isobath, listed by alongshelf region for the full water column and also listed for the 

entire shelf break at specified depth ranges . The downcoast region extends from 

LATEX-A mooring 4 to mooring 7 and includes the south Texas shelf. The midshelf 

region extends from mooring 7 to mooring 10, and the upcoast region includes the shelf 

break from mooring 10 to mooring 13 (see Figure 1). Offshore flow is positive. 

April 28, 1992 

May 8, 1992 

May 18, 1992 

May 28, 1992 

June 7, 1992 

June 17, 1992 

June 27, 1992 

October 3, 1992 

October 13, 1992 

October 23, 1992 

November 2, 1992 

November 12, 1992 

February 7, 1993 

February 17, 1993 

February 27, 1993 

March 9, 1993 

March 19, 1993 

March 29, 1993 

April 8, 1993 

April 18, 1993 

April 28, 1993 

0-200 meters 

Downcoast Midshelf Upcoast 

Shelf-wide 

0-70 meters 70-200 m 0-200 m 

0 .51 0 .08 -0.51 0 .46 -0 .37 0.08 

0 .21 0 .05 -0.26 0 .39 -0 .38 0.00 

0 .48 -0.38 -0.14 0 .23 -0 .26 -0.04 

0 .86 -0.31 -0.14 0 .46 -0 .05 0.41 

0 .55 -0.41 0.01 0 .36 -0 .22 0.15 

0.27 -0.45 0.08 0 .17 -0 .27 -0.10 

-0 .13 -0.77 0.02 -0.39 -0 .49 -0.88 

-0 .48 0.74 -0.11 -0.17 0.32 0.15 

-0 .26 0.47 -0.07 -0.15 0.29 0.14 

-0 .49 0.71 -0.12 -0.13 0.24 0.10 

-0 .40 0.67 0.26 0.13 0.40 0.53 

-0 .14 0.59 0.55 0.42 0.58 1 .00 

0.35 0.10 0.55 0.77 0.23 1 .00 

0.76 -0.03 0.70 1 .04 0.39 1 .43 

0.00 0.41 0 .55 0.80 0.16 0.96 

0.72 0.25 0 .35 1 .00 0.32 1 .32 

0.35 0.16 0 .19 0.56 0.14 0.70 

-0 .30 0.59 -0.10 0.32 -0.13 0.19 

-0 .52 0.72 -0.44 0.14 -0.37 -0.23 

-0 .48 0.61 -0.64 0.01 -0.52 -0.51 

-1 .38 1 .24 -0.69 -0.16 -0.68 -0.83 

20 
0 

a b c 

50 

E 

fi 100 

a 

D 

150 

200 

-8 -4 0 4 8 -8 -4 0 4 8 8 -4 0 4 8 

Transport/0 .5m (Sv"103) Transport/0.5m (Sv*103) Transport/0.5m (Sv"103) 

Figure 8. Mean offshore geostrophic transport per half meter depth across the Louisiana-Texas 

shelf break for (a) downcoast, (b) midshelf, and (c) upcoast Louisiana-Texas shelf 

break regions for time intervals treated in this study. Regions are defined as downcoast 

(mooring 4 to mooring 7), midshelf (mooring 7 to mooring 10), and upcoast (mooring 

10 to mooring 13). Flow in the offshore direction is positive. 

across the 200-m isobath was onshore in the upper 70 meters except in the midshelf region, and 

offshore in deeper waters except in the downcoast region . By November transport was offshore 

throughout the water column and remained so through February to mid-March. Onshore flow 

first appeared in deeper waters from 70-200 meters in late March and April 1993, and flow was 

onshore throughout the water column by the end of April 1993 . 

Flow across the shelf break is not unidirectional in the mean (Figures 8 and 9) or at any given 

time of year, but varies by depth and by location along the shelf break (Appendices A and B) . 

Seasonal average geostrophic transport profiles in the offshore direction across the LouisianaTexas shelf are provided in Figure 9 . In summer there is onshore mean transport in the deeper 

layers and offshore transport in the upper water column. In the fall, this offshore transport has 

deepened with a maximum around 82 meters decreasing to nearly zero at the bottom. Above 33 

meters mean fall cross-isobath transport is onshore. Winter mean transport is strongly offshore in 

near surface waters from zero to 70 meters. Substantial offshore mean transport does not 

penetrate as deeply in the water column in spring as in winter . 

Flow onto or off of the shelf in any given region may compensate for flow in the opposing 

direction elsewhere across the shelf break or for alongshelf flow. Often the direction of flow 

21 
0 0 

50 

Summer Fall 

50 

E 

100 

0 

150 

200 

-8 -4 0 4 8 

Transport/0 .5m (Sv"103) 

0 

Winter 

50 

E 

a 100 

aD 0 

200 

-8 -4 0 4 8 

Transport/0 .5m (Sv*103) 

E 

100 

0 

150 

200 ' 

-8 -4 0 4 8 

Transport/0.5m (Sv'103) 

0 

Spring 

50 

E 

100 

0 

200 

-8 -4 0 4 8 

Transport/0.5m (Sv`103) 

Figure 9 . Seasonal average geostrophic transport per half meter depth in the offshore direction 

across the Louisiana-Texas shelf break. Seasons are defined as summer (June, July, 

August), fall (September, October, November), winter (December, January, February), 

and spring (March, April, May). Offshore direction is positive . 

22 

150 150 
across the 200-m isobath appears to differ in the midshelf region from the direction in the 

upcoast and downcoast regions, and this is more frequently the case in the upper 70 meters of the 

water column. Flow in this upper layer was offshore in the midshelf region but onshore 

elsewhere from the beginning of October through mid-November 1992, and from the end of 

March through April 1993 . This was also true in the lower layer during the latter time period . 

Upper layer onshore flow occurred only in the midshelf region from mid-May through June 

1992 . The data coverage was insufficient to allow a complete mass balance for the shelf. 

Mean profiles of geostrophic transport in the offshore direction at the 200-m isobath were 

obtained for upcoast, midshelf, and downcoast regions of the Louisiana-Texas shelf break and 

are depicted in Figure 8. Comparing Figure 8 and Figure 9 it is apparent that cross-shelf 

transport differs by season more than it differs by alongshelf location. Upper layer offshelf 

geostrophic transport in the midshelf region was robust during October and April of 1992, when 

the direction of transport was towards the coast in other regions of the shelf. This may explain 

the overall offshore transport in the upper layer over the midshelf region . Offshore transport as a 

function of position on the 200-m isobath at 10 day intervals can be found in Appendix B, 

Figures B1 through B4, and as time series by region in Figure B5. The strongly offshore transport 

between moorings 9 and 10 in late April 1992 appears to move gradually downshelf and by early 

June occurs between moorings 8 and 9 (Figure B 1) . Transport on June 17 and June 27, 1992, was 

very large in the onshore direction at mooring 5 and in the offshore direction at mooring 6 . 

Summer heating may have allowed considerable energy to move into the higher baroclinic 

modes, resulting in aliasing of this energy and the consequently poorer fit obtained during the 

summer in the downcoast region when using only three modes. This may be partially responsible 

for the large transports in the downcoast region during June 1992. It appears that the strongly 

offshore geostrophic transport observed in April and May between moorings 5 and 6 moved 

further north between moorings 6 and 7 in mid-June and later June and this is accompanied by a 

reversal and very strongly onshore geostrophic transport between moorings 5 and 6. These 

changes are apparently associated with the alongshelf current reversal on the inner shelf that 

occurred during mid-June of 1992 (Nowlin et al . 1998). 

Vertical profiles of shelf-wide geostrophic mean transport at ten day intervals are provided in 

Appendix A. These sequences of high resolution profiles of offshore transport reveal smooth 

and gradual transitions between seasons. 

23 

IV. DISCUSSION 

The cyclonic circulation pattern suggested by Cochrane and Kelly (1986) for the Louisiana-Texas 

shelf includes offshelf flow at the southwestern (downcoast) part of the shelf break and onshore 

transport in the northeastern (upcoast) part, to complete the gyre, during non-summer months. 

During summer, the Cochrane and Kelly rendition of shelf-wide circulation specifies the break 

down and reversal of non-summer patterns . 

The LATEX program, funded by the MMS in the early 1990's, presented a new wealth of 

knowledge and detail pertaining to circulation patterns on the Louisiana-Texas continental shelf. 

Comparison of results obtained by the LATEX-A scientists with Cochrane and Kelly's earlier 

concept of overall circulation patterns on the Louisiana Texas shelf were equivocal, especially 

with respect to the outer shelf (Nowlin et al . 1998). Inner shelf currents were observed to flow in 

the downcoast direction during non-summer months, reversing to flow upcoast in the summer as 

outlined by Cochrane and Kelly. Monthly averaged currents along the outer shelf edge were not 

often in the same alongshore direction, although mean alongshore currents over the 32 months of 

LATEX-A data were upcoast over the outer shelf. However no coherent pattern to the annual 

signal on the outer shelf was determined in the LATEX analyses, and while cross-shelf currents 

at the shelf break must ultimately maintain continuity it is thought that they may be primarily due 

to offshore eddies. The large interannual variability on the outer shelf is attributed to anticyclonic 

rings such as Eddy Vazquez, which is thought to be responsible for the strong upcoast flow 

observed by the LATEX program in south Texas in 1993 (Nowlin et al. 1998). In a study based 

on LATEX-A data as well as M/V Gus III cruises and other historical data, high surface salinities 

exceeding 36.0 that extend through the water column were observed to move upcoast as far as 

central Texas in the summer (Li et al . 1997). In other seasons, water on the Texas shelf is fresher 

and this is presumably due to downcoast flow transporting fresh water outflow from the 

Mississippi and Atchafalaya River systems onto the Texas shelf. The annual cycle of freshwater 

volume on the Louisiana-Texas shelf is known to be dominated by the spring flood of the 

Mississippi/Atchafalaya rivers (Dinnel and Wiseman 1986). 

The results presented in this report provide an additional analysis of cross-shelf geostrophic 

transport at the shelf break during parts of 1992 and 1993, that is not generally consistent with 

Cochrane and Kelly's analysis of summer and non-summer mean cross-shelf transport. Mean 

seasonal cross-shelf geostrophic transports are listed in Table 5 . Although Cochrane and Kelly's 

summer was July-August, months that were not analyzed in the present study, June 1992 patterns 

of cross-shelf transport at the shelf break were used to estimate the summer mean transport. The 

downcoast region as defined here extends from mooring 4 to mooring 7, the midshelf region 

extends from mooring 7 to mooring 10, and the upcoast region extends from mooring 10 to 

mooring 13 (see Figure 1). 

In the downcoast (southern) shelf break region, mean non-summer cross-shelf flow was onshore 

although net flow in this region was small, and mean summer cross-shelf flow was offshore . This 

is not consistent with the analysis of Cochrane and Kelly (1986) in which flow across the shelf 

break was away from the coast in the non-summer. Analysis of the midshelf region reveals 

25 
Table 5 . Mean seasonal estimated offshore geostrophic transport at the Louisiana-Texas shelf 

break in Sverdrups (106 MI/S). 

Downcoast Midshelf Upcoast All 

Fall -0 .36 0.64 0.10 0 .38 

Winter 0 .37 0.16 0.60 1 .13 

Spring 0 .03 0.23 -0.18 0 .08 

Summer 0.23 -0 .54 0.03 -0.28 

Non-summer -0.01 0 .37 0.00 0.36 

Mean 0.02 0 .24 0 .00 0.26 

substantial mean non-summer offshore flow and summer onshore flow occurred. However the 

small mean summer offshore flow and near zero mean non-summer flow in the upcoast (eastern) 

regions is not consistent with Cochrane and Kelly's findings of flow towards the shore across the 

eastern shelf break during non-summer seasons . 

The LATEX-A box model study by Bender and Reid (Nowlin et al . 1998) provides information 

on cross-shelf transport (both geostrophic and ageostrophic) at the shelf break, whereas the 

present study quantifies geostrophic transport only. There is good qualitative agreement and 

some quantitative agreement between the two studies, which is not unreasonable considering that 

Chen (1995) found that 70% of the variance in the LATEX-A ADCP data was geostrophic and 

30% ageostrophic . The estimated correlation coefficient for monthly averaged shelf-wide 

transport across the 200-m isobath from the two studies is 0.72, corresponding to an rZ of 0.52 , 

and 100% of shelf-wide monthly average transports were of the same sign (onshore or offshore). 

Regional monthly averages from the two studies were not as well correlated. The estimated 

correlation of monthly averaged transports across the shelf break for downcoast, midshelf, and 

upcoast regions treated separately was 0.49 with a corresponding r2 of 0.24 . 

Qualitative agreement between the Bender and Reid study and the present study was still good 

for these regional monthly averages, having the same direction of monthly averaged regional 

cross-shelf flow 76% of the time (Table 6). The two differences in cross-shelf direction of flow 

in the upcoast region were the two smallest in absolute value of mean flow in the present study. 

The differences in the downcoast region are not as small, but the discrepancies occurred during 

months when data from mooring 4 was incomplete and the transport between moorings 4 and 5 

was not analyzed. The southernmost edge of the Louisiana-Texas shelf is often the site of strong, 

persistent offshore jets which could not be resolved in the transport estimates computed for the 

present study, and may be responsible for the discrepancies between these two studies in the 

October and November 1992 directions of transport. Flow directions between mooring 7 and 

mooring 10 in the midshelf region of the shelf break agreed 100% of the time . 

26 
Table 6. Monthly average direction of cross-shelf flow at the shelf break (200-meter isobath) for 

the present study and for the Bender study (in parentheses) . Direction of flow coincided 

76% of the time. Mean offshore flow is denoted (+), mean onshore flow is denoted (-). 

This study (Bender study) 

Downcoast Midshelf Upcoast 

May `92 + (+) 

Jun '92 + (-) - (-) + (-) 

Oct '92 - (+) + (+) - (+) 

Nov '92 - (+) + (+) + (+) 

Feb '93 + (+) + (+) + (+) 

Mar '93 + (+) + (+) + (+) 

Apr '93 - (-) + (+) - (-) 

The geostrophic transports reported in this study appear large, but are consistent in range with 

those computed by the LATEX-A program. Offshore transport over the entire shelf was highest 

in the winter months, with average shelf-wide offshore transport of 0.76, 1 .13, and 0.74 Sv for 

the months of November, February, and March respectively. Onshore transport was greatest in 

spring and summer, with average shelf-wide onshore transport of 0.28 Sv and 0.52 Sv during the 

months of June 1992 and April 1993 . Oey (1995) found his modeled westward transport 

integrated from shore to the 30-m isobath to be comparable to that observed, about 0.15 Sv from 

the coast to the 30 meter isobath, though as high as 0 .25 Sv in the fall . Bender and Reid report 

transport on the inner shelf is strongly downcoast averaging more than 0 .25 Sv, and considerably 

greater alongshore flow on the outer shelf than the inner shelf. Cross-shelf flux over the 200-m 

isobath is generally stronger than the outer shelf alongshore fluxes (Nowlin et al . 1998) . The 

mean transport across the shelf break in the present study is 0 .26 Sv (Table 5) . 

Bender and Reid saw no seasonality of outer shelf transport, either alongshore or cross-shelf. 

However the high resolution profiles of cross-shelf transport presented in Appendix A suggest a 

slow, smooth seasonal progression of vertical patterns of mean cross-shelf geostrophic flow. 

27 

V. SUMMARY AND CONCLUSIONS 

This study applies dynamic modal analysis techniques to LATEX-A mooring data to estimate 

time dependent, highly vertically resolved geostrophic transport across the Louisiana-Texas 

continental shelf break as well as dynamic height at mooring locations. Mean steric water level 

anomaly from the shelf break moorings is consistent with the smooth annual cycle determined 

from cruise hydrography by Current (1996), and is notably similar to the findings of Whitaker 

(1971) . Geostrophic transport profiles also vary smoothly with season . Offshore transport is 

strong in the upper 70 meters of the water column during the winter. During the summer, onshore 

transport develops first in deeper waters, penetrating higher in the water column until eventually 

the direction of transport is onshore from surface to seafloor. 

The scope of this study is limited to cross-shelf geostrophic flow on the 200-m isobath, and the 

implications of the present findings for the overall pattern of circulation on this continental shelf 

are as yet unknown. Transport profiles are influenced less by alongshore location on the shelf 

than by the time of year. Results of this study are consistent with the findings of the TexasLouisiana Shelf Circulation and Transport Processes Study (LATEX-A), but provide high 

resolution vertical profiles of geostrophic transport previously unavailable to MMS. 

Dinnel and Wiseman (1986) describe the large freshwater discharge of the MississippiAtchafalaya river systems onto the shelf in the springtime, where it travels downcoast with the 

prevailing currents . During the summer, this excess freshwater migrates upcoast with the summer 

reversal of winds over the Louisiana-Texas shelf. However, by November Dinnel and Wiseman 

observed that freshwater volume on the shelf is low. The very strong offshore flow east of 95°W 

observed in this study during November is not inconsistent with these prior observations . This 

offshore direction of flow east of 95°W was confirmed by the Bender and Reid study (Nowlin et 

al ., 1998). 

The circulation scheme proposed for the Louisiana-Texas shelf by Cochrane and Kelly (1986) 

has been confirmed for alongshore circulation on the inner shelf by several studies including 

Nowlin et al . (1998) . However, the cross-shelf transport patterns put forth by Cochrane and Kelly 

to attain mass balance for the shelf has not been substantiated by the present study or by the 

Bender and Reid study described in Nowlin et al . (1998) . This continental shelf continues far 

beyond the Mexican border . Future investigators may find it productive to examine the 

possibility that variability in alongshelf flow northward from Mexican waters on the outer shelf 

may be a key to understanding mass balance on the Louisiana-Texas shelf. 

29 

LITERATURE CITED 

Arango, H. and R. O. Reid. 1990. A generalized reduced-gravity ocean model. Atmos.-Ocean 

29 :256-287. 

Chen, H. -W. 1995 . Variability and structure of currents over the Texas-Louisiana shelf-a 

view from a shipboard acoustic Doppler current profiler. Ph.D. Dissertation . Texas A&M 

University, Dept. of Oceanography, College Station, TX. 117 pp . 

Cho, K., R. O. Reid, and W. D. Nowlin Jr. 1998 . Objectively mapped streamfunction fields on 

the Texas-Louisiana shelf based on 32 months of moored current meter data. J. Geophys. 

Res. 103 :10377-10390. 

Cochrane, J. D. and F . J. Kelly. 1986. Low-frequency circulation on the Texas-Louisiana 

continental shelf. J. Geophys. Res. 91 :10645-10659 . 

Csanady, G. T. 1979. The pressure field along the western margin of the North Atlantic . 

J. Geophys. Res. 84 :4905-4915. 

Current, C. L. 1993 . Empirical vertical structure of density anomaly in the Gulf of Mexico . M.S . 

Thesis . Texas A&M University, Dept . of Oceanography, College Station, TX. 143 pp. 

Current, C. L. 1996. Spectral model simulation of wind driven subinertial circulation on the 

inner Texas-Louisiana shelf. Ph.D. dissertation . Texas A&M University, Dept. of 

Oceanography, College Station, TX. 144 pp. 

Dinnel, S. P. and W. J. Wiseman. 1986 . Fresh water on the Louisiana and Texas shelf. Cont. 

Shelf Res. 6:765-784. 

Flierl, G. 1978. Models of vertical structure and the calibration of two-layer models. Dyn. 

Atmos. Oceans 2:341-381 . 

Fofonoff, N. P . 1985 . Physical properties of seawater: a new salinity scale and equation of state 

for seawater. J. Geophys. Res. 90 :3332-3342 . 

Kundu, P. K., J . S. Allen, and R. L. Smith. 1975 . Modal decomposition of the velocity field 

near the Oregon coast. J. Phys. Oceanogr. 5 :683-704. 

Li, Y., W. D. Nowlin Jr., and R. O. Reid. 1996 . Spatial-scale analysis of hydrographic data over 

the Texas-Louisiana continental shelf. J. Geophys. Res . 101 :20595-20605 . 

Li, Y., W. D. Nowlin Jr., and R. O. Reid. 1997 . Mean hydrographic fields and their interannual 

variability over the Texas-Louisiana continental shelf in spring, summer, and fall . J . 

Geophys. Res. 102 :1027-1049. 

31 
Nowlin, W. D. Jr ., A. E. Jochens, R. O. Reid, and S . F. DiMarco. 1998. Texas-Louisiana Shelf 

Circulation and Transport Processes Study: Synthesis Report, Volume I : Technical 

Report . OCS Study MMS 98-0035 . U.S . Dept. of the Interior, Minerals Management 

Service, Gulf of Mexico OCS Region, New Orleans, LA. 502 pp. 

Oey, L. -Y. 1995 . Eddy- and wind-forced shelf circulation. J. Geophys. Res . 100 :8621-8637 . 

Pattullo, J., W. Munk, R. Revelle, and E. Strong . 1955. The seasonal oscillations in sea level. 

J. Mar. Res. (14)1 :88-155 . 

Science Applications International Corporation (SAIC) . 1989. Gulf of Mexico Physical 

Oceanography Program, Final Report : Year 5. Volume II : Technical Report . OCS Report 

MMS 89-0068. U.S . Dept. of the Interior, Minerals Management Service, Gulf of Mexico 

OCS Region, New Orleans, LA. 333 pp. 

Sturges, W. and J. P. Blaha. 1976. A western boundary current in the Gulf of Mexico. 

Science (192)367-369 . 

Whitaker, R. E. 1971 . Seasonal variations of steric and recorded sea level of the Gulf of 

Mexico . M. S . Thesis. Texas A&M University, Dept. of Oceanography, College Station, 

TX. 111 pp. 

32 
APPENDIX A 

VERTICAL PROFILES OF SHELFWIDE TRANSPORT 

0 

50 

E 

100 

a a) 

D 

150 

April 28 May 8 May 18 

200 -, 

, 

r 

- 

. . r i 

0 

50 

E 

100 

a 

0 

150 

May 28 June 7 June 17 

200 

-10 0 10 -10 0 10 

0 

Transport/0 .5m (Sv*103) Transport/0 .5m (Sv`103) 

50 

E 

100 

a a) 

D 

150 

June 27 

200 

-10 0 10 

Transport/0 .5m (Sv"103) 

Fig. A1 . Shelfwide transports per half meter depth in the offshore direction across the 

Louisiana-Texas shelf break, 15-day means from April 21 - July 3, 1992 . 

35 
0 

50 

E 

100 

a 

D 

150 

Oct. 3 Oct. 13 Oct. 23 

200 

-10 0 10 

Transport/0 .5m (Sv'103) 

50 

E 

100 

a 

D 

150 

Nov. 2 Nov. 12 

200 

-10 0 10-10 0 10 

Transport/0 .5m (Sv''103) Transport/0 .5m (Sv`103) 

Fig. A2. Shelfwide transports per half meter depth in the offshore direction across the LouisianaTexas shelf break, 15-day means from September 26 - November 19, 1992 . 

36 
0 

Feb. 7 Feb. 17 Feb. 27 

50 

E 

100 

n 

N 

D 

150 

200 

0 Mar.9 Mar.19 Mar. 29 

50 

E 

100 

a a) 

D 

150 

200 

0 

Apr. 8 Apr. 18 Apr. 28 

50 

E 

100 

a a) 

D 

150 

200 

-10 0 10-10 0 10 -10 0 10 

Transport/0 .5m (Sv'103) Transport/0 .5m (Sv'103) Transport/0 .5m (Sv"103) 

Fig. A3 . Shelfwide transports per half meter depth in the offshore direction across the 

Louisiana-Texas shelf break, 15-day means from January 31 - May 5, 1993 . 

37 

APPENDIX B 

ESTIMATED TRANSPORT BY SHELF REGION 

0 .02 

April28, 1992 

0 

51- -0 .01 

-0 .02 

0.02 

> 0.01 May 8, 1992 

0 .00 0 

N -0 .01 

-0 .02 

0.02 

0.01 May 18, 1992 

0.00 - 

0 

-0 .01 

-0 .02 

0 .02 

0 .01 May 28, 1992 

0.00 0 

-0.01 

-0.02 

0.02 

> 0.01 June 7, 1992 

0.00 0 

-0.01 

-0.02 

0.04 

0.02 June 17, 1992 N 

0.00 

0 

-0. 

-0.04 

0.08 

0.04 c' nJune 27, 1992 

0.00 0 

c'- -0.04 

-0.08 

0 100 200 300 400 500 600 

Figure B 1 . Estimated offshelf transport per km from April 28 - June 17, 1992, as a function of 

distance north and east along the 200-meter isobath from LATEX-A mooring 4. 

Circles represent locations half way between moorings 5, 6, 7, 8, 9, 10, 11, and 48. 

Notice the difference in transport scales on June 17th and 27th. Although the upshelf 

flow at this time was strong along the 200-meter isobath, it is thought that there may 

have been considerable energy in higher baroclinic modes during late June, causing 

an aliasing problem in accurate determination of transport. 

41 
0.02 

0.01 

0 0.00 

-0.01 

-0.02 

0.02 

0.01 

0 0.00 

-0.01 

-0 .02 

0 .02 

0 .01 

0 .00 0 

C,- -0 .01 

-0 .02 

0.02 

0.01 

0.00 0 

-0.01 

-0.02 

E 0.02 

Y 

0.01 

0.00 0 

a -0.01 

c 

2 -0.02 

October 3, 1992 

October 13, 1992 

October 23, 1992 

November 2, 1992' 

November 12, 1992 

0 100 200 300 400 500 600 

Figure B2. Estimated offshelf transport per km from October 3 - November 12, 1992, as a 

function of distance north and east along the 200-meter isobath from LATEX-A 

mooring 4. Circles represent locations half way between moorings 5, 6, 10, 11, 

and 48. 

42 
0 .02 

> 0 .01 

0 .00 0 

-0 .01 

o 

-0 .02 

0.02 

> 0.01 

0.00 

-0.01 

-0.02 

0 .02 

> 0 .01 

0 .00 0 

-0 .01 

m 

-~ . 

0 .02 

0 .01 

0 .00 0 

-0 .01 

m F= -0 .02 

~-

February 7, 1993 . 

February 17, 1993 

February 27, 1993 

vMarch 9, 1993 

0 100 200 300 400 500 600 

Figure B3 . Estimated offshelf transport per km from February 7 - March 9, 1993, as a function 

of distance north and east along the 200-meter isobath from LATEX-A mooring 4. 

Circles represent locations half way between moorings 4, 6, 7, 8, 11, 13, and 48. 

43 
0 . 02 

0 .01 

0 .00 

-0 .01 

-0 .02 

0 .02 

0 .01 

0 .00 0 

-0 .01 

-0 .02 

0 .02 

0 .01 

0.00 0 

-0.01 

-0.02 

0.02 

0.01 

0.00 0 

-0.01 

-0.02 

0.02 

0.01 

0.00 0 

-0.01 

-0.02 

- March 19, 1993 - 

March 29. 1993 

April 8, 1993 

April 18, 1993 

- April 28, 1993 

0 100 200 300 400 500 600 

Figure B4. Estimated offshelf transport per km from March 19 - April 28, 1993, as a function of 

distance north and east along the 200-meter isobath from LATEX-A mooring 4. 

Circles represent locations half way between moorings 4, 6, 7, 8, 11, 13, and 48 . 

44 
1 .0 

--- O-Downcoast 

0.5 -0 -Midshelf 

-O-Upcoast 

0 . 0 

N 

C 

N 

E= -0.5 

a 

-1 .0 

April May June 

1 .0 

-0 -Downcoast 

0.5 0 Upcoast O- 

--0-Midshelf 

a 0.0 

c 

m 

f= -0.5 

b 

-1 .0 

September October November 

1 .5 

1 .0 C Oc'n 0.5 ~ 

- 

~ -- 

0 . 0 ~ 

-0 .5 

---O-Downcoast 

___Q__Upcoast 

1 .0 ~ -Midshelf 

-1 .5 

February March April 

Figure B5. Estimated offshore geostrophic transport across the Louisiana-Texas shelf break by 

alongshore shelf region . (a) April 28 - June 27, 1992; (b) October 3 - November 12, 

1992; and (c) February 7 - April 28, 1993 . Notice the expanded transport scale in (c) 

required to illustrate the large offshore transport in midshelf and the large onshore 

transport in the downcoast region in late April 1993 . 

45 

~PQSN,EHT OF ry~'y The Department of the Interior Mission 

o k p As the Nation's principal conservation agency, the Department of the Interior has responsibility for 

most of our nationally owned public lands and natural resources. This includes fostering sound use of 

our land and water resources ; protecting our fish, wildlife, and biological diversity; preserving the 

_ a9 environmental and cultural values of our national parks and historical places; and providing for the 

4ACH 9 ~~ enjoyment of life through outdoor recreation . The Department assesses our energy and mineral 

resources and works to ensure that their development is in the best interests of all our people by 

encouraging stewardship and citizen participation in their care. The Department also has a major 

responsibility for American Indian reservation communities and for people who live in island territories 

under U.S . administration . 

~o,,~ The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary 

responsibilities are to manage the mineral resources located on the Nation's Outer Continental helf 

~ 

(OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute 

C4'~ 

those revenues. 

~'~NAGEMT'~ Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program 

administers the OCS competitive leasing program and oversees the safe and environmentally sound 

exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The 

MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and 

accurate collection and disbursement of revenue from mineral leasing and production due to Indian 

tribes and allottees, States and the U.S . Treasury. 

The MMS strives to fulfill its responsibilities through the general guiding principles of : (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected 

parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for 

all Americans by lending MMS assistance and expertise to economic development and environmental 

protection .