ACCESS NUMBER: 30660-19948 

STUDY TITLE: Dynamic Height and Transport Across the Louisiana-Texas Shelf Break 

REPORT TITLE: Dynamic Height and Seawater Transport Across the Louisiana-Texas 

Shelf Break 

CONTRACT NUMBER: 14-12-0001-30660-19948 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREAS: Western and Central Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 1998, 1999 

COMPLETION DATE OF REPORT: July 2000 

COSTS: FY 1998: $31,679; FY 1999: $36,347; CUMULATIVE PROJECT COST: 

$68,026 

PROJECT MANAGERS: C. Current, W. Wiseman 

AFFILIATION: Louisiana State University, Coastal Studies Institute 

ADDRESS: 331 Howe-Russell Geosciences Complex, Baton Rouge, LA 70803 

PRINCIPAL INVESTIGATORS: C. Current, W. Wiseman 

KEY WORDS: Western Gulf, Central Gulf, Louisiana-Texas shelf, continental shelf, 

shelf break, circulation, dynamic height, transport, geostrophy, pressure, density, time 

series, seasonal, annual, dynamic modes, vertical structure, cross-shelf flow, coastal 

dynamics, upwelling 

BACKGROUND: The Louisiana-Texas continental shelf is a region of active oil and 

gas production and exploration. The U. S. Department of the Interior sponsored the 

Louisiana-Texas Shelf Physical Oceanography Program (LATEX) to understand the 

physical processes and circulation of the shelf water that influence the stability of 

structures, the transport of pollutants, and the ecosystem of regions that may be 

affected by oil and gas operations. This report contains an analysis of cross-shelf 

geostrophic transport at the shelf break employing LATEX data. 

OBJECTIVES: To use techniques of dynamic modal analysis to estimate the time 

evolution of mean dynamic height and high resolution profiles of geostrophic transport 

across the Louisiana-Texas shelf break from LATEX-A mooring data. 

DESCRIPTION: 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. Local dynamic vertical modes of pressure anomaly were 
ACCESS NUMBER: 30660-19948 

calculated with respect to smoothed, seasonally adjusted leveled state reference 

profiles obtained independently from LATEX hydrographic cruise data. 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. Pressure anomaly profiles were computed from temperature and salinity 

time series using linear interpolative techniques, and were then fitted to dynamic modes 

of pressure anomaly. Time series of high resolution pressure profiles reconstructed 

from the dynamic modes and modal amplitudes are the basis for estimation of the time 

evolution of dynamic height and (following application of geostrophy) cross-shelf 

geostrophic transport at the shelf break. Geostrophic transport profiles 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. 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). 

SIGNIFICANT CONCLUSIONS: Transport profiles are influenced less by alongshelf 

location on the shelf than by the time of year, with energetic offshore transport in the 

upper water column during winter and onshore transport during summer. A slow, 

smooth seasonal progression of vertical patterns of mean cross-shelf flow exists at the 

200-m isobath. Mean transport over the entire shelf for the seven months studied was 

0.26 Sv. The time dependent transport profiles suggest the possibility of upwelling on 

the outer Louisiana-Texas continental shelf, which could replenish nutrients depleted by 

biological demand. 

STUDY RESULTS: Results of this study are consistent with the findings of the MMSfunded Texas-Louisiana Shelf Circulation and Transport Processes Study (LATEX-A), 

but provide high resolution vertical profiles of geostrophic transport previously 

unavailable to MMS. These geostrophic transport profiles 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. 

Mean steric water level anomaly from the shelf break moorings is consistent with the 

smooth annual cycle determined from 1992-1994 LATEX A and C cruise hydrography 

by Current (1996), and is notably similar to the findings of Whitaker (1971) throughout 

the northern and western part of the Gulf of Mexico. The seasonal signal is 

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

maximum near the autumnal equinox. 

Circulation on the outer Louisiana-Texas shelf appears to be more complex than was 

thought prior to the LATEX program and this is consistent with the conclusions of the 
ACCESS NUMBER: 30660-19948 

LATEX-A scientists. Cross-shelf transport was as large as or larger than previous 

reports of alongshelf transport, varying greatly in time and by shelf location, confirming 

previous results based on other LATEX-A data. 

STUDY PRODUCTS: Current, C. L., and W. J. Wiseman, Jr. 2000. Dynamic height 

and seawater transport across the Louisiana-Texas shelf break. A final report for the 

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

Region, Metairie, LA. Contract No. 14-12-0001-30660-19948. 58 pp. OCS Study MMS 

2000-045. 

*P.I.’s affiliation may be different than that listed for Project Manager(s).