ACCESS NUMBER: 30724 

STUDY TITLE: Drifter Study in the Mississippi River Plume 

REPORT TITLE: Velocity and Transport Characteristics of the Louisiana-Texas 

Coastal Current during 1994

CONTRACT NUMBER: 14-35-0001-30724 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICATION PLANNING AREA: Western Gulf of Mexico 

FISCAL YEARSS OF PROJECT FUNDING: 1993/94; 1994/95; 1995/96; 1996/97; 

1997/98;1998/99 

COMPLETION DATA OF REPORT: November 2001 

COSTSS: FY1993; $68,335: FY1994; $34,316: FY1995; $60,535: FY 1996; $46,881: 

FY1997; $67,168: FY1998; $22,723: CUMMULATIVE: $299,958 

PROJECT MANAGER: Stephen P. Murray 

AFFILIATION: Coastal Studies Institute, Louisiana State University 

ADDRESS: 331 Howe-Russell Geoscience Complex 

PRINCIPAL INVESTIGATORS: Stephen P. Murray (replaced by Nan D. Walker in 

1999) 

KEY WORDS: coastal current, velocity, Gulf of Mexico, drifters 

BACKGROUND: The Gulf of Mexico is a semi-closed marine basin with major inflow 

and outflow openings in its southeastern corner. On its northern rim, it has a broad 

shallow shelf dominated by North American continental drainage, including the 

Mississippi and Atchafalaya River distributaries and an array of smaller drainages, alldischarging fresh water and sediment. The Atchafalaya–Mississippi River outflow 

produces buoyant, turbid plumes, which have been observed to respond to winds, 

impinging Loop Current rings, and high level runoff events. While a considerable but 

yet undetermined proportion of the Mississippi River effluent is directed offshore, 

studies of satellite imagery consistently show the Atchafalaya discharge is a dominant 

source of the major extended plume of low salinity and high turbidity that stretches from 

the central Louisiana coast, westward along the Texas coast and southward even at 

times to the U.S.-Mexican border. Initial results from the first two cruises of the 

Mississippi River-Atchafalaya Plume Study (sponsored by MMS, Contract No. 14-35-

0001-30632) brought into sharp detail the intense temporal and spatial variability of the 

coastal current emanating from the Atchafalaya discharge into the shallow inner shelf 

off central Louisiana. It also appeared that plume water was rapidly advected by the 
ACCESS NUMBER: 30724 

shifting wind field along the cruise track. Satellite images and previous drifter studies 

suggest the intermittent presence of a meandering, at times highly convoluted, 

baroclinic coastal jet. 

OBJECTIVES: Objectives of this study were (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 the 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. 

DESCRIPTION: 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 Atchafalaya River discharge into the eastward flowing Louisiana-Texas current. 

This description is 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. 

To determine the transport of the Louisiana-Texas coastal current (LTCC) and its 

temporal and cross-shore variability, a mooring line extending 80 km offshore was 

deployed on the cross-shore transect near Cameron, LA. In addition to current meter 

data, pressure, salinity, wind, and Atchafalaya River discharge were analyzed to better 

understand the variability of the LTCC. Based on availability and quality of the data, two 

regimes (summer upcoast flow and downcoast winter) were selected for detailed 

studies. 

SIGNIFICANT CONCLUSIONS: 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, confirm 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. 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. 

STUDY RESULTS: Energy spectra of the near surface currents showed expected 

semidiurnal and diurnal peaks associated with tidal and inertial motions for both 

summer and winter flow regimes. A broad range of energetic motion was also observed 

in the subtidal/weather frequency band during the winter regime. Summer spectra of 
ACCESS NUMBER: 30724 

the near surface currents in the subtidal band may be described as “red,” i.e., the 

energy increases with decreasing frequencies. For both regimes, the energy in this 

frequency band was 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 force. In summer high coherence of currents in 

the vertical was not observed. Transport computations show that in the winter season, 

the mean transport of the 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. Data also 

show that in the summer flow upcoast regime, the decorrelation time scale is greater 

than the downcoast flow regime with values of 80-290 hours and 40 to 80 hours, 

respectively. For both regimes, the decorrelation times of the cross-shore component 

are shorter than those of the alongshore, with values of 20-60 and 50-90 hours, 

respectively. 

STUDY PRODUCTS: 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. Department of the Interior, Minerals Management Service, 

Gulf of Mexico OCS Region, New Orleans, LA 106 pp. 

Jarosz, Ewa. 1997. Summer flow regime on the Louisiana innershelf, Master’s thesis, 

Louisiana State University, Baton Rouge, LA, 74 pp. 

*P.I.'s affiliation may be different than that listed for Project Managers.