ACCESS NUMBER: 30660-19922 

STUDY TITLE: Pressure Gauge and Moored CTD Array along the Louisiana Coastal 

Current 

REPORT TITLE: Investigation of Pressure and Pressure Gradients along the LA/TX 

Inner Shelf and their Relationships to Wind Forcing and Current Variability. 

CONTRACT NUMBER: 14-35-0001-30660-19922 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Northwestern Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 1994-2001 

COMPLETION DATE OF REPORT: August 2001 

CUMULATIVE PROJECT COST: $167,026 

PROJECT MANAGERS: S.P. Murray, N.D. Walker 

AFFILIATION: Coastal Studies Institute, Louisiana State University 

ADDRESS: Howe-Russell Geoscience Complex, Louisiana State University, Baton 

Rouge, LA 70803. 

PRINCIPAL INVESTIGATORS*: S.P. Murray, N.D. Walker 

KEY WORDS: physical oceanography, Louisiana Coastal Current, shelf circulation, 

water level changes, pressure gradients, air-sea interaction, coastal set-up, coastal 

trapped waves, coastal processes. 

BACKGROUND: The northwestern rim of the Gulf of Mexico is characterized by a 

broad, shallow shelf dominated by North American continental drainage, including the 

Mississippi and Atchafalaya Rivers and an array of smaller rivers and bays. The 

Mississippi/Atchafalaya River outflow produces buoyant, turbid plumes along the 

Louisiana coastline. This current exhibits intense spatial and temporal variability related 

to the volume of river discharge and nature of the wind forcing. Although circulation 

over the Louisiana/Texas shelf is complex and can vary rapidly, certain general 

circulation patterns have emerged from previous studies. On the inner shelf west of 

Atchafalaya Bay, surface circulation is primarily wind-driven and coherence has been 

found between alongshore wind-stress and alongshore currents for coastal locations. 

The wind-current relationship results in downcoast flow (westward/southward) along the 

Louisiana/Texas coast for much of the year with a coastal-type jet on the inner shelf. 

The predominant east to west flow on the inner shelf is enhanced by the Atchafalaya 

and Mississippi River discharges. Short-lived flow reversals occur when winds blow 

from the west, as occurs with the passage of winter storms with recurrence frequencies 
ACCESS NUMBER: 30660-19922 

of 3-10 days. Longer-lived flow reversals occur during summer when strong southerly 

winds prevail along the south Texas coast and weak southerly and southwesterly winds 

blow along the Louisiana coast. In this study, six pressure gauges were installed along 

the Louisiana/Texas inner shelf to enable a detailed investigation of the alongshore 

pressure and pressure gradients and their relationships to wind forcing and coastal 

circulation along the Louisiana/Texas coast. Lessons learned from observations, in the 

LCC and other studies suggest that alongshore variation in the pressure (water level) 

and pressure gradients may be of great importance in understanding the mechanisms 

controlling the Louisiana/Texas coastal current. 

OBJECTIVES: The primary objectives of this study were: 

1. To deploy an alongshore near coastal pressure gauge array in order to understand 

the role of sea level slopes in the dynamics controlling the Louisiana Coastal 

Current. 

2. To quantify the relationships between coastal wind stress and (1) sub-surface 

pressure (water level) and (2) alongshore pressure gradients. 

3. To obtain a better understanding of the role of alongshore pressure gradients in 

coastal circulation changes. 

4. To produce a data set of near coastal sea level and sea level slopes suitable for 

calibration and validation of numerical models of the Louisiana/Texas coastal 

current. 

DESCRIPTION: To analyze the sub-tidal temporal and spatial variability of sub-surface 

pressure and pressure gradients along the Louisiana-Texas coastline, six sub-surface 

pressure gauges were deployed in very shallow water (< 20m). These gauges were 

either SD 635 or SBE Seagauge instruments and they were set to record data with a 

15-minute sampling interval. Most of the instruments were attached to offshore oil and 

gas platforms. The gauges became operational in July 1994 and were kept in the 

specified locations until August 1995. Inspection of the recorded pressure time-series 

from this initial deployment revealed several periods of time with questionable or bad 

data that occurred mostly as a result of instrument malfunction. Faulty instrumentation 

was repaired and, for the most part, the sub-surface pressure gauges were returned to 

the same locations. The second recording period began in May 1996 and lasted 

through January 1998. To improve the spatial coverage of the pressure and pressure 

gradient changes along the Louisiana-Texas coast, four water level stations from the 

Texas coast were included in the analyses. Hourly wind measurements from the same 

CMAN stations, an airport at Brownsville, and an anemometer at Freshwater Bayou 

(see Table 1 and Figure 1 for their locations) were also analyzed together with the subsurface pressure time-series to investigate pressure and pressure gradient responses to 

wind stress components. To investigate time series variability and interdependence, the 

following statistical analyses were employed: spectra estimation, multiple, partial and 

ordinary coherence. 

SIGNIFICANT CONCLUSIONS: A semi-annual cycle in sub-surface pressure/water 

level was detected in the monthly mean pressure data along the Louisiana and Texas 
ACCESS NUMBER: 30660-19922 

inner shelves. Mean monthly pressure maxima were measured in October of each year 

with secondary maxima in April/May. The pressure minima were observed in 

January/February with secondary minima in July/June. These seasonal pressure 

changes were closely associated with seasonal changes in alongshore wind forcing. 

Pressure minima coincided with relatively strong upcoast wind stress and pressure 

maxima coincided with relatively strong downcoast wind stress. Strong westward wind 

stresses were measured in October. A more detailed analysis of the water level 

fluctuations on the synoptic scale revealed that the highs and lows in water levels were 

attributed to one or more distinct wind events within each month. Statistical analyses of 

the relationships between pressure and wind stress revealed that at all stations and in 

both summer and winter seasons, highest coherences were observed between the 

alongshore wind stress and sub-surface pressure variations. Along the Louisiana coast, 

coherences of 0.7-0.8 were found between pressure and alongshore wind stress at 

frequencies between 2 and 20 days. No large seasonal differences were observed in 

the levels of coherence between pressure and alongshore wind stress along the LA 

coast. Along the Texas coast, coherences between pressure and alongshore wind 

stress were also much higher than those with cross-shore wind stress. Coherences 

between alongshore wind stress and pressure were 0.7 to 0.8 during winter and 0.48 to 

0.70 during summer. Current meter measurements on the inner shelf near Cameron 

were compared with the pressure gradients along the Louisiana-Texas continental shelf. 

Visual inspection of graphical outputs revealed similarities between pressure gradients 

along the Texas coast and flow off Cameron. Further statistical investigation 

demonstrated the existence of significant relationships only during summer. This is an 

important result as it indicates that inner shelf currents along at least a portion of the 

Louisiana shelf during summer may be driven by pressure gradients and currents 

generated along the Texas coast. In general, however, the computed pressure 

gradients were consistent with gradients that would result from the wind stress forcing. 

Based on this study, we conclude that the barotropic pressure gradients along the coast 

do not provide a major control on current variability except in the west portion of the 

study region during the summer. Long period oscillations in pressure/water level were 

detected along the Louisiana-Texas coastline during the 1996 summer. They exhibited 

periods of about 10 days and amplitudes of at least 10 mb. Similar long-period 

oscillations were evident in the corresponding atmospheric pressure and alongshore 

component of the wind stress. The spatial vastness of the pressure events and the time 

lag between stations may suggest that these are long coastal-trapped waves. 

STUDY RESULTS: A detailed analysis of the water level fluctuations on the synoptic 

scale revealed that the highs and lows in water levels were attributed to one or more 

distinct wind events within each month. Westward (Louisiana) and southwestward 

(Texas) wind stress events produced the highest coastal water levels. Lowest water 

levels along the Texas coast were produced by northward wind stress. Along the 

Louisiana coast, lowest water levels occurred in close association with strong eastward 

wind stress. Although similarity in water level responses was observable in the water 

level records, there were events for which the coastal water levels across the region 

responded differently to a wind event. The change in coastline orientation from 

Louisiana to Texas in relationship to wind stress produced interesting differences in 
ACCESS NUMBER: 30660-19922 

water level responses on the synoptic scale. For example, a strong wind blowing 

towards the east along the Louisiana coast produced alongshore wind stress, upwelling 

and a reduction in coastal water levels due to Ekman set-down. This same wind stress 

along the Texas coast resulted in cross-shelf offshore wind and little water level change. 

A detailed analysis of pressure/water level fluctuations along the Louisiana/Texas 

coastline was performed for January 1997. This analysis demonstrated that water 

levels only fall when winds blow upcoast. Thus, the presence of strong winter winds 

from the northeast does not reduce coastal water levels, but increases them. Thus, all 

frontal passages do not have the same effect on coastal water levels. Winter storms 

that blow upcoast (west winds) have a much stronger flushing effect in coastal 

Louisiana than winds blowing downcoast (east winds). 

STUDY PRODUCTS: Walker, N.D. 2001, E. Jarosz and S. P. Murray.2001. An 

investigation of pressure and pressure gradients along the Louisiana/Texas inner shelf 

and their relationships to wind forcing and current variability. OCS Study MMS 2001-

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

Region, New Orleans, LA, 52 pp. 

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