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Velocity and Transport Characteristics of the Louisiana-Texas Coastal Current

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14. ABSTRACTThe Louisiana-Texas Coastal Current (LTCC) is a major dynamic feature responsible for the distribution of fresh water, sediment and nutrients on the northwesternshelf of the Gulf of Mexico. Earlier studies have indicated that this current exhibits a distinct although asymmetric annual cycle during which it flows downcoast, i.e.,westward along the Louisiana coast and then southward along the Texas coast in fall, winter, and spring; however, in summer, the flow reverses and moves upcoast.This annual cycle of the LTCC is clearly observed with measurements from a cross-shore current meter mooring array located south of Cameron, LA in 1996 and1997. Analyses of these data show that the currents are indeed downcoast between September 1996 and January 1997 with modest mean velocities up to 6 cm/s.During the expected upcoast regime of June-August 1996, our data show mean eastward speeds of 2-6cm/s. Cross-shelf spatial correlation length scales of thecurrents are well in excess of 60km during the downcoast regime but they are distinctly smaller (30-50 km) during the upcoast regime. Coherence analysis andpredictions from a wind-driven model indicate that the downcoast currents and volume transport associated with them are highly coherent with alongshore windstress. This wind stress also controls fluctuations of the upcoast currents and transport; however, it is not a dominant forcing. The data indicate that during theanalyzed summer season, the alongshore sea-surface slope was also an important driving force of the LTCC.

15. SUBJECT TERMS

Louisiana-Texas Coastal Current (LTCC), downcoast, upcoast, wind-driven model, sea-surface slope

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Title of Paper or PresentationVelocity and Transport Characteristics of the Louisana-Texas Coastal CurrentAuthor(s) Name(s) (First, MI,Last), Code, Affiliation if not NRL

Ewa Jarosz (Dr), S. P. Murray

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HQ-NRL 5511/6 (Rev. 12-98) (e) THIS FORM CANCELS AND SUPERSEDES ALL PREVIOUS VERSIONS

Velocity and Transport Characteristics of the

Louisiana-Texas Coastal Current

Ewa Jaroszt and Stephen P. Murray2

Coastal Studies hIstitute, Louisiana State Universit,, Baton Rouge. Louisiana, USA

The Louisiana-Texas Coastal Current (LTCC) is a major dynamic feature respon-sible for the distribution of fresh water, sediment and nutrients on the northwesternshelf of the Gulf of Mexico. Earlier studies have indicated that this current exhibitsa distinct although asymmetric annual cycle during which it flows downcoast, i.e.,westward along the Louisiana coast and then southward along the Texas coast infall, winter, and spring; however, in summer, the flow reverses and moves upcoast.This annual cycle of the LTCC is clearly observed with measurements from across-shore current meter mooring array located south of Cameron. LA in 1996and 1997. Analyses of these data show that the currents are indeed downcoastbetween September 1996 and January 1997 with modest mean velocities up to 6cm/s. During the expected upcoast regime of June-August 1996. our data showmean eastward speeds of 2-6 cm/s. Cross-shelf spatial correlation length scalesof the currents are well in excess of 60 km during the downcoast regime but theyare distinctly smaller (30-50 kin) during the upcoast regime. Coherence analysisand predictions from a wind-driven model indicate that the downcoast currentsand volume transport associated with them are highly coherent with alongshorewind stress. This wind stress also controls fluctuations of the upcoast currents andtransport; however, it is not a dominant forcing. The data indicate that during theanalyzed summer season, the alongshore sea-surface slope was also an importantdriving force of the LTCC.

"INTRODUCTION outflow usually peaks in spring and is at a minimum inearly fall. The river enters the Gulf of Mexico through the

The Mississippi River is the major source of fresh water. Mississippi River birdfoot delta and the Atchafalaya River.sediment, nutrients, and pollutants for the northern shelf which is a. distributary of the Mississippi River divertedof the Gulf of Mexico. It drains 429,,, of the continental from the main channel at Old River. north of Baton Rouge.watershed of the United States and has an annual average LA. Thirty percent of the total discharge is delivered to thedischarge rate of- 19,000 m3/s [lWisenan et al., 1997]. The Louisiana shelf by the Atchafalaya River. The remaining

seventy percent flows through the Mississippi River delta.

'Now at Oceanography Division. Naval Research Laboratory. Sten- Recent estimates. however. suggest that only 43% of thenis Space Center. Mississippi. USA. fresh water discharged by the birdfoot delta is carried to the

2Now at Office of Naval Research. Arlington. Virginia. USA. western Louisiana shelf [Etter et al.. 2004]. This portion of

Circulation in the Gulf of Mexico: Observations and Models the Mississippi River outflow joins with the AtchafalayaGeophysical Monograph Series 161 River discharge, mixes with the shelfswaters and ultimatelyThis paper is not subject to U.S. copyright. Published in 2005 forms a coastal current [Wisemnan and Klhs, 1994: Muraray etby the American Geophysical Union.10. 1029/16IGMII al.. 1998]. The coastal flow has been called various narnes in

143

20060605024

144 VELOCITY AND TRANSPORT CHARACTERSTICS OF THE LOUISIANA-TEXAS COASTAL CURRENT

the literature: for instance, it was referred to as the Louisiana Murray et al. [1998] showed that during the summer upcoastCoastal Current by Wiseman and Kelly [1994] and the Texas flow regime, current fluctuations in southern Texas are

current by Vastano et al. [1995]. The latest studies [Miurray significantly coherent with the alongshore sea-surface slopeet al., 1998; Nowlin et al., 1998] show that despite strong and wind stress; however, variability of the current in centralspatial and temporal variability, this current can generally Texas and central Louisiana is highly coherent primarilybe traced west of the Mississippi River delta along the entire with the alongshore sea-surface slope.Louisiana and Texas coasts. Therefore it seems reasonable to In contrast to the larger scale resolution of the previousrefer to it as the Louisiana-Texas Coastal Current (LTCC). studies, this paper utilizes a data set from a current meter

During much ofthe year, the LTCC flows westward along array specifically deployed to study cross-shore variabilitythe shore fromn Louisiana to Texas, consistent with a hal- and coherence at one location in the LTCC [Mtsurrco' et atl., -

ance between the cross-shelf pressure gradient set up by the 2001]. We focus on the variability of currents and volumeriver and the local Coriolis force. This may suggest that the transport observed within the LTCC on the Louisiana innerflow is in general agreement with geostrophic theory and is shelf west of both the Atchafalaya and Mississippi deltasdriven mainly by the pressure gradient. In such a scenario (approximately along the 93"W longitude line). Our primarythe river outflow is deflected to the right upon enterifig the objective is to use the current meter data from the cross-sea (the Gulfof Mexico) in the northern hemisphere. produc- shore array (a) to determine the seasonal cycle of the meaning a buoyant coastal current trapped against the coastline. flow in the LTCC and its fluctuations in different seasons,Observations and analyses do not, however, support such i.e. in summer (upcoast regirne) and fall/winter (downcoast ia simple behavior of the LTCC. Smith [1975, 1978] reports regime). (b) to estimate the correlation between the LTCCseasonal variability of the current on the central Texas shelf currents and alongshore wind stress in this region. and (c)with a strong south-southwesterly flow in winter and weak to estimate the transport of the LTCC and its seasonal vari-north-northeasterly or south-southwesterly flow in summer. ability as well as its dependence on driving mechanisms suchHe also implies that many features of the nearshore current as wind stress, sea-surface slope and buoyancy forcing Aspattern can be explained as a response to surface wind stress, represented by the Atchafalaya River discharge.C'out et al. [1984] also describe seasonal variability of thecurrent on the western Louisiana inner shellf and conclude DATAthat the summer current is generally disorganized and prob-ably driven by nonlocal forcing. In contrast to the sum- A map of the Loufisiana-Texas shclftFigure I) shows themer flow. the winter alongshore current is better organized locations where the observations were collected for this work:and is well correlated with alongshore wind stress. Similar (a) the inner-shelf transport resolving array off Cameron.conclusions regarding the wind stress and winter flow are LA. NbI subsurface pressure gauges. and (C) anieneomneterdrawn by lWisemanue al. [1986]. Coc-lrane sssd Kelli- [1986]. locations. The type ofa current meter available to this studvsynthesizing previous investigations, report that alongshore was the ducted impeller Endeco Model 174. which employswind stress is the major driving force of the coastal current a tether line to filter out surface wave velocities and allowson the Louisiana-Texas shelf in a region west of 92.5"W. the instrument to orient in the mean flow direction. Our data.During much of the year (usualiy from September through though obtained with ati instruiment array lacking the verti-May), the current flows downcoast (ii tile sense of Kelvin cal resolution ofan ADCP current meter, still provide signifi-wave phase propagation in the northern hemisphere), and is cant new insight into the characteristics of the LTCC. Tablehighly correlated with the downcoast wind stress component. I iives the location and total water depth at each mooringIn late summer. there is a reversal of the flow direction and along with the relative depth of each current meter. distansseupcoast flow, well correlated with the wind, prevails. The front the coast (hr each location and time coverage of thesummer upcoast flowv was observed as far east as 90.5"W current observations. Note tile mooring array extends over 65[.Ia'osz, 1997]. Based ott hydrographic and current meter km ini tile cross-shore direction. from about the Ill-it isobathdata analyses, this annual reversal of the nearshore circu- to the 22-m isobatht over a fairly uniforrilly sloping bottom.lation on the northern shelf of the Gulfo If Mexico and its The location for the transport array was selected based on -w

dependence on the alongshore wind stress is also reported by two considerations: ta) to be far enough downstream fromLi et al. [1997]. 1Miurray* t et al. [1998] , Nowlin et al.. [1998]. tile Atchaltlaya River mouth to escape its direct influenceClto et al.. [1998], -aowlin et ut. [2005], and Wulker [2005]. and the influence ofithe large Tiger and Trinity shoals loc tedThe asymmetric annual cycle of the nearshore circtuation just hest oi the riser mout: and (b) survisabilitv, since iSv

is also well reproduced by numerical models [Os'y, 1995: imooring in the water cohimnt ott the Louisiana inner shelfCurrent. 1996: Zsawla-Hidalgo et at.. 2003]. Additiottally. will survive oil industry traffic and Fishing pressure onlv ifit

JAROSZ AND MURRAY 145

30N-e, ~ LUSAA~TEXAS '.

-1E . Fres sater Bayuyt o,

B a 42035 10 +0~29N- A oyster Basns - 30reeport r. 20 S

20 30 gO0

50~ C Morings50 ~~+ Pressure Gaue

A WindStain

95W 94W 93W 92W 91W 90W 89W

Longitude

Figure 1. Regional map containing study transect (A througzh G moorings) ofthe LTCC south of Cameron. LA. Pressuregauges at Oyster Bayou and Freeport, and wind stations at Freshwater Bayou and the US huoy off Galveston (B42035)are also shown.

is locnted near an existing oil field structure (in our cane usu- Equipment malfunctions and severe weather limited theS ally a standpipe). We selected standpipes and small platforms data return in the 1995 pilot program. but better instrumenta-

(1-10 m in diam-eter) and placed the moorings 2-3 diameters tion deployed in May 1996 greatly improved data returns (seefrom them on a line as near to a coastal normal as possible. Table I for time coverage of this deployment) [.tturrcn et at.,Thec outer mooring is located on the 22-tm isobath. and, as 2001)1. A period extending from May 199610o April 1997 con-r. -atedl high-resolution hydrographic sections (with a sta- veniently allows the study of the two distinct flow regimes:

; ispacing of approximately 7 kml in this region [Mto'rar the upcoast (summer) flow regime of 1996 (a period betweenet A., 1998] save shown, this distance from the coast would I June and 14 August 19961 and the downcoast lfall/xvinter)

3 usually capture the lower salinity waters except during the flow regitme of 1996-97 (a period between 26 Septembersummer upcoast flow regimne when the low salinity waters 1996 and 4 January 1997). The time period analyzed forspread farther offshore, the Upcoast flow regime begins in June. in a month when

Tahle 1. Moorina configusratio'.Mooing Lonituel\I Ltitde)N) Total Instrument Distance Start Time-tpTm

_________ _________Depth (m) Depth (ml Offshore (kmsiAtop"' 93.1627711 29.081)33', 23.1 7.,S 76.92 o)5. 14.1996-019 19 199t,Amid'" 13.3 0______ 5.14,1996-0t9 19.19%6

4 . Bto 1l 93.182331 29.19233" 19.5 . 64.45 01) 017:1996-014,16 1997Bmid2' 101.01 09________ 107'l9t)6-04 16 1997Ctopl'2 ' 93.24219,1 29.3289' 17.7 6.4 49.54 05S 13. 1996-0911I 1997

(19 19,1996-014.16 1997Cmid''2 11.01 05,131;1996-01911. 1996

09'19,1996-01 '18,1997* 6 Chot'.

2' 15.11 05il3i11996-0119/L 1996

________ _________09Z19/1996-01,18/1997

Dtop"___ 93.243" 29.391671 15.0 40.5 0,13/196____99

Etop'-2 ) 93.20)4171 29.531133' 13.1 5.7 27.06 050W111996-018!31, 199609/25/1996-01: 24,1997

Ebot2' 101.0 0)9/25/1 996-111.14/1997

Flop'.2' 93.16433" 29.599671 12.0 6.5 19.79 015/13/1996-% 1& 81996

5 . 09,06,1996-1111)5:1 997Fbot'.2' 101.0 05113/1996-0)6 18,1996

________ __________ ________ ________________ _________ 09 106/1996-01.05.1997§itnpli 93.160S44' 29.66)3 0.1.5 13.401 05:14/1996-08;'27 1996

inostrumenits use,) for Ilpenast reginse analyses: 2/instruments used Cribr dnvcost regimle analvses.

146 VELOCITY AND TRANSPORT CHARACTERSTICS OF THE LOUISIANA-TEXAS COASTAL CURRENT

the LTCC usually changes its direction from downcoast to at all considered locations at speeds between 2-6 cm/s,upcoast, and it ends one day before a strong cold front, nor- while the direction of the flow was westward (downcoast)mally observed during the downcoast flow regime, moved throughout the water column in the fall and winter of the

through the mooring array. The time duration for the down- 1996-97 season with speeds up to approximsately 6 cm/s.coast flow regime encompasses the longest common time This annual cycle of the inner shelf circulation is also appar-period with good data return from the analyzed instruments. ent in progressive vector diagrams (Plate 1). These diagramsAdditionally, the numbers adjacent to each current meter in show eastward displacement for the summer and persistent

Table I indicate which instruments are used in the summer downcoast movement during the fall/winter season. The(1) and fall/winter (2) flow regime studies, magnitude of the mean flow when examined at different

Our intent here is to describe only the subinertial variabil- distances offshore for the same regime does not show muchity (below 0.6 cpd); therefore all analyses presented in the variability for flows recorded at similar depths. There is,subsequent sections used low-pass-filtered data with a cutoff however, a noticeable decrease in magnitude of the net dis-frequency of 0.6 cpd (Butterworth filter). Additionally, all placement and alongshore means with increasing depth,quantities are referred to a Cartesian coordinate system Ix, especially evident for the downcoast regime (Plate lb andy, z) with x positive eastward (upcoast), y positive northward Table 2). On inner shelves, i.e., in shallow water regions,(onshore), and z positive upward. where surface and bottom boundary layers typically overlap

and interact with each other, and the currents usually align i

MEANS AND VARIABILITY OF THE CURRENTS with the direction of forcing. this weakening of the meanflow with depth could probably be attributed to a decreasing

Figure 2 shows the variations of means at various depths influence of surface forcing wvith depth as well as to bottomas a function of distance from the shore for both current friction. Additionally, only one out of 17 estimated cross-components and flow regimes. Additionally. Table 2 lists shelf velocity means (Table 2) is statistically significant atthese means as well as standard deviations, root-mean-square the 95% significance level. Thus we conclude that the meanserrors of the means estimated as suggested by Ktondt and of the cross-shelf component do not show any clear patternAllen [19761, major axis orientations oftthe velocity variance, in the analyzed seasons.and orientations of the mean flow. The large standard deviations (Table 2). usually 2-3 times

An examination of the means and mean flow orientations larger than the means. and the meandering of the flow clearlyclearly shows two different patterns. i.e.. in tile suominer visible in Plate I document the sign ificant current variabilityof 1996. the mean flow was directed eastward (upCoast) in the LTCC duiring both upcoast and downcoast regimes.

(a) (b)

8 G F E D C B A G F E D C B A

6

4

.2 0 • , ,S

-4 M "t -4)- T p Meters 0

-6-- I- Mid-deth Meters 0-- Top Meters 0-8 - -- Mid-depth Meters D

-- Bottom Meters U-8c 1 d -d-,,, 1'e. .s

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

Distance Offshore (km) Distance Offshore (kim)Figure 2. Me:ins ofta tile alongshore (east-west) and (b) cross-shore (Inolt-south) current components Ibr the upcoast

(open sy a ibnls)an t downcoast (solid s vniholsi flow ren tinies: positive values are upcoast (eastward) rand onshotre (north-ward) bir the iahongshore and cross-shore ineatis. respectively.

S

JAROSZ AND MURRAY 147

Table 2. Mooring statistics.

_______ c/s c/i co VD u a•Vs2) Principle Axis OrientationU! I SD I) VSD I 1 S2

Mooring _cm/s) (cm/s) (cm/s) (cm/s) (cm/s) (cm/s) (dee) .) (deg) 3:__Upcoast RegimeL to 3.0 -2.7 14.1 12.6 6.6 6.6 - 51 132

Amid 4.4 -1.8 11.8 8.9 5.5 3.7 65 112to 4.8 0.9 10.3 5.8 2.5 1.5 93 79

Cmid 6.3 0.7 8.4 5.8 1.7 1.1 90 84Cbot 1.9 1.1 3.1 3.5 0.6 0.6 30 60

tDro. 2.5 -0.3 8.5 6.8 1.6 1.7 80 97

Etop 3.4 -0.6 8.2 5.7 1.7 1.2 91 100Gtop 3.6 -0.4 8.8 3.4 1.9 0.6 93 96

Downcoast RegimeBtop -6.2 1.8 13.5 5.0 2.4 1.2 86 286Bmid -4.4 0.8 12.0 5.9 2.1 0.9 90 280Ctop -5.8 -1.2 10.0 3.7 1.7 0.4 88 259Cmid -4.3 1.3 11.1 4.8 1.8 0.7 84 287Cbot -2.4 0.3 8.2 4.8 1.3 0.7 70 277Etop -5.5 3.6 10.9 4.8 1.6 0.6 101 304Ebot -1.2 1.2 5.9 3.3 0.9 0.4 93 314Ftop -6.3 1.4 14.7 4.8 2.3 0.5 98 282Fbot -2.6 1.2 9.3 3.3 1.3 0.5 91 295Standard deviations of respective currelnt conponents: " rms errors of mean estimates: ineasured clockwise from true north:NO 10" ambiguity should be added.

From Table 2 it is also apparent that the alongshore standard correlated at the different depths and locations. To deter-deviation usD consistently decreases with depth, whereas the mine whether this visual observation is valid, the complexcross-shore standard deviation vso remains nearly constant correlation coefficient [Kimndtr. 1976], whose magnitudewith depth for both summer and fall/winter seasons. As one gives the overall measure of correlation between two cur-approaches the coast, there is also a decreasing trend for both rent records, was computed separately for the upcoast andstandard deviations in summer, while nearly constant values downcoast flow regimes. The magnitudes of this coefficientin the same direction are found for the fall/winter period, for the top instruments as a function ofa mooring separation

rthermore, the fluctuations during the downcoast season distance are displayed in Figure 3. Using the algorithm forare anisotropic (usD > V,,) and their principle direction is a confidence level estimation suggested by Sciremaniinoalmost east-west, as is the direction of local isobaths and [1979], coefficients greater than 0.29 for the downcoastcoastline. During the upcoast season, however, the fluctua- flow and 0.33 for the upcoast flow are significant at the 95%o-tion field is more isotropic (usD 7 VSD). especially near the confidence level.bottom and at the moorings located farther offshore. Nearer It is very apparent from Figure 3 that the cross-shoreto the shore, these fluctuations again follow the direction spatial correlation length scale of the currents within theof local isobaths. Additionally. large standard deviations LTCC is different for downcoast and upcoast flows. For the!isually imply large standard errors of the means (Table 2) downcohst regime. the magnitude of the complex correlation.:ggesting that the mean flow speeds are not statistically dif- coefficient is in the 0.86-0.91 range and varies little across

ferent from 0. Thus one mnay conclude that the mean currents the mooring array. Such high values suggest that the cross-in the Gulfof Mexico off Cameron. LA were very sluggish. shore spatial length scale of the flow is undotibtedly muchespecially in summer 1996, larger than the maximumn available separation distance, i.e., it

is larger than 60 kim, and our mooring array is not resolvingSPATIAL CORRALATIONS OF THE CURRENTS this scale well in the fall/winter season. In contrast. magni-

IN THE LTCC tudes of the correlation coefficients found for the upcoastregime are smaller, and. for instance. their value at the 50-km

An inspection of the raw data as well as the progressive separation distance reaches only 0.32 in the summer seasonactor diagrams (Plate 1) suggest that the currents are visibly as opposed to 0.89 in the fillwinter period. Additionally, the

148 VELOCITY AND TRANSPORT CHARACTERSTICS OF THE LOUISIANA-TEXAS COASTAL CURRENT

zp 0

E 2

wo

coE

40

0 0 0 0 0

w)I

JAROSZ AND MURRAY 149

1.0 Tseason and to 0.75 for the fall/winter time period for larger0. vertical separations (9 m or more). The weaker correlation

of the summer currents at the separation distance of 9 m or. more could be related to summer stratification. The well-

developed summer pycnocline is able to isolate effectively. -flow observed in the lower layer of the water column from

U direct forcing such as winds, for example. Consequently, theA flow in the lower layer is usually very weak, disorganized

and often opposed to the flow observed above the pycnocline03 [C-rout et al., 1983; Murray et al., 1998]. The relationship

0. - between currents at different depths strengthens significantly0 U -pcoat Regime due to weaker stratification in fall/winter months and deeper

E influence of wind forcing.0 10 20 30 40 50 60 70 80

Separation Distance (km) VELOCITY CEOFS

"Figure 3. Correlation coefficients as a function of the cross-shoreseparation distance for the upcoast (open triangles) and downcoast To extract variability that is coherent across the mooring(solid circles) flow regimes: the coefficients are computed only for array and quantify major patterns in the LTCC currents, wethe top instruments. subjected the data to complex empirical orthogonal function

analysis (CEOF) [Kumdu aund Allen. 1976]. For each flowsummer coefficient does not display a monotonic decrease regime, the velocity CEOFs were computed by forming a'vith increasing separation distance as often observed in matrix containing demeaned complex velocities obtained.dter coastal regions [Kundu andlAllen. 1976: Dever. 1997]. from the low-frequency along- and cross-shore current com-

However, inspection of the correlation values and their errors ponents for all available depths in a given season. Complexsuggest that the summer cross-shore spatial length scale eigenvectors estimated from the associated covariance matrixcould be between 30 km and 50 km. were then converted into amplitudes and veering angles for

The distinct difference in spatial length scales of the LTCC each statistically significant mode (Table 3). The amplitudes'M currents between seasonal regimes may result from differ- of each mode were also scaled by the square root of the cor-

ences in the scale and strength of forcing mechanisms such responding eigenvalue. and the veering angles were relativeas winds [Giairre: die Velasco and lVinant, 1996: llWng et to the Btop and Atop currents for the downcoast and upcoastI., 1998]. During the downcoast regime, the spatial length flows. respectively.:ale is much larger than 60 km, meaning that it is greater For the falliwinter months, only the first CEOF mode

than the width of the LTCC (48 km as estimated by .lturrav is statistically significant when tested with an algorithmet al. [1998]). This large spatial scale probably results from proposed by North ei al. [1982]. It explains 81.1% of thecoherence imposed on the surface layer by the large and current variability, and its spatial structure shows very littleenergetic synoptic scale weather systems (cold fronts) variation in veering angles in the Cameron cross-sectionimpacting the northern Gulf of Mexico in fall. winter, and (Table 3). The spatial distribution of the amplitudes andspring. These systems usually induce strong vertical mixing veering angles suggests that the primary downcoast flowand drive flows inside and outside of the LTCC with similar pattern is in a form ofa surface-intensified coastal jet. Theefficiency [ Wiseman et al.. 1986: WVisenman and Kelly. 1995: first mode is also hiighly correlated with the alongshore windI urrui et al.. 1998]. In summer, more complicated spatial stress with a squared correlation coefficient (R 2) of 0.73

structure of the coastal current. its weaker and more variable that is significant at the 95% confidence level. This highspeed, smaller scale forcing generated by less organized and correlation clearly shows that the alongshore wind stress isweaker winds and/or well-developed stratification could be an important driving mechanism of the LTCC fluctuationspartly responsible for the much shorter cross-shelf spatial in the downcoast flow regime. Additionally. dominance oflength scale. the first mode and its high correlation with the wind stress

There is also a difference between values of the correlation suggest that this mode represents a predominant barotropiccoefficients ofthe upcoast and downcoast regimes as a funo- response ofthe inner shelf current to the strong fall/wintertion of vertical separation (results not shown). The correla- winds and storms. Plate 2 demonstrates how swiftly thetion coefficients are high (> 0.8) in both regimes for adjacent currents respond to changes in the alongshore wind stressinstruments and drop to approximately 0.54 for the summer forcing. This plate shows daily values of wind stress (Plate

150 VELOCITY AND TRANSPORT CHARACTERSTICS OF THE LOUISIANA-TEXAS COASTAL CURRENT

Table 3. Amplitudes" (cm/s) and veering angles2

l (deg) of the CEOF modes.Downcoast Regime U coast Regime

Mooring Amp I Angle I Mooring Amp I Dir I Amp 2 Dir2Btop 13.1 0 Atop 12.4 0 6.9 0Braid 12.1 356 Amid 10.4 3 5.1 27Ctop 8.7 356 Ctop 5.8 341 8.8 163Cmid 11.1 358 Cmid 6.1 331 6.5 167Cbot 8.4 344 Cbot 2.3 279 1.5 154Etop 10.6 352 Dtop 6.8 349 6.2 172

Ebot 5.2 3 Etop 4.1 357 0.6 255Ftop 14.0 8 Gtop 5.3 357 0.9 159Fbot 8.5 2

"Amp I and Dir I arc amplitudes and veering angles of mode I: Amp 2 and Dir 2 are amplitudes andveering angles of mode 2. 21 Veering angles are relative to currents measured at Btop and Atop mooringsfor the downcoast and upcoast regimes. respectively.

2a) and combined mean flow and mode I for each mooring waters (20-30 psutl. and (c) a near coastal westward flow ofon 6 and 9 October 1996 (Plates 2b and 2c. respectively). On low salinity waters (less 20 pst).6 October. a wind stress of 0.11 Pa was directed downcoast In contrast to the high correlation between the currents andas were the currents across the entire Cameron section. On 7 alongshore wind stress in the downcoast regime. the summerOctober, this downcoast stress started diminishing and on 8 current fluctuations are not very correlated with this forcing.October, changed direction to upcoast. The currents followed The squared correlation coefficient between summer CEOFthese changes in wind stress. On 9 October. the downcoast mode I and the alongshore wind stress is low and itsra mag-flow stopped and at most locations, the flow began to move nitude is 0.31 (R-" greater than 0.12 is significant at the 95%upcoast as shown in Plate 2c. Similar behavior of the currents confidence level which was computed from the algorithmin the fall/winter/spring months on the Louisiana-Texas inner proposed by Sciremtntnano [1979]). while there is no corre-shelf. i.e.. their swift response to the wind stress variations, lation between mode 2 and this component ofthe wind stressis reported by .htorrtt. et al.. [1998]. R

2 = 0.04). Tile complex spatial structure of the coastal

For the upcoast current data. CEOF modes I and 2 are current, its highly variable speed. and or well-developedthe only statistically significant modes. Mode I is not as stratification could be partly responsible for the low correla-dominant as in the downcoast flow. but it can explain 46t'i tion between the current and wind stress in summer.oftbe flow variance. while mode 2 accounts for 25% of thevariance. The amplitudes of these modes are smaller and A WIND-DRIVEN MODEL

the veering angles vary more (especially those of mode 2)than those of mode I estimated from tbe downcoast data. To further clarify the role of local wind stress as a majorThe angles of sutimer mode 2 imply two current reversals force controlling current variability within the LTCC offoccurring (a) offshore (between moorings A and C) and Cameron. LA. a wind-driven model, previously used suc-(b) near the coast (between moorings E and G) (Table 3). cessfully to study variability ofcurrents within the AmazonAdditionally. the spatial structure of the amplitudes and veer- River coastal plume [Lenw:. 1995]. is employed. This model Iing angles of mode I suggest that one of the current patterns assumes that current variability is caused solely by windin slnmmer could be a jet confined to the upper 10 to 14 m stress. Thus the input of energy from the wind into thie plumeof the water column. This conclusion agrees with summer layer is balanced only by a constant linear friction at the basecurrent observations collected on the Louisiana inner shelf ofthe plume and the temporal acceleration of the alongshoreand presented by VirrtO" et cal.. [1998]. The spatial structure flow. This alotngshore momentum balance is expressed asof the second mode could represent more complex summer followed:flow also described by VhtrraY et acl., [1998]. Their ADCP. alt ru r,CTD measurements, and SCULP drifters [.(ohnsotn and +- - (1)Niiler, 1994: Ohiutcnn et al., 2001] showed that in summer Dt h ph1994, there were three distinct zones in the region where our where ut is the alongshore current. p is the reference watermoorings were later deployed namely (a) a coherent eastward density. r is the linear friction coefficient. h is the plumeflow of waters with salinity of330 psu and higher, (b) an inte- thickness and r, is the alongshore wind stress. Integratingrior zone of slow. disorganized flow of intermediate salinity (1) in time and solving fir u yields

JAROSZ AND MURRAY 151

1-______________________ ________________ _______________________0.1_____________

(a)0.12

S0.08

S0.04-

C 0.00

o-0.04

j-0.00

10/02/96 10104 10/06 10/08 10/10 10112 10114 10/16

- .10-.,29'N -t Logtd Lo10ud

Plat 2.E)Diyao~hr vn testm eis(hnvrtcdlnsidct w ae:6ad9Otbr19) n

*_%ft B

152 VELOCITY AND TRANSPORT CHARACTERSTICS OF THE LOUISIANA-TEXAS COASTAL CURRENT

u(t) f exp(-r(t- t')h)dt'+u(t = exp(-rtlh) (2) observed between 3 and II October 1996, 22 October and4o ph November 1996, and 7 and 19 November 1996. Analoious

The alongshore wind stress component was computed conclusions are obtained for the other two moorings (moor-from the wind data measured at Freshwater Bayou using the ing C: R2 = 0.58; mooring B: R2 = 0.58). These resultsapproach proposed by Large and Pond [1981]. As in Lentz reinforce the earlier conclusion that alongshore wind stress,[1995], the linear friction coefficient was chosen to yield the which alone explains at least 58% of the current variance, isbest agreement with the observations. For the downcoast a dominant driving force of the current variability observedflow regime, the alongshore current component was cal- within the LTCC in the downcoast flow regime.culated from (2) at three different mooring locations (B, C When the wind stress observed in the summer regime isand F), and then compared with the observations collected applied to (2), amplitudes of the alongshore velocities areby the top meters at each locations. The best agreement poorly modeled for almost entire June but they are quite wellbetween the wind-driven alongshore velocities based on (2) reproduced for July and August 1996. The model estimatesand the observed velocities is found at mooring F (Figure are again compared with subsurface current observations at4a), with h = 12 m and the reference density of 1021 kg/in3. three different locations (moorings A, C, and G). The bestWe used h equal to the total water depth in moorings F and agreement is found at mooring G (Figure 4b), with a squaredG (data from mooring G were used fbr the upcoast regime correlation coefficient of 0.37 between the observed along-computations presented in the next paragraph) because the shore velocities and estimates, with r = 0.00035 m/s, h = 10historical hydrographic data suggested that the entire water m, and the reference density of 1010 kg/m3. It is obvious that

column near these mooring locations was filled out with light major velocity fluctuations in July and August, particularlywaters (salinity less then 30 psu). The squared correlation events observed between I and 6 August are wind driven.coefficient between the observations and predictions with r However, the predicted variability in June. still in transition= 0.0003 m/s is 0.66 (all correlations presented in this sec- from downcoast to upcoast dynamics, is different from thetion are significant at the 95% confidence level computed observations, with amplitude differences reaching even 30from the algorithm proposed by Sciremamniao [1979]). cm/s. which is quite large because lower speeds are usuallyVariability of the predicted wind-driven alongshore current measured within the LTCC in summer. Similar conclusions

is very similar to that observed at F. for instance, events are found when predictions are compared with subsurface

40o (a) /

20 -0

S09126196 10116 11/05 11125S 12115 0 1104/97

-40 - •

a.o0

-4 0 - Observations '

CL-60-•-_.M d!Rsu?

•0 i i80 1 l l . . . . . . . . .. . . . . .I.. . . . . . . . I.. . .. . . . . . . : . . . . . I . .

06I01196 0611 06/21 07/01 07111 0721 07131 0010

TimeFigure 4. (a) The alongzshore 6.5m current observed at mlooring F du risg the downcoast flow regime and I b) the along-shore 5.5Sm current observed at mooring Gi during the upenast flow regime compared to the model results ( equation 21"positive values indicate upcoast velocities. wlhile negatiye values indicate downcoast velocities.

JAROSZ AND MURRAY 153

currents at moorings C (R2 = 0.34) and A (R2

= 0.32). These meter. The data are then interpolated onto a grid (I-m depthresults suggest that alongshore wind stress, explaining by by 2-km cross-shore distance) at each hour, using linearitself as much as 37% of the current variance, is an impor- interpolation. A rigorous definition of the offshore extent

it driving force of the coastal current in the upcoast flow of the coastal current is obviously difficult because of itsr:gime. The unexplained variance of the currents implies inherent temporal variability; however, the limited salinitythat other driving forces, such as barotropic and baroclinic data recovered from the moorings suggest the presence ofpressure gradients, could be also important in summer, coastal plume water across the section even as far out as

The friction coefficients used for the simulations of the mooring A. Thus we make an operational definition of theLTCC currents are in the range of bottom friction coefficients coastal current transport by integrating across the sectionestimated for the Louisiana-Texas inner shelf by Chuang and between moorings A and G, recognizing that this estimate isWisenian [1983]. For both flow regimes, these friction coef- a lower bound. It does, however, have the advantage of beingficients are an order of magnitude larger than those utilized consistent with the ship-mounted ADCP transport estimates

r the Amazon plume (r = 0.00002 m/s) [Lentz, 1995], and of Murray et al. [1998].twis magnitude difference is mainly due to the vertical size The alongshore transport through the section during theof the Amazon plume and Mississippi-Atchafalaya outflow at downcoast regime of 1996-1997 is shown in Figure 5a.their respective locations. In the case ofthe Louisiana inner Note the persistent downcoast transport of-0.1 to -0.15 Svshelf, the low salinity waters at moorings F and G extend with a record length mean of-0.06 Sv (a root-mean-squaredfrom the surface to the bottom, and the friction coefficient error of 0.02 Sv). Significant bursts of elevated downcoastis simply the bottom friction coefficient, while it is the inter- transport occur on 6 October. 17 November, and 8 and 13layer friction coefficient for the Amazon plume. January. We will examine the spatial characteristics of some

of these bursts later. The corresponding transport duringCROSS-SECTIONAL TRANSPORT the preceding upcoast regime is shown in Figure 5b. As

expected, regional forcing conditions in the summer of 1996In order to calculate the transport within the LTCC through cause the coastal current to reverse directions, but it shifts

the section off Cameron, we first produce a time series of the predominantly upcoast by 9 June and remains upcoast forlow-passed alongshore current component at each current most of the summer. Note. however, the temporal variability

"0. .................................... .........

08126196 10116 11105 11125 12/15 01/040.02

-02

-0.4

-0.6

-0.8 '..... ........ n.....i.....t.....I.. .. i.....i,

06/01/96 06/11 06/21 07/01 0711/ 0721 07131 0810

TimeFigure 5. Transport in the LTCC daring (a) the downcoast flow regime and (h) upcoast flow regime: positive valuesindicate thle upcoast transport, while negative values indicate dowvncoast transport.

154 VELOCITY AND TRANSPORT CHARACTERSTICS OF TUE LOUISIANA-TEXAS COASTAL CURRENT

in the transport. These transport fluctuations, which rang-e and Freeport for both seasons, and buoyancy forcing is rep-from slightly below 010o maximums of 0.1510o0.23 Sv, occur resented by Atchafalaya River discharge.roughly on a 5-day time scale. The record length mean for Figure 6 shows multiple and partial coherence over thethis period is 0.05 Sv with a root-mean-squared error of 0.03 0.05 to 0.6 cpd frequency band between the transport andSv. Our mean seasonal transport results are in surprisingly possible driving mechanisms. For the fall/winter observation Vgood agreement with results from a high-resolution (a 1/200 period (Figure 6a), 78% to 96% of the total transport fluctua-grid in latitude and longitude) NCOM model of the Gulf of tions. shown by the multiple coherence curve, are explainedMexico driven with climatological monthly surface fluxes by the four forcing variables. Looking at individual partial[Zam'ola-Hidolgo et al., 2003, Figure 2A]. The model results coherences, the alongshore wind stress clearly accounts forfor a section close to our observations, integrated from the a majority (at least 65%) of the transport variance. There is25 mn isobath to the coast, show a 10-month transport of the also an indication that the cross-shore wind stress might bedowncoast flow reaching -0.1 Sv and a 2-month transport of considered a significant forcing for the transport fluctuationsthe sumnmer upcoast flow reaching approximately 0.04 Sv. with periods betwveen 3 and 5 days. Partial coherences of the

Thle most energetic transport oscillations during the fall. alongshore sea-surface slope and river discharge with thewinter period appear closely related to direct wind driving. downcoast transport are rarely statistically significant, thus -

as seen during the event between 2 and I I October (Figure their importance as possible sources of subtidial transport5al. Ten days of northeasterly winds exert downcoast wind fluctuations is extremely small. if any.stresses that produce downcoast Current speeds in excess of In the summer flow regime. 60% to 92% of the transport30 cmn/s across the entire section with two zones ofintensifi- fluctuations ( Figure 6b) are accounted for by the four pot-cation (Plates lb and 2b). A high-speed current (speeds of6O sible drivitng forces over the entire rrequency band examined,cm/s) occupies the outer (southern) edge of the section. and During this regime, the transport fluctuations are noi onlya second jet, nearly as strong. occupies the inshore (north- coherent wiih the alongshore wind siress. but also ss thern) end of the section. Previous data taken during LATEX the alongshore sea-suirface slope at frequencies lower thanB cruises [.1luirom c el(i., 1998] suggest that intense verti- 0.45 cpd. In addition, the cross-shore wind stress could becal mixing. combined with zones of intensified horizontal an important influence onl the transport fluctuations wvithdensity gradients (fronts). are associated with these jets in periods between -5 and 10 days. but its importance is clearlythe coastal current. The cessation of the northeasterly winds secondary compared to the alongshore wind stress and sea-drivitig the October 6 intensification of the downcoast flowv Surface slope. Partial coherence between the tranisport andevent is followed by an upcoast flow event, buoyancy forcing represented by the Atchafalaya discharge

The time series of transport during, the summer regime Onily appears saitalysignificant f~or frequencies loverI Figuire 5b) is similarly characterized by fluctuations with than 1.2 cpd. w\ith the percentage of variance expilained byseveral days to wveek time scales. The strongest upcoast this possible driving force reaching about 75`; at 0.1lcpd.intensification. centered onl 23 July. is notable for its rathermodest current speeds of2O cims observed along the mnoor- SUMMARY AND CONCLUSIONSing array. These witnd driven upcoast currents are upwelling,favorabl'e, leading to offsshore transport in the surface layer Temporal and spatial variability Of the Currents and vol-~and a consequent wveakening of cross-shiore density gradients utim transport of the Loulisianla-Texas Coastal Current wvere .

atid reduction ofbaroclinic alongshore velocities, Studied with data from a cross-shore transect located south ~of Cameron. Louisiana. In add it ion to the enurrent ninter

DRIVING FORCES OF TI-I LTCC data, subsurface pressure. wvind. and the Atchafalaiva Riverdischarge were also analyzed to better understand the local

To seek further insight into dynamical relationships con- behavior of the LTCC onl the wvestern Louisiana inner shelftrolling the transport Observed along the transect, we exam- Two subsets of the data. one for the uIPCOaSt ( summer) andtied multiple and partial coherence between thle transport -.mothier for the dovnmcoast (fall~winterl flow regimes. were

and four forcing mechanisms: (a) alongshore wvind stress. analyzed to determine whether there were any differences in(b) cross-shore wvind stress. (cl alongshore bttrotropic pre- the current characteristics atid transport betwveen these two4Sure gradient (a sea-surface slope), and Id) buoyancy Forcing regimies of the LTCC.proxied by the river discharge as in tihm'incowm find Galminie In the fall/winter season, the fluctu1ations of the crronts[1993]. Wind stress wvas computed from the amnemometer at amid transport are highly dependent onl the wviind forcing.Fresh Water Bayou. The sea-surface slope was calculated The overall meain flow is downcoast (wvestward) as expected *

fr oili sbsufaIce pressure gauges located at Oyster Bayou for this part of a year: however, thle rotating winds assoet-

JAROSZ AND MURRAY 155

(a) (b)

0.8

~0.

0.2 .'

/ ]-' -. - ---0

0.0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.1 0.2 0.3 0.4 0.5 0.6

Frequency (cpd) Frequency (cpd)Figure 6. Multiple (thick line) and partial coherence (symbol lines) between the transport observed during- (a) thedowncoast regime and (hi the upcoast regime and alongshore wind stress (solid line with circles), cross-shore windstress (dash-dot line), alonushore sea-surface slope (dashsed line), and tlse Atchsafalaya River disclsare (solid litne wvithdiamonds); the lower straiglt solid line is the 9P5% significatnce level.

d willh cold fronts do tetmporarily reverse its direction to season. in addition to the alongshore wind stress. the alon,-,upenast (eastwardl) for (8-36 hours. The doxvncoast flow is shore sea-surface slope was also an important drivinu forcing

clerl poarzedinth alnghor dretio. ighcorea- of the LTCC. which aerees with the conclusions reached bylions in thte vertical and large cross-shore spatial length scale .(iorrqov cil. [(998].of the observed currents as well as the spatial structure ofthe amplitudes and veeringage fiedoiatvlct :-lkiroivlcdntents. This study was funded by the Minerals Mats-

*1' CEOF mode (mode 1) strongly sugs ute-intensified agemient Service under contract 14-35-l)Ol-3(1724 awarded to Lost)-jet-like response to the wind forcing. Based on (sigh correla- siana State Utsiversitv.l ion of the currents and transport with the alongshore wind

ress, we conclude that this wvind stress component is the REFERENCES~' principal driving mechanism of the LTCC off Cameron. LA

during the downcoast flow, which is in agreement withs the Cho,. K.. R. 5). Rteid. and WI. D. Nox" in Jr. (194)8). Objecriseir mappedIi ning of the i nestealos [sotaL'Ial..(98: Lolt~tc' streamt Function lIeldon ithe Texas-Losiisiatsa siseithased on 32 moored

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and boths currents and their transport is wveaker. Thtus We Gutidrrez de Velasco (3.. tud C. 0. XWitsatt 09(l9,j. Seasonal pattertts of'Conclude that other forcing mechanisms such ats barotropic svind stress and wnind stress cutrl over the Guttifof Ntexico.d.. Geolhv~s.

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156 VELOCITY AND TRANSPORT CHARACTERSTICS OF THE LOU ISIANA-TEXAS COASTAL CURRENT

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