Research papers

Spatial variability of flow over a river-influenced inner shelf in coastalAlabama during spring

Brian Dzwonkowski a,n, Kyeong Park b, Jungwoo Lee b,1, Bret M. Webb c,Arnoldo Valle-Levinson d

a School of Marine Sciences, University of Maine, Orono, ME 04469, USAb Department of Marine Sciences, University of South Alabama, Dauphin Island Sea Lab, Dauphin Island, AL 36528, USAc Department of Civil Engineering, University of South Alabama, Mobile, AL 36688, USAd Department of Civil and Coastal Engineering, University of Florida, Gainesville, FL 32611, USA

a r t i c l e i n f o

Article history:Received 8 July 2013Received in revised form26 November 2013Accepted 9 December 2013Available online 19 December 2013

Keywords:Velocity structureDischargeCurrent asymmetryCoastal currentAlabamaGulf of Mexico

a b s t r a c t

Spring-time water column velocity data in 2011 and density data from a series of spring-time hydro-graphic surveys from 2008 to 2011 were used to examine the spatial variability of the circulation over theinner shelf of the Mississippi Bight off Mobile Bay. Spring-time depth-averaged currents were eastwardat all sites, but the vertical profiles were different. East of Mobile Bay the along-shelf flow was eastward,with an offshore component at the surface and an onshore component at depth, indicative of upwellingcirculation. West of Mobile Bay the along-shelf flow was also eastward, with a characteristic region ofnegative vertical shear in the upper layer of the water column. The deeper site had an across-shelf flowstructure similar to the east sites, while the shallower site exhibited onshore flow throughout the watercolumn. These spatial differences are attributed, in part, to the seasonally averaged effects of local windforcing and discharge. In terms of wind forcing, the depth-averaged along-shelf current responded to along-shelf wind asymmetrically in favor of upwelling (more transport in upwelling than during downwelling).Thus, weak seasonal downwelling favorable wind conditions did not inhibit the velocity profiles from havingupwelling circulation. West of Mobile Bay, negative vertical shears in the upper portion of the velocityprofiles were consistent with the influence of freshwater discharge. This freshwater influence is supportedwith available chlorophyll-a data (as a freshwater proxy), which showed an enhanced freshwater influencewest of Mobile Bay. In addition, across-shelf density data showed a shallow lens of freshwater west of MobileBay. These findings have implications for understanding the transport of river-derived nutrients on theMississippi–Alabama shelf.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Transport and circulation in the coastal zone are affected byspatial variations in the velocity field that are derived from a rangeof physical processes (e.g. land-induced changes of the wind field,and magnitudes and locations of discharge sources) as well as thephysical geometry of the coastal system (e.g. coastline orientationand bathymetric irregularities). Density gradients, in particular,can produce spatial variability in inner shelf currents. For example,Tilburg and Garvine (2003) found the along and across-shelfspatial scales associated with a buoyancy intrusion on the NewJersey shelf to be on the order of 20 km and 3–5 km, respectively.Other examples of the impacts of density gradients on spatial

structure of shelf circulation abound and result from a number ofprocesses such as wind-driven across-shelf advection in boundarylayers (e.g. Li and Weisberg, 1999a, 1999b; Weisberg et al., 2000;Kirincich et al., 2005), differential heating/cooling of water across asloping shelf (e.g. He andWeisberg, 2002, Weisberg et al., 2005; Lentz,2008a), and/or buoyancy discharge from riverine/estuarine sources(e.g. Chao, 1988; Chant et al., 2008). In the last case, pulses of fresherwater often form buoyant coastal currents that flow down-shelf(in the direction of Kelvin-wave propagation) adjacent to the coastdue to the influence of Coriolis force (Whitney and Garvine, 2005).In regions heavily influenced by riverine and estuarine discharge,such as the northern Gulf of Mexico, these buoyant coastal currentsare expected to have a prominent role in controlling the transportof material on the shelf (Wiseman and Garvine, 1995; Walker et al.,2005; Chant et al., 2008; Pringle et al., 2011). While the seasonaleffects of buoyant discharge plumes are often concentrated in thesurface layer of the water column, near-shore mixing of freshwater canlead to the development of an across-shelf density gradient affectingflow throughout the water column (e.g. Lentz, 2008b).

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Continental Shelf Research

0278-4343/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.csr.2013.12.005

n Corresponding author. Tel.: þ1 207 581 4391; fax: þ1 207 581 4990.E-mail address: briandz@maine.edu (B. Dzwonkowski).1 Present address: Environmental Engineering Research Division, Water

Resources & Environmental Research Department, Korea Institute of ConstructionTechnology, Korea.

Continental Shelf Research 74 (2014) 25–34

Given the multiple sources of freshwater contributing tothe seasonal freshening of the hydrographic conditions in theMississippi Bight (Morey et al., 2003; Dzwonkowski et al., 2011a),the spatial structure of the flow field on the inner to mid-shelfregion is likely complex. Most studies on shelf circulation in theMississippi Bight, the region between the Mississippi Delta andApalachicola, Florida, demonstrate an eastward flow during thespring season (He and Weisberg, 2002; Smith and Jacobs, 2005;Dzwonkowski and Park, 2010, 2012). This eastward flow wasconsistent with the pathway of a substantial portion of the surfaceoil during the Deep Water Horizon disaster in April–August 2010.However, eastward along-shelf flow is contrary to inner shelfforcing conditions expected to drive flow toward the west. That is,wind conditions during the spring season are predominantly fromthe southeast and discharge from river and estuary systems ismaximum during late winter and throughout spring; a 35-yr long(1976–2011) mean daily discharge from the Mobile River system is2656 m3 s�1 during spring in comparison to 816 m3 s�1 duringfall. Both wind and river discharge, together with Coriolis accel-eration, would dictate development of westward flow, particularlyin the shallower inner shelf. Spring westward flow over the shelfbetween 881 and 861W was shown with surface drifters (Golubevand Hsueh, 2002; Ohlmann and Niiler, 2005), despite this regionlying to the east of the largest freshwater sources in the Mis-sissippi Bight. However, these previous studies had limited data onthe inner to mid-shelf of the Mississippi Bight and/or have beenlimited in their spatial coverage.

The spatial variability in circulation is a particularly importanttopic due to the marine ecosystem in the northern Gulf of Mexico,as well as other river-influenced systems globally, being sustainedthrough terrestrial nutrients delivered by freshwater dischargeand its resulting distribution (Lohrenz et al., 1997; Delvin andBrodie, 2005; McPhee-Shaw et al., 2007). To address this issue,a study was conducted in the spring of 2011 (during the period ofpeak discharge) examining the flow structure on the Alabama shelfto the east and west of the mouth of Mobile Bay (Fig. 1),a primary source of freshwater discharge to the region. This studybuilds on recent work examining the velocity structure on the innershelf on the Alabama coast which has focused primarily on data froma long-term (7 years) time series of water column velocity andhydrographic measurements on the 20 m isobath (Dzwonkowski andPark, 2010, 2012; Dzwonkowski et al., 2011b). By collecting velocitydata at three additional sites in conjunction with hydrographic data

from an across-shelf survey line and satellite ocean color data, thisstudy provides new insight into the spring seasonal circulation. Thistopic is of particular interest in light of the Deep Water Horizon oilspill disaster (and continued industry activities), which demonstratedthat the understanding of inner shelf circulation is inadequate forresource and disaster managers to properly protect sensitive coastalregions. For example, a previous study shows that, depending on theday, there could be considerable differences in the oil locationforecasts between different estimate methods, particularly in thenear-shore regions (figure 1 in Mariano et al. 2011).

2. Data and methods

2.1. Data sources

During the spring (February 11– June 3) of 2011, water columnvelocity data were obtained from four sites on the inner shelf(Fig. 1), including an existing site with an RD Instruments 600 kHzADCP (site CP) and three sites with 600 kHz Nortek acoustic waveand current profilers (AWACs) (sites w12, e12 and e19). Site CP,maintained since 2005 as a component of the Fisheries Oceanogra-phy in Coastal Alabama (FOCAL) program, is located on the 20 misobath approximately 20 km southwest of the mouth of MobileBay, the 4th largest freshwater discharge source in the continentalU.S. (Schroeder, 1978). The three AWACs were deployed inshore andto the east of site CP on approximately 12.8 and 19.5 m isobaths, inorder to examine the relative importance of discharge on circulationeast and west of the bay mouth. The AWACs were programmed tocollect only velocity data due to the relatively long duration ofdeployment. The velocity data were collected with 5-min averagesevery 20 min at 0.5 m vertical bins (i.e. velocity data at a verticalinterval of 0.5 m), which were subsequently hourly averaged. CTDsurveys were conducted along a transect, also as a part of the FOCALprogram, to obtain hydrographic data from inside Mobile Bay out tothe 35 m isobath, approximately 50 km offshore (Fig. 1). At eachcast, the CTD was soaked for �1 min at a depth of �2 m, and then,to capture the near-surface density structure, raised until the topone-third of it was out of the water before starting the down-cast.This typically provided data as shallow as 0.5 m deep. During thespring, four surveys were conducted in 2011 on February 15, March15, April 13, and May 10, and data from these four surveys were

Fig. 1. Study area in the Mississippi Bight off– the Alabama coast showing the spring mean wind conditions (blue dashed arrows) and depth-averaged currents (black solidarrows); the top (bottom) reference arrow is for the wind (current) vectors. Wind data were obtained from two NDBC stations (∎) at station DPIA1 on Dauphin Island (DI) andstation 42012 offshore of Orange Beach, AL (OB). Current velocity data were obtained from 4 sites at CP, w12, e19, and e12. The CTD cast locations on the hydrographicsurveys (�) beginning just south of the Mobile Bay mouth (T09) and extending to the 35 m isobath (T35). The coloration represents the seasonally averaged chlorophyll-adata for the spring 2011 from Aqua MODIS mapped with a 4-km spatial resolution (http://oceancolor.gsfc.nasa.gov/). The inset shows the study site (black box) in relation tothe northern Gulf of Mexico. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

B. Dzwonkowski et al. / Continental Shelf Research 74 (2014) 25–3426

supplemented with those from eight additional surveys fromprevious spring seasons in 2008–2010.

Data for wind and MODIS Aqua chlorophyll-a were also used.Hourly wind data during the study period were collected from theNOAA National Data Buoy Center (NDBC) station 42012 offshore ofOrange Beach and station DPIA1 on Dauphin Island, AL (Fig. 1),from which wind stress was calculated using the method of Largeand Pond (1981). Additionally, freshwater discharge for the MobileRiver system was obtained at two U.S. Geological Survey gaugingstations for the Alabama River (31132048″N, 87130045″W: USGS02428400) and Tombigbee River (31145025″N, 88107030″W: USGS02469761). Their sum is used as a total freshwater discharge intoMobile Bay, following Park et al. (2007).

2.2. Data processing and analysis

The current and forcing time series data were generally con-tinuous and any short gaps of 12 h or less were filled using linearinterpolation. However, velocity data from site CP had a data gapfrom May 29, 2011 till the end of the study period. With theexception of daily freshwater discharge, a low pass 40-h Lanczosfilter was used to isolate the subtidal processes in the data.

In processing velocity data, several near-surface data bins wereexcluded from the analysis to avoid side-lobe contamination andpotential biasing due to wave effects, i.e. the effects due to thelowered water surface during the passage of wave troughs.Specifically, 4 bins at the inner sites (w12 and e12) and 6–7 binsat the outer sites (CP and e19) were excluded by assuming that adistance of approximately 15% of the water depth is lost as a resultof these effects. Different deployment frames of the AWACs andADCP resulted in slightly different bottom-most bin heights,1.5 and 2 m, respectively, and the bottom-most bin at site e19was removed due to poor data quality which resulted in the innersites collecting data between 1.5 and 10.5 m above the bottomwhile the outer sites collected data between 2–17 and 2–16 mabove the bottom, respectively.

Seasonal and depth-averaged currents were obtained followingShearman and Lentz (2003). The along and across-shelf axes weredetermined from the principal axis of the low-pass filtered depth-averaged data. The principal axes of the west sites (CP and w12 inFig. 1) were approximately oriented with the geographic north/south and east/west directions and those of the east sites (e12 ande19) were approximately 61 counterclockwise to that orientation.Standard error analysis, following Lentz (2008a), was used toquantify the uncertainty in the seasonal means. The along-shelf(across-shelf) uncertainty in the currents ranged from 1.5–2.5(0.8–1.2) cm s�1 at the surface to 0.6–0.9 (0.4–0.6) cm s�1 at thebottom. Correlations and scatterplots were used to examine therelationships between depth-averaged velocities at different sitelocations as well as between along-shelf wind fluctuations anddepth-averaged along-shelf velocity conditions. In particular, scat-terplots and the associated trend lines comparing the depth-averaged current response to wind forcing during upwelling anddownwelling events were used to assess any potential asymmetryin the local wind forcing response. Because of the limited numberof available data points at each site, the trends are provided toshow the general pattern as variations in the properties definingan ‘event’ will change the relative value of the slope and y-intercept to varying degrees. Velocity ‘events’ were obtained byidentifying a peak in along-shelf subtidal flow that exceeded5 cm s�1 and was separated from another event by at least thedecorrelation time scale of the velocity time series. A 5 cm s�1

threshold was used because this value was nearly as large as orlarger than the typical mean spring time velocity of 3–6 cm s�1,yet still allowed isolation of a reasonable number of wind-drivenupwelling and downwelling events (6–7 upwelling and 4–5

downwelling events at each site). Increasing the threshold valuefrom 5 to 10 cm s�1 altered the slopes and y-intercepts, but didnot change the results, i.e. upwelling events remained stronger ata given wind stress value. Using threshold values larger than10 cm s�1 resulted in too few events (2 downwelling events ateach offshore site) as well as a limited range of wind conditionsover which the events occurred, for example, no events withwinds less than 0.1 Pa at site e19. The time lag associated with thehighest correlation coefficient between the wind stress and thedepth-averaged current was used to select the wind stress thatcorresponded to a velocity event. The wind stress corresponding toan event was required to be greater than 0.005 (�0.005) Pa to beconsidered as a locally forced upwelling (downwelling) event;changing this threshold value did not qualitatively change theresults. The along-shelf wind stress at stations DI and OB was usedfor the west and east sites, respectively. Events associated withwind stress that did not meet the requirements were consideredto be forced by other processes such as remote wind forcing and/or river discharge. At site e19, a single outlier occurred, approxi-mately 0.01 Pa above the cutoff value (corresponding to approxi-mately 8% of the range of observed wind stress values) and wasexcluded from the trend line analysis. Including this event alteredthe slopes and y-intercepts, but did not change the results, i.e.upwelling events remained stronger at a given wind stress value.

The CTD data for temperature, salinity, and density wereinterpolated to a 0.5 m (vertical) by 1 km (across-shelf) planealong the transect from which seasonally averaged fields weregenerated. From the individual survey density fields, this studyexamined the thermal wind balance expressed as

∂u∂z

¼ gfρo

∂ρ∂y

ð1Þ

where ∂u/∂z is the vertical shear of the along-shelf velocity, g isgravity0s acceleration, f is the Coriolis parameter, ρo is a referencedensity, and ∂ρ/∂y is the across-shelf density gradient. The velocitystructures estimated using Eq. (1) with ∂ρ/∂y from each individualCTD survey data and a reference depth of zero at the bottom wereaveraged and compared with the observed velocity profiles duringthe time of the CTD survey. Dzwonkowski and Park (2012) demon-strated that thermal wind well represented the velocity shear at mid-depth during the stratified spring and summer of 2010 at site CP,consistent with the finding of others on other shallow stratifiedshelves (Lentz et al., 1999; Garvine, 2004). A point of concernwas thelimited number of CTD surveys (four times) available during thespring of 2011, all of which were conducted during generally light,variable wind conditions (Fig. 2). This may raise questions of theapplicability of estimates of shear from the thermal wind balance inderiving a seasonally averaged velocity profile. However, otherstudies have found that along-shelf circulation was well representedby the thermal wind balance on annual and seasonal time scales (e.g.Codiga, 2005; Lentz, 2008a, 2008b). Furthermore, the analysis wasstrengthened by use of data from eight additional surveys in 2008–2010 and by comparison with seasonal MODIS chlorophyll-a data,as discussed below.

The 12 CTD surveys during spring, four in 2011 and eight in2008–2010, were conducted over a range of discharge conditionsthat varied between approximately 700 and 7600 m3 s�1. Thisrange, typical of the spring season in the study area, providedsome confidence that the observed conditions were representativeof spring. In particular, the four surveys during the spring 2011spanned this range (Fig. 2a), with two surveys conducted duringlow discharge (600–800 m3 s�1), one at high discharge(7600 m3 s�1) and one at an immediate level (1400 m3 s�1),which suggests that the CTD data well represent spring hydro-graphic conditions. Furthermore, the micro-tidal nature of theAlabama shelf, with the tidal amplitude varying between �0.05

B. Dzwonkowski et al. / Continental Shelf Research 74 (2014) 25–34 27

and 0.40 m would be expected to have a limited impact on theobserved hydrographic conditions on the shelf around site CP.It should be noted that the CTD survey stations north of site CPwere not aligned exactly in the across-shelf direction but ratherapproached the site at an angle from the mouth of Mobile Bay(Fig. 1), which could result in some biasing of the estimatedacross-shelf density gradient. Using CTD data along this transectduring the spring and summer of 2010, Dzwonkowski and Park(2012) found that the thermal wind balance explained 67% of thevariance in the mid-depth velocity shear at site CP, suggesting thatthe across-shelf density gradient was approximated reasonablywith this transect.

3. Results and discussion

3.1. Intraseasonal variability

Time series data of spring river discharge, wind, and currentvelocities are shown for 2011 in Fig. 2. The discharge was fairlytypical of spring, having a few large pulses intermixed withperiods of below average discharge levels. The depth-averagedalong-shelf velocity from the four mooring sites showed a number

of downwelling (negative current) and upwelling (positive cur-rent) events in response to the wind forcing. The correlationsbetween the depth-averaged along-shelf velocity and along-shelfwind stress were between 0.55 and 0.63 with lags between 5 and23 h, and shallow sites had shorter lags than the correspond-ing deeper sites. Although the velocity fluctuations were usuallysimilar in magnitude (720 cm s�1), the associated downwellingwinds were typically stronger than upwelling winds. Focusing onspecific events where all four sites were acting in a similarmanner, the upwelling events (e.g. February 11 and March 25)had a stronger response to wind forcing than the downwellingevents (e.g. March 5 and 30). This was most clearly shown at theoffshore sites (CP and e19), where a scatterplot comparing thelagged wind stress to ‘event’ velocities showed that the upwellingevents tended to have stronger along-shelf velocities than down-welling events (Fig. 3).

There were a number of other events in along-shelf velocitythat were not associated with upwelling or downwelling winds(e.g. May 4), but occurred more frequently as the spring pro-gressed. Some of these other events were associated with across-shelf wind (e.g. April 30 and May 4), which could setup a pressuregradient against the Louisiana coastline; note that across-shelfwind for the Alabama coastline is along-shelf wind for the

Fig. 2. Time series data during the spring of 2011: river discharge (a), wind speed at stations DI and OB in Fig. 1(b), depth-averaged along-shelf current (c), and near-surfacealong-shelf current (d). The positive (negative) along-shelf wind and current is eastward (westward). The black vertical lines in (a) indicate the days of the CTD surveysshown in Fig. 4a. The symbols at the top of (c) indicate velocity ‘events’ for the CP site used in Fig. 3, and additional 75 cm s�1 horizontal lines are also included in (c).Filled symbols indicate upwelling (red) and downwelling (blue) events and open symbols indicate other events not associated with upwelling or downwelling wind. Velocityevents identified for the other sites are similar to those shown for CP. (For interpretation of the references to color in this figure legend, the reader is referred to the webversion of this article.)

B. Dzwonkowski et al. / Continental Shelf Research 74 (2014) 25–3428

Louisiana coastline. Assuming a balance between the barotropicalong-shelf pressure gradient and linear bottom stress,

gdηdx

¼ τbρoh

ð2Þ

τb ¼ ρoruda ð3Þ

where dη/dx is sea surface slope, τb is bottom shear stress, h iswater depth, r is resistance coefficient, and uda is depth-averaged

along-shelf velocity, estimates for pressure induced velocity fluc-tuation can be approximated. Values of barotropic pressure gra-dients of 3.3�10�6 m s�2 and a resistance coefficient (r) of3.5�10�4 m s�1 for the study area during spring/summer of2010 (Dzwonkowski and Park, 2012), yield estimates of udabetween 9 and 19 cm s�1 at depths of 10 and 20 m, respectively.These values are consistent with the observed velocity fluctuations(Fig. 2c).

Other events not associated with wind forcing may be relatedto additional mechanisms such as the influence of river discharge.Correlations between the Mobile Bay discharge and the depth-averaged along-shelf velocity at all sites did not demonstratemeaningful relationships. The near-surface currents (the upper-most bin at each site), where the effects of discharge might bemore pronounced, were not correlated with the Mobile Baydischarge either. However, as will be shown below (Section 3.2),this is due to the fact that local discharge from Mobile Bay entersthe shelf as a very shallow lens of freshwater. Discharge from otherregional sources such as the Mississippi River or other rivers (e.g.Pearl River, Pascagoula River, etc.) could affect the study regionthrough the development of larger scale pressure gradients.Dzwonkowski and Park (2010) speculated that this could be acontributing factor to the seasonal structure of along-shelf cur-rents. Furthermore, studies off the New Jersey shelf in summerfound that buoyancy discharge from an upshelf source led tocomplex features in the downshelf velocity field, approximately100 km from the source (e.g. Yankovshy et al., 2000; Tilburg andGarvine, 2003). The Escambia and Perdido Rivers with annualdischarge rates of 176 and 781 m3 s�1, respectively (http://gcoos.tamu.edu/products/data/riverdischarge/README_GOMRIVERS.txt)represent upshelf sources of discharge that may impact thepressure field of the study region as these rivers discharge intothe Mississippi Bight between Mobile Bay and Pensacola Bay.

Some spatial variability was observed between sites. Thedepth-averaged along-shelf velocity at the two outer sites (CPand e19) had the highest correlation (r¼0.88). However the

Fig. 3. Scatterplot of lagged along-shelf component of the wind stress and depth-averaged along-shelf velocity for upwelling (red) and downwelling (blue) events atCP (�) and e19 (▲).Open symbols indicate velocities 45 cm s�1 but associated withvery weak or inconsistent (relative to flow direction) wind conditions. The straightlines are trend lines for the upwelling (red) and downwelling (blue) events. Theopen symbols are not included in the trend. Note that the negative of the windstress is plotted for downwelling events. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Seasonal across-shelf density transect during the spring 2011 (a) and surface (1 m deep) density (∎), scaled salinity (þ) and satellite-derived chlorophyll-a (○) alongthe transect line (b). Note that (chlorophyll-a)�1 is plotted.

B. Dzwonkowski et al. / Continental Shelf Research 74 (2014) 25–34 29

correlations noticeably decreased between the inner sites (w12and e12) with an r-value of 0.57. The comparisons betweeninshore and offshore sites revealed correlations that decreasedtoward the west with site e19 and e12 having a stronger correla-tion (r¼0.65) than that of CP and w12 (r¼0.43). The decreasingcorrelations with distance to the coast were consistent withthe notion of decreasing spatial scales near the coast and inregions exposed to estuarine discharge, i.e. low correlationsbetween w12 and CP or e12. The spatial variability in flowbetween the shallow and deep sites, as well as between the eastand west sites, indicates the influence of small scale forcing, i.e.scales smaller than the separation between moorings, 20–50 km,in this region (Fig. 2c and d).

3.2. Seasonal conditions

The seasonal depth-averaged currents at all four locations wererelatively consistent, having predominately eastward flow with anonshore tendency, particularly at site w12 (Fig. 1). There was aslight weakening of the currents from west to east with the meanvelocities west of Mobile Bay being a few cm s�1 stronger thanthose to the east. The depth-averaged currents, even at themost shoreward sites, provided no evidence of a westwardcoastal current. Westward currents are typically expected to beassociated with lower density water derived from the coast asseen in other parts of the world. In addition, the eastward along-shelf flow was counter to the along-shelf easterly component ofthe winds.

Additional understanding of the flow structure can be gainedby examining the shelf density field. Information on the seasonalacross-shelf density structure was obtained directly from thehydrographic surveys. The seasonal density structure (Fig. 4a),

i.e. the average distribution using data from four CTD surveysduring the spring 2011, showed a shallow lens of buoyant waterextending offshore of site CP. This surface density structure wasassociated with low salinity water, and was well correlated to theinverse of the across-shelf structure of the chlorophyll-a data, i.e.low density was associated with high chlorophyll-a (Fig. 4b). Thisindicates that the seasonal chlorophyll-a map (Fig. 1) qualitativelyrepresented the region of freshwater influence. Assuming thefreshwater discharge was trapped in a very shallow surface layer(1–3 m) near the coast, as was the case west of Mobile Bay(Fig. 4a), it is not surprising that the depth-averaged velocitiesdid not capture the freshwater influence. However, the densitystructure has important implication on the vertical structure of thehorizontal velocities.

The seasonal mean velocity profiles (Fig. 5) revealed some subtle,yet appreciable, spatial structure among the four sites. There was anotable difference between the west and east site locations. At thewest sites, the along-shelf current at both sites had a subsurfaceeastward maximum, more pronounced at site CP than at site w12.On the other hand, the east sites had a vertically sheared along-shelfcurrent profile with the upper 60–70% of the water column directedeastward and the lower portion directed westward. Interestingly,the across-shelf structures at all sites, except w12, were generallyonshore at depth and very weakly offshore at the surface. Surpris-ingly, the spring mean velocity data did not show westward surfacecurrents typical of a coastal current. However the negative shear inthe upper layer of the water column at the west sites was indicativeof a shallow freshwater lens. Thus, the measurement capabilities ofthe ADCP/AWACs may limit the extent to which any coastal currentcan be observed in this study. The differences between deploymentlocations provide insight into the dynamics acting across the studyregion as discussed in the next section.

Fig. 5. Seasonal vertical structure of the along-shelf (a; positive eastward) and across-shelf (b; positive onshore) velocity at four sites, w12 (Δ), CP ( ), e12 (Δ), and e19 ( ).The velocity profiles are normalized by the site depth: 12.8 m for w12 and e12, 19.5 m for e19, and 20 m for CP. Also included in (a) are the along-shelf velocity profiles at siteCP derived from the thermal wind balance using the across-shelf density gradients from the spring of 2011 data (○) as well as the merged spring 2008–2011 data (þ).Note the difference in scales of the x-axis between (a) and (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version ofthis article.)

B. Dzwonkowski et al. / Continental Shelf Research 74 (2014) 25–3430

3.3. Forcing interactions

There are several features of the inner shelf circulation thatprovide an explanation for the observed spatial structure and sitevariability in the spring mean velocity patterns. At the east siteswhere the influence of discharge was expected to be limited relativeto the west sites, the seasonal velocity structure was indicative ofupwelling circulation despite a mean along-shelf wind componentthat was weakly downwelling favorable. This is illustrated in thealong and across-shelf profiles (Fig. 5) where there was verticallysheared eastward along-shelf flow (via thermal wind effects) withoffshore flow at the surface (v up to 1 cm s�1) and onshore flow atthe bottom (v¼2–3 cm s�1). Although the near-surface across-shelfvelocities were very close to (or below) their uncertainty measure-ments, the trends in the currents indicated that the current shouldbecome increasingly offshore at the surface.

A possible explanation for this behavior lies in an asymmetricresponse to along-shelf wind stress that favors upwelling circulation(Fig. 3). Previous work on the West Florida shelf showed anasymmetric response to wind forcing similar to our observations(e.g. Weisberg et al., 2001; Liu and Weisberg, 2005). Weisberg et al.(2001) provided an explanation for the observed asymmetry usingthe conceptual and analytical work of MacCready and Rhines (1991)and Garrett et al. (1993). They argued that stratification could limitacross-shelf transport in the bottom Ekman layer on the slopingbottom when the across-shelf baroclinic pressure gradient balancesthe Coriolis force. Specifically, as a stratified interior encounters asloping bottom, turbulence generates a mixed layer that will beuniform in the vertical but have a horizontal density gradient withlower density water closer to the coast. The resulting buoyancy forceacts toward the coast in the bottom boundary layer. During down-welling (upwelling) the resulting sea-level setup (setdown) drives anoffshore (onshore) pressure gradient force in the bottom Ekmanlayer. The two layer circulation associated with coastal Ekmancirculation leads to the bending of isopycnals into (away from) thebottom during downwelling (upwelling). The bending of isopycnalsduring downwelling events strengthens the buoyancy force that actscounter to the pressure gradient force limiting these events. Duringupwelling events fewer isopycnals bend toward the bottom, butthose that do still act constructively with the pressure gradientforcing (Weisberg et al., 2001). This generates an asymmetrybetween the upwelling and downwelling events. Garvine (2004)noted that the role of the interior density field (isopycnal bending) onthe buoyancy force in the bottom mixed layer becomes importantwhen the ratio of isopycnal slope (β) to bottom slope (α) is of O(1).The CTD data from four 2011 surveys had ratios (β/α) of O(1) asindicated by the sloping isopyncals in the lower layer of the watercolumn (Fig. 4a) as well as the spring ‘climatology’ based on all 12spring CTD surveys (not shown). Note that the lower layer isexpected to be similar on both the east and west sides of MobileBay because fresh water only affects a shallow lens of surface water.This mechanism is consistent with the previous work in the studyregion, which showed evidence of an asymmetric behavior in theacross-shelf circulation at site CP, with eastward upwelling windstress driving nearly twice the across-shelf velocity responseof downwelling wind during the stratified spring and summer(Dzwonkowski et al., 2011b).

The west sites were also influenced by the asymmetric response towind forcing. Similar to the east sites, the depth-average velocitieswere stronger during upwelling events (Fig. 3) and the direction of theshear between the east and west sites was consistent in the lowerhalf of the water column (Fig. 5), indicative of upwelling circulation.The effect of the upwelling circulation is evident in the seasonal CTDdata (Fig. 4a), with the lower portion of the water column havingisopycnals that sloped upward toward the coast, consistent withupwelling, despite a seasonally averaged easterly along-shelf wind

component that favors downwelling. In addition, the west sites wereinfluenced by discharge. The seasonally averaged surface chlorophyll-alevel was high west of Mobile Bay, particularly along the coastline,indicating the impact of discharge. The offshore extent of highchlorophyll-a steadily increased with distance to the west of Pensacola(881200W), consistent with the increasing number of freshwatersources westward from Pensacola, including Perdido River, MobileBay, Pascagoula River, Pearl River, and Lake Pontchatrain. The increasedfreshwater discharge impacts the surface as a thin density lens(Stumpf et al., 1993), in a manner consistent with a surface-advectedplume (Fig. 4a). As such, the impacts are locally concentrated at thesurface. This effect was evident in the velocity structure of the westsites with the weakening of eastward flow in the upper third of thewater column (Fig. 5a). Extending these trends to the surfacesuggested very weak currents and even possibly westward currents,provided that the trend in the velocity shear remains or increasesapproaching the surface.

This interpretation was independently assessed with the velo-city structure obtained from the across-shelf density gradientusing Eq. (1), assuming a reference velocity of zero at the bottom.While there are some clear differences between the two profiles(not surprising considering the limited number of available sur-veys), the density gradient-derived velocity structure does providesome potential insight into the general features at site CP (Fig. 5a).Both profiles show a positively sheared eastward flow in the lowerportion of the water column, a subsurface eastward maximum atmid-depth (between 0.5–0.6 normalized depth), and a negativelysheared current in the upper portion of the water column.Although the estimated density gradient-derived velocity indi-cated the presence of strong westward flow near the surface, themeasured velocity data only showed a change in the velocity shearof the upper layer. This difference might be attributable to thelimited number (four) of CTD surveys available during the springof 2011. That is, data from only four CTD surveys may not representthe seasonal density conditions, and the resulting velocity profilemay not be a robust representation of the seasonal mean struc-ture. Therefore, additional CTD survey data from other springseasons (2008–2010) have been included in the seasonal profilecalculation since the observed velocity structure at site CP wasconsistent with observations in previous spring seasons (2005,2006 and 2010) (Dzwonkowski and Park, 2010, 2012). The result-ing velocity profile derived from the thermal wind balance wasvery similar to the one for the spring 2011. This similarity certainlyconfirmed the robustness of the estimated velocity profile (Fig. 5a)and demonstrated that the mean of the four CTD surveys duringthe spring 2011 reasonably represented the mean spring condi-tions (see Section 2.2).

The profile, however, still indicated a westward surface flow.This difference between the estimated thermal wind balance (geos-trophic) and observed velocity profiles likely arose from the ageos-trophic contribution of the wind forcing that would be expected tooccur in the frictional surface Ekman layer. That is, the surfacecurrent will have a contribution from direct wind forcing throughthe frictional diffusivity of momentum as well as a contribution fromthe across-shelf density gradient. Additionally, the shallow depths ofthe study sites may lead to periods when there was interactionbetween the surface and bottom Ekman layers, which would lead tovertical shears not represented by velocity profiles derived from asimple thermal wind balance, e.g., high wind conditions when CTDsurveys were not conducted. An approximation for the frequency ofinteraction between boundary layers can be obtained for stratifiedconditions (Weatherly and Martin, 1978) modified for the surfaceEkman depth (δE, Lentz, 1992):

δE ¼1:3qnffiffiffiffiffiffi

f Np ð4Þ

B. Dzwonkowski et al. / Continental Shelf Research 74 (2014) 25–34 31

where qn is the friction velocity (¼τs/ρ)1/2, τs is wind stressmagnitude, ρ is average density, f is the Coriolis parameter, and Nis the buoyancy frequency. Using a seasonal surface to bottomdensity difference of approximately 6 kg m�3 at site CP (Fig. 4a), ρof 1023 kg m�3, and wind stress values from station DPIA1 (Fig. 1),the depth of the surface Ekman layer can be estimated. Based on thework of Mitchum and Clarke (1986) boundary layer overlap begins at3δE. Estimates at site CP indicated that the interaction betweenboundary layers would be expected to occur �9% of the periodobserved (i.e. δE exceeded 6.7 m �9% of the time). Although theapplicability of Eq. (4) has been questioned (Garvine 2004), particu-larly for areas influenced by discharge such as site CP, this estimatedemonstrated that ageostropic current derived from the boundarylayers would influence the entire water column at times during thespring.

Consequently, the observed seasonal flow structure at the westsites was the result of the superposition of the asymmetric response toupwelling favorable wind forcing and of surface advected dischargeplume events. The net effects of these synoptic processes can be seenin the seasonal across-shelf density structure (Fig. 4a); the lowerportion of the water column had isopycnals sloping upward towardthe coast, which is expected during upwelling circulation, while theupper portion of the density field had a lens of buoyant waterconsistent with the effect of surface advected discharge plumes.Dzwonkowski and Park (2010) hypothesized that the typical springvelocity structure could have resulted from the interaction of an along-shelf pressure gradient and the easterly wind component. However,the spatial locations of the velocity profiles and the available watercolumn density data in this study as well as insight from several recentafore-mentioned studies, provided an improved explanation to theobserved velocity structure. In particular, the observed spatial differ-ences between the sites east and west of Mobile Bay mouth wereunlikely to arise solely from the seasonal along-shelf pressure andwind field as these forcings typically have spatial scales larger than�45 km that separated the study sites. Furthermore, the surfacepressure fields over smaller spatial scales, due to the plumeoutflow, should exhibit a large gradient just offshore of MobileBay mouth, and thus cause the observed spatial variabilitybetween the stations west and east of the outflow.

3.4. Data limitations

The data from this study leave several unanswered questions.Recent work in the shallow inner shelf found that across-shelf windand wave-driven across-shelf transport are other sources of offshoreflow (Fewings et al., 2008; Lentz et al., 2008). In terms of transport,Fewings et al. (2008) found that under well mixed conditions the roleof across-shelf wind forcing increases with decreasing depth andbecomes more important than along-shelf wind for a water depth ofapproximately 10–30 m. The role of across-shelf wind in drivingacross-shelf transport was explored in Dzwonkowski et al. (2011b) atsite CP with earlier data. Across-shelf wind was of very limitedimportance in driving across-shelf transport at this site (depth of20 m) during stratified periods. The role of across-shelf winds at theshallower sites (12.8 m at w12 and e12) could not be clearly assessedwith the data available in this study, i.e. spring 2011. However, shouldthis mechanism be significant, the seasonal onshore wind compo-nent would increase onshore (offshore) flow in the surface (bottom)layer of the across-shelf velocity profiles. Given the similarity of theacross-shelf structure between the east sites (where the deep sitewas not expected to be impacted by this process), both havingoffshore flow at the surface (Fig. 5b), any effect on the shallower sitewas likely small. While site w12 did have onshore flow at the surface,across-shelf wind-driven flowwas not likely significant on account ofstronger stratification associated with Mobile Bay discharge. Riverdischarge would further limit the impact of across-shelf winds

relative to the east site. The onshore flow was likely related to thegeographic location of the site relative to the mouth of Mobile Bay asdiscussed later in this section.

In terms of wave-driven forcing, outside the surf-zone, offshoreflow is concentrated in the near-surface of the water column asfound by Fewings et al. (2008) and Lentz et al. (2008). To estimatethe potential impact of this transport mechanism, wave data fromthe NOAA NDBC station 42012 offshore of Orange Beach (Fig. 1)were used. Data from this location likely represent a high estimatefor the wave conditions associated with the study site as this buoyis in deeper water and closer to Desoto Canyon. A first orderestimate of the wave forced current (uw) at the shallow sites wasassessed using a compensating depth-averaged offshore flowderived from the Stokes transport (Lentz et al., 2008):

uw � � gh16c

Hsig

h

� �2

cos ðθwÞ ð5Þ

where Hsig is the significant wave height, c is the phase speed of thewaves, and θw is the wave direction relative to offshore. Assumingshallow water waves, propagating with speed cE(gh)1/2 directlyonshore (θw¼1801), and using the observed significant wave heightduring the study period (Hsig¼0.75 m) and the site depth(h¼12.8 m) gave uw smaller than 0.25 cm s�1. While this effectmay not always be negligible, this conservative estimate suggestedthat uw was relatively small during the spring of 2011. Furthermore,Kirincich et al. (2009) proposed that increased inner-shelf stratifica-tion would increase the vertical structure of the wind-drivencirculation relative to the wave-driven circulation off the Oregoninner shelf. The role of strong stratification on wave-forced across-shelf circulation remains unclear. However, wave-induced circulationmay be important during the winter/fall when stratification isreduced and wave conditions are larger in the study area.

The inshore sites of the study region exhibited other features.Assuming the seasonal chlorophyll-a distribution is a good proxy ofthe surface density field, site e12 would be expected to show signs ofwestward flow near the surface (Fig. 1). However, there were noindications of any near-surface flow reversal. This could reflect thefact that the qualitative relationship between density and chloro-phyll-a data observed in the region of Mobile Bay (Fig. 4a) may not beas representative for other parts of the Mississippi Bight, potentiallybecause of changes in suspended sediment types and amounts,changes in colored dissolved organic matter (CDOM) content,and/or other shallow water effects. Despite this potential limitation,the chlorophyll-a pattern is consistent with the expected spring/summer surface salinity in the Mississippi Bight (e.g. figure 7 inMorey et al., 2003). Thus, even without the density/chlorophyll-arelationship, the extent of the influence of river discharge during thespring season would not be expected to be qualitatively differentfrom that of the chlorophyll-a distribution.

In addition, site w12 stood out for having a distinctly differentacross-shelf flow in the upper portion of the water column (Fig. 5b).The onshore nature of this profile throughout the water column maysuggest this site could be influenced by the anticyclonic portion ofsurface advected plumes as the surface current would be expected torotate back toward the coast. The CTD monitoring data (unpublished)from the mouth of Mobile Bay provided an estimate of the plumedeformation radius (Ri) of approximately 6 km. This radius places sitee12 well within the theoretically estimated spreading distance of 4 Riin an inertial bugle region of a surface advected plume (Yankovsky andChapman, 1997). Although previous satellite-based studies of theMobile Bay plume (Dinnel et al., 1990; Stumpf et al., 1993) did notobserve the presence of a ‘bulge’ region, the limited number ofobservations used in these studies makes it entirely possible that thistype of plume behavior could be present under certain environmentalconditions (e.g. low wind conditions). Circulation associated with a

B. Dzwonkowski et al. / Continental Shelf Research 74 (2014) 25–3432

plume bulge is known to have complicated structures (Whitney andGarvine, 2005). An additional site further downstream from themouthof Mobile Bay (to the west) may be better to observe the expectedseasonal westward flow usually associated with the net effect ofdischarge plumes. Given the importance of the density field in thisregion and the relatively shallow nature of the discharge plumes,increased insight would certainly be gained using instrumentationthat better captures the very near-surface flow (i.e. the top portion ofthe water column not captured by acoustic Doppler profiling instru-ments), such as a standard range high frequency radar system, surfacedrifters, and downward looking acoustic Doppler profilers withadditional horizontal transducers (e.g. Nortek Aquadopp Z-Cell).

It is also important to keep in mind that while the seasonallyaveraged chlorophyll-a data give the impression of a continuousband of high chlorophyll concentration westward from Pensacola,this distribution is a net result of sporadic discharge pulses. Thesepulses occur over the course of the spring from multiple pointsources in the northern Gulf, which likely drive material westward ina very discontinuous manner and result in complex across-shelf flowpatterns. The geographic constraints of the system (i.e. a series ofbarrier islands) also present an important boundary condition that isnot well understood. Simultaneous drifter releases from several ofthe local outlets would provide important information on therelationships between outlets and the resultant flow pathways ofthese types of systems. These topics are beyond the scope of thiswork, but represent relevant areas of future research.

4. Conclusions

Velocity structure on the inner shelf of the Alabama coast showedmarked spatial variability in the seasonal along- and across-shelfvelocity as observed by four acoustic Doppler profiling instruments.At sites east of Mobile Bay (i.e. to the east of the primary source ofregional discharge), the vertical structure of the seasonal mean currentwas consistent with an asymmetric response to wind forcing thatfavors upwelling circulation despite the observed downwelling favor-able wind conditions. At sites west of Mobile Bay (i.e. to the west ofthe primary source of regional discharge), however, there was adistinctive change in the upper water column velocity structure withthe flow trending toward the west. The velocity structure at thewestern sites was related to both freshwater discharge and to anasymmetric response to wind forcing. Effects of freshwater dischargewere indicated by negative vertical shears in the upper third of thewater column. The asymmetric response to wind forcing was repre-sented by the positive vertical shear in the lower portion of the watercolumn. The presence of lighter water in a very shallow lens offshoreof Mobile Bay was confirmed with CTD data. The resulting geostrophicvelocity estimates (via thermal wind balance) were consistent withthe main features of the observed velocity profile at site CP. Theasymmetric response to wind forcing is hypothesized to be a result ofincreased buoyancy forcing in the bottom boundary layer due toisopycnal bending that preferentially limits across-shelf transport inthe bottom Ekman layer during downwelling (relative to upwelling),which in turn reduces the intensity of along-shelf flow, in accordancewith Weisberg et al. (2001). Extrapolating (up to the surface) thenegative shear in the upper layer of the west sites resulted in surfacecurrents that would limit the eastward spread of freshwater on theinner shelf, as was evident in the seasonal chlorophyll-a data. Thefindings of this study show that the balance between discharge eventsand an asymmetric response to upwelling favorable winds plays acritical role in driving the mean structure and spatial variability of thecurrents on the inner shelf. Furthermore, the study notes that missingthe upper 10–15% of the water column, typical of acoustic Dopplerprofiling instruments, represents a critical undersampling of the watercolumn in highly stratified regions of the coastal ocean.

Acknowledgments

This work would not have been possible without the help ofthe Tech Support Group at the Dauphin Island Sea Lab, includingKyle Weis, Roxanne Robertson, Alan Gunter, Mike Dardeau, andLaura Linn. In addition, we would like to thank Mimi Tzeng for thepreliminary processing of the data. We would also like to thankthe Ocean Biology Processing Group (Code 614.2) at the GSFC,Greenbelt, MD 20771, for the production and distribution of theocean color data. Finally, we thank Rich Pawlowicz at the Uni-versity of British Columbia for the freely available MATLAB m_maptoolbox. This work was supported by the Gulf of Mexico ResearchInitiative (GoMRI) Rapid Response and the Fisheries Oceanographyin Coastal Alabama funded by Alabama Department of Conserva-tion and Natural Resources - Marine Resources Division.

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