Subtidal circulation on the Alabama shelf during the Deepwater Horizon oil spill

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2011jc007664, 2012 Subtidal circulation on the Alabama shelf during the Deepwater Horizon oil spill Brian Dzwonkowski 1 and Kyeong Park 2 Received 7 October 2011; revised 16 January 2012; accepted 27 January 2012; published 17 March [1] Water column velocity and hydrographic measurements on the inner Alabama shelf are used to examine the flow field and its forcing dynamics during the Deepwater Horizon oil spill disaster in the spring and summer of Comparison between two sites provides insight into the flow variability and dynamics of a shallow, highly stratified shelf in the presence of complicating geographic and bathymetric features. Seasonal currents reveal a convergent flow with strong, highly sheared offshore flow near a submarine bank just outside of Mobile Bay. At synoptic time scales, the flow is relatively consistent with typical characteristics of wind-driven Ekman coastal circulation. Analysis of the depth-averaged along-shelf momentum balance indicates that both bottom stress and along-shelf pressure gradient act to counter wind stress. As a consequence of the along-shelf pressure gradient and thermal wind shear, flow reversals in the bottom currents can occur during periods of transitional winds. Despite the relatively short distance between the two sites (14 km), significant spatial variability is observed. This spatial variability is argued to be a result of local variations in the bathymetry and density field as the study region encompasses a submarine bank near the mouth of a major freshwater source. Given the physical parameters of the system, along-shelf flow in this region would be expected to separate from the local isobaths, generating a mean offshore flow. The local, highly variable density field is expected to be, in part, responsible for the differences in the vertical variability in the current profiles. Citation: Dzwonkowski, B., and K. Park (2012), Subtidal circulation on the Alabama shelf during the Deepwater Horizon oil spill, J. Geophys. Res., 117,, doi: /2011jc Introduction [2] Coastal circulation is a critical component for many biogeochemical processes in marine ecosystems. For example, regional primary productivity on continental shelves is often dependent on the delivery of nutrients into the euphotic zone through physical processes such as coastal upwelling [Barth et al., 2007] or gravity currents associated with river discharge [Chant et al., 2008]. Alternatively, physical processes can amplify or mitigate the coastal impacts of anthropogenic pollution released or transported into the coastal zone [Morey et al., 2005]. [3] While a detailed understanding of many aspects of coastal circulation exists, the temporal and spatial variability of coastal forcing functions and their interaction with each other as well as the system geography, results in unresolved complexities in flow fields. To this extent, many studies have focused on the role of wind stress in driving coastal transport as this is a primary forcing function on continental shelves [e.g., Huyer, 1983; Brink, 1991; Hickey, 1998]. 1 Dauphin Island Sea Lab, Dauphin Island, Alabama, USA. 2 Department of Marine Sciences, University of South Alabama, Dauphin Island Sea Lab, Dauphin Island, Alabama, USA. Copyright 2012 by the American Geophysical Union /12/2011JC Simple conceptual models of wind-driven coastal circulation typically invoke a two-dimensional picture involving Ekman dynamics where along-shelf wind drives net surface transport resulting in an across-shelf pressure gradient which drives along-shelf flow in the direction of the wind and a subsequent return flow at depth. However, deviations from this conceptual model have been observed, and understanding the factors that cause deviations is critical to determining the transport and fate of material in the coastal zone. [4] Studies of wind-driven circulation have demonstrated that a number of processes and conditions can generate highly variable three-dimensional flows in the coastal zone. As stated by Chant et al. [2004], along-shelf variability can be generated by coastal promontories [Kosro, 1987], underlying bathymetry [Glenn et al., 1996; Song et al., 2001; Castelao and Barth, 2006] and flow instabilities resulting from internal dynamics [Barth, 1994]. Buoyancy currents can also drive increases in along-shelf flow at their leading edge [Yankovsky and Garvine, 1998; Lentz et al., 2003] and can often result in complex mesoscale flow features [Yankovsky et al., 2000].In fact, only a difference in surface and bottom stress at any given location is required to drive along-shelf divergence [Tilburg and Garvine, 2003]. [5] As demonstrated by the transport and dispersion of surface oil during the Deepwater Horizon (DWH) disaster in the spring and summer of 2010, the coastal circulation in the 1of15

2 Figure 1. The study area off the Alabama coast with the mean seasonal wind conditions and depthaveraged currents. The spring (summer) vectors are indicated by black (red) arrows. The wind data are obtained from two NDBC stations (black squares) at DPIA1 on Dauphin Island (DI) and offshore of Orange Beach, Ala. (OB). The current data are obtained from two sites (large black circles) at CP and buoy M. The top (bottom) reference arrow is for the wind (current) vectors. Coastal sea level data are obtained from three regional NOAA water level stations (green triangles) at Pascagoula (Pas), Dauphin Island (DI), and Pensacola (Pen). Locations of the conductivity-temperature-depth (CTD) surveys are shown with black dots beginning just south of the Mobile Bay mouth (T09) and extending to the 35 m isobath (T35). The inset shows the study site (black box) in relation to the Gulf of Mexico and the location of the Deepwater Horizon oil spill (red dot). northern Gulf of Mexico is complex [Liu et al.,2011;mariano et al., 2011]. This was particularly evident on the Mississippi/ Alabama/Florida Panhandle coastline as these regions experienced significantly different exposure to coastal oiling over the course of the DWH spill (I. MacDonald, personal communication, 2011). Only a few previous studies have examined coastal circulation in this area. Regional-scale modeling studies, lacking a detailed focus on the coastal zone, have mainly focused on monthly or seasonal currents [Morey et al., 2003a, 2003b, 2005; He and Weisberg, 2002, 2003; Smith and Jacobs, 2005]. A limited number of observational studies have been conducted off the Alabama shelf based on satellites images [Schroeder et al., 1985; Abston et al., 1987; Dinnel et al., 1990; Stumpf et al., 1993], a near-bottom current meter [Chuang et al., 1982], or surface drifters [Schroeder et al., 1987;Golubev and Hsueh, 2002;Ohlmann and Niiler, 2005]. These limited observational studies indicate that shelf current variability is generally wind driven. More recent work from a single long-term mooring site on the 20 m isobath has focused on the vertical structure of the water column with Dzwonkowski and Park [2010] demonstrating that the seasonal currents are not solely forced by wind stress and Dzwonkowski et al. [2011a] examining the time varying role of along-shelf and across-shelf wind stress in driving acrossshelf surface transport. However, all of these past observational efforts lack horizontal and/or vertical coverage on the middle to inner shelf region of the coast. [6] The purpose of this study is to explore the spatial variability of circulation on a shallow, stratified shelf. As such, water column velocity and hydrographic measurements on the Alabama shelf are used to examine the flow field at seasonal and synoptic time scales. This is a particularly interesting study region as it is associated with complicating geographic orientation (90 change in coastline), bathymetric features (subsurface bank), and large-scale estuarine outflow (Mobile River system). The geographic constraints and highly stratified conditions that result from a combination of seasonal warming as well as buoyancy intrusions [Dzwonkowski et al., 2011b], represent a complex environment in which this study determines the primary forcing terms that drive the along-shelf flow. While this study only compares two sites on the Alabama shelf, this is the first time any observational data set has been used to examine the spatial variability of the water column velocity structure on the Mississippi/Alabama shelf. Furthermore, the collection period of this data set, spanning the full duration of the DWH oil spill event, represents a unique opportunity to characterize circulation patterns in this region during the oil spill and thus provides some insight as to reasons for the differential oil exposure experienced by the Alabama coastline. The findings from this study indicate that the seasonal currents during high-discharge periods are not wind driven and can experience high spatial variability. The synoptic-scale currents are consistent with a wind driven system that is influenced by bathymetry, coastal geography, and density gradients. 2. Data and Methods 2.1. Data Sources [7] To analyze the flow field and its associated forcing mechanisms, water column velocity data are obtained from two sites on the shelf. The western site, maintained by Dauphin Island Sea Lab, is on the 20 m isobath located approximately 20 km offshore and approximately 25 km west southwest of the Mobile Bay mouth (site CP in Figure 1). This site is equipped 2of15

3 with an upward looking RDI 600 khz ADCP which collects 5 min averages every 20 min at 0.5 m bins between 2 and 19 m above the bottom (mab), of which 2 17 mab are consistently reliable. These data are subsequently averaged hourly. The eastern site, maintained by the NOAA PORTS program, is on the 17.4 m isobath located 14 km offshore of the mouth of Mobile Bay (buoy M in Figure 1). This site is equipped with a Nortek downward looking ADP which collects data hourly at 1.0 m bins between 3 and 13 mab. The two sites are separated by approximately 14 km. [8] In conjunction with the velocity data, two types of hydrographic data are available: time series and conductivitytemperature-depth (CTD) transect surveys. The hydrographic time series exist at site CP and consist of 20 min data, subsequently hourly averaged, from two SeaBird MicroCATs (SBE37-SMP) for salinity and temperature at the near surface (15.7 mab) and bottom (0.2 mab). Hydrographic survey data are collected along a transect that extends from the middle of Mobile Bay out to the 35 m isobath, of which only the stations outside of the Bay are used in this study (Figure 1). During the course of the DWH oil spill event, seven CTD casting surveys were conducted on 28 April; 7 May; 2, 15, and 29 June; 14 July; and 27 August in Due to small variations in the vertical location of the instrument measurements and the irregular distances between the survey stations, the data for temperature, salinity, and density from these seven surveys are interpolated to a 0.5 m (vertical) by 1 km (across shelf) plane along the transect. [9] Additional physical data, including wind velocity, freshwater discharge, and sea level, are obtained from local sources. Hourly wind data during the study period are collected from the NOAA National Data Buoy Center station DPIA1 at Dauphin Island (DI) and station offshore of Orange Beach, AL (OB) (Figure 1). Daily discharge data for the Mobile River system are obtained at two U.S. Geological Survey gauging stations for the Alabama River ( N, W: USGS ) and the Tombigbee River ( N, W: USGS ). Their sum is used as a total freshwater discharge into Mobile Bay, following Park et al. [2007]. Hourly sea level data are obtained from three regional NOAA tide stations at Pascagoula, MS (Station ID: ), Dauphin Island, AL (Station ID: ), and Pensacola, FL (Station ID: ) (Figure 1) Data Processing and Analysis [10] Several basic procedures are applied to the current, hydrographic, and forcing data. The data are generally continuous and any short gaps of 12 h or less are filled using linear interpolation. However, two large gaps occur in the velocity data (approximately 10 days at site CP and 1 day at buoy M), during which analyses are not conducted. With the exception of daily freshwater discharge, a low-pass 40 h Lanczos filter is used to isolate the subtidal processes in the data. [11] The regional coastline and bathymetry experience a change in orientation to the west of the study site as well as an irregular feature around buoy M (Figure 1). As such, the principal components of the subtidal depth-averaged currents are examined in determining the along-shelf and across-shelf directions. Both sites exhibit rectilinear currents with site CP having a major (minor) axis of 14.4 (2.9) cm s 1 and buoy M having a major (minor) axis of 16.0 (3.6) cm s 1. The orientation of site CP is consistent with the east/ west coastline of Alabama (90/270 axis), while the orientation of buoy M is slightly rotated clockwise (98/278 axis), likely as a result of the local bathymetry. In this study, east west and north south orientations, with eastward and northward directions being positive, are used for the alongshelf and across-shelf directions, respectively. Only minor differences arise by using the principal component reference frame at buoy M. [12] To the extent possible, this study examines the terms of the along-shelf momentum balance. Following Lentz et al. [1999], assuming minor sea surface elevation compared with water depth and hydrostatic flow, the depth-averaged alongshelf momentum equation is: u t þ 1 h Z 0 u 2 dz þ 1 x h h Z 0 uvdz f v ¼ 1 p y h r o x þ t s r o h t b r o h ð1þ where (u, v) are the along-shelf and across-shelf components of the velocity, (ū, v) are the corresponding depth-averaged velocities, z is the height about mean sea level, h is the water depth, f is the Coriolis parameter, r o is a reference density, P/ x is the depth-averaged pressure gradient, and t s and t b are the wind and bottom stresses, respectively. [13] Depth-averaged currents in equation (1) are calculated by integrating the subtidal velocity over the water column using the trapezoidal method. The velocity data not available near the surface and bottom are obtained by assuming a constant value from the uppermost (lowermost) value to the surface (bottom) [Shearman and Lentz, 2003]. A linear trend from the upper (lower) value to the surface (bottom) is also performed resulting in only minor differences. [14] The first term in equation (1) is the local acceleration term, which is estimated as the centered differences over 2 h intervals. The second term is the advection term, which cannot be properly estimated from the data set where only a very coarse differencing between the two sites is discussed u (i.e., u M u CP CP Dx where the subscripts CP and M indicate site CP and buoy M, respectively). The third term is the across-shelf momentum flux divergence, which is estimated following Lentz and Chapman [2004]. In short, to be consistent with two-dimensional wind-driven flow theory, v is removed from v prior to filtering giving v [Dever, 1997]. Then, in conjunction with the total u component, the product uv is integrated from the surface to the bottom (in the same method as the depth-averaged calculation). This result is divided by site depth and the distance to the coastline where the across-shelf momentum flux is zero. The fourth term is the Coriolis term. [15] The forcing terms are on the right hand side (RHS) of equation (1) with the first term being the pressure gradient. Given the shallow depth of the study site, this term is assumed to be barotropic and is approximated by g Dh Dx where g is gravity, Dh is the difference in coastal sea level between Pascagoula (Pas in Figure 1) and Pensacola (Pen), with sea level being relative to mean sea level at each site, and Dx is the distance between the stations. Prior to filtering, the sea level data are detided using NOAA/NOS/CO-OPS harmonic predictions to remove any long-term tidal effects. After filtering, the subtidal sea level is adjusted for atmospheric pressure using an inverse barometer correction [Berwin, 3of15

4 Figure 2. Vertical structure of the seasonal mean (a) alongshelf and (b) across-shelf velocity at site CP (black symbols) and buoy M (red symbols) for spring and summer. 2003]. This is the same approach used in a number of studies [e.g., Wong, 1999; Liu and Weisberg, 2005]. [16] The second term on the RHS is the wind stress term, which is estimated at 10 m above sea level from the wind velocity data using the height of the sensor (13.5 m at DI and 5 m at OB) and the bulk formula of Large and Pond [1981]. The last term of equation (1) is bottom stress, which is estimated in two ways, as direct bottom stress measurements are not available. Both the linear and quadratic drag laws are used with the bottommost (z = 2 mab) velocity values. In the case of the linear drag law, a value of is used for the resistance coefficient (r) as this is consistent with the frictional time scale (t = h/r where t is the lag time between wind and depth-averaged along-shelf current) and falls in the range determined by Liu and Weisberg [2005] for the west Florida shelf. The quadratic formula uses a bottom drag coefficient (C d ) value of , following the findings of Liu and Weisberg [2005]. The stress terms are first calculated using the hourly data and then low pass filtered. Note that the analysis of the terms in equation (1) focuses on site CP because of the better vertical coverage with velocity data available for 75% of water column compared to 57% at buoy M. [17] In addition, this study examines the thermal wind balance expressed as: u z ¼ g r f r o y ; where u/ z is the vertical shear and r/ y is the across-shelf density gradient. Vertical shear estimated using equation (2), with r/ y calculated from the CTD transect survey data, is compared with the observed profiles of u/ z, which is estimated from a fifth-order polynomial fit to a vertical profile of the horizontal velocity following the procedures of Garvine [2004]. It should be noted that the CTD survey stations north of site CP are not aligned exactly in the acrossshelf direction but rather approach the site at an angle from ð2þ the mouth of Mobile Bay (Figure 1), which could result in some biasing of the estimated r/ y (see section 4.4). Two of the seven CTD casting surveys are only extended out to site CP, so the across-shelf gradient at site CP is linearly extrapolated from the gradient between site CP and the closest onshore casting site. Additionally, the magnitude of middepth velocity shear is determined by averaging the shear in the middle 50% of the water column, equal distance from the surface and bottom boundary minimizing any shear associated with the boundary layers. [18] At the largest time scale in this study, the data are grouped by season using typical definitions of spring (March May) and summer (June August) for temperate climates, although the spring period only has data beginning on 12 April At synoptic time scales, basic analysis techniques, including correlation and regression, are performed. Unless specifically stated, the correlations are significant at the 95% confidence level (using a minimum decorrelation time scale of 24 h in determining the effective degrees of freedom). It should be noted that the along-shelf and across-shelf wind stress are significantly correlated at weather band time scales ( days) in the study area [Dzwonkowski et al., 2011b], which can complicate interpretation of the wind forced response in well-mixed coastal regions. However, Dzwonkowski et al. [2011a] demonstrate that during highly stratified periods, the along-shelf wind is the primary forcing function driving wind-driven acrossshelf transport. As such, this study focuses on along-shelf wind forcing. 3. Results 3.1. Seasonal Time Scales [19] The environmental conditions during the DWH oil spill event are relatively typical for the study region. During the spring and summer season, the seasonal discharge values from the Mobile River system (1694 and 689 m 3 s 1, respectively) are consistent with historical seasonal averages (1975 and 832 m 3 s 1, respectively), which are large compared to many other coastal systems. Thus, the spring discharge in conjunction with the increasing solar insolation generate a highly stratified shelf environment having an O(0.9) slope Burger number, a critical parameter in determining the importance of stratification in shelf dynamics. Furthermore, the southeasterly/southerly wind over the study region during the spring and summer is also relatively typical (Figure 1). The along-shelf component in the summer season is historically more variable with seasonal values fluctuating between west and east directions [Chuang et al., 1982]. [20] During the DWH oil spill the seasonal flow at site CP and buoy M is surprisingly different. The mean depth-averaged flow is generally eastward (southward) at site CP (buoy M) in both the spring and summer periods (Figure 1). This is in sharp contrast to the seasonal wind conditions which are generally opposed to the current flow at both sites. Furthermore, the vertical structure of the shelf flow reveals several notable features (Figure 2). The vertical structure of the along-shelf flow at site CP, consistent with the previous observation in [Dzwonkowski and Park, 2010, Figure 4], has spring and summer profiles that are highly sheared having a near surface trend tending toward the west and a subsurface maximum toward the east (Figure 2a). The 4of15

5 Figure 3. Time series data: (a) 11 day lagged river discharge, (b) along-shelf wind, (c) depth-averaged along-shelf current, (d) depth-averaged across-shelf current, de-meaned surface and bottom across-shelf current at (e) site CP and (f) buoy M, (g) sea level, and (h) surface and bottom density at site CP. The positive (negative) along-shelf wind and current are eastward (westward), and the positive (negative) acrossshelf current is northward (southward). In Figure 3a, the cross marks the day of the Deepwater Horizon accident (20 April 2010), and inverted triangles mark subsequent CTD surveys. 5of15

6 Table 1. Current and Sea Level Correlations With Along-Shelf Wind at Orange Beach (Dauphin Island) Stations a Variable Site CP Buoy M r b Lag c Confidence Level d r b Lag c Confidence Level d ū 0.8 (0.77) 18 (17) 0.29 (0.28) 0.86 (0.83) 15 (15) 0.27 (0.26) v 0.17 (0.16) 7 (39) 0.19 (0.19) 0.51 ( 0.5) 17 (21) 0.23 (0.21) v surface 0.71 ( 0.72) 1 (0) 0.24 (0.23) e v surface_s 0.58 ( 0.58) 0 (0) 0.22 (0.21) 0.56 ( 0.55) 0 (0) 0.21 (0.21) v bottom 0.71 (0.7) 6 (3) 0.27 (0.26) e v bottom_s 0.67 (0.67) 1 (0) 0.25 (0.24) 0.7 (0.68) 8 (6) 0.23 (0.22) f h DI 0.73 ( 0.72) 22 (20) 0.25 (0.25) a The surface and bottom across-shelf velocities are de-meaned. b Correlation coefficient. c Lag in hours. d Confidence level at 95%. e Here s indicates the site CP depth equivalent to the surface or bottom at buoy M. f Sea level at the Dauphin Island station. profile at buoy M is notably different, having a reduced eastward flow at middepth and a westward flow near the bottom third of the water column. Although there is a similar flow pattern between the two sites in summer, the upper 60% of the water column at buoy M, relative to site CP, appears to have an additional westward current, resulting in westward currents at a much shallower depth. [21] The differences in the vertical structure of the velocity become even more striking in the across-shelf profiles. There is minimal shear and generally weak onshore velocities throughout the water column at site CP during both spring and summer. In contrast, buoy M has relatively large offshore surface velocities that are strongly sheared throughout the profile during both spring and summer. It should be noted that some of the differences in the spring seasonal currents may be the result of slightly different averaging periods with different data gaps at site CP and buoy M (Figure 3c) Synoptic Time Scales [22] Times series of the environmental variables, including river discharge, along-shelf wind velocity, sea level and stratification, are shown with current velocity at site CP and buoy M (Figure 3). Although the seasonal conditions are relatively typical, the Alabama shelf experiences a wide range of environmental conditions during the DHW spill event. In particular, intraseasonal variability in the discharge results in several large events in the late spring/early summer, with peaks exceeding 3000 m s 1 for two events on lagged discharge dates of 9 May and 3 June (Figure 3a). However, the late summer values are at or below the 35 year average discharge level. Consequently, late spring shelf stratification is high and remains high throughout the study period despite several mixing events, e.g., on 4 July and 30 August (Figure 3h). The along-shelf wind velocities fluctuate between 9ms 1 with numerous upwelling (positive values) and downwelling (negative values) events of a wide range of magnitudes and durations (Figure 3b). The OB site has slightly larger wind velocities, likely due to the site being further offshore. [23] Not surprisingly, the depth-averaged along-shelf current at both sites (Figure 3c) is highly coherent with the along-shelf wind, whereas the across-shelf counterpart (Figure 3d) is generally not (Table 1). A significant negative correlation exists between the along-shelf wind and the depth-averaged across-shelf current at site M, but it is physically meaningless (section 4.2). The depth-averaged across-shelf current at the two sites is relatively small and shows little relationship to each other. The depth-dependent across-shelf current (de-meaned), at the surface and bottommost available bins, generally shows similar trends at both sites, with the surface and bottom currents typically flowing in opposite directions to each other and having similar magnitudes (Figures 3e and 3f). However, there are some clear instances when the surface (or bottom) currents between the two sites have significant differences. For example, there are times when the surface (or bottom) currents act in opposite directions between site CP and buoy M, e.g., 17 April; 1, 8, and 26 June; and July. Except for these occasional differences, the across-shelf current structure is generally coherent with the along-shelf wind where the surface current is offshore (negative) during upwelling favorable (positive) wind and onshore (positive) during downwelling favorable (negative) wind with bottom current acting in an opposite manner. Consequently, sea level responds to along-shelf wind as the resulting across-shelf surface flow drives sea level change. These relationships are quantified using correlation analysis (Table 1), where the depth-averaged along-shelf current and bottom across-shelf current is positively correlated with the along-shelf wind, and the coastal sea level at DI and the surface across-shelf current is negatively correlated with the along-shelf wind. [24] A more detailed view of the vertical flow structure is provided in Figure 4. The along-shelf and across-shelf currents at site CP are shown over a 24 day time period that extends over typical downwelling and upwelling events observed during the study period. Over both downwelling (27 June to 8 July) and upwelling (9 15 June) periods, strong shear is present in the along-shelf current, having peak flow at the surface in the direction of the wind forcing and decreasing with depth to the point where a bottom flow reversal is observed at times. Interestingly, these bottom current reversals often occur at or near transition periods in the wind forcing from which animations of flow events show the vertically sheared profiles being pushed in the direction opposite to the wind forcing at these times (see Animation S1 in the auxiliary material). 1 The across-shelf current 1 Auxiliary materials are available in the HTML. doi: / 2011JC of15

7 Figure 4. (a) The along-shelf wind at OB (thick line) and DI (thin line) stations. Vertical structure of the (b) along-shelf and (c) the across-shelf currents (cm s 1 ) at site CP during 27 June to 20 July The vertical black lines indicate times of CTD surveys. Note the difference in the color bar values. is consistent with the above mentioned comments in regards to the surface and bottommost current time series (Figures 3e and 3f) and generally has onshore (offshore) flow near the surface and offshore (onshore) flow near the bottom during downwelling (upwelling) favorable wind. The depth extent of these opposing flow regimes is somewhat variable over the course of an event. [25] The net effect of the observed relationships is evident in the hydrographic data, where the across-shelf transects of density illustrate typical isopycnal patterns during plume, downwelling, and upwelling conditions. The high-discharge events in the late spring and early summer strongly stratify the surface layer forming a shallow surface lens of light water as can be seen during the hydrographic survey on 2 June 2011 (Figure 5a), a period of relatively light upwelling wind and high discharge. The hydrographic survey on 29 June occurs during the initial onset of a strong downwelling event. Even at this early stage, the isopycnals near the coast (between 5 and 10 km from station T09) are bent downward into the seafloor (Figure 5b). In contrast, the late stage of a strong upwelling event is captured on 14 July, where isopycnals are sloping upward as far offshore as km from station T09 (Figure 5c) Along-Shelf Flow Dynamics Depth-Averaged Along-Shelf Momentum [26] Given the prominence of the along-shelf component in the flow variability of the study region, the along-shelf momentum balance is examined to better discern the contributing role of various forcing functions. A first-order view of the primary actors in driving along-shelf variability can be obtained using the linearized depth-integrated along-shelf momentum balance [Wong, 1999]: u h ¼ g t x þ t s r o h t b r o h where the first term is local acceleration, and the terms on the RHS are the barotropic pressure gradient, wind stress and bottom stress. The standard derivations of these terms suggest the pressure gradient term is notably larger than the other three terms (Table 2). This likely results from the fact that this term is calculated using data from coastal sea level stations rather than those from pressure sensors along the 20 m isobath. Other studies [e.g., Hickey, 1984; Liu and Weisberg, 2005] using this same methodology have applied a downscale factor to account for an assumed across-shelf decay scale associated with coastal sea level anomalies. Following Liu and Weisberg [2005], whose study region is the west Florida shelf in 15 m of water, a downscale factor of 0.7 is applied to the along-shelf pressure gradient term. This scaling results in a standard deviation that is comparable to the wind stress term and improves the correlation between terms as discussed below. The local acceleration and bottom stress terms in equation (3) are about half the standard deviation of the wind stress term. The variability of the forcing terms, the terms on the RHS of equation (3), have a good degree of consistency among them, with bottom stress (r = 0.67, lag = 12 h) and pressure gradient (r = 0.63, lag = 8 h) terms being correlated with ð3þ 7of15

8 Figure 5. Across-shelf density transects during (a) plume, (b) downwelling, and (c) upwelling conditions. The black contour lines are isopycnals at 1 s t intervals. The location of the transect line is shown in Figure 1. The times of the CTD surveys are marked in Figure 3a. wind stress at station OB (Figure 6a). The sum of the bottom stress and pressure gradient terms further improves the correlation with wind stress (r = 0.73, lag = 10 h) (Figure 6b). However, a comparison of the sum of the forcing terms with the local acceleration term shows notable discrepancies between the time series, with a large reduction in the correlation (r = 0.36, lag = 7 h) (Figure 6c). [27] Expanding the analysis to include the additional terms in equation (1) does not improve the results. The Coriolis term, the 4th term in equation (1), has a standard deviation comparable to the wind stress (Table 2), however addition of this term to the local acceleration term only increases the discrepancy between the forcing terms and the acceleration terms (not shown). Furthermore, the estimate of the nonlinear across-shelf momentum flux divergence term is much smaller than the other terms (Table 2 and Figure 6d). A crude estimate of the advective term has a standard deviation nearly equal to that of bottom friction (Table 2). Despite the larger magnitude of the advective term, neither estimate of the nonlinear terms provides an improvement to correlations with the other terms in the momentum balance Thermal Wind Balance [28] The repeated CTD casting surveys during the DWH spill event allow for an examination of an additional aspect of the along-shelf flow dynamics: the forcing of the vertical flow structure. The presence of strong across-shelf density gradients observed in the CTD surveys (Figure 5) suggests that the synoptic-scale currents may be strongly influenced by the thermal wind balance despite the shallow depth of the study site as found in other similarly shallow water sites [e.g., Lentz et al., 1999; Münchow and Chant, 2000; Sanders and Garvine, 2001; Garvine, 2004]. The along-shelf velocity profiles and their associated shear at both site CP and Table 2. Standard Deviations of the Terms in the Along-Shelf Momentum Balance at Site CP (Units of 10 6 ms 2 ) ū/ t fv gdh/dx t s /(r o h) t b /(r o h) 1/h ( R uvdz)/ y ū ū/ x (1.6) a 1.1 (1.3) b a Wind stress estimates at the Orange Beach (Dauphin Island) stations. b Bottom stress estimates using linear (quadratic) method. 8of15

9 Figure 6. (a) Low-pass filtered estimates of the terms in the along-shelf momentum balance showing OB along-shelf wind stress (black line), downscaled along-shelf pressure gradient (red line), and linear bottom stress (blue line). The cross marks the day of the Deepwater Horizon accident (20 April 2010), and inverted triangles mark subsequent CTD surveys. (b) Wind stress (black line) and the negative of the sum of pressure gradient and bottom stress (blue line). (c) The sum of the wind stress, pressure gradient, and bottom stress (black line) and the local acceleration (blue line). (d) The nonlinear across-shelf momentum flux divergence term. buoy M are obtained on the hours closest to the times of the CTD casting at site CP. In general, the velocity data and the thermal wind balance derived velocity (using equation (2) with the assumption of a reference velocity of zero at the bottom) show similar features, with particularly good comparisons shown in Figure 7. The examples in Figure 7 correspond to the plume, downwelling and upwelling events shown in Figure 4. At site CP, some differences exist between the along-shelf velocity and the thermal wind balance derived velocity profile, in part because of the reference velocity assumption, which can shift the position of the vertical profile. However, the shear profiles, which are not affected by the assumption, are often well correlated throughout the middle of the water column where thermal wind balance would be expected to dominate, i.e., away from the shear associated with the surface and bottom boundary layers. The velocity and shear profiles at buoy M can be quite different from both site CP and the thermal wind balance derived profiles as illustrated in the downwelling case (Figures 7d and 7e). However, there are times when the spatial deviations between the two sites are minimal and the three profiles are nearly identical as in the upwelling case (Figures 7g and 7h). [29] A quantitative measure of the representativeness of the thermal wind balance in explaining the shear can be obtained by comparing the average middepth thermal wind shear to the shear derived from the velocity data at site CP (Figure 8). Because of a data gap in the velocity at site CP during one of the CTD surveys (28 April), the shear is derived using the velocity data at buoy M for this one case. Despite the limited number of the CTD surveys, the comparison has a notable linear trend. The slope is not in perfect agreement, but the significant regression does explain a good portion of the observed shear demonstrating that the 9of15

10 thermal wind balance is a significant player in the shelf dynamics during the spring and summer. 4. Discussion 4.1. Spatial Variability in Seasonal Flow [30] The mean flow does not appear to be entirely driven by local wind stress, in agreement with Dzwonkowski and Park [2010] who hypothesize that the spring/summer discharge into the region combined with easterly along-shelf wind forcing generates the observed profiles. The differences between the depth-averaged velocities (Figure 1) and the vertical profiles (Figure 2) at the two sites suggest that smaller-scale forcing functions are also playing a role in controlling the spatial variability of the current structure, i.e., processes experiencing variability on the order of km. As suggested by the reduction (spring) or reversal (summer) of the along-shelf component in the depth-averaged current and the strongly enhanced offshore flow at buoy M, this region just offshore of Mobile Bay appears to generate alongshelf convergence and subsequent across-shelf transport. [31] There are several possible reasons for the reduced/ reversed along-shelf flow and strong offshore flow at buoy M. One likely candidate is the local bathymetry, which is often associated with enhanced offshore flow during synoptic flow events. Another possible cause could be associated with stronger density gradients, i.e., stronger thermal wind balance effects, closer to the mouth of Mobile Bay which would result in a stronger seasonal flow reversal at depth and thus cause the observed reduction/reversal of the depth-averaged along-shelf flow. Both of these possible causes are discussed in sections 4.3 and 4.4. [32] The difference in the seasonal vertical structure of the across-shelf flow (Figure 2b) is an additional point worth commenting on. In particular, the across-shelf current structure at buoy M can be viewed as a two-layer estuarine circulation with outward flow at the surface and inward flow near the bottom that has been shifted by a depth-averaged offshore current (as discussed in the previous two paragraphs and sections 4.3 and 4.4). This profile likely occurs due to the proximity to the mouth of Mobile Bay, where both the discharge plume and the baroclinic pressure gradient associated with shelf/estuary exchange would be expected to generate significant shear in the water column flow structure. Given the seasonal change in discharge, the acrossshelf water column shear would be expected to have a seasonal change. Unfortunately, the upper 25% of the water column, where the discharge plume would be primarily residing, is not captured in the velocity data at buoy M. However, the velocity shear trend in the uppermost bins of the spring and summer profiles suggests the upper layer of the spring profile likely experiences more shear than the summer profile as the surface is approached (Figure 2b). 10 of 15 Figure 7. The vertical profiles of (left) along-shelf current, (middle) along-shelf shear, and (right) across-shelf current during (a c) plume, (d f) downwelling, and (g i) upwelling events at site CP (crosses) and buoy M (circles) and derived from thermal balance assuming zero velocity at bottom (dots). The across-shelf density structure for each of these events is shown in Figure 4.

11 Figure 8. Comparison of the average middepth shear derived from the thermal wind balance with the average middepth vertical shear of the along-shelf current at site CP (pluses). The black line is the linear fit to the data including shear from buoy M (red star) as data from site CP are not available during this CTD survey on 28 April The slope and r 2 values in the parentheses are without the buoy M value Synoptic-Scale Coastal Circulation [33] To the first order, the synoptic-scale shelf circulation is relatively consistent with a wind-driven system in the presence of a geographic boundary. The current response to wind forcing follows Ekman dynamics acting in the coastal zone where upwelling (downwelling) wind drives surface flow offshore (onshore), which reduces (increases) coastal sea level and consequently drives along-shelf flow in the direction of the wind and a compensating onshore (offshore) return flow at depth. As demonstrated in the across-shelf density transects (Figure 5), the resulting shift in the shelf mass properties is consistent with upwelling and downwelling events and consequently generates strongly sheared velocity profiles via thermal wind balance (Figure 7). [34] A significant correlation occurs between the alongshelf wind and depth-averaged across-shelf current at buoy M (Table 1); however this is likely a spurious result. The relationship is neither consistent between sites (i.e., not significant at site CP) nor between coordinate systems (i.e., not significant when the principal axes of buoy M are used). More importantly, there is no clear physical reason to expect the depth-averaged across-shelf current to be affected by the along-shelf wind. [35] From the depth-averaged along-shelf momentum balance, both bottom stress and along-shelf pressure gradient act to counter wind stress as seen by their negative correlation with wind stress (Figure 6a). The importance of the along-shelf pressure gradient results from the 90 shift in the coastline to the immediately west of the study region (Figure 1). This is similar to other findings, such as those by Wong [1999] and Yankovsky [2003], who report that a sharp change in coastline results in an along-shelf pressure gradient over 100 km from a geographic corner. Interestingly, as a consequence of the thermal wind balance generated shear, the along-shelf pressure gradient drives a flow reversal in the bottom current during transitional periods prior to many wind stress reversals (Figure 4). Flow reversals have been observed in other coastal regions [e.g., Chant et al., 2004; Gutierrez et al., 2006], although the details of the reversals vary with the geographic setting to some extent. [36] The data set shows that there are times when the flow field is three dimensional. This can be seen in the vertical structure of the along-shelf and across-shelf profiles at the two sites. For example, during the plume event under light upwelling favorable wind (Figures 7a and 7c), site CP has generally eastward along-shelf current and very weak onshore flow, whereas buoy M has eastward along-shelf current only at the surface and strong offshore flow with both components being associated with strong shear. Other periods of contrasting across-shelf currents at the two sites are also shown in Figure 7. Given the proximity to the mouth of Mobile Bay, estuarine discharge maintains large vertical and horizontal density gradients in the coastal zone (Figure 5). As such, the presence of the Mobile Bay plume could drive some of the observed site differences. In fact, Tilburg and Garvine [2003] find along-shelf (across-shelf) spatial scales as small as (3 5) km in the presence of buoyancy discharge on the New Jersey shelf. In addition, a number of studies suggest that site bathymetry may also contribute to localized flow reversals [e.g., Song et al., 2001; Chant et al., 2004], a topic that will be discussed in section 4.3. [37] This complex flow structure is likely a contributing reason for the terms in the linearized along-shelf momentum equation to be unbalanced and suggests that nonlinearity in the flow field is important in this region. Despite the inability of the nonlinear terms to enhance correlations with the forcing terms in equation (3), the standard deviation of the crude estimate of the along-shelf advective term, which is of the order of bottom friction (Table 2), suggests that this term could potentially be important. A better sampling design is certainly required to better estimate this term. It is interesting to note that the across-shelf momentum flux divergence term appears to be of little importance in this shallow depth. Often overlooked (and apparently, rightfully so in our case), Lentz and Chapman [2004] demonstrate that in high-slope Burger number ( 1 or larger) environments this term can account for 50% of the wind stress term and as a result, return flow occurs above the bottom boundary layer. Given the environmental and geographical conditions of the study region (i.e., shelf slope = and buoyancy frequency of s 1 ), the shelf system experiences a high-slope Burger number, O(0.9), throughout the spring and summer. Thus, it is surprising that this term contributes minimally to the along-shelf momentum balance. Despite the strong stratification, the shallow depth of the study region allows bottom friction to be a significant along-shelf momentum sink. In addition, the presence of an along-shelf pressure gradient may also contribute to the limited importance of this term. It is not clear whether the across-shelf momentum flux divergence term will be more important in deeper water, where the frictional term is further reduced, as some of the assumptions in the work of Lentz and Chapman [2004] do not hold in an environment where wind conditions fluctuate from upwelling to downwelling (e.g., not consistently upwelling favorable), 11 of 15

12 local acceleration term is not insignificant, along-shelf divergence may be present, etc Flow Interactions With Bathymetry [38] The difference in the depth-averaged across-shelf velocity between the two sites is somewhat surprising, given the relatively short distance (approximately 14 km) between them. A source of this spatial variation may be the difference in the local bathymetry, with buoy M located where the 10 m isobath bulges southward (Figure 1). A number of studies provide possible explanations for enhanced offshore flow resulting from submarine banks. For example, Chant et al. [2004] apply MacCready and Pawlak s [2001] findings of increased form drag over a bathymetric ridge, which is expected to generate additional bottom friction over the ridge, to explain along-shelf convergence and subsequent offshore transport over a submarine bank on the New Jersey coast. Several other studies have found nondimensional relationships that dictate when flow is expected to separate from a bathymetric contour. For example, Klinger [1994] observes flow separation in laboratory experiments when U/f exceeds the radius of curvature (R) of a bathymetric feature. Given typical along-shelf velocity fluctuations (U) of 20 cm s 1, U/f is approximately 2.7 km, which is on the order of R (approximately 5 km), suggesting that flow separation around this feature is possible. Similarly, Castelao and Barth [2006], using a numerical model, find that maximum flow separation of an upwelling jet is associated with flows that have an O(1) Burger number (NH/fR) and an increasing Rossby number (U/fL) where N is the buoyancy frequency, H is the depth of the water column, and L is the length scale. Using stability derived from the mooring site N = O( s 1 ), H = 10 m (approximate depth of the local feature), and R = 5 km (approximate radius of curvature at 10 m depth), the Burger number is of the order of 1.8. The Rossby number is approximately 0.2, using U =20cm s 1 and L equaling the internal Rossby radius (approximately 14 km). While these values do not put the system in the parameter space of maximum flow separation [Castelao and Barth, 2006, Figure 5], they are indicative of some separation and enhanced offshore transport. Thus, the preferential offshore transport at synoptic time scale can, in part, be attributable to the along-shelf flow interacting with the local bathymetry Influence of the Density Field [39] An additional component contributing to the spatial variability in the flow field is the local density distribution. The across-shelf density transects demonstrate the role of thermal wind balance on the vertical structure of the velocity at site CP and suggest that spatial variability in the shelf density field could be responsible for the variations in vertical structure between the sites at the seasonal (Figure 2) and synoptic time scales (Figure 7). As noted in section 2.2, the thermal wind balance estimates are determined for a transect in which site CP is a corner point at a change in direction of the survey line (Figure 1). This would be expected to generate some error in the velocity shear estimates. However, the effect of the directional change is difficult to assess due to the very high regional variability in the density field associated with high discharge from Mobile Bay and intermittent gaps in the regional barrier islands (i.e., sources of freshwater outflow). [40] Many substances, including nutrients, colored dissolved organic matter, and suspended sediment, are associated with low salinity river discharge, and thus satellitederived chlorophyll-a data is often used as a proxy for tracking river discharge [e.g., Walker et al., 2005]. As such, the small-scale variations in water mass properties can be observed using high-resolution imagery. Data obtained from the Medium Resolution Imaging Spectrometer (MERIS, 300 m resolution) during an upwelling and downwelling event illustrate strong small-scale gradients in chlorophyll-a, which indicate the presence of a very complex density field throughout the coastal zone in the northern Gulf of Mexico (Figure 9). Similarly, Jochens et al. [2002] present evidence of a highly variable surface salinity field further offshore in the Mississippi/Alabama/Florida Panhandle region. [41] Consequently, the seasonal and event-scale evolution of the density field in this region likely contributes to smallscale variability in the along-shelf velocity structure with stronger gradients closer to the estuary mouth. On a seasonal scale, this would be consistent with the spatial variability in the vertical structure of the along-shelf currents where there is a stronger flow reversal at depth at buoy M in both seasons (Figure 2a). At the event scale, more detailed density mapping is needed due to the short-term mobility of density fronts (Figure 9) Implication on the Deepwater Horizon Oil Spill [42] A range of instrumentation was fortuitously in place on the Alabama inner shelf during the course of the DWH oil spill event. This data provide new insight on the circulation of a region that has been clearly understudied and provide a rare glimpse of a flow field in a region directly impacted by a major oil spill event. The mean currents are in general agreement with the path of surface oil (west to east) during May through June of the event. More interestingly, the strong offshore flow at buoy M, near the mouth of Mobile Bay, is consistent with and likely a major reason for the reduced impact of surface oil in Mobile Bay as compared to the adjacent Alabama/Mississippi/Florida Panhandle coastlines. The data also suggest that deficiencies in model simulation of oil trajectories in the inner coastal zone may have resulted from inaccuracies in simulating variations in the fine-scale density field as baroclinic forcing is important in the regional circulation during the DWH oil spill: a good topic for future investigation. 5. Conclusions [43] The subtidal circulation on the Alabama shelf is examined using hydrographic, sea level, and velocity data collected in the spring and summer of 2010, i.e., during the Deepwater Horizon oil spill disaster. At the seasonal time scale, a comparison between two sites demonstrates surprising variability and reveals a convergent flow with strong, highly sheared offshore flow near a submarine bank just outside of Mobile Bay. The synoptic time scale flow variability at the sites is consistent with typical wind-driven flow dynamics in which local along-shelf wind drives surface transport, across-shelf sea level fluctuations, along-shelf flow, and a compensatory bottom return flow. The along- 12 of 15

13 Figure 9. Medium Resolution Imaging Spectrometer (MERIS) imagery of chlorophyll-a patterns in the Alabama coastal zone on (a) 26 April 2010 and (b) 8 July 2010 with wind (15 h lag) vectors from DI and OB stations and surface (red) and bottom (black) current vectors at site CP and buoy M when available. Note the scales change in the reference vectors between images. The top (bottom) reference arrow is for the wind (current) vectors. shelf flow structure is strongly sheared at both sites and is, in part, consistent with the thermal wind balance at site CP, despite its shallow depth (20 m). Furthermore, flow reversals in the along-shelf bottom currents, occurring during transitional periods between upwelling and downwelling alongshelf winds, are a direct result of the vertical velocity shear derived from the thermal wind balance being translated opposite to the direction of the wind in response to the along-shelf pressure gradient overwhelming the wind forcing (see the animation in the supplemental material). The importance of the pressure gradient is a result of the regional coastline geography which rotates by 90 within 150 km of the study region. Despite the high levels of stratification with a slope Burger number of O(0.9) and the presence of an along-shelf pressure gradient, bottom friction remains an important sink for along-shelf momentum (approximately 50% of the wind stress term) and contributions from the potentially significant nonlinear across-shelf momentum flux divergence are surprisingly minimal. [44] Comparisons between the two sites demonstrate a rich three-dimensional environment as evidenced by the notable variability between the two sites despite the relatively short distance separating them. The time series of the current velocity reveal a notable lack of coherency in the depth-averaged across-shelf current between the two sites. Similarly, there are some times when the depth-dependent across-shelf current is not coherent between the two sites, either at the surface or bottom. The observed spatial variability is argued to be a result of local variations in the bathymetry and density field as buoy M is located at the tip of a submarine bank near the mouth of Mobile Bay. Given the physical parameters of the region, along-shelf flow 13 of 15