Jan 7, 2012 - weaker than that due to tidal pumping and subtidal barotropic transport. ... Columbia River [e.g., Jay and Smith, 1990; MacCready et al.,. 2009].
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, C01003, doi:10.1029/2011JC007124, 2012
Circulation dynamics and salt balance in a lagoonal estuary Peng Jia1 and Ming Li1 Received 10 March 2011; revised 26 October 2011; accepted 8 November 2011; published 7 January 2012.
[1] Albemarle-Pamlico Sound (APS) is a shallow lagoonal estuary connected to the Atlantic Ocean through narrow inlets. The circulation dynamics and salt balance in this estuary are investigated using a numerical model. Although the vertical stratification is weak, the mean flow features a two-layer gravitational circulation with speeds reaching several centimeters per second. Analysis of the momentum budget shows a primary balance among the barotropic pressure gradient as a result of sea level slope, the baroclinic pressure gradient due to horizontal salinity gradients, and stress divergence. The salt budget for APS is determined by the balance between river flow and salt exchange through the inlets. At the inlets, the salt flux resulting from estuarine shear flow is much weaker than that due to tidal pumping and subtidal barotropic transport. Tidal pumping produces a net influx of salt into APS: Strong flood currents push oceanic water into the estuary through a propagating density front, whereas ebb currents advect lighter estuarine water over denser bottom water. The salt flux due to the subtidal barotropic transport across the inlets shows large temporal fluctuations associated with wind events. This transport can be either a source or sink of salt to APS and correlates well with the sea level difference across each inlet. Higher–sea level on the shelf leads to an intrusion of oceanic water into APS whereas higher–sea level inside APS leads to a withdrawal of estuarine water to the shelf. Citation: Jia, P., and M. Li (2012), Circulation dynamics and salt balance in a lagoonal estuary, J. Geophys. Res., 117, C01003, doi:10.1029/2011JC007124.
1. Introduction [2] Estuaries feature a wide range of topographic shapes, including coastal plain estuaries, fjords, and lagoonal/bar-built estuaries. Much progress has been made in understanding circulation dynamics and salinity distribution in coastal-plain estuaries (see MacCready and Geyer [2010] for a review), such as James River [e.g., Pritchard, 1955, 1956; Valle-Levinson et al., 2000], Hudson River [e.g., Peters, 1999; Geyer et al., 2000; Lerczak et al., 2006; Ralston et al., 2008; Scully et al., 2009], and Columbia River [e.g., Jay and Smith, 1990; MacCready et al., 2009]. In comparison, few studies have been devoted to coastal lagoons. Lagoons are shallow bays or sounds that are separated from the ocean by barrier islands. Their horizontal scale ranges from several to hundreds of kilometers whereas their vertical scale is only a few meters. Coastal lagoons represent a special class of estuaries where the exchange between the lagoons and the ocean is often restricted to one or several narrow inlets. Lagoons are common features along the East and Gulf Coasts of the United States, and constitute about 13% of the world’s coastline [Cromwell, 1973]. Examples of lagoonal estuaries include Indian River Bay, Delaware [Janzen and Wong, 1998], Albemarle-Pamlico Sound (APS), North Carolina [Roelofs and Bumpus, 1953], St. Andrew Bay, 1 Horn Point Laboratory, University of Maryland Center for Environmental Science, Cambridge, Maryland, USA.
Copyright 2012 by the American Geophysical Union. 0148-0227/12/2011JC007124
Florida [Murphy and Valle-Levinson, 2008], Apalachicola Bay, Florida [Huang et al., 2002], Mobile Bay, Alabama [Noble et al., 1996], Laguna San Ignacio, Baja California, Mexico [de Velasco and Winant, 2004], Havre-aux-Maisons Lagoon, Gulf of Saint-Lawrence, Canada [Guyondet and Koutitonsky, 2008], and the Venice Lagoon, Italy [Solidoro et al., 2004]. Previous investigations of the lagoons have mainly focused on tidal- and wind-driven exchanges across the inlets [Stommel and Farmer, 1952; Wong, 1991; Geyer and Signell, 1992; Churchill et al., 1999; Luettich et al., 1999; Hench et al., 2002; Hench and Luettich, 2003]. Less attention has been paid to circulation dynamics and salt balance inside the lagoonal estuaries. [3] APS is representative of a class of shallow and wide lagoonal estuaries, and thus provides a good case study for the lagoonal circulations. APS consists of two major basins: Albemarle Sound to the north and Pamlico Sound to the south (Figure 1a). Pamlico Sound is further split into the southern and northern basins by the shallow Bluff Shoals (Figure 1b). Croatan and Roanoke sounds are two narrow waterways connecting Albemarle and Pamlico sounds. Compared with its broadness, APS is shallow with an average depth of 4.5 m, varying from less than 2 m on the shoals to more than 7.5 m in the center of Pamlico Sound. The Neuse and Pamlico rivers drain into Pamlico Sound while the Chowan and Roanoke rivers drain into Albemarle Sound. The North Carolina Outer Banks separate APS from the Atlantic Ocean, with three major inlets (Ocracoke, Hatteras, and Oregon inlets) serving as the main passage of saline
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Figure 1. (a) Map of APS: Chowan, Roanoke, Pamlico, and Neuse rivers discharge freshwater into APS while oceanic water enters it through Oregon, Hatteras, and Ocracoke inlets at the Outer Banks. Modeldata comparisons of salinity are conducted at three stations (black open circles) and along two ferry routes (blue and green lines). Section S1 in red is used for momentum analysis. (b) The rectangular model domain and bathymetry. The black dashed line indicates the location of Bluff Shoal, which divides Pamlico Sound into southern and northern parts. Open triangles show locations across Oregon Inlet used for the water level analysis. oceanic water into the estuary [Pietrafesa et al., 1986; Lin et al., 2007]. [4] Because of restrictive connections with the Atlantic Ocean, tides inside APS are weak, but winds have been recognized as a major forcing for the system. The observed vertical stratification is weak in APS: Bottom-to-top salinity difference ranges between 0.1 and 2 practical salinity unit (psu) in most of Pamlico Sound [Roelofs and Bumpus, 1953]. This weak stratification has been attributed to the effectiveness of winds in mixing the water column throughout the shallow sound [Luettich et al., 2002]. Previous modeling studies have indicated that winds drive circulation in APS at synoptic time scales [Pietrafesa et al., 1986]. Xie and Pietrafesa [1999] developed a 3-D baroclinic model for APS (based on the Princeton Ocean Model) and examined APS’ response to a sudden shift of wind direction from southwesterly to northwesterly. Xie and Eggleston [1999] used the model to investigate how the circulation and salinity distributions in APS respond to idealized winds in eight different directions. In another modeling study, Luettich et al. [2002] found that winds generate semidiurnal seiching within APS and the Neuse River estuary. [5] Despite these interesting studies, several questions concerning the circulation dynamics and salt balance of APS remain unanswered. Although it is well known that the stratification is weak in APS, the nature and spatial pattern of the mean circulation are not known. Although baroclinic forcing has been suggested to play a role in APS [Pietrafesa et al., 1986], it is unclear if the mean circulation is driven by the winds or by the gravitational force created by geographically separated sources of freshwater (rivers) and saline water (inlets). According to the classic theory, the estuarine
circulation is primarily determined by the momentum balance between the along-channel pressure gradient and vertical stress divergence [Pritchard, 1956; Hansen and Rattray, 1965; Chatwin, 1976]. Can this theory be applied to interpret circulation dynamics in lagoonal estuaries such as APS? Understanding the circulation pattern in APS is not only interesting from the viewpoint of estuarine dynamics but also important for understanding ecosystem productivity and water quality in a lagoonal estuary. Because of the excessive nutrient loads from its drainage basins, APS has experienced various ecological problems such as hypoxia, harmful algal blooms, and fish kills [Paerl et al., 1998; Mallin et al., 2000; Buzzelli et al., 2002]. Freshwater carries various forms of nutrients, organic matter, and toxic materials into the upper tributaries of APS. These inorganic and organic materials are subsequently transported and delivered to various regions of the APS estuary. As shown by Paerl et al. [2003, 2006], phytoplankton in APS exhibit species and group-specific growth responses to different physical forcing and nutrient formulations. Fish species such as Atlantic menhaden spawn in continental shelf waters but spend their juvenile phase in estuarine nursery grounds. They depend critically on physical processes that transport larval fish from spawning areas to nurseries inside APS [e.g., Luettich et al., 1999]. [6] Besides the circulation dynamics, it is also important to understand physical processes that control the salt balance in this lagoonal system. APS receives freshwater from four rivers and exchanges salt with the Atlantic Ocean through three inlets. The river discharges have a well-defined seasonal cycle: high flows during spring and low flows during summer. How does the river discharge and salt flux through the inlets affect the seasonal evolution of salinity distribution
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in APS? Salt flux in an estuary can be decomposed into tidal pumping, subtidal shear dispersion, and subtidal barotropic components. In long coastal-plain estuaries, subtidal vertical shear dispersion, resulting from the estuarine exchange flow, is usually the dominant mechanism driving the salt flux [e.g., Lerczak et al., 2006]. However, tidal dispersion/ pumping may dominate in regions of abrupt topographic changes such as headlands and inlets [Geyer and Signell, 1992]. What is the relative importance of tidal pumping and shear-dispersion mechanisms in supplying salt to the lagoonal estuary? Wind-driven flows have been shown to be an important mechanism for the estuary-shelf exchange at the mouth of some estuaries [e.g., Valle-Levinson and Bosley, 2001]. What is the contribution of the wind-driven flows to the salt flux into APS? [7] To answer these questions, we developed a 3-D hydrodynamic model for APS using Regional Ocean Modeling System (ROMS) [Haidvogel et al., 2000]. ROMS is a hydrostatic, primitive equation model using a curvilinear/ rectangular grid in the horizontal directions and a stretched, terrain-following coordinate in the vertical direction [Song and Haidvogel, 1994]. It has been successfully applied to several estuaries, including Hudson River [Warner et al., 2005b], Chesapeake Bay [Li et al., 2005; Li and Zhong, 2009], Columbia River and Oregon shelf [MacCready et al., 2009], and Puget Sound [Sutherland et al., 2011]. [8] In this article, we conduct hindcast simulations of APS estuary using the year 2003 as an example. Our objectives are twofold: (1) to investigate the dynamics of mean circulation and seek its driving mechanism and (2) to examine the salt balance in APS and study the processes that regulate salt flux through the inlets. In section 2, we introduce the ROMS model for APS and describe the validation of model results against observations. Section 3 is devoted to the dynamics of the mean circulation. Section 4 focuses on the salt budget in APS and the salt fluxes through the inlets. Concluding remarks are made in section 5.
2. Model Configuration and Validation [9] We have configured ROMS for APS. Bathymetry is extracted from high-resolution Coastal Relief Model data archived at NOAA’s National Geophysical Data Center. The model domain covers Albemarle Sound, Pamlico Sound, four major tributaries (Neuse, Pamlico, Chowan, and Roanoke rivers), and a part of the coastal ocean to facilitate free exchange between the estuary and adjacent shelves (Figure 1b). Coastal boundaries are specified as a finitediscretized grid via land/sea masking. The total number of grid points is 142 162. The grid spacing is about 1.4 km in most places, but finer resolution (about 200 m) is placed inside the three inlets that connect APS to the Atlantic Ocean (about 8 2 grid points in each inlet). The model has 20 vertical layers. A quadratic stress is exerted at the bed, assuming that the bottom boundary layer is logarithmic over a roughness height of 1 mm [Reyns et al., 2006; Xu et al., 2008]. The vertical eddy viscosity and diffusivity are computed using the k-kl turbulence mixing scheme with the background viscosity and diffusivity at 105 m2 s1 [Warner et al., 2005a]. The horizontal eddy viscosity and diffusivity are set to 1 m2 s1.
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[10] The model is forced by freshwater inflows at river heads, by tidal and nontidal flows at the offshore boundary, and by wind, heat, and freshwater exchanges across the water surface. At the upstream boundary in the four tributaries, daily freshwater inflows with zero salinity and time-varying temperatures are prescribed. The river flows obtained at the U.S. Geological Survey (USGS) gauging stations are multiplied by the ratio of the entire river’s drainage to the drainage area of the monitoring station [Lin et al., 2007]. Figures 2a and 2b show the time series of river flows in 2003: The Chowan and Roanoke rivers, which drain into Albemarle Sound; and the Neuse and Pamlico rivers, which drain into Pamlico Sound. [11] At the offshore open boundary, the condition for the barotropic component consists of a Chapman’s condition for surface elevation and a Flather’s condition for barotropic velocity. The boundary condition for the baroclinic component includes an Orlanski-type radiation condition for baroclinic velocity and a combination of radiation condition and nudging (with a relaxation time scale of 1 day) for temperature and salinity [Marchesiello et al., 2001]. Tidal forcing at the open ocean boundary is decomposed into 10 constituents (M2, S2, N2, K2, K1, O1, P1, Q1, Mf, and Mm) using the harmonic constants linearly interpolated from the Oregon State University global inverse tidal model of TPXO7 [Egbert et al., 1994; Egbert and Erofeeva, 2002]. For nontidal forcing at the open ocean boundary, we use monthly sea levels constructed from Simple Ocean Data Assimilation (SODA) [Carton et al., 2000a, 2000b]. The de-tided daily sea level observations acquired at NOAA Duck and Beaufort stations are added to the monthly SODA boundary conditions at the northern and southern boundaries, respectively. Baroclinic velocity is prescribed using the monthly data from SODA. Salinity and temperature fields at the open boundary are obtained from monthly Levitus climatology [Levitus, 1982]. [12] Air-sea fluxes of momentum, heat, and freshwater across the surface of APS are computed by applying standard bulk formulae [Fairall et al., 2003] to North America Regional Reanalysis (NARR; from National Center for Environmental Prediction) products [Mesinger et al., 2006]. Hourly winds are interpolated from NARR 3 hour winds and have been validated against observations at 14 weather stations scattered across APS (see http://www.wunderground. com). Figure 2c shows weekly mean wind vectors in 2003. Southeastward winds dominate during winter whereas winds are predominately northeastward during summer. Wind directions are variable during the spring and fall transition periods. Figure 2d shows the precipitation and evaporation rates at a location near Cape Hatteras. Although the evaporation and precipitation rates are in an approximate balance over a year, there are seasonal imbalances. [13] We conducted a hindcast simulation for 2003. To initialize the model for 2003, we ran the model for 2002 using the observed forcing data. Model outputs at the end of 2002 were used to set the initial condition for the salinity and temperature fields. The initial velocity field was taken to be zero, and the water surface was set at the mean sea level. [14] The model results have been validated against surface salinity measurements acquired by the Ferry Monitoring program (FerryMon hereafter). The state of North Carolina
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Figure 2. Discharges from the (a) Chowan River (dotted) and Roanoke River (solid), (b) the Pamlico River (dotted) and Neuse River (solid) during 2003. (c) Weekly mean wind speed vector at a central location in APS. (d) 5 day mean precipitation and evaporation rate near Cape Hatteras. uses the Department of Transportation Ferry Service to record water-quality parameters (such as salinity) aboard ferry boats [Buzzelli et al., 2003; Paerl et al., 2009]. Two routes span across Pamlico Sound, connecting the Outer Bank posts to the mainland locations, as shown in Figure 1a. Salinity data collected from FerryMon were processed for
quality control according to National Data Buoy Center’s (NDBC) standard procedures (NDBC Technical Document 09–02). Three stations (marked in Figure 1a) were selected for comparing the salinity time series (Figure 3). They occupy different salinity regimes: Station 1 lies halfway between the mouth of the Pamlico River and Ocracoke Inlet;
Figure 3. Comparison of model-predicted (gray lines) and observed (open circles) surface salinity time series at (a) station 1, (b) station 2, and (c) station 3. 4 of 16
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station 2 is located in southern Pamlico Sound; and station 3 is in the lower Neuse River. Because of the operational constraints of ferries and applications of data-quality control, the observational data were sparse during some periods. Nevertheless, there is reasonable agreement between the observed and modeled salinity time series. In particular, the model captures spring freshening at station 1 accurately. It also reasonably reproduces the seasonal variation at station 2, although the model overpredicts salinity in the second half of the year. At station 3, the few available data points fall onto the modeled salinity time series. We have also used the FerryMon data to test the model predictions for the horizontal salinity distribution along the ferry route between Ocracoke Inlet at the Outer Banks and Swan Quarter at the Pamlico River. There is reasonable model-data agreement under conditions of both strong and weak horizontal salinity gradients. [15] In addition to the salinity comparison, we compared the model-predicted subtidal sea level fluctuations against observations near tide gauges inside and outside APS: NOAA station at Duck, North Carolina; USGS station at Washington, North Carolina in the Pamlico River; and USGS station Fort Barnwell in the Neuse River. There is good agreement between the predicted and observed sea level at the three stations (figure not shown). The correlation coefficient is 0.98 and the root-mean-square error is 5 cm. [16] These model-data comparisons have shown that the ROMS model does a reasonable job in capturing the observations in APS. Some of the discrepancies between the predicted and observed salinities may be attributed to inaccuracies in the atmospheric forcing and river runoff data and the use of monthly climatological data at the offshore open boundary. In particular, the atmospheric model forecasts for evaporation and precipitation are prone to errors and would directly affect the surface salinity that is being compared against the FerryMon measurements. Developing a hindcasting model would require the use of observations of rainfall and offshore conditions for 2003, but the data were not available. To check if the model results were sensitive to grid resolution, we conducted another model run with coarser grids (about 400 m at the inlets) and found that the modelpredicted salinities were only slightly different. Therefore, the fine-resolution ROMS model resolves major physical processes in APS and is suitable for investigating physical mechanisms governing the circulation dynamics and salt balance in this lagoonal estuary.
3. Dynamics of the Mean Circulation [17] Previous investigations have shown that the stratification in APS is weak except in and near the tributaries [e.g., Roelofs and Bumpus, 1953; Pietrafesa et al., 1986]. However, the mean circulation pattern in this weakly stratified estuary is not well known. It is not clear if the circulation is driven by winds or by the horizontal density gradients set up by geographically separated freshwater (rivers) and saltwater sources (inlets). In this section, we investigate the dynamics of the mean circulation and seek its driving mechanism. [18] The annual mean surface salinity is shown in Figure 4a. Albemarle Sound receives freshwater from the Chowan and Roanoke rivers and does not have direct exchanges with the Atlantic Ocean. Hence, salinity in Albemarle Sound is low (
