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Effects of Winds on Stratification and Circulation in a Partially Mixed Estuary

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Effects of Winds on Stratification and Circulation in a Partially Mixed Estuary JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, C12012, doi:10.1029/2010JC006893, 2011 Effects of winds on stratification and circulation in a partially mixed estuary Yun Li1 and Ming Li1 Received 15 December 2010; revised 9 August 2011; accepted 30 September 2011; published 13 December 2011. [1] Numerical experiments are conducted to investigate how axial winds affect stratification and circulation in the partially mixed estuary of Chesapeake Bay. In the absence of rotational effects, stratification in the estuary decreases following both down-estuary and up-estuary winds, but stratification experiences larger reduction and takes longer to recover under up-estuary winds. In the presence of rotational effects, wind-driven lateral circulations cause the lateral straining of density field and weaken the shear in the along-channel flows. Under the down-estuary winds, a counterclockwise lateral circulation steepens isopycnals in the cross-channel sections, while the Coriolis force acting on it decelerates the downwind current in the surface layer and the upwind-directed current in the bottom layer. Under the up-estuary winds, a clockwise lateral circulation flattens isopycnals in the cross-channel sections and reduces the shear between the surface and bottom currents. Hence, in the presence of rotational effects, the lateral straining offsets the effects of longitudinal straining such that the asymmetry in stratification reduction is significantly reduced between the down-estuary and up-estuary winds. Regime diagrams based on Wedderburn (W) and Kelvin (Ke) numbers are constructed to summarize the net effects of winds on estuarine stratification during both wind perturbation and postwind adjustment periods. Citation: Li, Y., and M. Li (2011), Effects of winds on stratification and circulation in a partially mixed estuary, J. Geophys. Res., 116, C12012, doi:10.1029/2010JC006893. 1. Introduction vertical shear, thus reducing stratification. Wilson et al. [2008] and O’Donnell et al. [2008] suggested that along- [2] Most of the research in estuarine dynamics has focused channel wind straining regulates stratification and turbulent on the effects of tides. Relatively little attention has been mixing, thereby influencing the flux of oxygen into hypoxic paid to the role of winds in estuarine circulation, despite regions of western Long Island Sound. early predictions of first-order effects [Bowden, 1953; [4] Using a numerical model of an idealized estuarine Rattray and Hansen, 1962] and observational evidence of channel featuring a triangular cross section, Chen and strong wind driven flows [e.g., Wang, 1979a, 1979b; Sanford [2009] found that the net effect of winds on estua- Goodrich et al., 1987; Wong and Moses-Hall, 1998; Wong rine stratification depends on the competition between and Valle-Levinson, 2002]. Recent studies have suggested wind-driven mixing and wind-induced straining: moderate that wind effects are not limited to mixing in the vertical down-estuary winds enhance estuarine stratification whereas direction. Since estuaries typically have strong horizontal strong down-estuary winds and all up-estuary winds reduce density gradients, wind-driven currents can significantly stratification. They proposed a hypothetical diagram to alter estuarine stratification through the straining of density classify the wind effects on estuarine stratification and sug- field. gested that the Wedderburn number and the ratio of the [3] North et al. [2004] and Scully et al. [2005] observed surface mixed layer to the water depth are two important stratification and exchange flows that increased during nondimensional parameters. How do the results from this moderate down-estuary winds but decreased during moder- idealized estuary apply to real estuaries with complex ate up-estuary winds. Scully et al. [2005] proposed a wind bathymetry? The Chesapeake Bay features broad shallow straining mechanism analogous to Simpson’s tidal straining: shoals and a narrow, deep center channel. What will be the down-estuary wind enhances subtidal vertical shear and net effects of wind-induced mixing and straining? Chen and strains the along-channel density gradient to increase strati- Sanford [2009] did not consider the effects of Coriolis force fication; up-estuary wind reduces or even reverses the in their modeling study. The width of Chesapeake Bay and other similar estuaries is comparable to or larger than the 1 internal Rossby radius of deformation. How does the Earth’s Horn Point Laboratory, University of Maryland Center for Environmental Science, Cambridge, Maryland, USA. rotation affect the estuarine response to wind forcing? [5] The response of wind-driven circulation in the along- Copyright 2011 by the American Geophysical Union. channel direction has previously been interpreted in terms of 0148-0227/11/2010JC006893 C12012 1of16 C12012 LI AND LI: WIND EFFECTS ON STRATIFICATION C12012 the competition between the wind stress and barotropic Chesapeake Bay and validated against observational data [Li pressure gradient due to sea level setup [Wang, 1979b; et al., 2005, 2007; Li and Zhong, 2009; Zhong and Li, 2006; Garvine, 1985; Chuang and Boicourt, 1989; Janzen and Zhong et al., 2008]. We use this model to investigate the Wong, 2002]. While this two-layer theory seems well estab- effects of winds on the circulation and stratification in lished, a number of studies in Chesapeake Bay have shown Chesapeake Bay. that along-channel winds can drive strong lateral Ekman [9] The model domain includes eight major tributaries and flows and isopycnal movements, generating upwelling/ a part of the coastal ocean to facilitate free exchange across downwelling at shallow shoals [Malone et al., 1986; Sanford the bay mouth (Figure 1). The total number of grid points is et al., 1990; Scully, 2010]. The lateral flows can interact with 120 Â 80. The model has 20 layers in the vertical direction. cross-channel density gradient in a way analogous to the A quadratic stress is exerted at the bed, assuming that the straining of density field in the along-channel direction. bottom boundary layer is logarithmic over a roughness Without wind forcing, a freshwater plume hugs the western height of 0.5 mm. The vertical eddy viscosity and diffusivity shore as it moves seaward and isopycnals tilt downward on are computed using the k-kl turbulence closure scheme the western side of a cross-channel section. Southward [Warner et al., 2005] with the background diffusivity and (down-estuary) winds generate downwelling on the western viscosity set at 10À5 m2 sÀ1. Coefficients of horizontal eddy shore and may tilt the isopycnals toward the vertical direc- viscosity and diffusivity are set to 1 m2 sÀ1, which produce tion, reducing stratification. On the other hand, moderate little dissipation of the resolved flow energy [Zhong and Li, northward (up-estuary) winds may flatten isopycnals in 2006]. The model is forced by sea level fluctuations, tem- cross-channel sections, enhancing stratification in the water perature and salinity at the open ocean boundary, by fresh- column. These lateral processes may offset the effects of water inflows at river heads and by winds across the water wind-driven straining in the along-channel direction. More- surface. The open-ocean boundary condition for the baro- over, recent modeling investigations of secondary flows in tropic component consists of Chapman’s condition for sur- tidally driven estuaries have shown that lateral advection can face elevation and Flather’s condition for barotropic be of first-order importance in the along-channel momentum velocity. The boundary condition for the baroclinic compo- balance [Lerczak and Geyer, 2004; Scully et al., 2009]. It is nent includes an Orlanski-type radiation condition for bar- likely that wind-driven lateral circulations will also affect the oclinic velocity. To deal with both inward and outward dynamics and structure of along-channel flows, thereby scalar fluxes across the open boundary, we use a combina- indirectly affecting the density straining in the longitudinal tion of radiation condition and nudging (with a relaxation direction. time scale of 1 day) for temperature and salinity [6] Several recent papers have investigated the dynamics [Marchesiello et al., 2001]. and effects of wind-driven lateral circulations. In the absence [10] We focus on winds in the along-channel (south– of rotational effects, Chen et al. [2009] showed that differ- north) direction since winds in the cross-channel (east–west) ential advection of the axial salinity gradient by wind-driven direction have short fetches. Weather systems passing over axial flow drives bottom-divergent/convergent lateral circu- the Chesapeake Bay have typical periods of 2 to 5 days lation during down-estuary/up-estuary winds. In an idealized [Wang, 1979a]. In this study, we impose a spatially uniform rotating basin, Reyes-Hernández and Valle-Levinson [2010] wind forcing, explored wind modifications on the lateral structure of den- sity-driven flow. Guo and Valle-Levinson [2008] examined A sin½wðÞt À 25 25 ≤ t ≤ 27:5 t ¼ ; ð1Þ how winds affect the lateral structure of density-driven cir- W 0 other times culation in Chesapeake Bay. Using a simplified oxygen model, Scully [2010] investigated wind-driven ventilation of t hypoxic waters in Chesapeake Bay and found that northward where W is the along-channel wind stress, t is the time w ¼ 2p winds were most effective at supplying oxygen to hypoxic (days), 5day is the frequency of the wind forcing, and A regions whereas eastward winds were least effective. These is the peak wind stress. We study both down-estuary interesting papers motivate the current research which is (southward) and up-estuary (northward) winds: positive tW directed at understanding how wind-driven along-channel corresponds to up-estuary winds. The maximum wind stress and cross-channel flows affect the stratification response in magnitude A ranges from 0.005 to 0.25 Pa, with the corre- À the partially mixed estuary of Chesapeake Bay. sponding range of 2.35 to 12.27 m s 1 for the wind speed [7] The plan for this paper is as follows.
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