Dynamics of wind-driven circulation in a shallow lagoon with strong horizontal density gradient

Albemarle-Pamlico Sound (APS) is a shallow lagoonal estuary with weak stratification but strong horizontal density gradient. Idealized and realistic model simulations are conducted to examine the dynamics of wind-driven circulation in APS. Two dimensionless parameters, the Ekman (Ek) and Wedderburn (W) numbers, are found to be two key parameters in controlling the flow pattern and dynamics. Circulation in Albemarle Sound and northern Pamlico Sound is primarily driven by wind-forcing, and shows laterally sheared flows with Ek approaching 1 and W exceeding 1. Analysis of the along-channel momentum equation shows a balance among the wind stress, barotropic pressure gradient due to sea level setup, and bottom friction, while the vertical vorticity is primarily determined by the balance between the wind stress curl and bottom stress curl. In contrast, circulation in southern Pamlico Sound is driven by both wind and gravitational forces, and shows vertically sheared two-layer flows with low Ek and W around 1. Both baroclinic and barotropic pressure gradients act against the wind stress to drive the two-layer flows. A regime diagram in the parameter space of Ek and W is used to summarize variability of circulation pattern on the seasonal time scale. Northern Pamlico Sound is characterized by high Ek and high W values and laterally sheared currents. Southern Pamlico Sound is usually characterized by low Ek and low W values and vertically sheared flows but featuring laterally sheared flows during the fall when Ek averages to over 0.5.