Rotation and Vertical Mixing Effects on Volume Exchange in Eastern Long Island Sound

A series of three-dimensional numerical simulations are carried out to illustrate the effects of the Earthʹs rotation, tidal forcing and vertical mixing on the volume exchange in an idealized estuary with sill bathymetry and in Long Island Sound (LIS). Simulations over a simplified sill and in LIS show that the effects of rotation on flow over a sill are to enhance recirculation and to reduce volume exchange. Near-surface recirculation allows net inflow of dense water over the deepest part of a sill located near LIS mouth and also generates a ‘ Z-shaped ’ flow pattern consisting of a seaward boundary current along the Long Island coast, weak outflow along Connecticut, and southward and westward circulation between the two outflow regions. Near-bottom inflow tends to follow isobaths around the sill and form a landward current along the northern slope of LIS. Incorporation of bottom friction during neap tides generates a bottom-mixed layer. Vertical mixing decreases the reduced gravity and, therefore, the strength of both near-surface and near-bottom boundary currents. Vertical mixing also reduces volume exchange across any given cross-section. During spring tides, boundary current formation is suppressed by increased vertical mixing and the effects of rotation become less evident. In neap tides, flow tends to follow isobaths and inertial effects are large. In contrast, during spring tides, flow crosses isobaths and is dominated by the tidal prism advecting a horizontal density gradient. Also in spring tides, the bottom mixed layer is thicker and exchange is reduced with respect to neap tides, thus hindering bottom water renewal to the landward side of the sill.