Tidal and subtidal lateral structures of density and velocity in the Chesapeake Bay Entrance

Hydrographic and velocity data collected along the Chesapeake Bay Entrance during seven 25-hours cruises centered around neap or spring tides, were used to assess the fortnightly variability in the transverse structure of the tidal and subtidal density and velocity fields. Amplitude and phase of the semidiurnal and diurnal harmonics reflected the influence of bathymetry, buoyancy conditions and fortnightly variability. Tidal amplitudes were almost twice as large in springs than in neaps. Both Chesapeake and North Channels had similar along-channel amplitudes, which suggested a higher vertically integrated mixing energy in North Channel. Tidal velocity convergences were more intense for springs than for neaps, and also for low than high buoyancy input conditions. In terms of subtidal properties, the vertical stratification showed fortnightly variability in North Channel (increased during neaps) but not in Chesapeake Channel. Consequently, the transverse density gradients and the transverse velocities were stronger during springs than during neaps. These patterns were likely linked to stronger gravitational circulation in Chesapeake Channel during springs and in north Channel during neaps. The shallow regions, Middle Ground and Six-Meter Shoal, usually stayed vertically homogeneous and velocity was unidirectional through the water column. The A simple 2-dimensional linear model based on the balance between the horizontal density gradient and vertical friction along the channel, and a balance among horizontal density gradient vertical friction and Coriolis acceleration was solved for an analytical bathymetry similar to that of the Chesapeake Bay. Results from the model were consistent with the main characteristics of the observed subtidal velocity.