Spatial and temporal variability of the velocity and hydrographic structure in a weakly stratified system, Broad Sound, Casco Bay, Maine

Abstract

AbstractThe velocity and hydrographic structure across Broad Sound, a north‐south orientated subsystem of Casco Bay, ME that lacks continuous coastal boundaries, were characterized using velocity observations from two moorings in late summer/fall of 2013 and velocity and density observations from a repeat‐transect ship survey conducted over a tidal cycle during the same period. At tidal time scales, the system is dominated by a barotropic semidiurnal standing wave with a west to east decrease in tidal amplitude and relatively minimal phase change across the majority of the transect. The stratification (vertical differences of 0.5–1.0 kg m−3) was generally laterally uniform and stronger during the flood phase which is hypothesized to result from stronger offshore stratification. The mean circulation had strong lateral shear with inflow over the deepest point in the bathymetric cross section and eastern slope and outflow over the western slope. There was also vertical shearing of the horizontal velocities with stronger northward (or northward trending) velocities at depth. The depth‐averaged subtidal fluctuations were relatively small (∼2–3 cm s−1) and uncorrelated between mooring sites suggesting the vertically uniform current response associated with remote wind forcing is of limited importance. On the other hand, the depth‐dependent velocity fluctuations at the subtidal time scale were, in large part (∼36–72%), driven by wind forcing. The net flux ratio, a means of quantifying the relative importance of the vertical and lateral shear in the flow field, was typically ∼0.44 indicating the structure of the local wind response favored vertically sheared flow.