SPATIAL AND TEMPORAL VARIABILITY OF THE VELOCITY STRUCTURE IN A WEAKLY STRATIFIED SYSTEM, BROAD SOUND, CASCO BAY, MAINE

Abstract:

The circulation structure across Broad Sound, a weakly stratified system was characterized using velocity data from two moorings in late summer/fall of 2013 and velocity and density data from a ship survey conducted over a tidal cycle during the same period. The tidal signal demonstrated a strong lateral gradient in the along-sound flow structure and complex patterns were observed in the intratidal velocity data. Local wind forcing in large part, drove fluctuations at the subtidal time scales. Despite having a triangular bathymetric cross section, the wind-driven circulation was vertically sheared as a result of the relatively deep nature of the sound, reducing the importance of bottom friction and hence bottom bathymetry. In contrast to the slopes, the thalweg had a three-layer response to wind forcing with near-bottom fluctuations being positively correlated with wind. This response is speculated to result from alterations in the density gradient that could arise from downwelling (upwelling) of freshwater (denser) water at the head of the bay. Furthermore, Coriolis force was an important factor in structuring the mean circulation having strong lateral shear with inflow (outflow) over the thalweg and eastern slope (western slope). The qualitative similarities (i.e., lateral and vertical shear structure) to analytical models indicate density-driven circulation is a significant component of the mean circulation.