Circulation and exchange processes between Casco Bay and the Adjacent Gulf of Maine

by: Janzen *, Pettigrew, and Fisher (University of Maine, School of Marine Sciences, Orono, Maine, USA)
* presenter

Circulation and exchange processes between Casco Bay and adjacent Gulf of Maine are examined using moored and shipboard observations collected over the 2004 seasonal cycle. In this study, supported by the Maine Oil Spill Advisory Committee and the Maine Department of Environmental Protection, three Oceanographic Data Acquisitions System (ODAS) buoys were deployed for 10 months in the principal exchange channels separating Casco Bay interior from the Gulf of Maine. Each buoy was equipped with temperature/salinity sensors, and acoustic Doppler current profiler, a global positioning system, a data logger and cellular phone telemetry. Lateral transects of Doppler and CTD profiles were also collected over a tidal period across the channels to examine the spatial and temporal details of the exchange flows.

Broad Sound Buoy with wind sensor
 

Moored data indicate that subtidal flow characteristics are distinct between the channels, and suggest a mean cyclonic Bay/shelf circulation throughout the year.  Subtidal currents appear to be barotropic and directed out of the bay throughout the watercolumn in Portland Channel. In contrast, Hussey Sound exhibits a two-layered circulation similar to gravitational circulation with subsurface flows exiting the bay, and deeper flows entering the bay. Notice how the inflow amplifies with depth in Hussey Sound. Broad Sound also indicates a depth-dependent flow structure,  but differs from Hussey Sound in that the subsurface flows (6m) reverse periodically, while showing a net inflow with depth. Again, amplified inflows occur deeper in the water column in Broad Sound. March - June 2004 subtidal currents at multiple depths can be viewed at the following links:

Portland Channel Subtidal Currents
Hussey Sound  Subtidal Currents
Broad Sound Subtidal Currents

Transport estimates for each channel are being made to determine if there is a near volume exchange balance between the three channels, or if possibly, as indicated in preliminary results, Portland Channel and Hussey Sound nearly balance themselves. A near transport balance between Portland Channel and Hussey Sound could indicate a potential for two cyclonic circulations occurring in the Bay, where the southern circulation cell is somewhat isolated from a circulation cell in the northern bay. This possibility is being explored using the spatially sampled shipboard ADCP transect data, and with newly planned observations in the conduit adjacent to Little John Island that connects the northern and southern portions of the Bay.

The temperature/salinity time series records from each mooring (near surface and near bed) indicate that Portland Channel and Hussey Sound export more freshwater than Broad Sound. During the spring freshet, there does not appear to significant influxes of freshwater via Broad Sound, as might be expected from the Kennebec River influence on the adjacent shelf; however, correlations between discharge and TS data need to be quantified to confirm this hypothesis.  Freshwater sources within the northern Bay are minimal by comparison to the external supply of freshened water on the adjacent shelf. Though previous studies indicate Casco Bay freshening during northeasterly winds and high discharge conditions from the Kennebec/Androscoggin Rivers (see Janzen et al., 2005, DSR II - listed on CV), there is no strong indication of such during this study. Further analysis is required to determine the role of wind forcing on the TS time series during the spring freshet to see if this might have influenced the import, or lack thereof, of external sources of freshwater into Casco Bay.

Links to TSD time series for each channel during the freshet period (March - June) are provided at the following links. The date/time stamp is in Julian Days at the moment.

Portland  ChannelTSD Time Series
Hussey Sound TSD Time Series
Broad Sound TSD Time Series

Ongoing analysis includes spectral time series and coherence statistics to examine the subtidal forcing of currents in the three channels. The main focus of this portion of the study is to examine the wind forcing, both remotely forcing coastal shelf dynamics, and locally directly over the bay. Coastal winds and wind stress experienced within the Bay in Broad Sound are not significantly different. Click on the following link to see the North/South and East West Components of the winds from a buoy located offshore of Casco Bay (coastal GoMOOS Buoy C) and the Broad Sound Buoy (bs).  The coastal and bay interior wind vector records show a complex correlation coefficient of 0.9565, with a slight veering of -12.05 (Broad Sound veers to the right of the coastal winds by -12.05 degrees) and amplitude reduction  inside the Bay (0.84 m/s Broad Sound wind speed per 1 m/s coastal wind speed).
However, caution is advised in assuming the coastal wind field can represent the bay wide wind conditions. Broad Sound is wider and is less disrupted by islands which can modify local wind fields dramatically. Being that Portland Channel and Hussey Sound buoys were located near the mouth of the Bay channels, we are going to assume the coastal winds can be used to examine local forcing at these two sites.