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Physical Setting

Massachusetts Bay is a semi-enclosed embayment that opens into the Gulf of Maine at its eastern boundary (Figure 1.1).

It is roughly 100 km long, 50 km wide and has an average water depth of 35 m. The bay is bounded on the east by Stellwagen Bank, which rises to within 20 m of sea surface. Boston Harbor empties into western Massachusetts Bay, providing a significant source of contaminants to the region.

Circulation in Massachusetts Bay is driven by a combination of local and remote processes that vary with season (Geyer et al, 1992). Throughout the year water flows southward in the western Gulf of Maine, and although most of this current continues flowing southward over the eastern flank of Stellwagen Bank (largely bypassing Massachusetts Bay), a small branch flows into the bay and drives a weak counter-clockwise flow that enters at Cape Ann and exits at Race Point. The magnitude of this flow varies from less than 1 cm s off Boston to about 3-4 cm s along the western shore of Massachusetts Bay.

The remotely driven mean circulation pattern is modified by seasonal current regimes. During the winter, the bay is well-mixed vertically, and the wind is principally from the northwest, reinforcing the counterclockwise circulation. In the spring, surface warming and freshwater intrusions from Gulf of Maine rivers cause the bay to become stratified, and the surface currents in the bay are dominated by the strong and variable density driven flow associated with these intrusions. By summer, the stratification has intensified due to further surface warming. The dominant wind direction in summer is from the southwest, which drives strong upwelling events along the western and northern shores of the bay. In the fall, rapid cooling on the shallow western side of the bay results in a density field that temporarily reverses the surface mean flow.

Tidal currents in Massachusetts Bay are dominated by the semi-diurnal M constituent (period = 12.42 hours). The currents are largely bidirectional and the magnitude ranges from about 10 cm s in the interior of Massachusetts Bay to more than 50 cm s off the tip of Cape Cod and in the entrances to Boston Harbor and Plymouth Harbor (Blumberg et al, 1993, Irish and Signell, 1992). Although the transport due to tides is generally oscillatory and therefore doesn't give rise to net transport of material, they are an important source of bottom-generated turbulent mixing.