On the dynamics of shallow water currents in Massachusetts Bay and on the New England continental shelf

Abstract

Massachusetts Bay is a coastal Bay 100 km long and 40 km wide located in the western Gulf of Maine. The Bay is closed by land to the north, west and south, but is open to the Gulf to the east; the opening is partially blocked by a shallow bank. The bottom sediment distribution in the Bay is complex; fine grained material is found in the deep basin, sand and gravel on the shallow bank, and mixtures of sand, gravel and fine material nearshore. Richardson current meters were moored 1 m from the bottom over a one year period at several locations in the Bay to study the bottom currents and the equilibrium between current and sediments. The current measurements suggest that the bottom sediments can be expected to move only occasionally in certain areas. The maximum bottom speeds are principally determined by the strong tidal currents in the basin. In winter, the near bottom currents are dominated by wind stress associated with strong storms. Bottom currents in the shallow areas are generally in the direction of the wind while currents in the deep portion of the basin are often opposite to the direction of the wind. Sea surface setup in the direction of the wind is observed, as well as absolute changes in sea level as the Bay adjusts to changes in the level of the adjacent Gulf of Maine. Adjustment of the bottom currents to wind events requires approximately 12 hours. Moored current meter measurements and synoptic hydrographic observations made in Massachusetts Bay show that freshening from the spring runoff dominates the low frequency currents and the hydrography of the Bay in the spring months. The major freshening is attributed to the Merrimack River which empties into the Gulf of Maine 30 km to the north of the Bay; discharge of the Merrimack increases by at least a factor of two in spring. Flow directly into the basin from several smaller rivers is not important. Two major features are found: a fresh surface plume confined to the upper 10 m of the water column which becomes more distinct as the seasonal thermocline develops, and a large deep fresh lens. Flow is clockwise around the deep lens and is consistent with the thermal wind relation. Sustained currents of 10-20 cm sec -1 with time scales of 5-10 days were observed as the deep lens (or lenses) slowly advected through the basin. Current observations made in the previous spring show similar low frequency behavior. Two simple linear models of the semidiurnal tide on the continental shelf are used to estimate the vertical turbulent eddy viscosity, a linear bottom drag coefficient, and the change in the bottom drag coefficient during storms. The analytic solution for the response of a homogeneous water column with constant eddy viscosity to a sinusoidal body force with a slip bottom boundary condition is presented. with measurements of the tidal current at two depths, four parameters are shown to be independent of the body force: the ratio of the clockwise current at two depths, the ratio of counterclockwise current at two depths, the change in the tidal ellipse orientation, and the change in phase of the tidal ellipse. Observations of the semidiurnal tidal current on the New England continental shelf are consistent with a vertical eddy viscosity of 20-50 cm2 sec -1 and a bottom drag coefficient of .02-.05 cm sec -1. The Ekman depth is thus 10 m and the integrated adjustment time is approximately 28 hours. An integrated linear model with linear damping of the semidiurnal tide on the continental shelf, forced uniformly at the shelf edge, shows an increasing phase lag of the tide at the coast with increased damping; amplitude remains relatively constant over a wide range of damping coefficient. Observations of the tide at the coast during storms shows a phase lag of as much as 10 degrees for the semidiurnal tide. For approximate dimensions of the New England shelf, this implies an increase by a factor of 3-5 of the bottom drag coefficient and an integrated motion adjustment time of 6-9 hours. Waves may be an important contribution to the increased bottom stress.