Effect of Wave-Enhanced Bottom Friction on Storm-Driven Circulation in Massachusetts Bay

Abstract

Massachusetts Bay is a shallow (35 m average depth) semienclosed embayment, roughly 100 × 50 km, which opens into the Gulf of Maine at its eastern boundary. Surface waves associated with winter storm winds from the northeast cause large sediment resuspension events, and wave and circulation fields during these events have a quasi-steady response to the wind stress. Coupled wave, circulation, and boundary layer models indicate that wave-enhanced bottom friction has a significant damping effect on storm-driven circulation in Massachusetts Bay. The simulated response exhibits significant three-dimensional structure, but still can be fundamentally understood using idealized models. The depth-integrated momentum balance is dominated by along-bay stress, pressure gradient, and bottom stress. The effective bottom drag coefficient during typical storm conditions is increased by a factor of 2–5 when wave effects are included, but the mean bottom stress is relatively unaffected by wave effects due to a reduction in bottom currents by 30–50. The vertical mixing is also relatively unaffected by the waves, and the result is that the increased drag causes a nearly depth-independent offset of the vertical current profiles. The alongshore transport in the bay is reduced 10–50, depending on wind direction.