The M2 Tide and Its Residual on the Outer Banks of the Gulf of Maine

Abstract

Tidal computations are reported for the Gulf of Maine with emphasis on its seaward banks. Georges Bank, Browns Bank, Nantucket Shoals, and the nearshore region off Cape Sable. The model is 3D and nonlinear, with quadratic vertical viscosity; stratification is ignored. The objective is to establish a theoretical baseline for tidal and residual currents on detailed topography against which seasonal changes may be examined. Two triangular meshes are used. The base case corresponds to earlier studies with roughly 6-km resolution on the banks. A more refined and extended mesh provides resolution at 2?3 km in critical areas and has boundaries farther removed from the banks. Both meshes provide very good fits with observed M2 elevations. Comparison of computed and observed M2 current profiles at 76 sites reveals general agreement that improves with resolution. For the refined mesh and the 48 sites with records exceeding 60 days, average deviations for M2 ellipse parameters are (6.1, 4.7) cm s?1 for the major and minor axes, and (10, 8) deg for phase and orientation. Ellipse deviations on the coarser mesh were larger by 17% to 50%. Detailed vertical profile comparisons at 11 representative sites illustrate the general level of agreement with this baseline result. The Eulerian residual current is significantly altered in key arm by mesh refinement from 6 to 3 km. Notably, speeds on the finer mesh are strengthened by roughly 40% on Browns Bank and on the northern flank of Georges Bank. Enhanced resolution also admits details of recirculation on Browns Bank that were poorly resolved at 6 km and an Nantucket Shoals. A closed gyre over Georges Basin, which has not been present in previous model results, occurs with the fine mesh. Seaward extension of the boundary provides noticeable relief of an otherwise constrained solution in the Northeast Channel. Transects across key sections reveal complex vertical residual structure including cross-bank and interbank flow patterns. Unresolved local bathymetric features appear to influence the local residual and deserve further attention, as does the characterization of Lagrangian circulation.