Barotropic Tides on the North Carolina Shelf

Abstract

Observations from 18 near-bottom pressure sensors and 8 current meter moorings provide a characterization of the barotropic tides over the North Carolina continental shelf between Chesapeake Bay and Cape Hatteras. The largest tidal constituents in this region are the M2 (sea level amplitude 47 cm), N2 (11 cm), and S2 (10 cm) semidiurnal tides and the K1/P1 (7 cm) and O1 (5 cm) diurnal tides. The barotropic tidal currents are small, less than 3 cm s?1, except for the M2 current which reaches 10 cm s?1 at mid shelf. Cross-shelf currents increase linearly from approximately zero near the coast to a maximum over the mid and outer shelf. Alongshelf currents are nonzero near the coast and increase to roughly twice the coastal value over the mid and outer shelf. While bottom friction is weak over most of the shelf, in water depths of less than 10 m bottom friction results in a rapid phase decrease toward the coast in the semidiurnal alongshelf velocities. This phase difference is not observed in the K1/P1 alongshelf flow, possibly because diurnal wind forcing causes an opposing phase increase toward the coast. Alongshelf variations in tidal amplitudes and phases near the coast are much larger than expected based on variations along the shelf break inferred from basin-scale altimeter estimates. The shelf width in this region increases from 25 to 100 km over an alongshelf distance of about 150 km. Results from an analytic, flat-bottom, two-dimensional model indicate that widening of the shelf toward the north causes the observed alongshelf increase in the semidiurnal amplitude and phase, but does not explain alongshelf variations in the weaker diurnal constituents.