Barotropic tides on the North Carolina shelf

Observations from 18 near-bottom pressure sensors and 8 current meter moori ngs provide a characterization of the barotropic tides over the North Carol ina continental shelf between Chesapeake Bay and Cape Hatteras. The largest tidal constituents in this region are the M-2 (sea level amplitude 47 cm), N-2 (11 cm), and S-2 (10 cm) semidiurnal tides and the K-1/P-1 (7 cm) and O-1 (5 cm) diurnal tides. The barotropic tidal currents are small, less tha n 3 cm s(-1), except for the M-2 current which reaches 10 cm s(-1) at mid s helf. Cross-shelf currents increase linearly from approximately zero near t he 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 ove r the mid and outer shelf. While bottom friction is weak over most of the s helf, in water depths of less than 10 m bottom friction results in a rapid phase decrease toward the coast in the semidiurnal alongshelf velocities. T his phase difference is not observed in the K-1/P-1 alongshelf flow, possib ly because diurnal wind forcing causes an opposing phase increase toward th e coast. Alongshelf variations in tidal amplitudes and phases near the coast are muc h larger than expected based on variations along the shelf break inferred f rom basin-scale altimeter estimates. The shelf width in this region increas es from 25 to 100 km over an alongshelf distance of about 150 km. Results f rom an analytic, flat-bottom, two-dimensional model indicate that widening of the shelf toward the north causes the observed alongshelf increase in th e semidiurnal amplitude and phase, but does not explain alongshelf variatio ns in the weaker diurnal constituents.