Enhancement of Semidiurnal Tidal Currents in the Tidal Inlets to Mississippi Sound

Abstract

Harmonic analysis of data collected in Mississippi Sound and the adjacent continental shelf in 1980 and 1981 indicates a pronounced difference between offshore and inshore tidal current regimes. On the shelf, the form number of the water levels and currents are approximately equal (6–7), while currents in the tidal inlets to Mississippi Sound have form numbers less than half that of the corresponding water levels. Most of the variation in current form number can be explained by a change from a two- to a one-dimensional flow field. Theoretical form numbers derived for Sverdrup waves and uniform flow through an inlet indicate continuity causes semidiurnal currents to increase relative to diurnal currents at the inlets and results in the reduced current form number. Water levels are not affected by the continuity constraint. Variations in the observed current form numbers from theoretical values are caused by compound tides and overtides of the diurnal constituents, which have contaminated the harmonic constant estimates of semidiurnal currents. The K₂/K₁ ratio (K₂ representing the first harmonic of K₁ generated locally) is estimated to be 0.3 in the inlet currents, and the two waves differ in phase by 180°. The enhancement of semidiurnal currents should occur in most tidal inlets bordering mixed or diurnal seas.