The Strength of the M2 Tide at the Chesapeake Bay Mouth

Abstract

As part of the Naval Research Laboratory and Office of Naval Research sponsored Physics of Coastal Remote Sensing Research Program, an experiment was conducted in September?October 1996 off Virginia Beach. Ocean surface currents were measured using the high-frequency (25.4 MHz) mode of the Ocean Surface Current Radar at 20-min intervals at a horizontal resolution of 1 km over an approximate 30 km ? 44 km domain. Comparisons to subsurface current measurements at 1?2 m beneath the surface from two broadband acoustic Doppler current profilers (ADCP) revealed good agreement to the surface currents. Regression analyses indicated biases of 4 and ?3 cm s?1 for cross-shelf and along-shelf currents, respectively, where slopes were O(1) with correlation coefficients of 0.8. Nine months of sea level heights from the NOAA National Ocean Survey Chesapeake Bay Bridge Tunnel tidal station revealed an energetic M2 tidal component having an amplitude of 37.5 cm and a phase of 357°. The S2 tidal constituent had an amplitude of 7 cm and a phase of 49°. By contrast, the diurnal band (K1,?O1) tidal constituents were considerably weaker with amplitudes of 1?5 cm. From 19 days of HF-derived surface currents, the M2 and S2 tidal current amplitudes had a maximum of about 50 and 8 cm s?1 at the Chesapeake Bay mouth, respectively. Explained variances associated with these four tidal current constituents were a maximum of 60% at the mouth and decreased southward. Analyses at the ADCP moorings indicated that the semidiurnal tidal currents were predominantly barotropic with cross-shelf and along-shelf currents of 18 and 10 cm s?1. Energetic semidiurnal tidal currents were highly correlated over the HF-radar domain, and the phase angles indicated a consistent anticyclonic veering of the M2 tidal current with along-shelf distance from the mouth. Normalized tidal current vorticities by the local Coriolis parameter (f), which represent a proxy for the Rossby number, were ±0.25f near the mouth and ±0.05f in the southern part of the domain for the M2 constituent. Simulations from a linear, barotropic model were highly correlated with observed M2 tidal currents at 85 points within the HF-radar domain, consistent with the premise of weakly nonlinear flows.