Storm-induced semidiurnal perturbations to surges on the US Eastern Seaboard

• Tide–storm interactions trigger semidiurnal surges in the mid-South Atlantic Bight.
• Semidiurnal surges are associated with tropical and extra-tropical cyclones.
• Intense along-shelf currents can cause tidal amplitude reduction and delay.
• Storm-increased bottom friction and Coriolis force during storms modify the tidal propagation.

Analysis of 19-year-long tidal gauge records along the US East Coast has revealed the appearance of semidiurnal perturbations to storm surges in the South Atlantic Bight. A total of 85 events with semidiurnal-surge amplitudes higher than 20% of the astronomic tidal amplitude and durations longer than two days were identified. These semidiurnal surge events were triggered by the passage of tropical storms and cold fronts. As a consequence of the storm-induced forcing, observed tides were delayed and partially damped with respect to the predictions. Such delay and damping resulted in a semidiurnal signal on the surge. Parallel-to-shore winds in the shelf region between Cape Hatteras and the South Atlantic Bight were highly correlated with the generation of the semidiurnal perturbations. Increased bottom friction combined with Coriolis acceleration, resulting from enhanced wind-driven alongshore currents, are proposed to be the primary factors delaying and attenuating astronomic tides.