Large Seasonal Modulation of Tides due to Ice Cover Friction in a Midlatitude Estuary*

Seasonal episodes of significant tidal damping (reductions of tidal amplitudes as much as 50%) and tidal modulation were observed in the Hudson River estuary in the course of three consecutive winters from multiple tide gages. Through comparisons with United States Coast Guard ice reports, it was hypothesized that these events correspond with an increase in ice concentration and the development of a seasonal ice field within the upper 170 km of the tidal Hudson north of Peekskill, New York. Using stationary (tidal harmonic) and nonstationary (wavelet) analyses as well as numerical modeling, it is shown that under-ice friction is the primary cause of the observed modulations in tidal circulation (water levels and currents) throughout the 240-km-long estuary. Upstream of the ice field edge at Peekskill, depth-averaged tidal currents are greatly reduced under the ice cover through first-order damping, and vertical current profiles under the ice become parabolic. Tidal ranges increase near the edge of the ice field, and, south of that, on Manhattan’s western shores, currents increase because of tidal wave reflection. These amplified currents create stronger vertical mixing leading to a less stratified estuary and decreasing salt front intrusion. At the other end, near Troy, tidal flows become smaller relative to the river’s streamflow, leading to increased ebb predominance and ebbdirected flows down to the port of Albany. Also, the increased friction leads to a higher sea level setup there. During such episodes, astronomical tide–based and operational forecast model predictions that neglected ice are severely compromised.