Scientists Solve Decades-Old Mystery Behind New York’s ‘Phantom Tsunamis’

New York’s Hurricane Danger Doesn’t End When the Wind Stops. Here’s Why

In 1938, and again in 1944, powerful hurricanes tore across Long Island. Once the winds died down, witnesses watched the water rush back toward the coast hours later, hard enough that some thought they’d seen a tsunami.

Those two events were not tsunamis. A new study led by researchers at Stevens Institute of Technology, with colleagues at the University of Western Australia and California Polytechnic State University, identifies what actually returned to shore: a slow, sloshing rebound of water called a continental shelf seiche.

Published in the journal Continental Shelf Research, the paper shows this delayed second surge, known as a resurgence, is not rare after hurricanes that pass near New York Harbor. The harbor sits inside the New York Bight, the curved stretch of coastline running from Cape May, New Jersey, to Montauk Point on Long Island. Of 112 hurricanes that passed near New York Harbor since 1860, 29, roughly one in four, triggered a measurable resurgence, some with water level swings reaching 1.5 meters, about five feet, rivaling the original storm surge. A milder version struck more recently: in 2020, after Hurricane Isaias’ center had passed and winds had eased, a resurgence caused minor flooding in New York Harbor and nearby communities.

A Continental Shelf Seiche Behaves Like Water Sloshing in a Bathtub

Strong wind from an approaching hurricane pushes seawater toward shore, building up a storm surge. Once the storm passes and the wind eases, that water does not simply settle back down. “The motion is similar to water rocking inside a bathtub after being pushed,” says Philip Orton, a research associate professor in Stevens’ Department of Civil, Environmental and Ocean Engineering. “Instead of passing through only once and dissipating, the wave bounces off or reflects between the sides of the tub until it peters out.”

In a harbor or bay, the coastline provides the far wall the water bounces against. Farther offshore, on the open continental shelf, there is no obvious wall in sight. “Continental shelf seiches are much less widely known than the classic type of seiches that occur in enclosed or semi-closed bodies of water,” Orton explains, “largely because there is no obvious reflecting boundary on the deep ocean side.” It’s there, just much farther out. “So when the wave goes to the edge of the continental shelf, about 100 miles offshore, and hits this stationary deep ocean that doesn’t want to move, the wave bounces off it and comes back to shore,” Orton says.

New York Harbor Is a Continental Shelf Seiche Hotspot

To confirm the mechanism, the team, led by Orton and PhD candidate Tam Trinh, combined 164 years of hourly tide-gauge records with computer simulations. They compared 17 coastal stations stretching from Nantucket Island, Massachusetts, to Duck, North Carolina, using a mathematical technique that spots brief, repeating ripples hidden inside messy data. New York Harbor stood out sharply: its post-storm water swings dwarfed those at the other 16 stations. “In our study, we found that these oscillations are triggered by abrupt changes in wind forcing,” Trinh says. “Strong onshore winds can push large volumes of water toward the coast as a storm surge, and when the wind weakens, changes direction, or the storm moves away, the water can be released and then oscillate.”

Fast-Moving Storms Are Linked to Stronger Seiches

Comparing decades of storm records against the tide-gauge data showed that the size of a resurgence tracked closely with the strength of the storm surge, a hurricane’s top wind speed, and how fast it moved north along the coast. Computer simulations using an idealized wind pattern, ramping up then abruptly cutting off the way real winds do once a fast-moving storm clears out, reproduced the same oscillation pattern at matching intervals.

A Seiche’s Timing With High Tide Raises Flood Risk

Timing matters as much as size. “In New York Harbor and the neighboring coastal areas, the seiche period is approximately 7 to 8 hours, meaning that resurgences of high-water levels arrive every 7 to 8 hours after the initial storm surge,” Trinh says. “Due to this timing, continental shelf seiches may coincide with high tide and amplify coastal water levels, potentially leading to secondary coastal flooding many hours after a storm has passed. And, unlike short-duration storm-surge peaks, these seiche oscillations may persist for several tidal cycles.”

The events mistaken for tsunamis in 1938 and 1944, Orton says, probably weren’t the seiche acting alone. “Where mistaken for tsunamis, there was likely a coincidence of high tide, the continental shelf seiche, and very large storm swells all superimposed, causing an additional period of dangerous conditions,” he says.

That combination is risky for a region that assumes danger ends once the wind stops. Rising sea levels could sharpen the stakes: a resurgence that once fell short of flood thresholds may be enough to push water over them. Orton, who has spent years on flood forecasting for the region, says the goal now is turning this discovery into better warnings. “Continental shelf seiches are less studied and harder to forecast than ordinary storm surges,” he says. “Our research aims to better understand the risks and improve flood forecasting and emergency management.”

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