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Full-moon tides under a slow nor'easter gave Atlantic City its highest water since Sandy
Atlantic City's water in the late-September nor'easter hit its highest level since Superstorm Sandy in 2012, as peak surge met a full-moon high tide. A storm researcher calls the El Niño outlook unclear, but rising seas alone will make equally strong storms more damaging.
The Scientist · Science desk
What happened
- The nor'easter pushed ocean water into neighborhoods from North Carolina to New York as heavy rain filled streets and near-hurricane-force winds cut power.
- Surges ran largest where wind drove water into shallow bays and inlets, including Raritan Bay in New Jersey and Delaware Bay.
- The storm's slow track up the coast kept the surge going, so flooding came back over several high tides.
- Sea level is rising faster on parts of the East Coast mainly because land at the edge of the former Laurentide ice sheet is still sinking back.
- One study has found that strong East Coast nor'easters are more likely during El Niño years.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- exposure With the baseline about 2.8 inches above its 2012 level, a nor'easter no stronger than earlier ones can put water into Atlantic City homes that those storms missed.
- decision Judging a winter storm's flood threat to Atlantic City now means reading the tide calendar alongside the wind forecast, since peak surge on full-moon tides produced this flood.
- constraint Few El Niños of this strength exist to compare against, so a forecast of a flood-heavy winter has little history to be checked on.
The explanation comes from a scientist who studies storm systems, writing in phys.org [16], and it depends on keeping two quantities apart. Surge is water that onshore wind piles against the coast. Its height at a given spot depends on wind strength, storm size, coastline shape and water depth [3]. Inundation, the water actually standing on the ground, depends on surge and tide together [14].
Both were high at once. As the storm deepened and its pressure fell, gale-force winds pushed water across hundreds of miles of the Atlantic toward the coast [17]. The peak arrived during the larger tides around a full moon, when the pulls of the moon and sun add together [6]. The same surge at an ordinary high tide would have put less water in the streets [14].
The thing the Sandy comparison doesn't tell you is how the two surges compare. Atlantic City's sea level is rising about 2 inches a decade [7]. At a steady rate, that puts the baseline roughly 2.8 inches higher than when Sandy hit in 2012 [1]. Part of this September's high water was sea that was not there 14 years ago [1]. The researcher's account does not give the measured peak or the surge height on its own. "A couple of inches can mean the difference between water staying out of one's house or pouring in," the researcher wrote [13].
On whether the storm previews the winter, the researcher wrote: "It's difficult to say." [15] The case for more trouble rests on two pieces of evidence. The study linking strong nor'easters to El Niño years [10] describes an association across seasons. It cannot say that this El Niño will produce a particular storm.
The other piece is seasonal model output, which gives better odds of above-normal East Coast precipitation this winter [11]. The researcher reads that as a sign that wet storms with damaging surges may be more likely [18]. A precipitation outlook, though, bears directly on none of the four factors that set surge height [3] [11]. The sample is also thin: every El Niño is different, and few past ones come close to the strength expected this time [12].
In my view the firmer forecast needs no El Niño at all. Even if nor'easters do not get stronger, the researcher wrote, they will become more damaging because sea level is already higher, and more places will flood [9].
What to watch
- Atlantic City's measured peak and surge height for the storm, to separate the surge's share of the record from the higher baseline.
- Updated seasonal outlooks for East Coast winter precipitation as the El Niño strengthens.
- Whether a winter nor'easter's peak surge again lands on the larger tides around a full moon.