News•October 29, 2012

Sandy's Storm Surge Explained and Why It Matters

As Hurricane Sandy comes ashore and grinds its way inland, the hurricane’s storm surge — the pulse of seawater pushed ashore by Sandy’s winds and low atmospheric pressure — will almost certainly cause more concentrated damage than the hurricane’s powerful surface winds, torrential rains and mountain snows.

Those other factors will be bad enough, especially since Sandy’s landfall in New Jersey — the nation’s most densely populated state, located smack in the middle of its most densely populated region — puts at least 60 million people, and probably more, at risk. Flash floods, falling trees, hurricane-force winds and power outages could cause billions of dollars in damage, and kill or injure people, from the Carolinas to Maine, and even into Canada.

Flooding in Sea Isle, N.J.
Credit: @tinacan25/twitpic

A single cubic yard of water, however, weighs nearly a ton — and when you add up the untold billions of cubic yards’ worth of ocean moving inexorably onto land in a pulse that could last a day or more, the crushing force involved is nearly impossible to imagine. Flooding along a thousand miles or more of coast could be nothing less than catastrophic. The threat to New York City alone is huge, as Climate Central’s Andrew Freedman has reported:

“. . . is particularly dire, since storm surge models are projecting that the surge will be considerably larger than during Hurricane Irene in 2011, when there was only minor coastal flooding. The city’s subway tunnels are extremely vulnerable to coastal flooding, and a disaster was averted by mere inches during Irene, making it more likely that flooding will occur in some sections of the system this time, particularly in parts of Brooklyn and Lower Manhattan. If the tunnels do flood, as suggested by the storm surge forecasts, it could cripple the lifeblood of transportation in and around the city for an extended period.”

This nightmare scenario — for New York and the rest of the northeast — is especially likely because Sandy’s storm surge is peaking through three high tides, which magnifies its effect. At the moment, high tide itself is higher than normal because we’re right at the full moon, when tidal effects are at a maximum.

As a result, according to National Weather Service projections and an analysis by Climate Central, it is likely that The Battery in Lower Manhattan, Sandy Hook, N.J., and Atlantic City will see the highest storm tides on record during Monday evening’s high tide. Those records go back to 1893, 1932, and 1911, respectively.

At The Battery, the prior record occurred during Hurricane Donna in 1960. As of noon ET, the central estimate for The Battery was slightly more than 6 feet above the Mean High Water level (mean high tide), which would top the storm surge from Hurricane Irene by more than a foot. For Sandy Hook, Hurricane Sandy could top Irene's surge by 1-to-2 feet.

In Bridgeport, Conn., the central estimate for surge is about 7.5 feet, which corresponds to a storm tide more than 5 feet above mean high tide. In Philadelphia, the surge may peak at 1 foot, but the tide could reach more than 2 feet above average. In Atlantic City, the surge may peak at 5.5 feet, reaching about 5 feet above mean high tide. And so on. (To figure out the particular risks for your local area, visit Climate Central’s interactive map.)

As for the surge itself, even that is more complicated than just the wall-of-water metaphor that’s so tempting to use. As explained more fully by Weather Underground’s Jeff Masters, it’s more of a bulge of water, spread out over a wide area in front of the storm. Winds account for about 85 percent of the height of the average surge, and the tremendous wind field surrounding Hurricane Sandy is maximizing that component of storm surge.

Another 5-to-10 percent comes from what’s called “wave set-up”— the fact that water can’t drain back off into the ocean because there’s more water pushing from behind. The final 5-to-10 percent comes from a tropical storm’s low atmospheric pressure, which literally sucks the ocean skyward.

But even that isn’t the whole story. Global sea level is now about 8 inches higher, on average, than it was in 1900, in connection with global warming. Sinking land has added several inches more of local sea level rise in the Mid-Atlantic. That means the storm tides from Sandy are that much higher than they would have been if the identical storm had come along back then.

And as sea level continues to rise in a warming world, a Sandy that arrives in 2100, when average sea level is likely to be about 3 feet higher than it is today, would be correspondingly more destructive.

For a more detailed look at how the ocean is responding to Sandy in particular, this recent report from Climate Central lays it all out.

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