The NYC Storm Surge Threat

New York City is highly vulnerable to a hurricane strike due to its location near the coast where winds and storm surges are usually at their maximum.  On one hand, we are fortunate that direct hurricane strikes are extremely rare – four hurricanes have struck NYC since 1600. On the other hand, residents have been lulled to complacency by this recent long period without a hit. Storm surges in these hurricanes were 10-13 feet, which flooded about half of Manhattan below 34th Street and large swaths of East Harlem, Queens, Brooklyn and Staten Island.

Flooding in the Hoboken PATH station during a 1992 noreaster, which shut down the entire NYC subway system (Metropolitan NY Hurricane Transportation Study 1995).

Even a powerful nor’easter can cause serious damage in NYC, and the most recent severe flooding incident occurred in December, 1992.  Seawalls around the city are mostly only a few feet above normal high tide levels, so a relatively modest peak storm surge of 4.3 ft during that storm flooded into and shut down the subway system for several days.  The funnel-shaped coastline offshore can focus and build a storm surge to a greater height, and the two water pathways through New York Bay and Western Long Island Sound can cause a merging surge that is difficult to predict.

As one part of a project called Consortium for Climate Risk in the Urban Northeast, we are quantifying storm surge risk in NYC, Philadelphia and Boston, in our current climate as well as future climate with sea level rise.  Climate change is likely to increase the storm surge threat due to sea level rise and also potentially due to ocean warming, which may (or may not) increase the number of intense coastal storms. Sea level rise has proceeded at a rate of 1.8 cm per decade over the past century, but is projected to be between 5 and 30 cm per decade in the 2080s. Even conservative sea level rise projections, when combined with historical storms, can triple the frequency of key planning metrics such as the 1 in 10 year coastal flood event (Horton et al., 2010).

Storms occur infrequently, so it is useful to use computer simulations of thousands of storms and the ocean’s response, to understand flood probabilities.  We are running storm surge simulations using the ocean model sECOM, the Stevens Institute version of the popular ECOM (Estuary and Coastal Ocean Model).  Coastal water level predictions are available for the New York and New Jersey, and Connecticut coastlines through the New York Harbor Observation and Prediction System (NYHOPS) and the Stevens Storm Surge Warning System.

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Did the Oil Spill Stop Hurricanes?

It has been known for centuries that oil reduces the ability for wind to create sea surface waves. Benjamin Franklin famously hid oil in the bottom of his cane, so he could shake it over a pond like a sorcerer, and stop the wind’s creation of ripples. More recently, scientists have demonstrated the mechanism that is at work – oil films reduce water’s surface tension, reducing the air-to-water flux of momentum, also known as wind stress.  Studies have even shown that natural oil slicks above seafloor seeps in many parts of the world are visible from space due to their effects on sea surface roughness.

credit: Washington Post

The widespread and persistent surface oil on the Gulf of Mexico caused by the 2010 oil spill provided an unprecedented opportunity last summer to explore the effect of oils on atmosphere-ocean exchanges of momentum, as well as heat and gases.  I wrote a proposal with Wade McGillis (Columbia University) that was funded by the National Science Foundation.  The plan was to quantify these effects using an anchored catamaran on the Alabama continental shelf, as well as large-scale mapping aboard a ship with air-sea flux measurements.

The research also has many applications related to understanding the Gulf oil spill and its consequences, because modified air-sea exchanges of heat, moisture and momentum could impact oil spill transport, atmospheric delivery of moisture to the Southeastern United States, and transfer of heat from the ocean to the atmosphere, an important factor during hurricane season.  The hurricane season was quiet in the Gulf, and it is possible that the oil films left over from the spill reduced the availability of the ocean’s heat and moisture for growing tropical storms.

The mooring study was conducted from July 30 through August 10, examining the water column heat budget and air-sea heat and CO2 fluxes. The study included one hydrographic profile mooring, vessel-based measurements, and a moored catamaran with measurements of oxygen, chlorophyll, turbidity, atmosphere and water pCO2, air-sea fluxes of CO2, heat, momentum, and moisture using the gradient flux (atmospheric profile) technique (McGillis et al. 2001), and net shortwave and longwave radiation.

The oil spill was capped right before we began the field work, so — fortunately for those who live in and around the Gulf — we never got the opportunity to study how oils affected the air-sea fluxes.  And there is only so much you can do with three months of post-doc funding, so deeper analysis of our field data will require additional funding.  In retrospect, it was an amazing adventure and great experience for Wade, four undergraduate students, and myself, learning how to make automated field measurements of the coastal ocean heat budget.

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