What is a Storm Surge?

Storm Surge NOAA NWS

Storm Surge is the abnormal sea level elevations (or depressions) caused by winds and atmospheric pressure. The components are:

1. Coastal set up (down) by the along shore wind stress. 
In deep water, the Earth’s rotation causes a water to move at a right angle the wind stress. This sets up a sea level slope against the coast and an alongshore current in geostrophic balance. With the current limited by friction the sea level set up is less than a meter.
2. Coastal set up (down) by atmospheric pressure.
Atmospheric pressure operates like an inverted barometer. Each mb of pressure drop (increase) raises (lowers) sea level by 1 cm. The largest hurricanes with pressure drops of 100 mb can cause a 1 m surge by this mechanism.
3. Coastal set up (down) by the across shore wind stress. 
In shallow water, and because of friction, the wind stress drives water downwind and piles it up against the coastline. The resulting sea surface slope (tending to balance the across shore wind stress) is the largest contributor to coastal storm surge and can exceed several m.

Other Factors

4. Coastal geometry.
By varying fetch and direction relative to a hurricane the embayment geometry is very important, as are the water depths and land elevations.
5. Continental shelf width.
In shallow water the sea surface slope required to balance the across shelf wind stress is inversely proportional to water depth. Hence wide, shallow shelves are prone to larger storm surges.
6. Tides.
Water level will be higher (lower) at high (low) tide. Since tides in Tampa Bay are about plus and minus 1.5’ this is small relative to the storm surge.
7. Water density.
By being lighter, warmer water in summer stands higher than colder water in winter. This can amount to about 1’.
8. Waves.
Waves are additive to surge. Theoretically a solitary wave can be 1.8 times the water depth. While this is not naturally realized, waves can have a huge impact. Imagine the surf zone on a very rough day displaced to Gulf Blvd.
Sources:
R.H. Weisberg and L. Zheng (College of Marine Science at USF St. Petersburg, Ocean Circulation Group)
http://ocgweb.marine.usf.edu/
National Hurricane Center: Storm Surge Overview https://www.nhc.noaa.gov/surge/

About Fred Pickhardt

I am a marine meteorologist and sailed briefly with American Export Lines in the Far East trade after graduating from State University of New York Maritime College. I have extensive experience in weather analysis, weather forecasting, optimum ship routing, vessel performance evaluations and forensic weather event reconstructions. I founded Ocean Weather Services and as Owner and Chief Consultant currently provide optimum ship routing services and forensic marine weather reports to the maritime industry.
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