Long Waves
Storm Surges
Storm surges are bulges of water generated by a hurricane. They can add up to 7.5 m to coastal sea level.
Image source: Superstorm Sandy and the Discussion from SBU Experts that Followed
Standing waves
Ever try to carry a bucket that is very full of water? Did you notice that, as you began to walk, the water started sloshing out of the bucket? That sloshing is a standing wave. Standing waves are two waves with same wavelength moving in opposite directions. Water particles move vertically and horizontally and the water sloshes back and forth.
A Seiche is a special type of standing wave. These waves form when water in a confined/small space is disturbed - usually by changes in atmospheric pressure, but seismic waves can also be a cause. The water sloshes back and forth out of the basin. Seiches were first studied in Lake Geneva.
Spectacular video of Devil’s Hole seiche
Tsunami
First, tsunami are NOT “tidal waves”! They are seismic sea waves that are caused by a major displacement of the ocean due to Earthquakes, Volcanic Eruptions, Submarine landslides or meteor impacts.
Tsunami travel 435 mph across the Pacific ocean basin and rise to maximum height as they rush into shallow water. The wave height is approximately 1-2m high in the open ocean, so they usually aren't visible until they arrive on the coast as a series of waves. Tsunami are the most dangerous when they occur locally because there is no warning time.
Tsunami Generation
Tsunami are formed due to a displacement of ocean water. This most frequently occurs as a result of movement along a subducting plate. NOAA's JetStream website has a great description of How tsunami are generated. The images below are from their site.
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| First, two converging plates are placed under compression. Stress begins to build where the two plates meet (1). |
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| Stress continues to build, with the subducting plate being pulled down (2) while the overriding plate is pushed up (3). |
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| Both sides further deform as stress continues to build. |
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| Eventually, the stress is released as an earthquake, and the crust on both sides return to "normal". This means that the bulge that was building on the overriding plate snaps downward. The water above this area is also pulled down, and then rebounds back upwards, generating the waves that form the tsunami. |
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| The tsunami waves radiate outward from their point of origin, rushing onshore as a series of huge waves. |
Estimated Tsunami Travel Times to Coastal Locations
The movie below shows the scope of the December 26, 2004 Sumatra tsunami as the waves moved around the world (click image to go to the Quicktime Movie). More animations about this tsunami can be found at the NOAA Center for Tsunami Research.
NOAA has two warning centers - National Tsunami Warning Center and the Pacific Tsunami Warning Center. The National Tsunami Warning Center (formerly the West Coast and Alaska Tsunami Warning Center (WC/ATWC)) was founded in Palmer, Alaska in 1967 after the 1964 Alaskan earthquake and tsunami. The Pacific Tsunami Warning Center, located in Ewa Beach, Hawaii, was founded in 1949, following the 1946 Aleutian Island earthquake and a tsunami.
National Data Buoy Center: Deep-ocean Assessment and Reporting of Tsunamis (DART®) Description
Tsunami are common in tectonically active basins.
Historic Tsunami:
- Volcanoes
- Krakatoa in 1883, killed 35,000 people
- Earthquakes
- Hilo, Hawaii in 1946 as a result of an Alaskan earthquake
- Nicaragua 9/18/92
- Landslides
- The French Riviera (10/16/79)
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