Key points about waves. Surf s up! All about waves. Wave terminology. Idealized wave spectrum

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1 Surf s up! All about waves What are waves? Waves and wind How do waves get big? What happens when waves get close to shore? Storm surges Tsunami Key points about waves Waves move energy, not the water itself Ocean waves can take many forms: surface waves, storm surges, capillary waves, tsunami, tides (and others); these are generated by different forces Wave speed depends on wavelength: long waves are fastest Idealized wave spectrum Wave terminology 1

2 Water motion in a wave Water in a wave moves in a circle! How does water move in a wave? Wave base (no motion below) One-half wavelength Water motion is circular Strong at surface, very weak by half-wavelength depth Animation: Wave base Depth is half the wavelength If water deeper than wave base Orbits circular No interaction with bottom If water shallower than wave base Orbits elliptical and increasingly flattened towards bottom What causes waves? A disturbing force Surface waves: wind Tsunami: earthquakes or underwater landslides Tides: the gravitational attractions among Earth, moon, and Sun What causes waves to weaken? A restoring force : Mainly gravity! (except for very small waves) 2

3 Most ocean surface waves are wind-generated Size and type of wind-generated waves are controlled by: wind velocity wind duration fetch original state of the sea surface As wind velocity increases: Wavelength period height increase, but only if wind duration and fetch are sufficient. How do waves start? Capillary waves - the beginning How do waves grow? How big can waves get? Wind whips up the water = bigger waves Maximum wind effect - a fully developed sea 3

4 Waves in a fully developed sea Wind Speed Wave Speed Wave Height Wavelength Wave period Fetch Duration mph mph feet feet sec miles hours Wave speed about half of wind speed (slightly more) More wind gives bigger, faster waves Fully developed sea takes more time and space to develop for strong winds Biggest waves (highest, longest period and wavelength) occur when strong winds blow over long fetch and duration Waves in a fully developed sea B. A. Fully Developed Sea (red line) A. In a fully developed sea, waves have reached maximum theoretical size for wind of specific duration, strength,and fetch B. Waves in this region are larger than local winds would support: they developed elsewhere and moved in to the region where measured C. Waves in this region are smaller than local winds would support: local winds have insufficient fetch or duration to create max wave size C. In Southern Ocean, sustained winds of 35 mph and waves of ~18ft. Highest average waves in Southern Ocean because: Strongest winds Long duration Virtually unlimited fetch Bigger waves can develop in strong storms. WIND SPEED WAVE HEIGHT Wave height map Swell How to explain big waves on a calm (lowwind) day? 4

5 Animation Link at edu/surf/nepa.html Swell and wave trains Waves form where strong winds blow for days Travel out of the region of strong winds Propagate as wave trains Sets of waves travel at half the wave speed Sorted by size Big waves fast Smaller waves slow Swell Waves of similar size group together Large waves from distant origin on a lowwind day Waves approaching shore: Breaking 5

6 Waves approaching shore: Breaking Wave feels bottom when depth = 0.5* wavelength Motion slows as wave feels bottom Wavelength decreases Wave tends to become peaked Wave becomes too high for wavelength (1/7 rule) As wave steepens, water in crest is moving faster than wave supporting it, and it breaks Breaking waves Waves tend to break when depth reaches 4/3 of wave height Plunging waves form when shoreline rises steeply from depths Spilling waves form when bottom depth has shallow slope. Real-world waves Many sources Many directions Many sizes Interference: waves that overlap either augment or diminish each other Wave interference (this is common in nature!) Waves that cross paths influence each other, positively and negatively 6

7 Rogue waves Waves approaching shore: Refraction A dramatic example of constructive interference! One in every 300,000 waves is 4X average height Wave slows when it feels bottom The part still deep is still moving fast Wave bends to become more parallel to shore Wave refraction around a curving shore Wave diffraction 7

8 Wave energy focuses on headlands; straightens coasts Other kinds of waves Storm surges Sea level and weather/climate connection Tsunami Tides - Thursday Storm surge Bulge of water under low pressure - driven ashore by tropical cyclone/hurricanes - up to 6m high One of the most devastating aspects of tropical storms! Strength depends on tide levels, strength of storm, slope of continental shelf, and shape of coastline Storm surge Effect of storm surge of a category 4 slow-moving hurricane on St. Louis Cathedral in New Orleans 8

9 Effect of surge during Camille, LA, 1969 More storm surge photos Hurricane Carol: Old Lyme, CT, 1954 Storm surge - an increasing hazard? Sea level is rising (next week) Tropical storms may intensify with global warming Both will magnify storm surges Internal waves Form along zones of contrasting density Small density difference between the water masses above and below the pycnocline. They travel more slowly They can be much larger Internal waves display all the properties of surface progressive waves including: Reflection Refraction Interference breaking, etc. 9

10 Kelvin waves (review ENSO basics lecture) Travel along equator (why?) Key to development of ENSO anomalies Small changes in thermocline position = large changes in temperature at a given location. Large subsurface (10 s of m) = small surface (10 s of cm) displacements Other internal waves Can be generated by - flow of water related to the tides flow of water masses past each other storms submarine landslides Tsunami Shallow water waves in deep ocean Caused by abrupt movements of the sea floor Primarily earthquakes but also landslides or volcanic eruptions Left Hilo, HI after 1960 Chile Earthquake, Right 2004 Sumatra earthquake tsunami in Thailand. Tsunami Propagation Velocity and wavelength decrease as water shallows but wave height increases 10

11 Tsunami of April 1, 1946 The speed of the tsunami wave C = (g x d) 0.5 C = speed, g = acceleration due to gravity (9.8 m/sec/sec) d = depth (depth of Pacific ~4,600 m) Earthquake triggers tsunami with devastating local and distant effects C= (9.8 * 4,600) 0.5 (m 2 /sec 2 ) Speed = 212 meters per second; 472 mph Alaska to Hawaii in 5 hours! Hilo, Hawaii, 1946: Tsunami crossed the north Pacific to become one of Hawaii s worst natural disasters Tsunami breaking over main pier in Hilo, 1946 This man did not survive 11

12 Sumatra Tsunami Dec 26, 2004 Generated by 9.0 mag. earthquake at subduction zone plate boundary Tsunami wave measured in open ocean by satellites Maximum height of 60cm measured 2hr after the earthquake From NOAA Tsunamis: what can be done? Early warning system for evacuation (if EQ is distant) Coastal zoning. Get development out of the way. Example: Hilo, Hawaii Defense. Protective walls. Example: Taro, Japan Pacific Tsunami Warning Net -No such system currently exists in Atlantic or Indian Oceans 12