ELEMENTS OF OCEANOGRAPHY

 

TOPIC 9 - OCEAN WAVES AND TIDES [Lecture ppt | pdf]

Overheads:

Energy Transfer Between the Atmosphere and Oceans
The Atmosphere and oceans transfer mass and energy between then at all times and scales
Heat is one important form of energy transfer
Dissolved gasses is one important form of mass transfer
Kinetic energy transfer between wind and ocean surface is another form of energy Transfer - this drives surface ocean currents

Wave Motion at the Ocean Surface
Waves are generated by wind shear at the ocean/atmosphere interface
Waves start as ripples. After ripples form, there is a non-horizontal ocean surface for the wind to push against
As winds continue, larger waves are developed
Wave radiate away from source areas like radio waves

Water Particle Motion in Waves
Wave particles move in circular loops win the directions of wave motion
The amplitude of loops diminishes with depth in the oecan
At some depth, which is dependent on wavelength, the wave dies out - no motion due to waves occurs below

Factors that Control Wave Size
Wind strength - faster winds have more energy to push waves to bigger size
Duration - the longer prevailing winds blow from the same directions, the bigger the waves
Fetch - the larger the surface area over which waves are developed, the bigger the waves

Wave Motion in Shallow Water
As waves approach the shoreline, water particles moving in the deepest loops see the bottom
The base of the wave slows down and is deflected upward making the wave higher
When waves are too high, gravity forces take over and create breakers

Wave Refraction and Long Shore Currents
Waves rarely approach beaches head-on (line of wave crests parallel to beach)
As the wave nearest the beach sees the bottom and starts to slow down, the wave crest line starts to bend or refract
Waves typically arrive at the beach at an angle creating longshore currents

Wave Types
Wind waves - 60-150 m in wavelength, caused by wind action
Seiche - km or more wavelength, caused by atmospheric pressure variations
Tsunami - hundreds of km in wavelength, caused by mass movment on ocean floor
Tide - 1/2 Earth in wavelength, gravity attraction of Moon and Sun

Tsunamis
Tsunamis are long-wavelength shock waves generated by sudden changes in seafloor level in coastal areas
Tsunamis are caused by earthquakes, ocean landslides, or volcanic eruptions
Tsunamis are ~ 1m high in deep ocean with wavelength of 100's of km
At beaches, wave heights may become >20 m as waves slow down

Tides
Tides are local rise and fall of sea level over 25 hour period
Caused by gravitational attraction between Earth, Sun, and Moon
Also affected by Earth's rotation
Because the oceans are fluid, they react more strongly and quickly to changes in gravity and motion

 

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