Tides

TSUNAMIS, TIDES AND OTHER LONG WAVES

Storm surges

Caused by winds and low atmospheric pressure associated with large storms
Can raise sea surface by up to 25 ft, bottom contours magnify effect
Not a progressive wave as only has one crest, does not break
Comes ashore like a tide -- sometimes called a storm tide
Main cause of loss of life and property in low lying areas when hurricanes come ashore

Seiche: Wave that resonates (sloshes) in a basin like a lake after a storm or change in atmospheric pressure

Tsunami (are not tidal waves)

Caused by displacement, usually of the seabed during seismic events
Are progressive waves with wavelength ~200 km (125 miles)
Act as shallow water waves, velocity 472 mph in Pacific
Period ~ 16-20 minutes, height ~ 1-2 ft in the open ocean, too small to see
When reaches shallow water velocity drops, period stays the same, wave height increases
Rushes on shore as a flood of water, not a breaking wave
1946 Aleutian tsunami was 30 ft high at Hilo, was >118 ft high at point of origin in Alaska
May have been big local tsunamis from island landslides
Network of seabed pressure sensors give warning of approaching tsunamis
Half the time the trough of the tsunami arrives first, curiosity draws people to investigate exposed shoreline, following crest (~8 minutes later) drowns them

Tides

Are waves -- the real tidal waves!
Caused by the gravitational attraction of the Moon and the Sun
Unlike wind waves, tides are constantly under the influence of their disturbing force
Wavelength up to 12,000 miles

Tide creating forces

Inertia: The tendency of a body in motion to keep going in the same direction
Gravity: The mutual attraction of two bodies, is proportional to the mass of the bodies but inversely proportional to distance between them
Effect of the Moon:

Earth and Moon orbit around common centre of mass is ~1,000 miles below the surface of Earth
Inertia and gravity balanced on planetary scale; not balanced though at every point on the Earth's surface
Force of inertia is away from moon and equal everywhere on Earth
Force of gravity is towards moon and decreases with distance away from moon
On the moon-side of Earth net force is towards moon, gravity>inertia
On far side of Earth net force is away from moon, inertia>gravity
Net forces raise ocean water into a high tide, one each on the near and far side of the Earth
Water drawn for bulge of high tide causes troughs (low tide) in other regions
Heights of tide (bulge) small ~ 2 metres (6 ft)
Wavelength ~12,000 miles, tries to travel at ~ 1,000 mph to keep up with the moon, usually fails
Solar day is 24 hrs, lunar day is 24h 51 minutes so high tides are 51 minutes later each day
Moon's monthly cycle from 28¡N to 28¡S of the equator affects height of high and low tides

Sun is 27 million times larger than the moon but is also 368 times further away
Gravitational pull of the Sun raises tides, is ~ 46% strength of that of the moon
Sun has an annual cycle from 23^o N to 23^o S

Sun and moon on the same side or opposite side of the Earth, forces add up get high, high tides, and low, low tides get Spring Tides
When Sun and Moon are at right angles to each other forces cancel get low high tides etc. get Neap Tides
Complete cycle takes 28 days (lunar month)
Are other factors affecting tidal height such as Earth- Sun distance
Altogether 150 factors affect tides, 7 have most effect
Amplitude (height) of tide corresponds closely to Moon phases

Real tidal patterns

Expect two high and two low tides per lunar day (Semidiurnal tide)
Tide is a very large wave, its progression is affected by shapes of ocean basins and presence of continents
Get constructive and destructive interference of tidal waves
Local tides affected by these factors

Tide types

Tidal heights are zero at Amphidronic points (points of destructive interference)

Bay of Fundy (Canada) tides are up to 50 ft

Honolulu has a mixed tide which is ~ 6 ft or less

Effect of tides on day length

Gravitational energy causing tides is dissipated as friction this is slowing the rotation of the Earth
Day length is increasing, but number of days per year is decreasing
350 million years ago 22 hrs/day ~405 days per year
350 million years from nowÊ ~ 26 hrs/day ~ 320 days per year
With fewer days we will have to make OCN 201 classes longer! Graduate early!