Ocean Tides
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15 Terms
Equilibrium Model of Tides
idealized view of tides
1. Tide wave is treated as a deep-water wave in equilibrium with lunar/solar forcing
2. Assumes no interference of the tide wave’s propagation by continents
3 daily tidal patterns
1. Diurnal = one high tide and one low tide per day
2. Semidiurnal = two equal high tides and two equal low tides per day
3. Mixed Semidiurnal = two unequal high tides and two unequal low tides per day
tidal bulges
created by the pull of gravity by the moon and … creates the bulges on opposing sides of the earth. This causes the regular rise and fall of the tides over an approximately 24h hour period as the earth rotates on its axis.
When does the tide rise?
when a point on the earth is aligned under the moon — and also on the opposite side of the earth from the moon…
when does the tide fall?
when a point on the earth is 90˚ away from the moon…
Tiday day
24h + 50 min
why is tidal day longer than 24 hours
Over the 24 hours needed for earth to make a full rotation, the moon itself has orbited 12 degree of angle in its monthly 360 degree journey to orbit around the earth.
It takes 50 minutes for the earth to rotate an additional 12 degrees to get back under the moon.
2 force responsible for two tidal bulges
gravitational and centrifugal forces
Barycenter
The pull of gravity by the moon and the centrifugal force caused by rotation about the Barycenter causes two tidal bulges. The spin of the earth relative to these two bulges produces the rise and fall of the tides over an approximately 24h hour period
moon’s orbit incline
28 degrees relative to the earth’s equator, and this has important consequences for differences in the daily pattern of tides depending on the latitude.
The Monthly Pattern of Tidal Amplitude is Caused by the Combined Effects of Gravity From…
the moon AND the sun (The Tidal Force Exerted on the Earth by the Sun is About 46% of the Tidal Force Exerted on the Earth by the Moon)
Spring tides occur when …
the moon pulls along the same line as the sun (new and full moon)
Neap tides occur when…
the moon pulls at 90 degrees to the sun (first and last quarter moon, ie half moon)
daily patterns
diurnal, semidiurnal and mixed semidiurnal tidal patterns
monthly patterns
spring/neap pattern of tidal amplitude