ocean tides and sea level - tide – “daily rise and fall of sea level” (c&d) - tide –...
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Ocean Tides and Sea Level
- tide – “daily rise and fall of sea level” (C&D)
- tide – distortions of sea by gravitational attraction of Moon and Sun on every part of Earth
- tidal currents - small gravitational forces give rise to horizontal water movement
- horizontal movement of water causes rise and and fall of sea level
Geography 104 - “Physical Geography of the World’s Oceans”
daily tidal patterns
- horizontal movement of water causes rise and and fall of sea level
- can be represented as “waves” (periods ~12 or ~24 hours)
- classified by period
- diurnal tide (~1 cycle/day = 1 high, 1 low)
- semidiurnal tide (~2 cpd = 2 highs, 2 lows)
- mixed semidiurnal (2 highs and 2 lows not equal)
- tidal day – time for one complete revolution of Earth beneath tidal bulges, ~24 hrs and 50 minutes
daily tidal pattern details
- actual tides depend on response of ocean to forcing
- tidal currents can be very strong (~5 knots)
- strongest currents typically near mouth of bays (i.e. SF Bay)
20 Jan 25 30 1 Feb 5 10 15 20 25 1 Mar 5 10
16 Jan – 11 Mar 2007
1
spring-neap tides Santa Barbara (tidal currents a few cm/s)
diurnal tide semi-diurnal tide
tide generating and raising forces
- tides caused by gravitational attraction (tide generating force)
- mostly by moon, but also by sun, negligible contribution from other bodies in solar system
- technically, difference between gravitational force at Earth’s surface and Earth’s center gives rise to tides (tide raising forces)
- gravitational force between two masses
FG = G m1m2/d2
FG - gravitational force between two masses
G - gravitational constant
m1, m2 - masses
d - distance
barycenter - the center of gravity where two or more celestial bodies orbit each other. For example, the moon does not orbit the exact center of the earth, instead orbiting a point outside the earth's center (but well below the surface of the Earth) where their respective masses balance each other.
centripetal acceleration and gravitational force
apparent force due to centripetal acceleration
Centripetal acceleration
Centripetal acceleration tide raising force
-Tide Raising Force (due to moon)
TRFm = 2 r G mm me/d3 (equation on page 227)
FG - gravitational force between two masses
G - gravitational constant
mm, me - masses
d - earth-moon distance
r - difference in earth-moon distance from Earth’s center
horizontal component of tide raising force
horizontal component unbalanced – produces water movement
Tides are caused by the gravitational attraction between the Earth and other planetary bodies; primarily between the Earth and Moon, and the Earth and Sun.
equilibrium lunar tides
at equator velocity = 442 m/s
to remain under moon tide wave would have to propagate 442 m/s eastward at equator
lunar day
5353
- 1 lunar day = 24 hours + 53 minutes- 2 high & 2 low tides per lunar day- called the M2 tide with period of 12.42 hours (see Table 11.1)
tidal inequality
lunar hours =
large tidal inequality – diurnal tides
small tidal inequality - semi-diurnaltides
earth-moon system
center of mass of earth-moon system
center of mass of earth
(moon’s orbit around earth)
29.53 days = lunar month
earth, moon, & sun
around sun
Path of moon around earth
Path of eartharound sun
20 Jan 25 30 1 Feb 5 10 15 20 25 1 Mar 5 10
16 Jan – 11 Mar 2007
1
spring-neap tides Santa Barbara
diurnal tide semi-diurnal tide
dA = 405,800 kmdP = 375,200
changing earth-moon distance
TRF at perigeeTRF at apogee
= dA
dP( )
3= 1.07 = 1.21
21% variation in TRF due to changing earth-moon distance
3
153x106 km 149x106 km
~8% variation in TRF due to changing earth-sun distance
changing earth-sun distance
consideration of ocean basin geometry and the Coliolis force results in amphidromic systems
co-tidal lines amphidromic M2 tide
Tides for Santa Monica, Municipal Pier starting with December 5, 2008.Day High Tide Height % Moon /Low Time Feet VisibleF 5 High 3:37 AM 4.1 40 5 Low 9:09 AM 2.9 5 High 2:01 PM 3.7 5 Low 9:00 PM 1.0
Tides for Santa Barbara starting with December 5, 2008.Day High Tide Height % Moon /Low Time Feet VisibleF 5 High 3:57 AM 4.0 40 5 Low 9:27 AM 3.0 5 High 2:21 PM 3.6 5 Low 9:18 PM 1.0
F 12 Low 1:50 AM 2.2 99 12 High 8:16 AM 7.2 12 Low 3:38 PM -1.9 12 High 10:16 PM 3.8
20 minute difference; 150km/20 min = 125 m/s; sqrt(9.8m/sx1500m = 121 m/s)
Class Summary (it really does all fit together):
- position on earth, navigation- water properties- bathymetry (water depth; Geology)- sea water, solubility (Chemistry)- gasses & nutrients (oxygen & primary production; Biology)- seawater density, temperature and salinity effects- vertical structure of ocean (mixed layer, pycno, halo, and thermo –clines)- specific heat of water- solar radiation (where ocean circulation begins)- air-sea heat budget (heat from ocean drives atmosphere)- atmospheric circulation- Coriolis force- tropical cyclones and El Nino- direct wind driven Ekman flow (upper ocean mixed layer)- large scale wind driven circulation, sea level set up, subtropical gyres- western and eastern boundary currents (coastal upwelling/downwelling)- thermohaline circulation- waves (development, propagation, dissipation)- tides (forces, time scales, amphidromic systems, Kelvin waves)