tides

Tides

Tides

At earth’s center: centripetal acceleration = centrifugal acceleration (at common center of mass).

The period of rotation about this common center of mass is 27.32 days (sidereal month).

Everywhere else on the earth, there is an imbalance between the centripetal (inward) and centrifugal (outward) accelerations. The centrifugal acceleration is the same everywhere on the earth, but the gravitational force due to the moon varies over the surface of the earth.

This results in the tide-generating force, TGF, since on the side of the earth toward the moon the gravitational force exceeds the centrifugal force, and on the side of the earth away from the moon the centrifugal force exceeds the gravitational force. The

sideways forces, or horizontal � �component, of the TGF is called the tractive force.

Note: Centripetal Force and Centrifugal Force is the action-reaction force pair associated with circular motion (Newton’s Third Law).

The equilibrium tide is that which would result from the TGFs if the earth were completely covered by water and responded instantly to the changing forces (i.e. no inertia and no friction). As a result of the tractive forces, the equilibrium tide has two bulges, one on either side of the earth. Thus, you see 2 highs and 2 lows per lunar day (semidiurnal lunar tidal constituent). It has a period of 12.42 hours and is denoted by the symbol M2.

The lunar day (also known as a tidal day), 24.84 hrs, exceeds the solar day (24 h) since the moon is revolving around the earth with a period of 27.32 days.

Tides

High tides occur every 12 hr 25 min 1 lunar day = 24 hr 50 min• Gravitational forces of sun and moon add at full and new

moon to produce spring tides. • Gravitational forces of sun and moon compete at half moon to

produce neap tides.

Moon declination

Moon declination effect

A = Semidiurnal B = Mixed C = Diurnal

Tides

[14.79 days]

Earth orbit around the sun

Tides

There are three types of tides:Semidiurnal

Mixed

Diurnal

Tides: Moon declination effect

A = Semidiurnal B = Mixed C = Diurnal

Monthly tides

Global distribution of tides

Local influences on tides

• Shape of the land • Shape of the ocean floor (bathymetery) • Depth of water • Restrictions to flow (narrow inlets to bays, etc.) • Local winds

How do we predict tides?

XTide Tide Prediction Serverhttp://www.mobilegeographics.com:81/

http://www.mobilegeographics.com:81/zones/:America/Puerto_Rico

http://tidesonline.nos.noaa.gov/monitor.html

Tidal “Wave”

Tidal Waves (Tsunamis)

http://www.ssc.erc.msstate.edu/Tides2D/tides_synopsis.html

Tidal “Wave”

Tidal “Wave”

No rotation With rotation

http://earthguide.ucsd.edu/earthguide/diagrams/waves/swf/wave_seiche.html

Tides San Juan, Puerto Rico

Largest tidesBay of Fundy, Nova Scotia, Canada

Largest tidesBay of Fundy, Nova Scotia, Canada

Tidal range = 15 m (50 feet or a 3-story building)

The largest tidal bore occurred in Hangzhou Bay, China, 1993. The bore was 9.14 m (30 ft) high.

Internal Waves

Barotropic Wave

Baroclinic Wave(internal tide or wave)

Internal Waves

Red Sea True-color Terra MODIS, July 26, 2003.

Important terms:• Barycenter

• Centripetal force

• Lunar and solar

declination

• Tropics of Cancer and

Capricorn

• Apogee

• Perigee

• Aphelion

• Perihelion

• Tidal day

• Semidiurnal, mixed, and

diurnal tides

• Spring tides (syzygy)

• Neap tides (quadriture)

• Amphidromic point

Consequences of tidal fluctuations

• Navigation – depth and currents• Intertidal – range and type of organisms• Larval retention and dispersal• Internal waves• Migrations synced with tides

Tidal Resources

• http://tidesandcurrents.noaa.gov/

• http://oceanservice.noaa.gov/education/kits/tides/tides01_intro.html

• http://tidesonline.nos.noaa.gov/

• http://www.mobilegeographics.com:81/zones/:America/Puerto_Rico

Tides

High tides occur every 12 hr 25 min 1 lunar day = 24 hr 50 min• Gravitational forces of sun and moon add at full and new

moon to produce spring tides. • Gravitational forces of sun and moon compete at half moon to

produce neap tides.

Tides

[14.79 days]