OcSci07.Tides

7/29/2019 OcSci07.Tides

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Tides

Observations

Driving forces: gravitational pullsof moon and sun

Equilibrium theory of tides

Reality: dynamic tides, tidalpatterns, confined basins

Impacts and applications

Anchorage AK: 12m range

San Francisco CA: 3.5m range

Panama City FL: 1.4m range

December 2004

December 2004

December 2004

Tides:Daily patterns

1. Semidiurnal: two tidalcycles per day

3. Diurnal. Onetidal cycle per day

2. Mixed. Two cycles,uneven heights

Tides are

Periodic, short-term changes in the level of theocean at a particular place

An example of a wave phenomenon

a forced wave: never escapes the influence of the

disturbing force

A shallow water wave: influenced by ocean bottom

Caused by the balance of gravitational forces (frommoon and Sun) and the Earths motion

Complicated!

Equilibrium theory: on water-covered planet

Dynamic theory: in real world: continents, varying

ocean depths, and other complexities

Important! Not just for navigation but for ecology,power generation, other.


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Simple equilibriummodel of tides

Driven by balance of gravitationalattractions (moon, Sun, Earth) and

centrifugal force

Explains semidiurnal pattern andrelationship with phases of the moon

Does not explain actual observations atmost places

Examples show Earth-moon systemhere, but same principles will hold forthe Sun

Tides: driven by gravity andcentrifugal force

Gravity


Gravitational tide force

Magnitude is proportional to mass anddistance

Varies inversely as the cube of the distance

between objects Thus moon >> Sun in importance

Outer planets have negligible impact ontides

Effect of moon is about 2.5x the effectof the Sun

Not constant over Earths surface - ineither strength or direction (see

previous slide)

Tides: gravity and centrifugal force


Centrifugal force


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Centrifugal force

Consequence of rotation of Earth-moonsystem around the systems center ofmass

(Not around the Earths center)

Constant in strength and direction overEarths surface

Balances gravity over Earth as a whole,also at Earths center, but everywhereelse an imbalance with gravity

The imbalance drives tidal motions

Gravity and centrifugal force(net effect called tractive forces)

Together:The earth-moon system


Tides: gravity and centrifugal force

Side facing moon:

gravity > centrifugal force = tidal bulge

Side away from moon:

gravity < centrifugal force = tidal bulge

Tides and the Earths rotation


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High tides: tidal

bulges Low tides = tidal

troughs

Lunar day = one

rotation of moon

around Earth ~24

hours, 50 min.

High tide 50 min later

each day

Tides: Cycling with the lunar day Tides: The moon and the sun together

Spring tide

Neap tide

Tides: the moon and the sun together

Spring tides: when the moon and thesun are in the same plane

-full moon and new moon (twice each month)

-highest high tide and lowest low tide =largest tidal range

Neap tides: when the moon and thesun are at 90 degrees

- first and third quarter moons (twice eachmonth)

-lowest high tide and highest low tide =smallest tidal range

Tides: The moon and the sun together

Spring tide

Neap tide


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Tidal patterns:spring tides & neap tides

Tides: now it gets complicated!

Orbital variations of moon and sun

-angle of moon to equator

-angle of sun to equator

-distance to moon

-distance to sun

Dynamic theory of tides

-effect of topography and shape of ocean basins

-Coriolis effect

Effects of confined basins

resonance

Moons angle to equator varies - each month it passesfrom max north position to max south.

Max north and south are 23.5 5 and that varies onabout 19-yr time scale.

http://www.co-ops.nos.noaa.gov/restles4.html

Declinationdetermines latitude of tidal bulge


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Orbits are elliptical, not circular, and distanceto Sun and moon also vary

http://www.co-ops.nos.noaa.gov/restles4.html

Tides respond to Coriolis effect: they veer right as theyenter basin, and hug shore on right (in NH) as they circulate

Circulate around a point of no rising/falling motion:

Amphidromic point

Tides in an ocean basin: Amphidromic system

Dynamic theory of tides As Earth rotates west to east, the tidal bulge is forced

against the western edge of the basin.

It piles up, and causes a pressure gradient.

Water flows downslope and is deflected to the right by

the Coriolis effect. Deflection causes water to build up against the

southern edge of the basin.

The resulting pressure gradient causes currents to

reverse, and flow northward.

Water is deflected toward the eastern side of the basin.

Circulate around a point of no rising/falling motion:

Amphidromic point

Result: rotary system flowing counterclockwise within basin

Dynamic theory of tides: Rotary flow


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Tidal circulation: Amphidromic system

Lines indicate time of high tide (hours) Tides progress around basins, counterclockwise in N

hemisphere and (clockwise in S hemisphere)

Tide movie based on satellite data

Motion is complicated!

Counterclockwise in NH

Southern Ocean , W. US examples

Amphidromic system Tides rotate around fixed

nodes

Points of no tide =amphidromic points

Cotidal lines connectpoints where tides aresynchronous

Corange lines on somemaps (fig 8.9 in book)show lines of constantrange Range increases as you

move away from an

amphidromic point

How fast do tides move?

Shallow-water wave speed:

C = d0.5

(m/sec) where d is depth For mean ocean depth of 4 km,

C = 198 m/sec = 444 mph

What happens in deeper or shallowerwater?

Speed variations; wave refraction


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Tides in confined basins

Increase tidal range (the difference

between high and low tide) Examples

--Bay of Fundy, Canada (tidal resonance)

--Northern Gulf of California, Mexico

Tidal bores - wave of water moving

upstream - result of high-tide crestentering confined inlet

Bay of Fundy: map

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Bay of Fundy tides

Extreme tides (10m or more) found where smallmarine basin adjoins large ocean

Bay of Fundy, Nova Scotia

Gulf of California

(in most places, tides are 1-2 meters in range)

Gulf of California

Large tidal range - up to 8-9 meters

Semidiurnal - a lot of water in motion!


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Tidal bore: Severn River, England

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A wall of water that surges upriver with the high tide; requires

large tidal range, confined basin, and shallowing depthshttp://www.mellowwave.co.uk/images

And you thought bores were boring

Tidal ecosystems

Rise and fall of tides creates stressfulenvironments for intertidal marine organisms

Tidal ecosystems

Others take refuge in tide pools, wherewater remains even at low tide


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Tidal rhythms in ecology

Grunion comeashore in spring

tides of spring andsummer

Females lay eggs,males deposit miltto fertilize

Eggs develop for~10 days in sand,until the nextspring high tidewashes them backto the sea

http://www.cabrilloaq.org/grunion.html

Tidal energy

Requires large tidal range (5m) and aconstricted flow path into a large

confined bay/estuary

Electricity is generates both on ebb and

flood tides

Bay can be closed off to control outflow

La Rance,France -

oldest andmstsuccessful

site.

Generatespower forabout half thetidal cycle.

Long-term tide changes

OcSci07.Tides

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