TIDES

• periodic, short term changes in the height of the ocean surface at a particular place

What causes tides?

• a combination of the gravitational force of the moon and sun and the rotation of the earth

• moon exerts greater effect than the sun

Tides are waves

• Tides are simply specific types of “waves,” we now know that waves can be either deep or shallow and long or short…

• Thus, tides are the longest of all (shallow water) wave examples

• longest and largest of all waves in the ocean = tides…wavelengths of half the Earth’s circumference (max.)– High tide is the crest of the wave and low tide is the

trough

Studies

• 2 scientists (combined) explain the effects and daily occurrence of tides

• Newton• Laplace

Equilibrium Theory of Tides• Isaac Newton, 1600’s• Would accurately describe tides on a planet

uniformly covered with water through a gravitational model

• Does not consider the effects of continents, ocean basin shape and ocean depth

• Explains the effects of the gravitational attractions of Earth, moon and sun (with the influence of the moon being 50% greater than that of the sun)

Fig 10-2, p.229

Planets orbit the sun in balance between gravity and inertia: (a) Not moving, gravity pulls planet to sun; (b) if planetis moving inertia will maintain straight line; (c) togetheryou get a fixed path for planet orbit.

Newton: Equilibrium theory

Newton/Equilibrium theory

• Assumes the ocean conforms instantly to the forces that affect the position of its surface – i.e. the oceans surface is always in equilibrium (balance) with the forces acting on it.

What’s missing?

• There are continents in the way of the water moving!

• The ocean basin is not flat and/or uniform!• Average tidal range = 7’ for world (thus

NOT at equilibrium)!

Dynamic Theory of tides

• Laplace, 1775 (1 century later)• A modification to Newton’s gravitational

model = the Dynamic Theory• Now considers ocean depth, continents, the

shape of the basin AND what that means to long-wavelength tides/waves in shallow waters

Tides in basins, confined areas etc.

Different tides in different locations!

Put both theories together…

• Newton explains HOW the tides occur• Laplace explains how they INTERACT

with the surrounding environment (continental margins and shores and even ocean basin depth)

• Result: What we now know as “daily tides”

Moon doesn’t rotate around the center of earth but instead around its center of mass (illustrated); they, in turn, together rotate around this center point. (complete cycle every 27.3 days)

Moon’s gravity attracts the ocean toward it, tidal bulgeon opposite side(s).

How tides occur...

• The bulges stayes aligned with the moon as the earth rotates on its axis,

• as points on the earth pass through the bulges, they experience high tide

Bulges followthe moon

Fig 10-7, p.231

Fig. 10-8, p. 232

Other factors affecting tides...

• #1 - the moon rises 50 min. later each day– Thus high tides occur about 50 min later each

day.

Remember (in the NE) there are 2 high and 2 low tides per tidal cycle (roughly 6 hrs. apart)

Fnft

Other factors affecting tides...

• #2 - the moon moves each month from a position 28.5o above to 28.5o below the equator– leads to unequal tides – a high high and a lower

high that same day.

Tides can beeven higherand lower –All based on moon’s position.

Tidal regimes

• Semidiurnal tides = two high tides and two low tides of nearly equal level each lunar day

• Diurnal tides = one high and one low tide each lunar day

• Mixed tides = when successive high or low tides are significantly different in height

Influence of the Sun

• The effects of the sun and moon cause wave interference– constructive interference…higher high tides

and lower low tides = spring tides– destructive interference…lower high tides and

higher low tides = neap tides

Fig 10-11, p.233

fnft

Highest “high” tides = spring tides

Interesting “additions” to tides

• Tidal range is different/unique in each location depending upon the topograpgy of the ocean basin underneath the water AND the continental margin(s) shape.

• Tidal range = high-water to low-water height differences in an area

Tides in basins, confined areas etc.

A = narrow basin B= Bay of Fundy

France, 46’, Fig. 10-1a, p. 229

Fig. 10-1b, p. 229

Effects? …Tidal currents• Rise and fall in sea-level as the tidal crest

passes will cause currents• Water rushing toward shore due to rising

sea-level is called a flood current• Water rushing away from shore is called an

ebb current• Slack water – a time of no current occurs at

high or low tide

And finally – tides effect marine life!

• Tides produce severe zonation• Currents move fish/planktonic organisms to

extreme locations (even more severe during storms)

• Seasons effect temperature and that effects everything!

• Ex. Grunion: swim up on shore to deposit eggs during an extreme spring tide each year