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

Fig. 10-CO, p. 195

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 10-1, p.197 Planets orbit the sun in balance between gravity and inertia: (a)Not moving, gravity pulls planet to sun; (b) if planet is moving inertia will maintain straight line; (c) together you 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

Fig 10-14, pg. 205 Tides in basins, confined areas etc.

Fig 10-10, p.202 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”

Fig 10-2, p.197 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)

Fig 10-3, p.198 Moon’s gravity attracts the ocean toward it, tidal bulge on 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

Fig 10-6, p.199 Bulges follow the moon

Fig 10-4, p.198

Fig. 10-5, p. 199

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)

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Other factors affecting tides... #2 - the moon moves each month from a position 28.5 o above to 28.5 o below the equator –leads to unequal tides – a high high and a lower high that same day.

Fig 10-7, p.199 Tides can be even higher and 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

Fig 10-10, p.202

Fig 10-10, pg. 202

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 10-8, p.200

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Fig 10.9, pg. 201 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

Fig 10-14, p.205 Tides in basins, confined areas etc. A = narrow basin B= Bay of Fundy

Fig b, p. 206

Fig a, p. 206

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

Fig 10-16, p.208

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