Tides

• Tides are huge shallow water waves.

• They are caused by:- Gravitational force of moon and sun- Motion of the earth

Tides

• Tides are periodic, short-term changes in the height of the ocean surface at a particular place, due to gravitational attraction of the moon and sun and the motion of the earth.

• Wavelength can be one half of the circumference of the earth.

Tides

1) Equilibrium theory of tides (due to position and attraction of earth, moon and sun only)

2) Dynamic theory of tides (takes into account water depth and interference of continents)

Tides

• Moon has greatest effect on tides

• Sun has about one half the effect

Tides

• Lunar cycle - explanation of what we see• Equilibrium theory of tides - idealized view

(attraction of earth, moon and sun and their motion)

• Dynamic theory of tides - more realistic view (water depth, continents in the way, tide rocks back and forth in ocean basins)

-Lunar cycle-tidal day is 24 hrs, 50 min long

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Some points about the earth/moon system:

- Lunar day is 27 days, 7 hours, 43.2 minutes- Moon rises 50 minutes later each day- High tide is 50 minutes later each day

- Moon does not stay over the equator- Each month it moves 28.5 degreesabove and below the equator

View model of earth, moon and sun

Tides

1) Equilibrium theory of tides (due to position and attraction of earth, moon and sun only)

- assumes that the ocean surface is in

balance with the forces acting on it.

What is the affect of moon on the ocean surface?

• Gravity tends to pull earth and the moon together• Inertia tends to keep them apart (“centifugal force”)• Earth and moon are in a stable orbit.

Inertia keeps water in a bucket that swings overhead.

Imagine two people holding hands and spinningImagine earth and moon connected by a stick and

orbiting - center of mass is about 1000 miles into earth.

Newton, 1687

The pull of gravity between two bodies is proportional to the masses of the bodies butinversely proportional the square of the distancebetween them.

What does this mean?

Heavy bodies attract each other more strongly.

Gravitational attraction quickly weakens as the distance grows longer.

F= Gravitational attraction

F = G (m1m2/r2)

G= gravitational constantm1 = mass of object onem2 = mass of object twor = distance between their centers

moon does not stay over the equator- each month it moves 28.5 degreesabove and below the equator

Moon is closer to the earth sohas a bigger effect than the sun, even though the sun is much larger (moremassive).

Sun has about 1/2 effect on ocean surfaceas does the moon.

Continents interfere with tide movement

Gulf of California

Tide in a bathtub

Mode at centerof Gulf - least tide

Extreme tides in N & SGulf

North - shallower, mostextreme tides

“seiche”

How are tides measured?

•Depth guages measure depth change at a site•One year of data per site is minimum necessary•Need 18.5 years to most astronomical data and most accurate record for tides at a spot.

Types of tides

Diurnal (one low, one high per day)

Semidiurnal (two lows, two highs per day)

Mixed (lows and highs are unequal)

Spring tides (springen - “to move quickly”)

During full and new moonsMost extreme tides

Neap tide (naepa - “hardly disturbed”)

During quarter moonsLeast extreme tides

Tides

• Tides are huge shallow water waves.• Caused by gravitational force of moon and

sun, & motion of the earth• Moon exerts a greater tidal effect than does

sun• Ocean basins and water depth influence

tides

Ocean currents and upwelling

Ocean currents and upwelling

Why does water pile up in the western Pacific and how does this lead to upwelling in the eastern

Pacific?

- Sun drives wind-Wind drives currents

Ocean currents and upwelling

Strong southeasterly winds prevail along S. America Coast.Coriolis Effect due to earth’s rotation drag water to the left(northwest). Cold deep water comes to surface to replace the surface water = upwelling.

Ocean currents and upwelling

- Sun drives wind-Wind drives currents

-Earth’s rotation to the east causes wind and water to:-Deflect to the right in the

Northern hemisphere - Deflect to the left in the

Southern hemisphere

-Leads to gyres (huge circular currents) in the ocean basins-Leads to Trade Winds at Equator (Easterlies)

-Leads to Westerlies at Poles-Water piles up on the western boundaries of oceans

-Water blown from shore on the eastern boundaries of oceans-Upwelling occurs when water blown from shore

Effects of tides on life in the intertidal

• Intertidal zone - what is it?

• What is it like to live there?

Effects of tides on life in the intertidal

• Intertidal zone - what is it?Area of the shore that is uncovered and covered by the tides each day.

• What is it like to live there?Good! (soup of food, gases, mobility for larval forms, etc)Challenging at the interface of land and sea

What are some of challenges?

Life in the intertidal

Challenges:

Physical factorsDesiccation (drying), heat, wind, waves

Biological factorsCompetition, predation, mutualism, commensalism

life in the intertidal

Diversity- most phyla are marine35-39 phyla of animals, most are marine, majority exclusively

marine (insects dominate on land)

Differences between life in sea versus land:Lots of suspension, filter and deposit feedersLarval stages are mobileRadial symmetry not uncommonMany sessile animals

Research (“Intertidal Stories”) at Rocky Point

– Sponge isopod and three male forms– Seastar plague from El Nino (1978- present)– Angelic tooth snail and barnacles– Nerite snail “arms race” with lumpy claw crab– Hermit crab zonation and clumping– Invasion of the killer hydroid