Chapter 11 Tides

Tides

• Tides Are the Longest of All Ocean Waves, hence they are Long waves

• Tides Are Forced Waves Formed by Gravity and Inertia

• They are responsible for the rise and fall in sea level.

• Tides occur as shallow water waves and depend heavily upon topography.

• One low-tide/high-tide cycle takes about 12 hrs and 25 min.

• Tidal range is the difference in ocean level between high-tide and low-tide

What is the Tidal Range?

• HT = 30 ft, LT = 20 ft• HT = 20 ft, LT = 12 ft• HT = 50 ft, LT = 20 ft

2 high tides = flood tides2 low tides = ebb tides

each day

The gravitational attractions of the Moon and Sun acting on the rotating

Earth.

What Causes Tides

Tides• Tides result from the gravitational pull of

the moon and, to a lesser degree, the sun.

Gravity and inertia

A planet orbits the sun in balance between gravity and inertia. (a) If the planet is not moving, gravity will pull it into the sun. (b) If the planet is moving, the inertia of the planet will keep it moving in a straight line. (c) In a stable orbit, gravity and inertia together cause the planet to travel in a fixed path around the sun.

The moon’s gravity pulls the ocean toward it. The motion of Earth around the center of mass of the Earth-moon system causes a bulge on the side of Earth opposite the moon. The combination of the two effects creates two tidal bulges.

Tidal bulges

Gravitational Effect of the Moon

• Two big bulges of water form on the Earth:–one directly under the moon –another on the exact opposite side

• As the Earth spins, the bulges follow the moon.

Tidal bulges follow the moon. When the moon’s position is north of the equator, the gravitational bulge toward the moon is also located north of the equator and the opposite inertia bulge is below the equator.

Lunar tidal bulge

Sun and Moon Influence Tides Together

Relative positions of the sun, moon, and Earth during spring and neap tides. (a) At the new and full moons, the solar and lunar tides reinforce each other, making spring tides. (b) At the first-and third-quarter moons, the sun, Earth, and moon form a right angle, creating neap tides.

Spring tides• When the sun and the moon are aligned, their

gravitational effects combine to increase the planetary water bulge.

Gravitational Effect of the Sun and Moon

• Spring Tides -every 2 weeks

–Earth, Moon, and Sun are lined up

–High Tides ~20% higher than normal

–Low Tides are lower than normal

Neap tides

• When the moon and sun are at right angles (quarter moon stages), tides are weaker = neap tides.

Gravitational Effect of the Sun and Moon

• Neap Tides- every 2 weeks

–Earth, Moon, and Sun form right angles

–High Tides are ~20% lower than normal and Low Tides are higher than normal, opposite interval to Spring Tide

Tidal records for a typical month at (a) New York and (b) Port Adelaide, Australia. Note the relationship of spring and neap tides to the phases of the moon.

Sun and Moon Influence Tides Together

The magnitude of tides varies

• Spring tide - higher high and lower low tides (more extreme)

• Neap tide - weaker tide• Timing of spring and neap tides are

important for forecasting hurricane storm surge – why?

NOAA tide demonstration

• http://www.montereyinstitute.org/noaa/lesson10.html

Ignite learning media

The Dynamic Theory of Tides

• The dynamic theory of tides explains the characteristics of ocean tides based on celestial mechanics (the gravity of the sun and moon acting on Earth) and the characteristics of fluid motion.

• Semidiurnal tides occur twice in a lunar day• Diurnal tides occur once each lunar day• Mixed tides describe a tidal pattern of significantly

different heights through the cycle• Amphidromic points are nodes at the center of ocean

basins; these are no-tide points.

• Diurnal One high and one low tide each lunar day

• Semidiurnal Two high and two low tides of about the same height daily

•

Tidal patterns

Tidal Patterns of world coastlines

Common tide types.

(a)A mixed tide pattern at Los Angeles, California.

(b)A diurnal tide pattern at Mobile, Alabama.

(c)A semidiurnal tide pattern at Cape Cod, Massachusetts.

(d)The worldwide geographical distribution of the three tidal patterns. Most of the world’s ocean coasts have semidiurnal tides.

• Co-tidal map shows tides rotate around amphidromic points. vertical movement)

• There are 140 amphidromic points in the world’s oceans.

• More down-to-earth pattern of tides in the ocean

Tides in the Ocean

Amphidromic Points

Tidal Ranges increase with increasing distance from Amphidromatic points. Coriolis drives the process counter-clockwise in the northern

hemisphere and clockwise in the southern hemisphere.

Red = most extreme Blue = least extreme

White lines indicate tide waves that circulate around these points due to Coriolis.

Tidal currents occur in some bays and rivers due to a change in tides

– Ebb currents - produced by outgoing tides

– Flood currents - produced by incoming tide

– LSW means Lower Slack Water

– HSW means Higher Slack Water

Tidal Currents