Tides

Tides• Tides are the rhythmic rise and fall of the

ocean’s water at a fixed location• Tides are extremely large waves

– High tide is the crest of the wave– Low tide is the trough of the wave

• Tides have a profound effect on marine organisms. Changing water levels creates hardships for coastal organisms, alternately exposing them to desiccation (drying out) and inundation (submerging in water)

Causes of Tides• Gravitational attraction of the Moon (Lunar

Tide) and to a lesser degree the Sun (Solar Tide)– There would still be a tide even without the

moon, it would be smaller and the difference between high tide and low tide would be less. The Sun is 27 million times larger than the Moon, but 400 times farther from the Earth

• Centrifugal force – created by the Earth rotating on its axis

Tidal Differences

• The Earth spins once every 24 hours, but the moon travels around the Earth as we spin. It takes a particular point on the Earth 52 min extra each day to “catch up” to the moon.– Tidal Period is 24 hrs 52 min

Differences in Tides are caused by

• Size, shape and depth of the oceans basins

• Season (more to do with solar tides)• Time of the month (effects mostly lunar

tides)• Coriolis effect

Amphidromic points – a point that does not rise and fall with the tides (the water rotates around it)

Tidal Range

• The tidal range is the vertical distance between the high tide and low tide– Example: if the high tide is 6.3 m and low tide

is 1.2 m would make the range 5.1 m

• The tidal range is at its greatest during the SPRING tide. This corresponds with the full moon and the new moon phases (not the season). This occurs twice every lunar month (29.5 days) when the Moon, Earth, and Sun are aligned.

– The attractive forces of the sun and moon pull water in the same direction and we observe higher high tides and lower low tides than normal

Full Moon New Moon

• Tidal range is at its least during the NEAP tide, which occurs during the first and third quarter phases of the moon (twice every lunar month). This is when the Moon and Sun are at a 90° angle with respect to the Earth.– The attractive forces of the sun and moon pull

water in different directions and we observe a lower high tide and a higher low tide than normal.

Third-quarter

First-quarter

Types of Tides• Diurnal Tide – only 1 high tide and one low

tide each day– Example: Northern parts of the Gulf of Mexico

and Vietnam

• Semidiurnal Tide – 2 high tides (of fairly equal height) and 2 low tides (of fairly equal height)– East coast of United States

• Mixed Tides – 2 high tides (of different heights) and 2 low tides (of different heights)– West coast of United States