LECTURE NOTES:OCEANOGRAPHY (MARSC 100),SNYDER, L.
Chapter 11: Tides
Be sure to study the diagrams in your text in conjunction with these notes in order to understand these concepts
Tides are Waves
Regular rise & fall of sea that occurs: 1X day (24.8 hrs) OR 2X day (12.4 hrs)
Crest (high tide), trough (low tide)
Waves with very long:Period (24.8 or 12.4 hrs)
Wavelength (½ of Earth’s circumference)
Shallow-water waves (long wavelength)
Never in H2O deeper than ½ their wavelength
Cause of Tides
Gravitational attraction between: Earth & moon, Earth & sun
Pulls ocean toward moon & sun
Inertia (Centrifugal): Moving objects continue in a straight line
Earth’s motion around center of mass of Earth-moon system
Earth's orbiting around sun
Push outward from center of rotation
The moon is smaller than the sun, but is closer to Earth: Moon exerts 2x gravitational attraction as Sun
Forces (Gravity & Inertia) produces 2 tidal bulges
1. Excess gravity on moon-side of Earth ‘s: ocean bulges toward moon
2. Deficiency of gravity on side facing away from moon: Centrifugal force (Inertia) pulls ocean (bulges) away from moon
Earth rotates between bulges daily: 2 high & 2 low tides each day (most places)
3 Types of Daily Tides
1. Diurnal: 1 high & 1 low tide a day
High Latitudes (~90°), bays
2. Semidiurnal: 2 high & 2 low tides of ~ same height daily
Most common
3. Mixed: Successive high &/or low tides have different heights
Common in low latitudes (~0-35°)
Dynamic Tidal Theory: Tidal pattern affected by basin shape, continental masses, & Coriolis Effect
Amphidromic points (nodes): Where tide waves converge in ocean basin (No tide points)
Tide waves move around these points & rotate due to Coriolis Effect
Monthly Tides: Sun & Moon Together
Spring tides: Earth, sun, & moon are aligned (during new & full moon lunar phases)
Solar tide has additive effect on lunar tide: Extra-high, high tides & very low, low tides
Neap tides: Sun & moon are at right angles (1st quarter & 3rd quarter moon)
Solar tide partially cancels out lunar tide & Produces moderate tides
Movement of tidal water towards & away from land:
Generates tidal currents
Flood current: flow of water towards land with approaching high tide
Ebb current: flow of water away from land with approaching low tide
Tidal Datum measured at average lowest, low tide (Mean Low, Low Water = MLLW)
Tides & Marine Organisms
Intertidal species must adapt to stresses:
Crashing waves, abraison (High tide)
Heat, sun, dryness, No O2 (Low tide)
Tides & Marine Organisms
Mix nutrients & gases (CO2 & O2)
Important to movement & migration:
- Plankton (floaters & drifters) carried into tidepools
- Filter feeders (sea anemones, sand crabs) rely on tides to bring plankton & nutrients in with rising tides
- Animal Reproduction: Larvae require tides to move them toward or away from coast (Recruitment)
California Grunion Reproduction
Come out of water to spawn (lay & fertilize eggs) on beach (Female digs tail in sand to lay eggs, Male deposits milt (sperm) around her)
At night from Feb-Sep
At highest spring tides (full or new moon)
In ~10 days babies hatch & wash out to sea at next high spring tide