What are Tides?
• Waves (long L) generated by the gravitational pull of the moon &
sun and forces from the rotation of the earth
• Noticed as a daily change in sea level in coastal areas
– (every 12 hrs, 25 min, and 24 hrs, 50 min.)
• Once daily rise = diurnal
• Twice daily rise = semidiurnal
What are Tides?
• Tidal ranges
• Microtidal - maximum tide is < 2 m
• Mesotidal - 2-4 m
• Macrotidal > 4 m
• maximum = spring tides.
• Minimum= neap tides.
What Causes Tides
• Gravity
– Mass - Distance
• What affects tides more, the Sun or the Moon?
• equilibrium model of tides
What Causes Tides
• rotation of earth and the moon around common center of mass
• Centrifugal force
• Why do semidiurnal tides not occur at all coastlines?
What Causes Tides
• travels a path that is at an angle to the earth.
• lunar declination
• maximum declination of 28.5¾ N and S of equator
– High latitude = diurnal
– Low latitudes = semidiurnal
– mid latitudes = mixed tides (1 high, 2 low, then 2 high, 1 low per day
What Causes Tides
• What is the influence of the sun on this system?
• Full and New Moons = Spring Tides
• First and Third Quarter = Neap Tides
What Causes Tides
• Other influences - Sun/Earth system
– Shape of the earths orbit around the sun
– Perihelion = closest to the sun
– Aphelion = furthest
– Highest spring tides in the winter in the northern hemisphere
Tides
Equilibrium Model
• DAILY TIDAL CYCLES
• MONTHLY TIDAL CYCLES
• YEARLY TIDAL CYCLES
• Complications - Too simplistic
– varying depth of the ocean
– complex shapes of continents
– Coriolis effect
Dynamic Model of Tides
• the ocean basins and Coriolis effect
– Rotation of Earth > Moon = friction
– NH = COUNTERCLOCKWISE FLOW
– SH = CLOCKWISE FLOW
Dynamic Model of Tides
• amphidromic point
• Cotidal lines
Tides in small elongate basins
• Elongate basins
– No rotation - wash in (flood tide) and wash out (ebb tide)
– tidal resonance
• EX: the Bay of Fundy in Canada
• range of 13.6 m
Tides in small elongate basins
• tidal bore
– Amazon in Brazil - 5 m high, and travels 20 km/hr.
– Fu-Ch’un river in China - 7-8 m - 25 km/hr
• Tidal currents
– St. John River
Measuring Tides
• Tidal gauging stations
• Satellite imagery
Tides and the life cycles of organisms
• Grunions
– Egg laying & hatching
• Christmas Crabs