Tides Tides are waves Gravity waves Restoring: Surface Wind Long period tension waves waves Tides Tsunami Disturbance: Seiche and storm surge Wind Sun and moon Energy 10 1 10-1 10-2 10-3 10-4 10-5 Wave frequency (s-1) 0.1s 1s 10s 30s 5min 12h 24h Period 0.017m 1m 10m 1000m Wavelength Tides 2vs wind waves Wind waves 0 Amplitude [m] -2 0 50 100 150 200 Time [seconds] 2 0 Tides -2 Amplitude [m] 0 10 20 30 Time [days] Tides are • the longest ocean waves. • have wavelengths of up to 12,000 miles. • caused by the gravitational forces and motion of the Earth-moon-sun system. • never free of the forces that cause them -- they are forced waves. Using the force is a change in velocity Acceleration: (speed and/or direction) Looking down from space, need an acceleration towards center to move around the circle. Using the force is a change in velocity Acceleration: (speed and/or direction) On orbiting rock, feel motion away from the center. Inertia / centrifugal “force” Simplest starting point To start with Consider the Earth as a ball uniformly covered by water. (i.e., no land). Consider just the Earth and Moon. Earth-Moon system Gravitational forces want to pull the earth and moon together. but because of inertia from the rotation of the Earth-moon system the distance between the Earth and moon is constant. Earth-Moon system rotates around center of gravity If moon was same size as Earth Center of gravity Earth-Moon system rotates around center of gravity but the earth is 81 times heavier than moon Center of gravity Tide producing force Centrifugal or Inertial “force” is the same everywhere on or within Earth’s Center the Earth. Gravitational force of the moon decreases with the square of the distance from the moon. Forces balance for the planet as a whole, but not on the surface. Tide producing force Difference between gravity and inertia produces a net force on the oceans (and land). Earth’s Center Extra inertia Extra gravity Tide producing force Vertical part of net force (gravity minus inertia) is much weaker than Earth’s gravity, but horizontal part moves water resulting in tidal bulge (~2m high). Almost two tides a day Earth spins on it’s axis once a day. Moon orbits earth once a lunar month. H H H 24 hours later 24 hrs 50 min Moon’sis not inorbit the same plane as the Earth’s equator. Over a month the moon cycle between 28oN and 28oS, affecting heights of low and high tide. Now include the sun • Sun is 27 million times larger than the moon. • Sun is 368 times further from the Earth than the moon. • Upshot is the sun’s affect on tides is about 40% of the moons. Spring and neap tides Spring tide higher highs and lower lows Neap tide Dynamic Tidal Theory Real tides are more complicated than equilibrium tides -Presence of land -Tides travel as shallow-water waves -Bottom friction -Coriolis Force Amphidromes • Result from combination of basin geometry and Coriolis. Brooks/Cole Main semidiurnal Main diurnal Types of tides Bay of Fundy (Canada) has the world’s largest tides, up to 50 ft . Funnel shaped bay has the same naturally resonates (seiches) at the same period as the lunar tide. Tidal bores blue_planet_bore.mov BBC Blue Planet Storm surges Tides influence biology Some key ideas • Tides are the longest ocean waves, and can have wavelengths of up to 12,000 miles. • Tides are shallow water waves. • Tides are caused by the gravitational forces and motion of the Earth-moon-sun system. • Presence of land, and bottom friction makes global tidal patterns complicated. • High tides generally occur almost twice a day. • Spring tides occur almost twice a month. • Tides influences many things..