Tides Bay of Fundy (Canada) Tides Measuring tides General description Physical mechanism Characteristics in different locations Tidal resonance and tidal bore Measuring tides Records of the position of sea surface with respect to a fixed level (a datum) tidal pole float tidal gauge Measuring tides Time series of sea surface height Hilo tide observations and predictions (PacIOOS) Measuring tides Examples of tidal time series General description Up and down motion of sea level Back and forth currents Periodic (~12 hours and 24 hours) Amplitude and timing vary from one location to another Propagating shallow-water wave of very long wavelength (~10,000km) Physical mechanism due to the gravitational attraction of Moon and Sun on Earth and its ocean and the rotation of Moon around Earth and the Earth-Moon system around the Sun Moon Sun Earth Physical mechanism due to the gravitational attraction of Moon and Sun on Earth and its ocean and the rotation of Moon around Earth and the Earth-Moon system around the Sun Moon Sun “It's complicated!” Earth Theory of equilibrium tides Simplified problem Earth-Moon system only No rotation of Earth around its own axis Earth covered with an ocean (no continents) No friction Theory of equilibrium tides Centrifugal Force (varies with r) r d Gravitational Force (varies with 1/d2) Theory of equilibrium tides Theory of equilibrium tides Theory of equilibrium tides Theory of equilibrium tides Theory of equilibrium tides Tidal Force Theory of equilibrium tides Tidal Force Theory of equilibrium tides Tidal Force Two Lunar Bulges With Earth's rotation Semi-diurnal Tide B A C ~ 1 day D Two Lunar Bulges A B C D A Lunar day Lunar day is about 24 hours and 50 min Semi-diurnal tide is 12 hours and 25 min With Earth's rotation Semi-diurnal Tide B A C 1 lunar day D Two Lunar Bulges A B C D A With Earth's rotation Semi-diurnal Tide C D A equator 1 lunar day Two Lunar Bulges A B C D A With Earth's declination Diurnal Tide C A B tor ua eq 1 lunar day Two Lunar Bulges A B C D A With Earth's declination Mixed Tide C B A tor ua eq 1 lunar day Two Lunar Bulges A B C D A With Earth's declination Semi-diurnal Tide C B tor ua eq 1 lunar day A Two Lunar Bulges A B C D A Three types of tides Diurnal Tide Semi-diurnal Tide Mixed Tide Spring and neap tides Spring Tide Neap Tide Two Spring/Neap Tides every 29 ½ days Spring and neap tides Additional variations Greatest tidal range Lowest tidal range Additional variations Spring Tide + Perigee = Proxigean Tides Greatest tidal range Lowest tidal range Real tides Tides are shallow-water waves (period 12-24 hours, size of an ocean basin) Propagates counterclockwise in N.H. and clockwise in S.H. Amphidromic points (weak tides at these locations) Real tides Bay of Fundy: Case of tidal resonance Off-shore tides excite a natural mode of variability of the continental shelf or the bay Tacoma Narrows Bridge Collapse Tidal bore Incoming tide forms a wave that travels up a river against the river current Watch the "Bono" in Indonesia: A case of river surfing (Kampar River) .