Tides GEOL 012 Physical Oceanography Tides New Brunswick

Tides GEOL 012: Physical Oceanography

Tides New Brunswick Hopewell Rocks July, 2004

Tides • Periodic, short-term changes in the height of the ocean surface at a particular place • Caused by a combination of the gravitational force of the moon and the sun and the motion of the earth • Tidal wavelength can equal half of the earth’s circumference • Tides are the longest of all waves • Two main theories to explain tidal movement: the equilibrium theory and the dynamic theory

Equilibrium Theory of Tides • Explains ocean tides by examining the position and attraction of the earth, the moon and the sun • Theory assumes that the seafloor does not influence tides, and that the ocean conforms instantly to the forces that affect the position of its surface • The ocean is presumed to always be in equilibrium with the forces acting on it

The Moon • The earth - moon system revolves once a month (27. 3 days) • The moon’s gravity attracts the ocean surface toward the moon creating a “bulge” • Earth's motion (centrifugal force) creates another “bulge” on the opposite side of earth

The Bulges • The key to understanding the equilibrium theory is to visualize the Earth turning beneath the bulges • The bulges tend to stay aligned with the sun, moon, and Earth

Bulges and Tides • These bulges are the crests of the “planet size” waves that cause high tides • Low tides correspond to the troughs, the areas between the bulges • The wave crest and troughs that cause high and low tides are very small – 2 meter (7 foot) change in sea level

Timing • Lunar tides also vary in strength each month as the moon shifts 28. 5° N and S of the Earth’s equator • Lunar tides: tides caused by the gravitation and centrifugal force of the moon and earth complete a cycle every 24 hours and 50 mins – Why? Because the moon rises 50 minutes later each day • Therefore the highest tide also arrives 50 mins later each day

Solar Tides • The sun’s gravity also attracts the earth • Closeness accounts for much in determining the strength of gravity • So the sun’s “pull” is much less than the moon’s “pull” • Solar tides are smaller bulges that follow the sun through the day • The sun shifts 23. 5° N and S of the Equator (causing the seasons) which changes the position of the solar bulge

Earth-Sun Relations

Solar + Lunar Tides • The ocean responds at once to the pull of centrifugal force, lunar, and solar tides • If the earth, sun, and moon are all in a line than lunar and solar tides will be additive, resulting in higher and lower tides (spring tides) • If the moon sun, and earth form a right angle, the solar tide will diminish the lunar tide (neap tides)

Solar and Lunar Tides Figure 16. 7

Monthly Tidal Cycle

Spring and Neap tides • Spring Tides: large tides caused by the linear arrangement of the sun, Earth, and moon – High tides are very high and low tides are very low – These tides occur at two week intervals corresponding to the new and full moons • Neap Tides: are those in between spring tides each month

Annual Extreme Spring Tides • The sun and moon have orbits that are ellipses • Moon close to Earth (perigee) farthest away from Earth (apogee) • Earth close to sun (perihelion) and greatest distance from the sun (aphelion) • Extreme spring tides: if the moon and sun are over the same latitude and at perigee and perihelion • Extreme spring tides will have greater range in the N Hemisphere winter than in the N Hem summer because Earth is closest to sun during the northern winter

• http: //www. youtube. com/watch? v=1 bi. SSrc 6 q. VA • Grunion Run

Spring and Neap Tides

In Review • Equilibrium Theory of Tides • Low and High Tides • Spring and Neap Tides

The Dynamic Theory Of Tides • The equilibrium theory of tides has several problems – The predictions do not match the observed tide behaviors – See Natural Inquirer… – The Earth is not an idealized water covered Planet – Ocean depth is variable

• If Equilibrium theory holds true then: • Highest Spring Tides: midnight or noon during full or new moons – Effects greatest near equator – Least toward poles • Lowest tides at sunrise and sunset Highest Spring Tides: Full and New Moons Noon And Midnight

12 noon Midnight

Tidal Patterns • Landmasses obstruct tidal crests • Divert, slow, and complicating their movements • Shape of ocean basins and coastlines is variable • Funneling in bays, waves pile up • Therefore, coastlines experience different tidal patterns

Tidal Patterns • Semidiurnal tides: (twice daily) 2 low and high tides of nearly equal level each lunar day • Diurnal (Daily) tides: (daily) one high and one low tide each lunar day • Mixed: pattern where successive high and low tides have significantly different heights throughout the cycle

http: //www. oceanservice. noaa. gov/ Clear Westward Passage of Tidal bulges= Semidiurnal Blocked Westward Passage of tidal bulges = Diurnal Partially Blocked Westward passage of bulges = Mixed

Patterns • Diurnal: Gulf of Mexico • Mixed: West coast • Semidiurnal: East Coast

Tidal Datum • The reference level to which tidal height is compared • This point is not always set at mean sea level (the height of the ocean surface averaged over a few years time) • Mixed tide coasts: zero tide is the average level of the lower low daily tide • Diurnal and semi diurnal coasts: zero tide level is average level of all low tides

Tidal Range • Varies with basin configuration • Largest ranges occur at the edges of ocean basins, especially in bays or inlets that concentrate tidal energy because of their shape – Bay of Fundy, Canada

Tidal Currents

Cook Straight in New Zealand

Tidal Friction • Daily rise and fall of tides consumes energy dissipated as heat • Tidal friction is gradually slowing Earth’s rotation by a few hundredths of a second per century • Even this small change has long term planetary effects • Growth rings in fossil corals and clams: the days have grown longer, so the # days/ year has decreased as planetary rotation has slowed • 350 my BP a year contained 400 -410 days, each day being 22 hours long

Earth’s Rotation

Tides and Marine organisms • Tidal zonation • Tidal reproduction

Tidal Zonation

Tidal Reproduction: Grunion • As a wave breaks on the beach, grunion swim as far up the slope as possible • female grunion twists body and digs until half buried in the sand with her head sticking up. • then deposits her eggs in the nest. • Males curve around the female and release milt. A grunion run.

Tidal Reproduction: Coral Porites Spawning Great barrier Reef, AU Coral Spawning Video

Tides…In Review 1. Dynamic Theory of Tides 1. 2. 3. 4. 5. Tidal patterns Tidal datum Tides in confined basins Tidal currents Tidal friction 2. Tides and marine organisms