Tides Overview and Causes in Marine Science

Marine Science - Tides Overview

  • Definition of Tides
    • Tides are periodic rises and falls of sea level.
    • They can be viewed as very long and regular shallow-water waves.

Causes of Tides

  • Gravitational Attraction

    • Primary cause of tides is the gravitational attraction exerted by the Moon and the Sun.
    • The gravitational force (F<em>gF<em>g) is defined by Newton's Law of Universal Gravitation: F</em>g=Gm<em>1m</em>2r2F</em>g = \frac{G m<em>1 m</em>2}{r^2}
    • where:
      • GG = gravitational constant
      • m<em>1,m</em>2m<em>1, m</em>2 = masses of the two objects
      • rr = distance between their centers

  • Barycenter Motion

    • The center of mass for the Earth-Moon system (barycenter) orbits around the Sun.
    • The positions and motions of the Earth, Moon, and Sun jointly influence tidal forces.

Types of Tidal Forces

  • Tide-Generating Forces (TGF)

    • Resultant forces that create tidal bulges by exerting a significant horizontal component.
    • TGF are inversely proportional to the cube of the distance between points on Earth and the center of the tide-generating body.
    • This means that distance has a larger effect on tidal forces than gravity does.

  • Lunar Tidal Bulges

    • Two distinct tidal bulges form: one towards the Moon and one opposite.
    • The lunar tidal bulge has a periodicity of 12 hours and 25 minutes.

Tidal Patterns

  • High and Low Tides

    • High tide (flood tide): seawater moves onshore.
    • Low tide (ebb tide): seawater moves offshore.

  • Tidal Cycles

    • The tidal cycle is approximately 29.5 days, with spring and neap tides occurring roughly every two weeks.

Declination and Orbits

  • Declination

    • Refers to the angular distance of the Moon or Sun above or below the equator.
    • The Moon is tilted at 5° relative to the ecliptic.

  • Elliptical Orbits

    • The Earth's and Moon's elliptical orbits cause variations in distance that affect tidal ranges.
    • Higher tidal ranges occur at perihelion (closest to the Sun) and perigee (closest to the Moon).

Tidal Predictions and Patterns

  • Idealized Predictions

    • There are two high tides and two low tides per lunar day with a 6-hour difference between them.
    • Tides can be semidiurnal, diurnal, or mixed based on geographical location and lunar cycle influences.

  • Coastal Currents

    • Tidal currents can include rotary currents in open basins and reversing currents in nearshore areas.
    • Flood current moves water into bays/river during high tide, while ebb current drains the water during low tide.

Example Study: Bay of Fundy

  • Known for its significant tidal range, which can reach up to 17 meters (56 feet).
  • The maximum spring tidal range is amplified due to the geographic characteristics of the region, highlighting the influence of local geography on tidal patterns.

Additional Key Terms

  • Zenith: The point on Earth closest to the Moon.
  • Nadir: The point on Earth furthest from the Moon.
  • Centripetal Force: The force that keeps a body moving in a circular path, crucial for maintaining orbital stability.
  • Spring Tide: Tides with the largest ranges, occurring during full and new moons.
  • Neap Tide: Tides with the smallest ranges, occurring during quarter moons.