Marine Science I Honors - Tides

Tides

Overview

• Tides result from the gravitational pull of the moon and, to a lesser degree, the sun.

• They are very long-period waves originating in the ocean, appearing as the regular rise and fall of the sea surface along coastlines.

Theories of Tides

• Equilibrium Theory (Isaac Newton):

  • Assumes a perfectly uniform Earth, very deep water, and no landmasses.

  • Too simplistic.

• Dynamic Theory (Pierre-Simon Laplace):

  • Modified Newton’s model to account for tidal variations.

  • Shows multiple tidal bulges, not just two.

Gravitational Pull

• The moon's gravitational pull is the primary driver of tides.

• The sun also exerts a gravitational influence, though to a lesser extent.

• High tide occurs when the crest of the tidal wave reaches the coast.

• Low tide occurs when the trough of the tidal wave reaches the coast.

• The tidal range is the difference in height between high and low tide.

Tidal Currents

• A tidal current is the horizontal movement of water accompanying the rising and falling tide.

• Flood Current: The incoming tide along the coast and into bays/estuaries.

• Ebb (or Slack) Current: The outgoing tidal current.

• Daily tides create currents that flow into and out of bays, rivers, harbors, and other restricted areas.

  • Inflow: Flood current

  • Outflow: Slack current

  • Midpoint: Slack tide

Factors Affecting Tides

1. The Moon’s Gravitational Pull

• The moon's gravitational field pulls the Earth’s oceans on opposite sides, creating tidal bulges.

• The Earth rotates in and out of these tidal bulges.

• Most coastlines experience two high tides and two low tides each day.

• High tides can be unequal in height due to the axial angle of the Earth’s rotation.

• Gravity and inertia are opposing forces creating tidal bulges on opposite sides of the planet.

2. The Sun, Moon, and Types of Tides

• Spring Tides:

  • Occur during new moon (no moon visible) and full moon (sun and moon aligned on opposite sides of the Earth).

  • Result in the highest and lowest tides.

• Neap Tides:

  • Occur when the moon is in a quarter phase, forming a right angle with the sun relative to the Earth.

  • The sun's gravity pulls to the side of the moon’s tidal bulge, raising low tide and lowering high tide.

  • Weaker tides.

3. Declination of the Moon

• The Moon’s declination (angle of the Moon’s orbit with respect to the Earth’s equator) varies over an 18.6-year cycle from ~28^\circ to ~18^\circ.

• The Earth’s tidal bulges track the position of the Moon and influence the height of tides.

4. Eccentricity of the Earth’s Orbit

• The Earth’s orbit around the Sun is slightly eccentric.

• Perihelion: Earth’s closest approach to the Sun at 147.1 \times 10^6 km.

• Aphelion: Earth’s farthest distance from the Sun at 152.1 \times 10^6 km.

• The difference between perihelion and aphelion is ~5 \times 10^6 km.

• This small change results in a small change in the solar tidal component (height of tide) because the tidal influence of the Sun is a function of distance.

5. Eccentricity of the Moon’s Orbit

• The orbit of the Moon around the Earth is slightly eccentric.

• Perigee: Moon’s closest approach to the Earth is 363,396 km.

• Apogee: Moon’s farthest distance from the Earth is 405,504 km.

• The difference between perigee and apogee is ~42,000 km.

• The tide-generating influence of the Moon increases during perigee.

• When the Moon is at perigee, it appears ~14% larger and is known as a super moon.

• The effect of eccentric orbits of the Earth and Moon have a very small influence on tides.

• This effect may be only a few inches but can be additive with other factors.

• “King tide” describes extra-large high tides, occurring when the Earth, Moon, and Sun are aligned at perigee and perihelion.

6. Regional Factors Affecting Tides

• Tidal range varies considerably along different coastlines.

• Underwater bathymetry (shape of seafloor) and coastline shape dramatically affect the local tidal range.

• As the Earth rotates within the tidal bulges, continents disrupt the migration of the bulge in the oceans.

Tidal Patterns

Lunar Day and Frequency of Tides

• Most coastal areas experience two high tides and two low tides in a lunar day of 24 hours and 50 minutes.

• A lunar day is the time it takes for a specific point on Earth to rotate from an exact point under the Moon to the same point under the Moon.

Types of Tidal Patterns

• Diurnal Tide: Having a single low and high tide daily (e.g., Gulf of Mexico).

• Semidiurnal Tide: Having two roughly equal high and low tides daily (e.g., East coast of the US).

• Mixed Tide: Having two unequal high and low tides daily (e.g., West coast of the US).

• If the Earth had no continents and was uniformly covered by ocean, the entire planet would experience two equally proportioned high and low tides every lunar day.

• Continents block the migration of tidal bulges, resulting in complex tidal patterns depending on coastline shape and seafloor bathymetry.

Unique Tidal Patterns - Tidal Bores

• A tidal bore forms when the incoming tide produces a wave that flows into a river or other narrow area.

• This is a true tidal wave and can be several meters/feet high in places (e.g., Amazon River).

Tide Characteristics

• Besides lunar and solar gravity, the imperfect sphere of the Earth, the season, the shape of the ocean basin, and the Coriolis effect influence tides.

• Tides rotate around amphidromic points, where water doesn’t rise and fall with the tides.

Monitoring Tides

• Predicting tides is important for shipping, marine industries, and commercial fishing.

• Navigating ships through shallow water ports requires knowledge of the time and height of the tides, as well as the speed and direction of tidal currents.

• A tide gauge measures the change in sea level relative to a baseline.

• Sensors continuously record the height of the water level.

• Tide gauges are automated with electronic sensors, and real-time tide data are available online.

• Data are available from more than 1,750 tide gauges worldwide.

• Tide gauges have been an important source of data for documenting the increase in mean sea level due to climate change.

• Global sea level has risen by ~2.0 mm/year.