Chapter 11: Tides Notes: -Field Trip Wednesday -Projects Insert: Textbook cover photo

Tides Are the Longest of All Ocean Waves Tides Are Forced Waves Formed by Gravity and Inertia The Dynamic Theory of Tides Adds Fluid Motion Dynamics to the Equilibrium Theory Tides Can Be Accurately Predicted Tidal Patterns Can Affect Marine Organisms

Tides Main Concepts Tides are periodic short-term changes in ocean surface height. Tides are forced waves formed by gravity and inertia. The equilibrium theory of tides explains tides by examining the balance of and effects of forces that allow our planet to stay in orbit around the sun, or the moon to orbit Earth. Because of its nearness to Earth, our moon has a greater influence on tides than does the sun. The dynamic theory of tides takes into account seabed contour, water’s viscosity, and tide wave inertia. Together, the equilibrium and dynamic theories allow tides to be predicted years in advance.

Tides Are the Longest of All Ocean Waves Tides are periodic, short-term changes in the height of the ocean surface at a particular place. Tides are caused by the gravitational force of the moon and sun and the motion of earth. The wavelength of tides can be half the circumference of earth and are the longest of all waves. Tides are forced waves because they are never free of the forces that cause them.

The Movement of the Moon Generates Strong Tractive Forces A planet orbits the sun in balance between gravity and inertia. If the planet is not moving, gravity will pull it into the sun. If the planet is moving, the inertia of the planet will keep it moving in a straight line. In a stable orbit, gravity and inertia together cause the planet to travel in a fixed path around the sun.

The Movement of the Moon Generates Strong Tractive Forces The moon does not rotate around the center of Earth. Earth and moon together – the Earth-moon system – rotate around a common center of mass about 1,650 kilometers (1,023 miles) beneath Earth’s surface.

The Movement of the Moon Generates Strong Tractive Forces The moon’s gravity attracts the ocean toward it. The motion of Earth around the center of mass of the Earth-moon system throws up a bulge on the side of Earth opposite the moon. The combination of the two effects creates two tidal bulges.

The Movement of the Moon Generates Strong Tractive Forces The action of gravity and inertia on particles at five different locations on Earth. At points (1) and (2), the gravitational attraction of the moon slightly exceeds the outward-moving tendency of inertia. At points (3) and (4), inertia exceeds gravitational force. Forces are balanced only at the center of Earth (point CE).

The Movement of the Moon Generates Strong Tractive Forces The formation of tidal bulges at points toward and away from the moon.

The Movement of the Moon Generates Strong Tractive Forces How Earth’s rotation beneath the tidal bulges produces high and low tides. Notice that the tidal cycle is 24 hrs 50 minutes long because the moon rises 50 minutes later each day. A graph of the tides at the island in (a).

The Movement of the Moon Generates Strong Tractive Forces A lunar day is longer than a solar day. In a 24-hour solar day, the moon moves eastward about 12.2°. Earth must rotate another 12.2° - 50 minutes – to again place the moon at the highest position overhead. A lunar day is therefore 24 hours 50 minutes long.

The Movement of the Moon Generates Strong Tractive Forces Tidal bulges follow the moon. When the moon’s position is north of the equator, the gravitational bulge toward the moon is also located north of the equator. The opposite inertia bulge is below the equator.

The Movement of the Moon Generates Strong Tractive Forces How the changing position of the moon relative to Earth’s equator produces higher and lower high tides. Sometimes the moon is below the equator, and sometimes it is above.

Sun and Moon Influence Tides Together Relative positions of the sun, moon, and Earth during spring and neap tides. At the new and full moons, the solar and lunar tides reinforce each other, making spring tides, the highest high and lowest low tides. (b) At the first-and third-quarter moons, the sun, Earth, and moon form a right angle, creating neap tides, the lowest high and the highest low tides.

Sun and Moon Influence Tides Together Tidal records for a typical month at (a) New York and (b) Port Adelaide, Australia. Note the relationship of spring and neap tides to the phases of the moon.

The Dynamic Theory of Tides So far, we have only discussed the equilibrium theory of tides. This deals primarily with the position and attraction of the Earth, moon, and sun, and does not factor in the influence on tides of ocean depth or the positions of continental landmasses. The dynamic theory of tides explains the characteristics of ocean tides based on both celestial mechanics (the gravity of the sun and moon acting on Earth) and the characteristics of fluid motion. The dynamic theory explains the differences between predictions based on Newton’s model and the observed behaviors of tides.

The Dynamic Theory of Tides Common tidal patterns Semidiurnal tides occur twice in a lunar day. They have two high tides and two low tides per day of approximately equal height. Diurnal tides occur once each lunar day. Mixed tides describe a tidal pattern of significantly different heights through the cycle.

Tidal Patterns Center on Amphidromic Points A mixed tide pattern at Los Angeles, California. A diurnal tide pattern at Mobile, Alabama. A semidiurnal tide pattern at Cape Cod, Massachusetts. The worldwide geographical distribution of the three tidal patterns. Most of the world’s ocean coasts have semidiurnal tides.

Tidal Patterns Center on Amphidromic Points Amphidromic points are nodes at the center of ocean basins; these are no-tide points. (ABOVE) The development of amphidromic circulation around an ocean basin.

Tidal Patterns Center on Amphidromic Points Amphidromic points in the world ocean. Tidal ranges generally increase with increasing distance from amphidromic points. The colors indicate where tides are most extreme (highest highs, lowest lows), with blues being least extreme. White lines radiating from the points indicate tide waves moving around these points.

Tidal Patterns Vary with Ocean Basin Shape and Size Tides in a narrow basin. True amphidromic systems do not develop in narrow basins because there is no space for rotation. Tides in the Bay of Fundy, Nova Scotia, are extreme because water in the bay naturally resonates (seiche) at the same frequency as the lunar tide.

Tidal Waves Generate Tidal Currents Tidal waves are the rise or fall in sea level as a tide crest approaches and passes will cause a tidal current of water to flow into or out of bays and harbors. Water rushing into an enclosed area because of the rise in sea level as a tide crest approaches is called a flood current. Water rushing out because of the fall in sea level as the tide trough approaches is called an ebb current. Slack water, a time of no currents, occurs at high and low tides when the current changes direction.

The Importance of Tides Most Tides Can Be Accurately Predicted There are at least 140 tide-generating and tide- altering forces and factors. Experience permits mathematical prediction of tidal height to an accuracy of about 3 centimeters (1.2 inches) for years in advance. Tidal Patterns Can Affect Marine Organisms Organisms that live between the high-tide and low- tide marks experience very different conditions from those that reside below the low-tide line. A deal deal of biodiversity can be found there. Power Can Be Extracted from Tidal Motion Tidal power is the only marine energy source that has been successfully exploited on a large scale.