MAR 110: Introductory Oceanography

MAR 110: Introductory Oceanography Ocean waves and tides

Rogue waves, part 1 • On 11 September 1995, steamship Queen Elizabeth II was hit by a wave at least 29 m high. • The ship was in heavy seas as it changed course to avoid Hurricane Luis, but the waves averaged about 18 m in height. • A nearby buoy recorded a wave of 30 m. • Unusually large waves are called rogue waves. • Because of their size, they can be very destructive. • They have a steep forward face preceded by a deep trough. Ocean waves and tides

Rogue waves, part 2 • Rogue waves are especially common in the Agulhas Current, where swell from the Southern Ocean hits the fast-moving current. • Rogue waves are generated in two ways: • Constructive interference, in which a storm wave rides atop a swift current. • A focusing effect of eddies produced by strong currents. • Rogue waves may explain the disappearance of ships in places like the Bermuda Triangle. Ocean waves and tides

Rogue waves, part 3 • Other incidents involving rogue waves: • USS Ramapo hit by a 34 m wave during a storm in the South Pacific, 7 February 1933. • The Edmund Fitzgerald may have been sunk by a rogue wave on Lake Superior, 10 November 1976. • Mariners had talked of rogue waves for centuries, but their stories were often discounted by “experts.” Ocean waves and tides

Types of waves • Most waves are produced by the interaction of the atmosphere and ocean in which wind transfers some of its kinetic energy to the ocean surface. • These waves are called wind waves. • Movements of the Earth’s crust trigger another type of wave – tsunami. • The gravitational interactions of the sun and moon with the Earth’s surface produce another motion – tides. Ocean waves and tides

Wind-driven waves, part 1 • A wave is a regular oscillation that occurs in solids, liquids, or gases, as energy is transmitted through the medium. • Characteristics of waves: • The highest point reached by the water surface is the wave crest. • The lowest point reached by the water surface is the wave trough. • The distance between the crest and trough is the wave height. Ocean waves and tides

Wind-driven waves, part 2 • Characteristics of waves (continued): • The distance from crest to crest or trough to trough is the wavelength. • The time needed for a complete wave to pass a point is called the wave period. • The number of waves passing a given point during a interval of time is called the wave frequency. Ocean waves and tides

Ocean waves and tides

Wave generation, part 1 • Wind transfers some of its kinetic energy to the water surface, generating small ripples known as capillary waves. • Capillary waves have a wavelength of less than 1.7 cm. • The surface tension of water (a result of hydrogen bonds) provides a restoring force that smoothes out these waves. • As winds strengthen, larger waves are produced. • Gravity provides the restoring force, but the downward momentum creates a trough ahead of the wave. Ocean waves and tides

Wave generation, part 2 • Waves propagate away from the disturbance that creates them. • Where friction from the bottom is negligible, water molecules move in circular obits. • The radius of the orbits decreases with depth. • The depth at which wave motion ceases is called the wave base. • The wave base is typically one-half that of wavelength. • There is actually very little horizontal transport of water molecules in a wave. Ocean waves and tides

Wave generation, part 3 • Environmental conditions that contribute to wave formation includes: • Wind speed • Turbulence: Increasing wind speed generates turbulence in the form of eddies in the air. • Wind duration: The length of time wind blows from the same direction. • Wind fetch: The distance that wind blows over a water surface. • Speed, duration, and fetch determine the amount of kinetic energy transferred to the water surface. Ocean waves and tides

Wave generation, part 4 • Interference influences the growth and development of waves. • In constructive interference, wave crests from two or more storm systems coincide to form composite waves with heights greater than those formed by the individual storm systems alone. • In destructive interference, waves from one storm system coincide with troughs from another, forming a composite waves with crests less than those formed by the individual storm systems. Ocean waves and tides

Sea and swell • A sea is a confused mass of waves moving in many different directions. • Swell refers to lower, more rounded waves that propagate beyond the limit of the storm winds that generated the waves in the first place. Ocean waves and tides

Deep-water waves, part 1 • Waves that form in water deeper than their wave base are known as deep-water waves. • Celerity is the speed of the wave relative to the water. • Where: • C = celerity in m/sec • Wavelength is measured in m Ocean waves and tides

Deep-water waves, part 2 • In deep water, waves with longer wavelengths travel faster than those with shorter wavelengths. • Longer waves thus outpace shorter waves. • As a result, swell can propagate thousands of kilometer from its source. Ocean waves and tides

Shallow-water waves, part 1 • As a wave moves into shallow water, friction against the bottom affects the wave. • The orbits of water particles in shallow-water waves flatten out with depth. • Wave period remains the same, but wavelength shortens, and wave height increases. • A shallow-water wave occurs in depths less than one-twentieth of wavelength. • A transitional wave occurs in depths between one-half and one-twentieth of wavelength. Ocean waves and tides

Shallow-water waves, part 2 • Celerity of shallow-water waves is calculated by: • Where: • C = celerity in m/sec • g = gravity • Depth is in m Ocean waves and tides

Shallow-water waves, part 3 • Shallow-water waves slow as they enter shoaling water (water that becomes increasingly shallow). • As waves shoal, molecules of water at the crest move more quickly than the wave itself. • As the wave front steepens, it become unstable and breaks. • Waves break when the ratio of wave height to wavelength approaches 1/7. • The wave front (crest angle) is about 120 degrees. Ocean waves and tides

Shallow-water waves, part 4 • Breaking waves pack a wallop – a 2 m wave exerts a pressure of 15,000 kg/m2. • Surf is a continuous train of waves along a shoreline. Ocean waves and tides

Seiches, part 1 • A seiche is a rhythmic oscillation of water in an enclosed basin – such as a lake. • In a seiche, while water at one end rises, it sinks at the other. • A seiche is a standing wave crests and troughs alternate in place. • Wind-driven waves are progressive waves in that the crests and troughs move through the body of water. • Gravity provides the restoring force for both progressive and standing waves. Ocean waves and tides

Seiches, part 2 • In a typical seiche, there is a node near the center of the basin in which the water level does not change. • At the node, there is considerable horizontal movement of water. • Along with a node are antinodes which have the greatest vertical movement of water. • There is little horizontal movement at antinodes, however. • Wind, air pressure changes, seismic activity, and tides are capable of producing seiches. Ocean waves and tides

Seiches, part 3 • The period of a seiche is proportional to the basin length. • The period of a seiche is inversely proportional to basin depth. Ocean waves and tides

Atmosphere-ocean transfers • Waves accelerate transfers of energy and matter between the atmosphere and ocean. • Waves with shorter lengths are important in heat transfer from the ocean to the atmosphere. • Latent heat from the ocean surface is an important driver of storm development. • Waves with shorter lengths are important in transferring salt particles to the atmosphere. • Salt particles are an important source of cloud condensation nuclei. • Breaking waves trap air in bubbles that serve as an important source of dissolved oxygen and carbon dioxide. Ocean waves and tides

Astronomical tides, part 1 • Astronomical tides are the regular rise and fall of water level caused by the gravitational interactions of the Earth, sun, and moon. • The length of a tide wave is much greater than the depth of the ocean, so it behaves as a shallow-water wave! • On a theoretical non-rotating, ocean-covered Earth, tides can be thought of as waves with a length about half the circumference of Earth. • Astronomical tides are forced waves whose crests are directly below the celestial body that causes them. Ocean waves and tides

Astronomical tides, part 2 • The speed of tide wave propagation depends on the rotational velocity of Earth relative to the position of the sun and moon. • At the equator, the crest of a tide wave would travel at about 1,600 km. • Because continents break up the ocean into individual basins with finite depth; this affects celerity. • Tide waves travel faster in deep waters than in shallow waters. Ocean waves and tides

Astronomical tides, part 3 • The tidal range is the height difference between high and low tide. • The greatest tidal range in the world is in the Bay of Fundy. • The tidal period is the elapsed time between successive high tides. Ocean waves and tides

Tide-generating forces, part 1 • Two forces combine to generate tides: • The gravitational interaction of Earth, sun, and moon. • The rotation of the Earth-moon and Earth-sun systems. • Two sets of bulges are produced: • One bulge points toward the moon; it has a corresponding bulge on the opposite side of the Earth. • One bulge points toward the sun; it likewise has a corresponding bulge on the opposite side of the Earth. Ocean waves and tides

Tide-generating forces, part 2 • According to Newton’s laws of gravity, the strength of the gravitational attraction is proportional to the masses of the objects involved and inversely proportional to the distance between the objects. • Earth-moon interactions: • The gravitational pull of the moon is greatest on the side directly underneath the moon; the opposite bulge is produced by the rotation of the Earth-moon system. Ocean waves and tides

Tide-generating forces, part 3 • According to the equilibrium model of tides, ocean bulges would always align with the forces that create them. • This assumes a frictionless surface of the Earth. • If only one celestial body was involved in generating tides, low-latitude locations would experience two high tides and two low tides each day. Ocean waves and tides

Tide-generating forces, part 4 • The motion of the moon around the Earth complicates the situation, however. • It takes a little more than 24 hours to experience two high and two low tides. • A tidal day is 24 hours, 50 minutes long. • Tidal bulges produced by the moon are offset latitudinally as well. • The moon’s orbit is offset by about 5 degrees relative to the Earth’s equator. • The moon’s latitudinal position swings from about 28.5 degrees N to about 28.5 degrees S each lunar month. Ocean waves and tides

Tide-generating forces, part 5 • Motions of moon and Earth (continued): • At the maximum latitudinal range of the moon, the Tropic of Cancer and Tropic of Capricorn experience one tidal bulge per day. • Tidal patterns produced by the interaction of the Earth and sun are similar to those produced by the moon and Earth. Ocean waves and tides

Types of tides, part 1 • Tides are described as diurnal, mixed, or semidiurnal. • Semidiurnal tides are when a location experiences two equal high and two equal low tides per day. • They have a period of 12 hours, 25 minutes. • They typically occur at all but the highest latitudes when the moon is directly over the equator (thus, the bulges are centered over the equator). • When the moon is directly over the equator, it is also referred to as an equatorial tide. Ocean waves and tides

Types of tides, part 2 • Different tides develop when the moon is either north or south of the equator. • Mixed tides occur when a location has two unequal high tides and two unequal low tides per day. • The difference in heights between successive high tides or successive low tides is called the diurnal inequality. • When the moon is directly overhead the Tropic of Cancer, or directly over the Tropic of Capricorn, the tide is described as a tropical tide. Ocean waves and tides

Types of tides, part 3 • When the moon and its associated tidal bulges are either north or south of the equator, high-latitude locations experience one high tide and one low tide per day – this is a diurnal tide. • The period of a diurnal tide is 24 hours, 50 minutes. • The tidal bulges created by the sun and moon interact. • During a spring tide, the bulges line up to produce the greatest monthly tidal range. • When the gravitational pull of the sun and moon are at right angles to each other, resulting in the minimum monthly tidal range; This is called a neap tide. Ocean waves and tides

Tides in ocean basins, part 1 • The presence of continents, the Coriolis effect, winds, coastline configuration, water depth, and bottom topography all affect tides. • The only area where tidal bulges have relatively unrestricted motion is in the Southern Ocean. Ocean waves and tides

Tides in ocean basins, part 2 • According to the dynamic model of tides, tidal bulges move to the western boundary of ocean basins as the Earth rotates. • The water surface slopes downward to the east. • As the tidal bulge moves downslope, the Coriolis effect deflects the motion of water particles. • As a result, water slopes downward to the north in the Northern Hemisphere and to the south in the Southern Hemisphere; the crests of the tidal bulges are near the equator. • Tidal bulges continue the rotation around ocean basins – counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Ocean waves and tides

Tides in ocean basins, part 3 • Dynamic model of tides (continued): • The rotary motion can be indicated by cotidal lines – lines that experience high tide at the same time of day. • Diurnal tides make one complete circuit per day. • The period of a diurnal tide is 24 hours, 50 minutes. • Mixed and semidiurnal tides make two circuits per day. • The period of a semidiurnal tide is 12 hours, 25 minutes. Ocean waves and tides

Tides in ocean basins, part 4 • Shallow basins with the right length may have periods of oscillation that match that of the tide-generating force; This is called resonance. • Resonance explains the tremendous tidal range of the Bay of Fundy, which can reach 16 m during a spring tide. Ocean waves and tides

Tidal currents, part 1 • The alternating rise and fall of tides generates tidal currents in coastal areas. • Tidal currents are strongest along the margins (antinodes) of ocean basins. • When currents flow towards the land, water levels in harbors and rivers rise; these currents are called flood tides. • When currents flow toward the sea, water levels in harbors and rivers drop; these currents are called ebb tides. • Between flood and ebb tides there is little horizontal movement of water; these intervals are called slack water. Ocean waves and tides

Tidal currents, part 2 • Where the tidal range is large and the flood tide enters a narrow bay or channel, a tidal bore forms. • Tidal bores are turbulent; they form walls of water typically less than 1 m in height. • Tidal bores are common along the mouth of the Amazon, the Severn River in England, and in Turnagain Arm off the Cook Inlet, Alaska. Ocean waves and tides

MAR 110: Introductory Oceanography