OC103 Lesson #18: Waves in the Ocean An ocean wave is an undulation of the sea surface. Although a wave appears to travel across the ocean, it is only the energy within the waves that moves long distances. The water itself just moves around a bit as the wave passes. Some of the properties of ocean waves are shown in the figure below (from your textbook). · Wave crest is the peak of the wave · Wave trough is the low point of the wave · Wave height is the difference in height between the crest and the trough · Wave length is the horizontal distance between the peak of one wave and the peak of the next (or one trough to the next) · Wave period is the time interval between two crests (or two troughs) passing a fixed point · Wave speed (celerity) of the wave can be calculated by dividing the wavelength by the wave period. Wavelengths are in meters, and wave periods are in seconds, so wave speed is in meters per second (i.e., meters divided by seconds equals m/s). The connection between the size of a wave and its speed is one of the reasons that waves often travel together in trains of 5–10 similar-sized waves that surfers call "sets".What Causes Waves? By far the most common waves on the ocean surface are caused by winds pushing the water surface into a wave form. Waves increase in size depending on: · Wind velocity (stronger winds make bigger waves) · Wind duration (longer storms make bigger waves) · Fetch (distance over which the wind blows; longer distances make bigger waves) · Simply put, wave size increases as you increase the strength and duration of the wind, and distance over which it blows. A submarine disturbance, such as slumping or faulting that displaces the seafloor, causes a disturbance of the overlying water into one or more low, broad waves called a tsunami (see figure below). The gravitational pulls of the Sun and Moon cause low, very broad waves that travel across the oceans in synchronization with the positions of the Sun and Moon. These waves are called tides, and will be covered in a later lesson.Types of Wind-Generated Waves Wind-generated waves are divided into three related types based upon their characteristics and how far they traveled (see figure below, from your textbook): · Sea - irregular, chaotic waves in the area of generation, such as within a storm (the left side of the figure below) · Swell - more regularly spaced and shaped waves that have traveled beyond their area of generation and have sorted themselves into a uniform size and wavelength (the right side of the figure below). · Surf - waves as they reach the coast, grow in height, and break (see the photo below right).Wave Interference Waves originate in the fetch area (shown in the left half of the figure on the previous slide), which is characterized by a “confused” sea state with extensive wave interference as waves cross and pass each other. This interference may be: • Constructive wave interference, which occurs when two wave crests or two wave troughs coincide. The crests build up and the troughs build down (see figure below left). Constructive interference occasionally causes massive waves called rogue waves. • Destructive wave interference occurs when the crest of one wave coincides with the trough of another wave, resulting in partial cancellation of both waves (see figure below right).Wave Motions Two basic motions are associated with an ocean wave · The forward movement of the wave energy. · The orbital motion of water particles within and beneath the wave. The water moves in a back-and-forth or circular motion, and returns to pretty much where it started as the wave passes (see figure below, from your textbook). It is wave energy, not water molecules, that travels across the sea surface any significant distance. Notice also in the figure that this motion only affects the water down to a depth of about half of the wavelength, which means that the bigger the wave and the longer its wavelength, the deeper its affects are felt in the water. In deep water (defined as >1/2 the wavelength), most waves do not interact with the sea bottom, and the orbits of the water molecules are circular (see figure below left). But in shallow water (when water depths are less than about 1/20 the wave length), waves interact with the seafloor, causing the orbits of the water molecules to become elliptical (see figure below right).What Happens in Shallow Water In shallow water, interaction with the sea bottom transforms the wave’s properties: the wave refracts, slows down, and collapses, forming surf (breakers). Breaking Waves - As waves enter shallow water, bottom friction slows the wave down and alters its form. The wave speed decreases and its wavelength shortens, but its height increases, and it becomes steeper (see figure at right). When wave height ÷ wave length increases to greater than about 1/7 (i.e., when the height of the wave becomes more than one-seventh of the wavelength), the wave begins to break (collapse). Depending on the slope of the bottom, the waves break by spilling (gently sloping bottom), plunging (steep bottom), or surging (very steep bottom) (see Figure 7.9 in your textbook, and the photos that go with it of the three types of breakers). Wave Refraction - When you stand on a beach looking out to sea, the waves usually appear to be coming directly toward you. But waves can approach a coastline from almost any direction. It is only when they get close to the beach that they bend around to strike the beach almost head on. As a wave nears the coast, the end of the wave that encounters shallow areas first is slowed down by drag along the bottom, allowing the end of the wave still in deep water to catch up and make the wave bend so it is parallel to the shoreline (see figure at right). This bending of waves is called refraction, and can be assessed by drawing wave orthogonals, the imaginary blue lines perpendicular to the wave crests in the figure at right. Wave energy is focused where the lines converge, such as on headlands; and spread out where the lines diverge, such as in bays.Tsunami A tsunami is a wave caused by sudden shifting of the ocean floor due to an earthquake, volcanic eruption, or submarine slumping. In the open ocean, tsunamis have short heights (1 m or less), long wavelengths (>100 km), and long periods (20 minutes to 2 hours), and travel very fast in the deep ocean (usually at almost 500 mph!). When they reach shallow water