Waves PDF - General Oceanography (MARS 3000)

Waves General Oceanography (MARS 3000) Professor van Hall [email protected] Office hours: Thursday’s from 10:30-11:30 AM in Pioneer Plaza, PL 609 or by appointment Physical Oceanography Quiz 11 starts HW: Bring a computer with R. Download at 8:45 AM files and R script before class...

Waves General Oceanography (MARS 3000) Professor van Hall [email protected] Office hours: Thursday’s from 10:30-11:30 AM in Pioneer Plaza, PL 609 or by appointment Physical Oceanography Quiz 11 starts HW: Bring a computer with R. Download at 8:45 AM files and R script before class Start studying for the final! HW: Bring a computer with R. Download files and R script before class Please install packages before class Draw a circle with these four arrows Northern hemisphere- deflects to the right Southern hemisphere- deflects to the left  Meso-scale ocean circulation (eddies)  Western Intensification Subtropical gyres centered westward Sea Surface Height Gradient Western intensification; steep slope bounds Gulf Stream  Eddies: key features of ocean circulation Eddies at Gulf Stream's End Formed because of a narrow, constrained flow Gulf Stream channeled between high sea surface height and continent Chaotic movement into open ocean Wobbling and momentum drive rotation, forming eddies Formed from turbulence  Warm-core Ring Warm-Core Ring Dynamics Surface convergence → downwelling Water flows downhill → clockwise rotation (N. Hemisphere) *Opposite rotation in the Southern Hemisphere Turbulence-initiated eddies clockwise* Not wind-driven; formed at Gulf Stream's end Coriolis effect sustains circulation around height anomalies anti-cyclonic Warm-core Ring  Cold-core Ring Cold-Core Ring Dynamics Cooler sea surface temperature at the center Upwelling of deeper, colder water Deeper water mixes with surface water Sea surface height low → divergence and upwelling Counterclockwise rotation (N. Hemisphere) *Opposite rotation in the Southern counter-clockwise* Hemisphere Formation Process Gulf Stream momentum causes divergence Divergence → upwelling, similar to equatorial upwelling cyclonic Eddies: key features of ocean circulation *Northern hemisphere case wa r m -core rin g cold-core rin g counter-clockwise* clockwise* map view: Low High cyclonic anti-cyclonic pressure gradient Eddies: key features of ocean circulation *Northern hemisphere case *Coriolis to the right wa r m -core rin g cold-core rin g counter-clockwise* clockwise* map view: Low High cyclonic anti-cyclonic water movement Eddies: key features of ocean circulation *Northern hemisphere case *Coriolis to the right wa r m -core rin g cold-core rin g counter-clockwise* clockwise* map view: Low High cyclonic anti-cyclonic water movement Eddies: key features of ocean circulation *Northern hemisphere case *Coriolis to the right wa r m -core rin g cold-core rin g counter-clockwise* clockwise* map view: Low High cyclonic anti-cyclonic Surface layer transport  Eddies play a key role in Eddies and Nutrients transporting phytoplankton Phytoplankton concentrations linked to nutrients (chlorophyll as indicator) Warm-Core Rings Low nutrients due to water convergence and downwelling Cold-Core Rings Upwelling introduces nutrients into subtropical gyre Eddy Indicators Defined by temperature, sea surface height, or chlorophyll distribution Pair share: A ______ has characteristics of sea surface height low, divergence, counterclockwise rotation in the Northern hemisphere and upwelling a. Cold-core eddy b. Convergent water c. Warm-core eddy d. Gulf Stream Key concept  Some apex predators follow eddies Mako shark video by Camrin Braun (Woods Hole / MIT) Pair share: Is this a cool or warm center eddy? What direction is it turning? Is it divergent or convergent? Is there high or low chlorophyll-a?  Eddies formed from wind Island Leeward Eddies Driven by wind through island gaps Initiated by Ekman transport Divergence: Convergence: Ekman transport One-sided Ekman removes water, transport piles water creating a valley up Upwelling Downwelling  Eddies formed from wind Highly variable due to fluctuating wind speeds and conditions Loretta Divergence: Convergence: Hill (high sea Valley (low sea surface height): surface height): downwelling → upwelling → cool warm SST, low SST, high chlorophyll chlorophyll  What are waves? Waves: moving energy, traveling along the boundary between two fluids of different density - The medium itself does not travel with the wave (for the most part) - Waves involve energy transfer, not fluid transport - Orbital water motion occurs without significant fluid movement - Disturbing forces like wind generate ocean waves  What are waves? Waves form along interfaces between fluids with different densities Surface waves (air–water interface) Surface waves primarily driven by wind Wind blowing across the surface of the ocean generates most ocean waves  What are waves? Waves form along interfaces between fluids with different densities Internal waves (water–water interface) Internal waves occur at pycnocline density boundaries Internal waves can be much larger than surface waves Tidal movement, turbidity currents, wind stress, and even passing ships The types of ocean waves Ocean tides Tides are waves caused by the Moon and Sun’s gravitational pull Tsunami Geological events like underwater avalanches and volcanic eruptions cause seismic sea waves or tsunamis Tsunamis flood coastal areas and cause damage Wind-generated waves Most surface waves are generated by wind  Orbital wave motions Sine waves Orbital wave motions Orbital wave motions Floating objects in waves move in circular orbits Objects return close to their original position but have a slight forward movement, called wave drift Circular orbital motion allows wave shapes to move forward while water particles stay in place The orbital motion moves the energy Pair share: What physical feature of a wave is related to the depth of the wave base? What is the difference between the wave base and still water level?  Wave characteristics Wave height (H) is the vertical distance between crest and trough Wavelength (L) is the horizontal distance between successive crests or troughs Wave steepness is the ratio of height to wavelength; if it exceeds 1/7, the wave breaks A wave's maximum height is limited by the 1/7 steepness ratio  Wave characteristics Frequency: number of waves passing a point per time (units of 1/time) Period: the time it takes one full wave to pass a point (units of time) Period = 1/Frequency Frequency = 1/Period *Period and Frequency are inversely related* Example: If it takes 10 seconds for one wave to propagate, the period is 10 seconds. A wave with a 10-second period has a frequency of 6 waves per minute. Example #2: A wave with a frequency of 10 waves per minute has a period of 6 seconds. Wave speed: Wavelength / Period (units of distance / time)  Wave types Deep-water wave Occur when water depth exceeds half the wavelength (wave base) Have a wave base Don’t interact with the ocean floor Wind-generated waves in the open ocean are deep-water waves due to depth  Wave types Shallow-water waves Form when water depth is less than half the wavelength Interact with the seafloor, altering their motion Reduced wave velocity Pair share: What is the primary driver of internal waves? A) Wind stress B) Density differences at the pycnocline C) Coriolis effect D) Atmospheric pressure Pair share: What is the period of a wave with a frequency of 6/minute? A) 10/minute B) 10 seconds C) 6/minute D) 6 seconds  Shallow-water waves Deep-water waves transform into transitional and then shallow waves as they approach shore Shallow-water waves form when wave base disappears Orbital motion in shallow-water waves extends throughout the water column Speed of shallow-water waves depends on gravitational acceleration and depth Deep water waves slow down, compress, and steepen in shallower water Breaking waves occur when wave steepness exceeds the 1:7 ratio Pair share: Why do shallow-water waves slow down as they approach the shore? Shallow-water waves slow down because their orbital motion interacts with the seafloor, causing friction and reducing their speed.  Deep-water waves Not “deep” strictly in reference to the ocean’s bathymetry - Rather, “deep” refers to the wave’s wavelength relative to bathymetry Relation among wave speed, wavelength, and period for deep-water waves: Capillary waves are small, rounded waves caused by wind's surface tension interaction Gravity waves have symmetric crests and rounded troughs Deep-water waves  Wave steepness exceeding 1:7 causes waves to break When wave speed equals wind speed, the wave reaches its maximum size with no further energy exchange The sea is the area where wind-driven waves are generated, characterized by choppy waves in many directions with a variety of periods and wavelengths Deep-water waves  Formation of wind generated waves Factors affecting wave energy: Wind speed Duration Fetch Wave height is directly related to wave energy As wave energy increases, steepness increases until it reaches 1/7, causing whitecaps  Formation of wind generated waves Swell: uniform, symmetrical waves that have travelled beyond their generation area - On a calm day, there are still waves breaking at shore. Why? Swell, from far away. - Contrast with choppy conditions that develop during local winds Constructive interference Destructive interference Mixed interference  Deep-water waves We rarely observe neat and clean sinusoidal waves in the ocean. Why? Reality: waves at any one time are combinations of waves formed in many places Pair share: True or false: Deep- water waves interact with the ocean bottom Rogue waves We rarely observe neat and clean sinusoidal waves in the ocean. Why? Reality: waves at any one time are combinations of waves formed in many places Extreme examples: rogue waves Example of constructive interference Beaufort wind scale Pair share: Which of the following best describes the motion of water particles in a wave? A) Linear B) Orbital C) Upward and downward D) None of the above Pair share: Can a wave with a wavelength of 14 meters be more than 2 meters high?  Wave refraction As a wave enters shallow water, it slows down, resulting in: Decreased wave speed Decreased wavelength Increased wave height (due to conservation of energy) Increased wave steepness, leading to breaking waves (surf) Incoming waves orient themselves to the shoreline  Refraction: waves bending; occurs when waves approach an irregular shoreline, causing their Wave refraction energy to be distributed unevenly. Tendency to bend such that they are parallel to shore  Reflection: waves bouncing off a vertical Shallow-water feature waves Like a wall or steep topography, but not a gently sloping shore Why is surfing so much better along the West Coast of the Fetch= distance over United States than along the water that wind East Coast? blows in a single direction 1. Pacific Ocean has a larger fetch, allowing for bigger waves 2. West Coast beaches have steeper slopes, creating plunging breakers favorable for surfing 3. East Coast beaches have gentler slopes, resulting in spilling breakers less favorable for surfing 4. Prevailing westerlies enhance waves along the West Coast, while winds along the East Coast typically blow away from shore Tsunami Enormous shallow-water wave, often generated by seismic activity Tsunamis are shallow water waves, even in the open ocean As tsunamis approach shore, they slow, steepen, and grow Tsunamis are not tidal waves Tsunami Tsunami does not form a huge breaking wave, but a flood or surge of water Tsunami are usually a series of waves with alternating surges and withdrawals The first surge is rarely the largest; later surges can be larger and occur hours later Tsunami resembles an extremely high tide The crest of a tsunami can raise sea level by up to 131 feet Tsunami 86% of all great tsunamis occur in the Pacific Ocean (Pacific Ring of Fire) Large-magnitude earthquakes along subduction zones in the Pacific The Pacific Ring of Fire is an area with frequent volcanic activity and large earthquakes Earthquakes along the Pacific Ring of Fire Most significant tsunamis since 1990 have occurred along the Pacific Ring of Fire Tsunami Japan experiences the most tsunamis The largest recorded tsunami height occurred in the Ryukyu Islands in 1971, raising sea level by 278 feet  Tsunami Warning The Pacific Tsunami Warning Center (PTWC) was established after the 1946 Hawaii tsunami to monitor and warn about tsunamis Pressure sensors on the ocean floor to detect tsunamis DART buoys transmit data via satellite  Exam Review: Key Concepts/ Terms Eddies and Ocean Circulation Compare the characteristics of warm-core rings and cold-core rings in terms of rotation, sea surface temperature, divergent or convergent, and nutrient distribution. How do eddies influence marine ecosystems and phytoplankton distribution? Key Terms: Warm-core eddy, cold-core eddy, upwelling, downwelling, Ekman transport. Waves Describe how waves transfer energy without significant fluid movement. Compare surface waves (air-water interface) to internal waves (water-water interface). Explain the relationship between wave height, wavelength, steepness, period, and frequency. Compare and contrast deep-water waves and shallow-water waves. How do storm-generated waves transform as they approach shore? Describe how wind speed, duration, and fetch affect wave formation and energy. Compare constructive, destructive, and mixed wave interference. Key Terms: wind duration, wave refraction, wave steepness, crest, trough, wave height, wave base, period, deep-water wave, shallow-water wave, fetch, orbital motion, wave drift, pycnocline, wind, swell, whitecaps, interference patterns. Tsunamis Explain why tsunamis are classified as shallow-water waves and their behavior as they approach shore. What creates tsunamis? Key Terms: Seismic sea waves, Pacific Ring of Fire, DART buoys.