oceanlecture13wavessea2025

Waves at Sea

• Description from 'The Cruel Sea' by Nicholas Monsarrat (1951)

  • The Saltash bridge view reveals chaotic sea conditions.

  • Features include:

    • Huge waves resembling mountains.

    • Continuous haze of spray and spume.

    • Gullies forming as if the ocean is trying to engulf the ship.

Structure of the Study Material

• Key Topics:

  • Atmospheric circulation

  • Upper ocean circulation

  • Upwelling and ENSO

  • Deep ocean circulation

  • Waves at sea and the shore

  • Tides

• Exam Details:

  • Scheduled for Tuesday, March 11.

  • Same multiple-choice format as the first exam.

  • Focus on 'why' and 'how' rather than just 'what'.

  • Format may vary (in-person or online).

I. Ocean Waves

• Mechanical Waves:

  • Transmission of energy through matter with little/no material transfer.

  • Fluids can transmit:

    • Compressional Waves (e.g., sound in oceans).

    • Waves at fluid interfaces (e.g., ocean surface and air).

• Particle Motion in Waves:

  • Orbital motion for interface waves.

A. Wave Morphology (Definitions)

• Definitions:

  • Crest: Highest point of the wave.

  • Trough: Lowest point of the wave.

  • Height and Wavelength: Key physical characteristics of waves.

Wave Characteristics

• Frequency (f):

  • Number of wave crests passing a fixed point per second.

• Period (T):

  • Time between arrival of successive wave crests (T = 1/f).

  • Does not indicate wavelength or speed.

• Celerity (C):

  • Defined as speed of the wave (C = L / T).

B. Wave Orbits

• Particle Motion:

  • Circular motion of particles decreases in size with depth.

  • Orbits are negligible below approximately half the wavelength (1/2 L).

• Stokes Drift:

  • Orbits are not perfectly closed, resulting in slight mass movement in the wave's direction.

C. Classification of Waves

• Types of Surface Ocean Waves based on:

  • Wavelength and disturbing forces.

• Disturbing Forces:

  • Wind

  • Seismic events (e.g. earthquakes, landslides)

  • Gravitational attraction (Earth-moon-sun system).

Free vs. Forced Waves

• Once disturbed, waves can be:

  • Free Waves: Propagate away from disturbance without continued force.

  • Forced Waves: Maintained by continuous disturbing forces.

D. Restoring Forces

• Restoring forces act to return the sea surface to a flat state:

  • Gravity: Restores waves longer than 1.73 cm.

  • Cohesion: Restores capillary waves shorter than 1.73 cm.

  • Overcompensation by restoring forces causes oscillations around equilibrium.

II. Wave Characteristics (LG14)

• Most characteristics depend on:

  • Wavelength and water depth.

• Waves categorized into three classes:

A. Deep Water Wave

• Water depth > 1/2 wavelength (L).

• Characteristics:

  • Cannot interact with seafloor.

  • Characterized by smooth crests and circular orbits.

  • Celerity increases with wavelength (C = gL / 2π).

B. Shallow Water Wave

• Water depth < 1/20 wavelength (L).

• Characterized by:

  • Pointed crests and flattened orbits.

  • Back-and-forth particle movement at the seafloor.

  • Celerity increases with water depth (C = gd).

C. Transitional Wave

• Water depth is between 1/20 and 1/2 wavelength (L).

• Characteristics:

  • Semi-pointed crests and semi-flattened orbits.

  • Speed depends on both wavelength and depth.

Clicker Question

• Tsunami Example:

  • In December 2004, nations around the Indian Ocean experienced a tsunami.

  • Wavelength approximately 200 km.

  • Main factor affecting wave celerity: Water depth everywhere.

III. Wind Waves in the Open Ocean (Deep Water Waves)

• Characteristics governed by:

  • Wind speed, duration, and fetch.

• Emergence of a chaotic surface with various sizes of waves.

• Significant Wave Height:

  • Mean height of the largest one-third of waves.

Wind Speed Classification (Beaufort Scale)

• Scale Description (0-12):

  • Force 0: Wind less than 1 knot; sea like a mirror.

  • Force 4: Wind 11-16 knots; wave height 1-1.5m; small waves forming.

  • Force 3: Wind 7-10 knots; wave height 0.6-1m; large wavelets and whitecaps.

  • Force 7 - 10 extends the description to extremely high winds and their effects on sea conditions (Hurricane effects).

Wave Height and Dispersion

• Wave Height:

  • Maximum height is around 1/7 of the wavelength (L).

  • Waves break beyond this height.

  • Wind energy dissipates as turbulence.

• Dispersion:

  • Longest wavelength waves move away from the storm fastest, arriving first at distant shores.

  • Smooth waves are referred to as "swell" and can travel hundreds of kilometers ahead of a storm.

Wave Interference

• When waves from different sources intersect:

  • Can cause constructive interference (waves add together) or destructive interference (waves subtract from each other).

Rogue Waves

• Defined as waves exceeding twice the significant wave height.

• Formed through freak interference and have been recorded with a theoretical limit of about 60m.

• More common where winds meet ocean currents.

Waves and Global Warming

• Global wave energy is correlated with wind speeds.

• Wind and current speeds have been shown to be increasing.

• Overall wave power has increased by approximately 0.5% annually since 1948; 2.3% per year since 1994 (Reguero et al., 2019).