Wind Meets Water: The Jeffreys' Shelter Model
In 1925, Jeffreys introduced a model to explain how wind transfers energy to ocean waves. His key insight was that pressure differences caused by wind flow over wave shapes could amplify wave growth.When wind blows across the ocean, it transfers energy to the surface, forming ripples that grow into waves. Jeffreys' sheltering model explains how this energy transfer works:
- Wave shape matters: The wind hits the front (windward side) of a wave and flows smoothly, but it breaks away from the back (leeward side).
- Pressure difference: This creates higher pressure on the front and lower pressure on the back, pushing the wave forward and helping it grow.
- Sheltering effect: The wave shelters its leeward side, concentrating wind energy on the front and amplifying the wave.
How Wind Creates Waves
🌊 This model helps explain how waves grow rapidly under strong winds and why steep wave groups can evolve into larger, more powerful waves. This theory was then enhanced over the years.
How Storms Create Swells
Large swell events often begin far from shore, born in the heart of powerful storm systems. Here's how:
Ingredients for Swell Formation
- Wind Speed: Strong winds must blow faster than the wave crests to transfer energy.
- Wind Duration: The longer the wind blows, the more energy it imparts.
- Fetch: This is the uninterrupted stretch of open water over which the wind blows. Longer fetch = bigger waves.
- Storm Intensity: Hurricanes and mid-latitude cyclones are prime swell generators.
- Water Depth: Deep water allows waves to travel long distances without losing energy.
From Chaos to Order
- Near the storm, waves are short, steep, and chaotic.
- As they travel away, dispersion occurs: longer waves move faster and outrun shorter ones.
- This sorting creates organized wave sets - what we call swells - that can travel thousands of miles across the ocean.
What Waves Do Close to Shore: From Deep Swell to Surfable Break
As ocean waves approach the shore, they undergo dramatic changes. These transformations are governed by physics, seabed topography, and coastal geometry.
Wave Shoaling: Rising Energy
- In deep water, wave energy is spread vertically through the water column.
- As waves enter shallower depths, the seafloor begins to "squeeze" the wave from below.
- This causes:
- Decrease in wavelength (distance between crests)
- Increase in wave height
- Slower wave speed
👉 This process is called shoaling, and it concentrates energy upward — making waves taller and slower.
Wave Refraction: Steering Energy
- Waves don’t always hit the beach head-on.
- As one part of a wave reaches shallow water before another, it slows down.
- The result: the wave bends toward the shallower area — a process called refraction.
- Refraction focuses wave energy on headlands and disperses it in bays which can be great for enhancing surf conditions - see more details on Surf Forecasting here.
Bottom Friction and Breaking
- The wave’s base drags against the seafloor, while the crest continues moving faster.
- This causes the wave to pitch forward and eventually break.
- The type of break depends on:
- Seafloor slope: steep slopes produce plunging waves (great for surfing); gentle slopes produce spilling waves.
- Wave height and period: longer-period swells break more forcefully.
Rogue Waves: Ocean’s Wildcards
Rogue waves are giant, unpredictable waves that seem to appear out of nowhere. Once dismissed as sailor myths, they’re now backed by science.
What Makes a Wave "Rogue"?
- Height: More than twice the size of surrounding waves.
- Suddenness: They appear without warning.
- Direction: Often come from unexpected angles.
How Do They Form?
- Constructive Interference: When multiple wave crests align perfectly, they stack into one massive wave.
- Nonlinear Amplification: Natural wave asymmetry (steep crests, shallow troughs) can boost wave height by 15–20%.
- Storm Chaos: In stormy seas, overlapping swells from different directions can combine to form rogue waves.
⚠️ These waves can reach heights of 80–100 feet and have damaged ships and oil platforms. They’re rare, but real—and forecasting models are starting to catch up.
Wave Energy: Power Beneath the Surface
Ocean waves carry immense energy, and scientists are working to harness it as a renewable resource.
Where Does Wave Energy Come From?
- Kinetic Energy: The motion of water particles in waves.
- Potential Energy: Water lifted against gravity in wave crests.
👉 Together, these energies are equipartitioned — they constantly convert back and forth as waves move.
How Do We Capture It?
- Point Absorbers: Buoy-like devices that bob up and down, converting vertical motion into electricity.
- Attenuators: Long floating structures that flex with wave motion to generate power.
- PacWave Project: Off the Oregon coast, this test site is pioneering open-ocean wave energy research.
