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How Does Surf Swell Work?

Published in Oceanography 2 mins read

Surf swell works by wind blowing across the ocean surface, creating waves that travel long distances and eventually become organized swell. Here's a breakdown of the process:

The Birth of a Swell

Swell originates from storms far out at sea. Several factors determine the size and power of the resulting swell:

  • Fetch: The larger the area of ocean (the "fetch") over which the wind blows, the more energy is transferred to the water.
  • Wind Speed: Stronger winds generate larger waves.
  • Wind Duration: The longer the wind blows consistently, the more energy is transferred and the larger the waves become.

Wave Formation and Organization

  1. Wind Creates Waves: Initially, the wind creates choppy, disorganized waves.

  2. Energy Transfer and Growth: As the wind continues to blow, smaller waves catch up with and merge into larger waves. This process transfers energy and increases wave size.

  3. Organized Swell: Eventually, these waves travel away from the storm area, sorting themselves out based on speed and wavelength. This organized group of waves is called swell. Swell waves are more rounded and consistent than locally generated wind waves (often called "chop").

Traveling Across the Ocean

Swell can travel thousands of miles across the ocean with minimal energy loss. This is because the energy is distributed throughout the water column, not just on the surface.

Approaching the Shore

As swell approaches the shore, several things happen:

  1. Slowing Down: The waves begin to "feel" the bottom as they enter shallower water. This causes them to slow down.

  2. Wave Height Increases: As the waves slow down, the energy they contain is compressed, causing the wave height to increase.

  3. Breaking: Eventually, the wave becomes too steep to support itself and it breaks, creating the surf that surfers ride.

Factors Affecting Swell at the Shore

  • Swell Size: The size of the swell directly impacts the size of the surf.
  • Swell Direction: The angle at which the swell approaches the shore determines which beaches will receive the largest waves.
  • Swell Period: The time between successive wave crests in the swell (swell period) influences the power and consistency of the surf. Longer swell periods generally result in more powerful surf.
  • Bathymetry: The underwater topography of the coastline (bathymetry) shapes the waves as they approach the shore, influencing wave shape and size.