![]() ![]() When shallow bathymetry slows down a part of a wave, this causes the waves to refract. Just as light waves and sound waves will bend when they hit something or change speed – a process called refraction – so do ocean waves. The shape and depth of the ocean floor is called the bathymetry. There are reefs, sand banks and canyons that shape the underwater terrain. Most coasts do not have a smooth, evenly sloping bottom extending from the deep ocean to shore. It comes down to what’s at the bottom of the ocean. So why don’t we find large waves breaking on all shores? Why are there some spots like Nazaré in Portugal, Mavericks in California and Jaws in Maui that are notorious for having big waves? But when swells run into a coastline, waves at one beach can be many times bigger than waves at another beach a mere mile away. When a swell is traveling through the ocean, the waves are all more or less the same size. Rúdisicyon via Wikimedia Commons, CC BY-SA The Nazaré Canyon, the dark winding depression extending horizontally across this aerial map, funnels and focuses wave energy toward one spot on the Portuguese coast, producing some of the biggest waves on Earth. And finally the period – the time it takes for two consecutive waves to reach a fixed location. Then the wavelength – the distance between the top of one wave and the top of the wave behind it. First, the height – how tall a wave is from the bottom to the top. When describing a swell, oceanographers and surfers generally care about three attributes. This organization of the waves creates “swell,” or regularly spaced lines of waves. The waves within a storm are usually messy and chaotic, but they grow more organized as they propagate away from the storm and faster waves outrun slower waves. For instance, the wave that Gabeira surfed at Nazaré was likely generated by a storm somewhere between Greenland and Newfoundland a few days earlier. The waves that surfers ride originate in distant storms far across the ocean. Generally, the biggest and most powerful wind-generated waves are produced by strong storms that blow for a sustained period over a large area. On rare occasions earthquakes and landslides can generate waves, but usually waves are created by wind. ![]()
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