An intense storm with hurricane-force winds generated massive waves in the Roaring Forties (south of the 40th parallel in Southern Hemisphere). A wave rider buoy moored about 375 miles south of New Zealand near Campbell Island measured a 78-foot wave May 9, the highest individual wave ever recorded south of the equator.
By the way, in the Northern Hemisphere, one of the biggest waves ever seen occurred 1933 when the officer on watch aboard the U.S. Navy tanker Ramapo measured a wave with a height of 112 feet.
The first waves (forecast forerunners) from this Southern Hemisphere storm will travel about 7,500 miles across the Pacific and will start to arrive along Central Coast from the south Monday and increase in height Tuesday into Wednesday, and here’s why.
The higher the wind speed, the higher the waves. For example, a 20-knot wind can generate a 5-foot wave, while a 70-knot wind can make a 56-foot wave with the same wind duration and length of the wind fetch (length of water over which the wind blows). Wind fetches in the Pacific can exceed 500 miles. The longer the duration of wind, the more of its energy is transferred to the ocean, resulting in more massive waves.
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As seas develop, they can reach at most a 7-to-1 ratio of wavelength to wave height. In other words, a wave with a 7-foot length can rise only 1 foot before it breaks. When the wave breaks, longer wavelengths develop, allowing the seas to increase in height over time. When the seas move out from under these winds, they become swells, longer-period waves.
The longer the wavelength of the swell, the faster it will travel across the ocean. The wavelength is the distance from the crest of one wave to the crest of the next wave. The period of the wave is the time it takes for two consecutive peaks to pass a fixed point. The longer-period waves have higher amounts of energy and can travel longer distances.
Longer-period waves, also called forecast forerunners, have periods of more than 25 seconds and wavelengths more than 2,000 feet. Some of these waves may have periods longer than 30 seconds, but the accelerometers in the wave rider buoys can’t measure them. Like a marathon, a few of the longer-period waves (fastest runners) will pull ahead, leaving the bulk of the waves in the middle of the pack. The shorter wavelength waves will fall behind.
As a rule, the average speed of wave trains traveling across the deep waters of the Pacific is about 25 mph or about 600 miles per day. Hence, waves from 7,500 miles away will need between 12 and 13 days to reach the Golden State.
The speed of the swell slows as it begins to feel the bottom of the sea near the coastline. The swell will transition from a deep-water wave to a shallow-water wave when its wavelength equals one half of the water depth. As the swell continues to move toward the shallower waters near the beach, its wavelength will shorten, and its wave heights will increase until the wave breaks.
Interesting to note, the wave period will remain constant. Surfer lore will tell you, the highest waves come in the middle of the wave train. In the middle of the group, the wave crests and troughs are in phase with each other and add together for maximum height. This is the so-called seventh wave.
Along the Southern California coast, the Wedge, in Newport Beach could get up to 14 feet by Tuesday and Wednesday. Along the Central Coast, the southerly facing beaches, like Avila Beach, may see wave sets of over 7 feet with an 18- to 20-second period.
Oceanographers operate a vast network of buoys that dot the Pacific Ocean. The buoys, like the wave-rider buoy at Diablo Canyon Power Plant, measure “significant wave height,” which is defined as the average height of the waves in the top third of the wave record. That turns out to be very close to what an experienced mariner — an “old salt” — would perceive the wave heights to be.
This heavy and powerful late spring-time Southern Hemisphere swell train may produce significant beach erosion and strong rip currents. Please be especially careful if traveling to the coastline to observe the biggest Southern Hemisphere waves of the year. Never turn your back on the ocean, as sneaker waves can drag you into the sea.