Big waves are coming to the Central Coast this week. Here’s why

Intense storms with hurricane-force winds have been brewing in the roaring forties — the latitudes between 40 and 50 degrees south of the equator in the Southern Hemisphere.

Even though these storms are nearly 8,000 miles away from the California coastline, they could have a significant impact along our coast, and here is why.

These storms will generate the so-called three phases: sea, swell and surf.

The high seas within the cyclone will travel away from the fetch — the length of water over which the storm wind blows — and become the long-period Southern Hemisphere swell. Eventually, it will become the surf when it arrives along our beaches.

The long-period swell does not lose much of its energy on its long journey across the Pacific Ocean. This was discovered by renowned Scripps geophysicist and oceanographer Walter Munk, who died in 2019.

Munk helped pioneered wave forecasting during World War II. His method was used for the amphibious landings in North Africa and, more importantly, during the D-Day invasion of Normandy, France, in June 1944.

Gen. Dwight D. Eisenhower assembled the best meteorologists and oceanographers from the United States and the United Kingdom. However, unlike today, there were no “waverider buoys” or other devices to measure the English Channel’s wave heights.

Weather satellites and Doppler weather radar were still in the future, and numerical weather models were in their infancy. These weathermen based their forecast entirely on surface observations, as upper-level winds weren’t fully understood.

To make it even more of a challenge, surface observations out over the water — especially those near the French coastline — were few and far between.

Despite this, the scientists convinced Eisenhower to change the date of the landings. Had the invasion occurred on the originally scheduled day, the high winds, seas, and surf would have probably doomed the attack with dire consequences for Europe.

The wave forecasts used for the amphibious landings were calculated for relatively short distances of wave propagation across the Mediterranean Sea for the North Africa landings and the English Channel for the D-Day invasion of Normandy.

Munk became increasingly curious about the long journey of swells across the vast expanse of the oceans.

In the summer of 1963, he commissioned a study called “Waves Across the Pacific” to predict the decay of the swell as it traveled from the roaring forties in the Southern Hemisphere to the North Pacific and between locations such as California and Hawaii.

He incorrectly hypothesized that much of the swell energy would be scattered or lost in the region of the equatorial trade winds, preventing the waves from reaching the distant shores of the North Pacific.

To track the swell from the massive Southern Hemisphere storms, he established wave measurement stations throughout the Pacific Ocean on islands such as Palmyra Atoll, American Samoa and Hawaii; on mainland shore stations such as New Zealand and Alaska, and at sea on the Floating Instrument Platform (FLIP) oceanographic research vessel in the North Pacific.

As the long-period swell traveled along the great circle from the Southern Hemisphere to the northern Pacific, the wave measurements from these stations show little decay of wave energy with distance traveled.

Both sound waves and ocean waves are mechanical and share many of the same properties.

Think of a car driving through your neighborhood with its stereo blaring. The first thing you hear is the thump of the base.

The low-frequency base is composed of longer period waves that can travel longer distances. You generally don’t hear the higher frequencies or treble until the car is near you.

Today, oceanographers operate a vast network of buoys that dot the Pacific Ocean, like the wave-rider buoy at Diablo Canyon Power Plant.

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.

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.

A few of the longer-period waves will pull ahead, leaving the bulk of the waves in the middle of the pack. The shorter wavelength waves will fall behind.

The waves from last week’s storms will arrive along the Central Coast from the south on Tuesday into Wednesday, with an even more significant Southern Hemisphere swell event expected on Thursday into the Independence Day weekend.

This Southern Hemisphere swell event will correspond with some of the highest astronomical tides of 2020 along the Central Coast, a pair of 6.6-footers on Friday and Saturday evening.

These high-energy swell trains combined with the high tides will create periods of high surf, severe surge and rip currents along our beaches, especially those that are southerly facing, like Avila Beach in San Luis Obispo County.

Surfer lore will tell you that 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, surf at the Wedge in Newport Beach could get up to 12 feet by Friday and Saturday.

Along the Central Coast, southerly facing beaches such as Avila Beach may see wave sets of more than six feet with an 18- to 20-second period.

Please be especially careful if traveling to the coastline to observe the year’s biggest Southern Hemisphere waves.

Never turn your back on the ocean, as sneaker waves can drag you into the sea.

This story was originally published June 30, 2020 at 5:05 AM.