Weather Watch

A ‘wall of water’ is headed toward the Central Coast. But could we ever see a rogue wave?

Two intense storms with hurricane-force westerly winds marching across the northern Pacific are generating ever-increasing sea and swell that will produce waves with frightful amounts of energy along the Central Coast starting Sunday night.

The swell will peak Monday with another big wave event on Wednesday.

Waves are created by the friction or the dragging motion of the wind over the water. If you’re going to the coast to watch these big walls of water, please be careful as many have lost their lives to rogue or sneaker waves. The earliest writings from ancient civilizations have told stories of huge waves damaging or sinking ships.

Not even modern technology is immune from Neptune’s wrath.

During World War II, British Prime Minister Winston Churchill offered President Franklin Roosevelt the use of the Queen Mary and Queen Elizabeth ocean liners to transport large numbers of U.S. troops to the British Isles. At the time, the Queen Mary was the fastest ocean liner in the world and could outrun any German U-boat.

In late 1942, the Queen Mary with her thousands of cabins stuffed with soldiers set sail to Britain. Like sailors on submarines, soldiers were required to “hot rack,” or share beds on eight- to 12-hour shifts to accommodate as many troops as possible.

On her journey across the Atlantic, the Queen Mary sailed into heavy seas and began to pitch and roll. Sure enough, motion sickness struck the thousands of landlubbers on board.

As she continued across the stormy Atlantic, the liner was struck by a rogue wave several hundred miles northwest of Ireland. The gigantic wave caused the ship to roll within a few degrees of capsizing. A tiny bit farther and she would have taken more than 16,000 souls to Davy Jones’ locker, becoming the worst maritime disaster in history.

Rogue waves occur in the open ocean in several ways. One of the most common causes is when wind waves with different periods or wavelengths meet in a single spot and complement each other as part of a general process called interference.

Destructive interference occurs when the different wave trains are 180 degrees out of phase and cancel each other out. Constructive interference occurs when the different wave trains are in phase, and two smaller waves coalesce to produce a massive wave for a short time. This additive formation of the large crest and deep troughs can cause waves to form of enormous size suddenly.

In oceanography, rogue waves are defined as waves whose height is more than twice the significant wave height. Significant wave height 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.

For example, if a deep-water NOAA or waverider buoys with greater than 3,000 feet of depth reported the wave height at 50 feet, there is a chance, statistically speaking, that one wave in 1,175 could reach 100 feet in height.

Could we ever see wave heights like this along the beaches of San Luis Obispo and Santa Barbara counties?

Well, the answer is no. You see, as waves approach shallower water along our coastline, which is classified one half of their wavelength (about 1,000 feet for 20 seconds wave), they feel the bottom of the ocean and their wavelength decreases while their height builds even though their wave interval/period stays the same. When these waves reach a 7-to-1 ratio of wavelength to wave height, they break. In other words, a wave with a 7-foot length can rise only 1 foot before it breaks.

The deep-water waves with 20 second periods and wavelengths of over 2,000 feet will see their wavelengths decrease to about 150 feet by the time they reach the shallower waters along many of our beaches. Consequently, they will break at between 20 and 21 feet. However, farther out to sea and in deeper water you can see these waves breaking at over 30 or 40 feet.

It takes unique bottom topography on the ocean floor to create monster waves like those that are found at Mavericks, in Half Moon Bay, Nazaré, Portugal, or Hawaii’s North Shore.

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.

If you’re headed to the coast to check out the big waves, remember, remember, never turn your back on the ocean. Many people thought they were far enough from the surf to be safe. Some of these waves can surge more than 150 feet up the beach.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.
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