Weather Watch

Why waves go rogue — a vexing mystery of wind and currents

Last week three boaters were rescued unharmed from their overturned fishing skiff off the coast of Cambria. According to the fisherman, their boat was hit by a rogue wave close to the surf line.

Maybe because of this incident I received a number of e-mails from readers asking about rogue waves and just how big wind-driven ocean waves can get. This is a fascinating question and is often fraught with mystery, arguments and misconceptions.

In 1933, the officer of the deck on watch aboard the U.S. Navy tanker Ramapo measured a wave with a height of 112 feet. This was truly a monster. Even swell trains with much smaller heights and shorter periods contain a fearsome amount of power. In December 1942, the Queen Mary was carrying 16,082 American troops across the North Atlantic Ocean. She was hit broadside by a rogue wave that may have reached a height of 92 feet. It was calculated later that the ship tilted 52 degrees. Let me tell you, I was on a Navy frigate, the USS McCandless, that took an estimated 38 degree roll off the Azores Islands in the North Atlantic and it scared the heck out of us.

As I wrote last week, oceanographers operate a vast network of buoys that dot the Pacific Ocean.

The buoys, like the wave-rider buoy at Diablo Canyon nuclear power plant, measure “significant wave height,” which is defined as the average height of the waves in the top third of the wave record.

This turns out to be very close to what an experienced mariner — “old salt” — would perceive the wave heights to be.

Rogue waves (also known as sneaker or freak waves) are relatively large ocean surface waves. In oceanography, they are more precisely defined as waves whose height is more than twice the significant wave height. For example, if the wave-rider buoy at Diablo Canyon reported the wave height at 10 feet, there is a chance, statistically speaking, that one wave in 1,175 could reach 20 feet in height.

I have seen “significant swell heights” reach more than 55 feet at the SE PAPA marine buoy, moored about 600 nautical miles west of Eureka. That means rare waves over 110 feet in height could have occurred under those conditions. On the outer fringe of probability, one wave in 300,000 can be up to 2.5 times the significant wave height. These would certainly count as rogue waves.

The three main factors that determine wave height are wind speed, wind duration and the fetch — the distance over which the wind blows across the ocean without significant change in its direction.

Waves are created by the friction or the dragging motion of the wind over the water.

Rogue waves occur in the open ocean in a number of 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 very large wave for a short time. This additive formation of large crest and deep troughs can cause waves to suddenly form of enormous size.

Another cause of rogue waves is when wind-driven waves run into strong ocean currents that are running counter to the direction of the swell. When waves hit these currents they can suddenly increase in height, producing extremely steep forward wave faces and deep troughs. Old salts often refer to these waves as “holes in the sea.”

Bottom topography on the ocean floor can also play a part in producing large waves. While stationed on San Clemente Island with HS-85, a U.S. Navy helicopter squadron, we would sometimes fly our trusty SH-3H Sea King helicopter out to the Cortes Bank located about 45 miles southwest of the island. Two buoys indicate the location of the bank and when the conditions were right, enormous waves would suddenly erupt in the middle of the ocean.

Weather forecast

A 1,034-millibar Eastern Pacific High about 1,000 miles west of San Luis Obispo combined with a thermal low over the Central Valley will produce strong to gale (25 and 38 mph) force northwesterly winds today.

The marine low clouds will burn off later this morning, leaving behind mostly clear and mild temperatures this afternoon as the northwesterly (onshore) winds bring cool marine air further into the interior.

Today’s temperatures will range from the low to mid 80s in the interior and the low- to mid-70s in the coastal valleys and southerly (Avila Beach and Cayucos) facing beaches. Temperatures along the northwesterly (Los Osos) facing beaches will range from the high 50s to the mid 60s.

A very nice day is expected on Monday, with sharper warming across all locations as the northwesterly winds decrease and a weak high-pressure ridge builds across the state.

Temperatures on Monday will range from near 70 along the beaches to the mid-80s in the coastal valleys and the low-90s across the interior. The warmer temperatures will be short-lived, however, as the marine layer deepens and moves inland during the night and morning hours Tuesday, producing cooler temperatures.

Increasing northwesterly (onshore) winds Wednesday through Friday will bring below-normal temperatures along with night and morning marine low clouds across the Central Coast.

Still no indication of any prolonged period of excessively hot weather developing over the next few weeks.

Surf and sea report

The Coastal Data Information Program at the Scripps Institution of Oceanography in San Diego in collaboration with the University of Washington deployed a wave-rider buoy at the SE Papa station moored about 600 nautical miles west of Eureka earlier this week.

This buoy provides wave and seawater temperature data but not any direct atmospheric measurements such as wind or pressure.

Nevertheless, wave spectral information can provide a treasure trove of useful data that can be directly related to future weather and oceanographic conditions along the Central Coast.

This morning’s 4- to 5-foot northwesterly (305-degree deep-water) sea and swell (with a 7- to 11-second period) will increase to 5- to 7-foot this afternoon and will remain at this height and period through tonight.

A 5- to 6-foot northwesterly (290-degree deep-water) swell (with an 8- to 11-second period) is forecast along our coastline Monday, decreasing to 4- to 6-feet Tuesday.

Increasing northwesterly winds along Central California coastline will produce a 3- to 5-foot northwesterly (310-degree deep-water) sea and swell (with a 7- to 9-second period) Wednesday through Friday.

Arriving from the Southern Hemisphere:

A 1- to 2-foot Southern Hemisphere (230-degree deep-water) swell (with a 14- to 16-second period) is expected to arrive along our coastline Thursday through Friday.

Seawater temperatures will range between 51 and 53 degrees through Friday.

Conservation tip

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John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 23 years. If you have a question, send him an e-mail at pgeweather@pge.com.

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