Never turn your back on the ocean.
That's the pervasive message along the coasts of Northern California and parts of Oregon, and for good reason: On a seemingly perfect sunny day, when tame-looking waves are lapping the sand, the unwary beachgoer can be bowled over and pulled into the cold Pacific waters by an unexpectedly large wave surging up the beach.
Sneaker waves, as they are colloquially known, can strike seemingly without warning and have been responsible for numerous deaths in recent years.
"For much of the West Coast, sneaker waves kill more people than all other weather hazards combined," Troy Nicolini, a forecaster with the National Weather Service office in Eureka, Calif., said during a presentation on the threat at the annual meeting of the American Meteorological Society, held in Atlanta earlier this month.
Nicolini, a California native who has worked in the Eureka area for 15 years, initially thought that the deaths that resulted from these stealthy waves were a matter of people being careless near a dangerous ocean, but in time he started to notice a pattern in the accounts of those who survived the onslaught: The larger waves were always preceded and followed by a calm ocean, disappearing just as quickly as they came. Survivors always noted that they were completely caught off guard.
The troubling deaths and consistent pattern led Nicolini to look into the phenomenon and see if his office could forecast days that were more likely to see sneaker waves and warn those headed to the shore. [7 Ways the Earth Changes in the Blink of an Eye]
Illusion of safety
"Anybody that's been to the beach has probably had the water surprise them and get their feet wet," Nicolini said. But when this happens, say, on a Florida beach, the unsuspecting party just ends up with soggy shoes.
In the frigid waters off the coast of Northern California, beachgoers aren't dressed in bikinis ready to jump in the water. They're often fully clothed, and in the fall and winter may have heavy boots and coats on, Nicolini said. They stay on shore and may just walk near the water, especially when the surf looks calm — until a sneaker waves hits and suddenly they're up to their waist or chest and being pulled out to sea. All that clothing weighs people down, and in addition to fighting the surging waters, the victim is up against the cold, he added. Cold paralysis in the limbs can set in in minutes.
A rough wave might knock a wader over in San Diego, but the water there is much warmer. "It doesn't have the consequences," Nicolini told Live Science.
Steep beaches are also more likely to have this hazard than flatter ones, which can attenuate the incoming wave, and Northern California's shores are more likely to be steep.
"You have to have the whole package for it to be this dangerous phenomenon," Nicolini said.
Human behavior also plays a part in the hazard. When surf is rough, people instinctively stay farther away from the water, Nicolini said, but when it is gentle, they move closer in (Nicolini and his colleagues have interviewed those who have experienced near misses or have survived a sneaker wave about this over the last decade.). It is this calm that lulls people into a sense of security. "People always think that they're very safe and very far from the water's edge," Nicolini said.
But the sneaker waves can come after a relatively long period of quiet (anywhere from five to 20 minutes) — by that time, the unsuspecting person has his or her towel down and is reading a book or otherwise preoccupied. Then the quiet period ends and "the larger wave comes and catches them off guard," Nicolini said.
Some of these waves can wash more than 150 feet (45 meters) up the beach, he added. "If they're paying attention, they may have a chance" to outrun the wave, as some potential victims have, but usually they're not paying attention, Nicolini said.
Which means the threat doesn't come just from the water, but from the ignorance of those who visit the coast. "Clearly, education is a critical component of this," Nicolini said.
Respect the ocean
While general warnings to be wary while near the ocean have long been part of local lore, Nicolini wanted to be able to issue focused forecasts to make sure that beachgoers were paying attention when they really needed to.
"There are certain days when there is a much higher likelihood that waves can surprise people," he said.
These large waves seem to form due to a basic phenomenon that occurs with waves (whether of the ocean or sound variety). All waves have peaks and troughs — when waves encounter each other and a peak lines up with a trough, they cancel each other out, but when peaks combine, they are amplified to create a higher peak. [Album: Monster Waves]
Of course, both of these things happen in the ocean all the time, as well as combinations in between. Sneaker waves occur under very particular circumstances, when the peaks of waves of the same period combine. The period, or frequency, of a wave refers to the distance from peak to trough (or peak to peak).
Nicolini and an intern played around with modeling different waves and combinations and found what they think is happening when sneaker waves are created. Storms far out at sea churn up waves that travel across the vastness of the ocean, separating out into groups of long-period and short-period waves. (Nicolini likens it to runners separating out into groups after the start of a race, with faster runners in a pack out front and slower runners lumped together far behind.)
When the waves are reaching the coast of California "they can cancel each other out and stay like that for quite awhile," making the surf small and tricking people, before the wave phase changes and the peaks meet, causing that huge, surprise wave.
This gave Nicolini something to look for: a faraway storm churning up the ocean, combined with calm conditions at home (as a storm near the coast will generate its own wave and cause interference) seemed to be the perfect scenario.
Though they hadn't verified the scenario or put it through the usual rigorous scientific paces, sneaker waves were a deadly enough problem that in 2011, "we sort of went out on a limb and started forecasting these," Nicolini said. (He's looking to publish a scientific paper on their work within the next year.)
When the forecasters think the conditions are ripe for sneaker waves and people are likely to be out on the beach (on a sunny weekend, for example), the National Weather Service office puts out a full court press, issuing warnings on social media, putting PSAs on the local media and contacting park rangers and other agencies that work along the coast. On rainy days or when waves might hit in the middle of the night, the office issues a forecast, but doesn't pound the drums quite as loudly, to avoid overplaying its message.
"We ramp up or down based on the situation," Nicolini said.
Nicolini has also pushed the term sneaker waves over other locally used expressions like sleeper waves or rogue waves. He thinks that "sleeper" sounds too benign and "rogue" too much like the waves are truly unpredictable, as well as the fact that rogue waves are a separate phenomenon.
So far, the forecasts seem to be doing well. While the number of deaths from sneaker waves have held steady, the number of visitors to the coast has risen, Nicolini said. And he has heard from people in the area that when the sea has seemed unusually calm, they have moved their families farther back on the beach.
Nicolini also spoke with an unfortunate person caught by a sneaker wave since the forecasts have been issued (they were from out of town and unaware of the threat) and the person described the exact conditions the team had forecast.
Nicolini said he hopes that the threat of sneaker waves doesn't keep people from enjoying the majesty of the Northern California shore, but rather that it teaches them to respect the power of the water.
"Our mission has never been to scare people away from the coast," he said. "It's a beautiful place."
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Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.