Whether you use the term "sneaker wave" could depend entirely on your point of view.
People intimately familiar with the ocean's movements — like lifeguards, surfers or oceanographers — might simply call them "set waves," or in the words of lifeguard supervisor Eric Abma of California State Parks in Monterey, "waves that are larger than others."
But if you're pummeled by a sudden wave while carelessly strolling along the beach, the term "sneaker wave" might strike a chord.
That's what happened two years ago, when spectators of the Mavericks International big-wave surf competition in Half Moon Bay were caught off-guard as a set of large waves suddenly came ashore, injuring 12 people. This year, viewing of Sunday's competition will be restricted to the safety of jumbo screens displayed down the street from the beach.
The large surf prediction that triggered a green light for this weekend's contest is also an indicator of prime sneaker waves, which occur every year along the Central Coast and north into Oregon and Washington.
The sudden waves can strike with a force strong enough to knock unsuspecting beachgoers, novice fishermen or tide pool seekers unconscious, and then retreat quickly back into the sea, sometimes dragging people along rocks before they have a chance to swim back to shore.
The waves have killed three people in the San Francisco Bay Area within the past month, including a man and his 9-year-old son who were swept off the rocks while fishing in the Marin Headlands. So far this year, no one from Monterey County has been taken by a wave.
Yet while the larger-than-usual waves seem to come out of nowhere, their formation is a regular part of ocean dynamics, said Curt Storlazzi, an oceanographer and expert on coastal ocean dynamics at the U.S. Geological Survey in Santa Cruz. The waves are the result of wave trains from different storms coming together, he said.
"When you have waves of different frequencies, and when both of their crests line up, the waves are bigger than normal," Storlazzi said. These are set waves. On the other hand, he said, when the waves are out of sync with each other, the two will cancel each other out instead, forming a lull.
While surfers live for set waves, beachgoers could find them a threat instead.
"When people get swept off of rocks on the coastlines, it's usually these set waves," Storlazzi said.
In winter, storms from the northern Pacific Ocean — as far away as Siberia — send large swells to California's shores. If the swells happen to meet up with those from a storm in Hawaii, for example, the combination can make for strong surf conditions here, said Storlazzi, with two 5-foot swells combining to generate a powerful 10-foot wave.
In summer, large waves triggered by winter storms in the southern hemisphere can also lead to large waves here.
But knowledge about the source of large waves won't protect people from being struck by one. Rather, conditions on the beach are the most important things to watch out for, said lifeguard Abma.
"My suggestion is that people do their homework before going to the beach," Abma said.
He warned that steep beaches like Monastery Beach or Carmel River State Beach make for particularly dangerous conditions.
"When waves break offshore, they lose a lot of their energy slowly," Abma said. "But when they hit a steep beach they lose all of their energy at once, and they just explode onto shore."
To make matters worse, Abma said, the steep conditions also cause the waves to retreat quickly, which can make getting back out of the water a tricky proposition.
"At Monastery, people have coined the term 'Monastery crawl,' because crawling up the beach is much easier than actually walking," Abma said.
But the easiest option by far, Abma said, is to avoid getting close to the water at steep beaches altogether.
"We don't recommend swimming or even really going near the water at those beaches," Abma said.
"When you get to the beach, take a look at the signage. Monastery beach has some very specific signs recommending not going near the water."
Abma cautioned, however, that a gently sloping beach doesn't guarantee safe water conditions either, pointing out that swimmers still have rip currents to contend with.
"On a typical beach, a person will be in the water, get stuck in a rip current and not realize it," said Abma. "They'll swim against the current and not be able to get back to shore."
Abma stressed that learning how to get out of a rip current — by swimming to the side — is an essential skill for ocean swimmers.
For those who aren't swimming (or don't intend to swim), the best course of action is to avoid standing on steep beaches, rocks that jut out into the water or sea cliffs that push up against the surf.
From a safe distance, sneakers are just large waves, after all.
Jessica Shugart can be reached at 646-1188 or firstname.lastname@example.org.