Waves are getting stronger along SLO County’s coast. Here’s why

It’s no surprise that so many Central Coast piers are failing.

The data tells the story: The red trendline on my chart shows a 78% increase in wave energy — measured in foot-pounds — from the earliest recorded event in 1983 to the most recent in 2024.

This striking rise underscores a long-term intensification in swell energy.

To better understand the threat facing our coastal infrastructure, I charted the 19 most energetic wave events recorded between June 1983 — when the Diablo Canyon Waverider Buoy was first deployed — and December 2024.

Each event is expressed in foot-pounds of energy per foot of wave crest, calculated using the wave’s length and the square of its height in 80 feet of water — the mooring depth of the buoy.

These high-energy events represent the real-world conditions most likely to compromise pier structures along our shoreline.

I’ve long understood that average swell energy has increased due to more intense mid-latitude cyclones forming in the Gulf of Alaska. But I hadn’t fully appreciated just how much more powerful these long-wavelength swell events have become — especially those generated by “bomb cyclones,” which rapidly intensify by dropping 24 millibars or more in just 24 hours.

As oceanographer Walter Munk of Scripps Institution of Oceanography discovered in the early 1960s, long-period swells attenuate far less as they travel across open water than shorter-period waves.

This all ties back to the warming oceans.

Oceans cover over two-thirds of the Earth’s surface and absorb roughly 90% of the excess heat from human-caused global warming. That’s an immense reservoir of energy. As sea surface temperatures rise, the atmosphere above warms, holding more water vapor and fueling greater evaporation.

This, in turn, supercharges storms — especially those mid-latitude cyclones in the Gulf of Alaska that now routinely set new low pressure records each decade.

When water vapor condenses in the atmosphere, it releases latent heat — the very engine that powers these cyclones. The release of that energy causes air masses to rise faster and higher, driving surface pressures even lower and intensifying wind speeds and rainfall. It’s a classic positive feedback loop.

These powerful winds agitate the ocean surface, generating higher seas and longer-wavelength waves within the storm’s fetch. Once these waves leave the fetch, they become deep-ocean swells that can travel thousands of miles across the Pacific. As they approach shallower coastal waters, the swells “feel bottom” at half their wavelength.

Thanks to energy conservation, they maintain their period, but their wavelengths shorten while their wave heights increase. The result? The longer the wave period, the taller — and more forceful — the wave becomes as it nears shore.

This amplifies the energy exerted on piers and other marine structures, explaining not only the excitement among big-wave surfers but also the growing vulnerability of our coastal infrastructure.

Combine that with rising sea levels — caused in part by the thermal expansion of seawater column — and we’re seeing tides run over a foot higher than predicted during strong El Niño years.

Add in storm surge, more intense rainfall, and higher runoff, and you’ve got a recipe for increased coastal flooding —a threat that’s only growing as the Earth continues to warm.

John Lindsey is a retired PG&E marine meteorologist. Email him at JohnLindseyLosOsos@gmail.com or follow him on X @PGE_John.