Powerful waves are headed to the Central Coast this week. Here’s why they pose a threat

In late November, a 974-millibar “bomb cyclone” slammed into the Oregon coastline, with sustained southerly winds that reached 85 mph with gusts to 106 mph at Cape Blanco.

The term “bomb cyclone” is used to describe a rapidly intensifying low-pressure system over a 24-hour time frame. To be classified as a meteorological bomb, a storm needs to fall 24 millibars in 24 hours. The Nov. 26 tempest off Oregon dropped 43 millibars in 24 hours.

These winds produced significant wave heights of 43 feet with a maximum wave height of 74 feet at the Cape Mendocino buoy (No. 094), run by Scripps’ Coastal Data Information Program (CDIP) on Nov. 26.

Last Friday night, another bomb 947-millibar mid-latitude cyclone with hurricane-force winds developed near the International Date Line. This intense mid-latitude cyclone moved eastward toward the Washington and Oregon coastline with a traveling or dynamic fetch (fetch is the distance over the water that the wind blows in a single direction).

This immense storm generated a very long-period west-northwesterly (290-degree, deep-water) swell that will arrive along our coastline on Tuesday afternoon at 7 to 9 feet (with a 25- to 28-second period), increasing to 9 to 11 feet (with a 20- to 22-second period) on Wednesday into Thursday.

While the height of this swell event may not be that impressive, the wave period is extraordinary, and here is why.

I’ve been forecasting ocean waves along the Central Coast since 1992, and I don’t recall NOAA’s Wavewatch III model ever predicting waves with periods or wavelengths (around 3,000 feet) of this magnitude.

The longer the wavelength of the swell, the faster it will travel across the ocean. Like a marathon, a few of the longer-period waves (fastest runners) will pull ahead, leaving the bulk of the waves in the middle of the pack. The shorter wavelength waves will fall behind.

The wavelength is the distance from the crest of one wave to the crest of the next wave. The period of the wave is the time it takes for two consecutive peaks to pass a fixed point. The longer-period waves have exponentially higher amounts of energy and can travel longer distances with little loss of energy.

For example, both sound waves and ocean waves are mechanical and share many of the same properties. Think of a car driving through your neighborhood with its stereo blaring. The first thing you hear is the thump of the bass. The low-frequency bass is composed of longer-period waves that can travel longer distances. You generally don’t hear the higher frequencies until the car is near you.

Some of these waves on Tuesday and Wednesday may have periods longer than 30 seconds; however, the accelerometers in the NOAA marine and CDIP waverider buoys can’t measure them, they aren’t engineered to measure such long wavelengths.

When these waves arrive along our coastline on Tuesday, Wednesday and Thursday, they will produce strong rip currents, massive surge, sneaker waves and coastal erosion. If you’re going to the coast to see these long-period waves, please remember, never turn your back to the ocean and keep a safe distance.

Many readers have asked if the swells along our coastline have become more powerful over the years.

Locally, an interesting way to measure the increase in wave power is to study wave data from Diablo Canyon Power Plant’s Waverider buoy.

This wave measurement station has been in existence since June 1983 and is one of the longest continuous-wave monitoring stations along the West Coast. You can view the historical wave data archive from this station at the CDIP database at the Scripps Institute of Oceanography at cdip.ucsd.edu/m/. CDIP maintains an extensive network of buoys that monitor waves along the coastlines of the United States.

In the 35-plus years that the waverider buoy has been deployed off the Pecho Coast, the wave archive indicates about a 5 percent increase in longer-period wave events, linked directly to a pattern of more intense storms in the Pacific with lower air pressures and stronger winds.

You see, as the atmosphere warms, it’s able to hold more water vapor. When this water vapor condenses over the Pacific, it liberates enormous amounts of latent heat and causes a rapid and sharp drop in air pressure that can create storms with hurricane-force winds.

Over the decades, these North Pacific storms have become more intense. In fact, a storm from the remnants of Typhoon Nuri in November 2014 intensified to 924 millibars in the Bering Sea. This was the lowest pressure ever recorded in the North Pacific region. The previous record was 925 millibars recorded at Dutch Harbor in October 1977. Often, the lower the pressure, the stronger the winds that blow across the Pacific, and then they generate higher seas and eventually a longer-period swell.

NOAA’s Marine Buoy (No. 72), 230 miles southwest of Dutch Harbor and located within the Aleutian Islands of Alaska, reached 56 feet with a 17-second period on Dec. 12, 2016, as a 948-millibar storm with hurricane-force winds moved through the Bering Sea. Understandably, the buoy stopped transmitting wave data later that day.

Regrettably, we continue to dump millions of tons of carbon dioxide into the atmosphere every hour from the burning of fossil fuels. “The increase in the rate at which the average global sea level is rising is just what is expected from climate models,” astronomer Ray Weymann said. “Many low-lying areas, like Miami, are already feeling the impacts. It will only get worse unless we quickly control CO2 emissions.”

To learn more about climate change and how you can reduce your carbon footprint, please visit PG&E at www.pge.com, The Central Coast Climate Science Education at www.centralcoastclimatescience.org/ or Climate Central at www.climatecentral.org/.