Waves off Central Coast contain clues about changing climate. Is California due for drought?

The waves along the Central Coast can tell you a lot about our changing climate, and here’s why.

The Diablo Canyon Power Plant’s Waverider Buoy has measured wave heights and periods since June 1983 and directions since June 1996 and is one of the longest continuous-wave monitoring stations along the West Coast.

Scripps Institute of Oceanography’s Coastal Data Information Program (CDIP) maintains an extensive network of buoys that monitor waves along the coastlines of the United States. You can view the historical wave data archive from Diablo Canyon and other stations at the CDIP database at cdip.ucsd.edu.

In the 36-plus years that the waverider buoys have been deployed off the Pecho Coast, the wave archive indicates about a 5% increase in longer-period wave events, linked directly to a pattern of more intense storms in the northern Pacific Ocean with lower air pressures and stronger winds due to climate change.

However, the number of swell events with significant wave heights — the average height of the waves in the top third of the wave record — reaching more than 20 feet has decreased at this station over the past few decades.

The highest measurable wave event occurred on Jan. 11, 2001, when significant wave heights hit 21.3 feet with a 17-second period.

The most recent time a swell event reached more than 20 feet happened on Jan. 21, 2017, when the buoy reported a swell height of 20.1 feet with a 20-second period.

The Diablo Canyon Waverider Buoy is partially shaded from the high-energy northwesterly swell trains coming from the Gulf of Alaska by Point Buchon. However, it’s not sheltered from southwesterly wave events that are more common during El Niño years, when the jet stream moves further south in latitude and steers a greater number of storms that tend to generate the southwesterly wave events and heavy rains along the Central Coast.

Consequently, the highest waves ever recorded along much of the Central Coast are usually from the southwest, like the El Niño-driven wave event on March 2, 1983, that collapsed the Union Oil Pier in Avila Beach.

On the other hand, a La Niña condition tends to keep the jet stream further north, carrying mid-Pacific cyclones into the Pacific Northwest and reducing the number of low-pressure systems that generate southwesterly wave events and rain from reaching our part of California, resulting in drier conditions.

Locally, rainfall amounts this rain season — July 1, 2020, through June 30, 2021 — have dropped to 7% of the average at San Luis Obispo County Regional Airport. Both the Paso Robles and Santa Maria airports have seen about a third of an inch or about 11% of normal.

In Los Angeles, rainfall is a meager 0.11 of an inch and 4% of normal.

These numbers point to another below-average rainfall season in Central and Southern California, but what about the years ahead? Will it continue to be dryer than usual? The wave measurements from the Diablo Canyon Waverider Buoy have been indicating a decreasing amount of southwesterly wave trans that are often associated with big storms and heavy precipitation.

To help verify this hypothesis, I decided to ask one of the foremost experts on California’s climate: Bill Patzert, a retired oceanographer and climatologist at NASA’s Jet Propulsion Laboratory in Pasadena.

“The past couple of decades have certainly been drier than wet for Central and Southern California,” Patzert said. “So far this year, the U.S. Drought Monitor is lit up like a Christmas tree. Nowhere in California are the rainfall totals near average for mid-December. Water managers, farmers, and firefighters are anxious.”

“La Niña is one of the largest signals in the climate system. Its impact on rainfall and temperature patterns across the planet shouldn’t be underestimated,” he said. “La Niña is good for farmers in some places and bad for others elsewhere.

“La Nina is already shaking up agricultural markets in Asia and the Americas. This fall, dry conditions have been reported in Brazil, Argentina, and parts of the U.S., and bouts of excessive rain in Australia and parts of Southeast Asia.”

“Another large footprint of La Niña was the record-breaking Atlantic hurricane season. La Nina is the real deal, and her impact has been severe and widespread,” Patzert said

The current La Niña seems to be peaking, increasing the odds that the Pacific Northwest will have more stormy weather this winter and spring, while the southwestern and southern U.S. will be dryer than usual.

This La Niña is occurring within the context of a larger climate event, a resurgence of a cool phase of the basin-wide Pacific Decadal Oscillation (PDO). The PDO is a long-term fluctuation of the Pacific Ocean that waxes and wanes between cool and warm phases approximately every five to 20 years.

During most of the 1980s and 1990s, the Pacific was locked in the oscillation’s warm phase. But conditions changed in 1999, when the cool phase, which resembles a large La Niña, has tended to dominate the Pacific Ocean.

For the past two decades, La Niña events have been more frequent than warmer El Niño occurrences.

“This multi-year Pacific Decadal Oscillation “cool” trend can intensify La Niña or diminish El Niño impacts around the Pacific basin,” Patzert said. “The reemergence of this large-scale PDO pattern tells us there is much more than an isolated La Niña occurring in the Pacific Ocean.”

“These shifts can trigger more extended droughts in some regions and damaging floods elsewhere,” Patzert noted.

Overall, a warmer climate is expected to produce less rainfall and fewer big southwesterly wave events along the Central and Southern California coastline in the years ahead.