Top climate scientists in California are already warning of exceptionally high seas and coastal flooding this winter.
Looking ahead, they see a confluence of three forces raising the ocean along California's shores: this year's extra potent El Niño, climate change and peak high tides.
"We're expecting likely the highest sea levels ever recorded on the California coast associated with this event," Mark Merrifield, director of the UC San Diego Scripps Institution of Oceanography's Center for Climate Change Impacts and Adaptation, said during a recent virtual town hall about El Niño.
El Niño, which the National Weather Service's Climate Prediction Center recently reported is likely to reach historic strength this fall, is the wild card at play. El Niño, a climate pattern that drives primarily winter weather in California, involves above-average water temperatures in the tropical Pacific. As it heats up, the water expands and rises. Various oceanic forces are currently driving those taller waters from the equator toward California, where a separate marine heat wave has already driven up temperatures.
"As we go up through Central America and up the west coast of Mexico and into the Gulf of California, water levels are similarly now approaching or even exceeding a foot above average in this region," Daniel Swain, a climate scientist for the University of California's Agriculture and Natural Resources group, said during an online briefing. He added, "We're seeing already about half a foot of sea level increase off the coast of California."Not only does El Niño bump up the volume of the eastern Pacific, but research shows it significantly increases winter wave energy and the likelihood of storms bringing above-average rainfall to California.
"That El Niño event will not only temporarily elevate sea level, but it will also increase the likelihood of major storms that can cause additional storm surge, which can amplify extreme tides," Swain said. "So this is why I think folks who really think about coastal risks are very concerned that the single highest sea levels on record are quite possible this winter in California. And that is going to be a problem."
This El Niño is also occurring in a warming world. Already, the National Weather Service has recorded about a 10-inch rise in bay waters since 1900. Climate change is projected to cause about 3 to 7 feet of sea-level rise in the Bay Area, compared with 2000 levels, by the end of the century.
The last factor is that some of the year's highest tides will hit right around Thanksgiving and Christmas. These so-called king tides occur ever year. But this winter, by chance, those royal tides are expected to peak in an approximately 18-year cycle. The difference from previous years may only come down to a couple of inches, but that could be significant during storms.
"During El Niños, we have higher background sea levels, and we may have bigger storms and bigger waves," Reinhard Flick, a research associate for Scripps Institution of Oceanography, told SFGATE. "Now, if those big storms and big waves happen to occur on or around Thanksgiving or Christmas or the end of January, then you have a situation like we had in 1982 and 1983."
El Niño conditions in the winter of 1982 to 1983 ranked as the strongest on record, at a peak near 2.5 degrees Celsius. Since 1950, only a handful of El Niño events have fallen into the "very strong" category, which the National Weather Service measures as temperatures in certain areas of the tropical Pacific Ocean that reach greater than 2 degrees Celsius above normal. An outlook the Climate Prediction Center published this month gives a 69% chance that this year's El Niño will become "historic" this fall, exceeding the strength of all previous events.
"Piers collapsed," Flick said about the winter of 1982 and 1983. "Places were flooded. People drowned. There were huge amounts of rain, and so the rivers flooded. It was just a huge mess, and at that point, El Niño was not a household name like it is now."
