Forecasters see biggest storm risk arriving in January through March
California communities are preparing for an incoming Kelvin wave — a massive band of warm ocean water that swells tides from beneath the surface — that is expected to raise sea levels along the state’s coast by a foot as it moves north toward Alaska. The wave, driven by El Niño conditions, is nearing Southern California and expected to reach the San Francisco Bay Area by early October, where emergency managers are distributing sandbags and securing flood barriers in advance.
Forecasters warn the wave’s effects will compound climate-change-driven sea level rise and overlap with a “super” El Niño that scientists said could become the most severe ever recorded. The El Niño climate pattern, characterized by high sea surface temperatures in the central and eastern tropical Pacific, is already spurring extreme weather around the world, fueling destructive storms in Hawaii and unleashing Pacific hurricanes that have hammered beach towns. Pacific Ocean surface temperatures are now warmer than those recorded before the destructive 1997-1998 winter that left 21 of California’s 58 counties declared disaster areas, raising the likelihood that elevated sea levels will coincide with severe storms during the year’s highest tides in November and December.
The Kelvin wave behaves differently from a surface wave or tsunami. Rather than climbing and crashing, it moves as a slosh of warm water across the Pacific, driven by El Niño, before colliding with the South American coastline and surging north. Its effects are subtle on their own but can amplify during high tides and storm surges.
On their own, Kelvin waves aren’t always hazardous, said Mike Jacox, a research oceanographer with the National Marine Fisheries Service, though they can reduce the growth of phytoplankton that form the base of the marine food web. “But when there are very high tides or storm surges, their effects can be amplified,” he said. Where rising waters coincide with rainstorms and high tides, the phenomenon could swallow beaches, chew away bluffs and coastlines, and submerge streets, homes and businesses.
The wave’s effects would compound roughly 8 to 12 inches of sea-level rise California has already experienced from climate change over the past century. That long-running rise, layered with a “super” El Niño that may become the most severe on record, has increased the likelihood that elevated sea levels will overlap with severe storms.
California will likely see the worst effects this winter when the state could get pummeled by moisture. Storms do damage on their own, but the most destructive occur alongside high tides, according to a recent memo from the California Coastal Commission, and the “super” El Niño will increase the severity of both. The highest tides of the year in parts of California will arrive in November and December, the months when the state tends to be hit by wet weather.
“Coastal flooding is at this point practically certain,” climate scientist Daniel Swain said in an interview with Inside Climate News. A wetter-than-average winter would also bring increased inland flooding risk, he added. “Both of these in some places can combine where you have freshwater outflows and saltwater inflows mingling,” he said. “The damage can be devastating.”
Swain pointed to the winter of 1997-1998, which exhibited similar hazards. Heavy storms caused massive coastal erosion, enormous landslides ruptured and buried highways, and huge waves swept into harbors across Santa Cruz and the San Francisco Bay Area. Levees failed north of Sacramento and seawalls were overwhelmed; 21 of California’s 58 counties were declared disaster areas. Surface temperatures in the Pacific Ocean are now warmer than they were before that season’s onslaught.
Several California areas are already feeling the effects. “Just the first few inches of elevation in southern California have already caused enough erosion to close the Amtrak line along the coast, and this is just the beginning,” Swain said. The San Francisco Bay Area is particularly vulnerable because of the critical transportation arteries, population centers and big tech firms at risk of flooding. “There’s a huge amount of people and infrastructure within 6ft of sea level there,” Swain said.
Across the Bay Area, sandbags are already being distributed as emergency managers and local governments work to secure infrastructure and inform businesses and residents about the incoming dangers.
Nick Brubaker, the chief resilience officer for the office of emergency management in Marin County, just north of San Francisco, said preparations were already under way for El Niño when reports arrived about how high sea levels will rise. “It certainly changes our baseline assumptions on how we’re going to be planning for impact,” he said, noting that Marin saw flooding from high tides in January without any added effects from a Kelvin wave. Engorged creeks and streams will be less able to drain as sea levels rise, he said. “We could see some significant flooding.”
Marin County and other jurisdictions have cleared drains and culverts, evaluated infrastructure and rushed through temporary mitigation measures including securing flood barriers. Checklists have been issued and emergency alert signups encouraged.
“We’re using all of the data that we can, but we don’t have a perfect measure,” Brubaker said. He has focused on outreach and communication “to make sure that people are aware that this may be one of the most significant El Nino years that we’ve seen.”
The biggest risks, scientists said, may still be ahead. “This coming Kelvin wave is just the beginning,” said Dr Jonathan Warrick, a research geologist at the US Geological Survey. “The thing to look out for is what will happen in January, February and March – that’s when we get the biggest storms, we get our largest waves and we have some very large tides.”
If those events align, their effects will compound, setting the stage for a tumultuous winter, Warrick said. Together, the systems will pack a far bigger punch, he added. “When it comes to these storms and the tides,” he said, “we really are kind of rolling the dice.”