Water from heavy rainstorms breeches a levee in Novato, Calif., on Feb. 14. (Terry Chea/AP)

Considering it’s been long known that El Niño conditions often bring about flooding precipitation to California, a ripe field for study would be a thorough examination of the damage wreaked.

And who knows catastrophic damage better than insurers? Their specialized knowledge prompted a pair of San Diego researchers to compare 40 years of insurance data against climate and water data to quantify the effect of El Niño on flood damage in the western United States.

The findings of Tom Corringham and Daniel Cayan, both of the Scripps Institution of Oceanography at the University of California at San Diego, confirm the connections between extreme weather events and El Niño, which is the periodic warming of equatorial waters in the Pacific Ocean.

In coastal Southern California and across the Southwest, El Niño conditions have had a strong effect in producing more frequent and higher magnitudes of insured losses, while La Niña conditions, the opposite of El Niño (when ocean temperatures are colder than normal), significantly reduce both the frequency and magnitude of losses. In the Pacific Northwest, the opposite pattern appears, although the effect is weaker and varies more from location to location.

The data was mined from a database of National Flood Insurance Program (NFIP) claims and losses collected from 1978 to 2017.

“We’ve known about the impacts of El Niño on weather patterns and flooding in California for years, but actually putting a dollar figure on the impacts has been challenging,” Corringham told The Washington Post. “Once we had the insurance data, it seemed like a natural question to ask.”

The dollar figures found by Corringham and Cayan were staggering: In Southern California, El Niño winters saw $170 million in insured losses during the study period -- which because not everyone carries flood insurance translates roughly into $5 billion in estimated total damages -- versus $17 million in insured losses during La Niña winters, or $500 million total damages.

A small number of events during the four decades under study had outsize impacts, as 1 percent of flood events caused more than two-thirds of total losses. This highlights the importance of the severe West Coast storms known as atmospheric rivers (ARs), especially the most damaging Category 4 and 5 ARs.

The biggest losses occurred north of San Francisco, along the lower Russian River in Sonoma County, which has expensive residences in an area prone to flooding. Another high-impact area was Malibu, where mansions are constructed literally at the edges of Santa Monica Bay.

But the research also confirmed that the persistent evolution of El Niño — Southern Oscillation (ENSO) offers the possibility that unusually high (or low) flood damage could be predicted in advance of the West Coast’s primary winter storm period.

Within the 40-year NFIP history, the study finds the multivariate ENSO index (MEI) would have provided an eight-month look ahead for heightened damage in Southern California. Such lead time would assist property owners, policymakers, and emergency planners and responders in planning for these high-impact events and reducing their toll.

El Niño conditions are in effect, as equatorial ocean temperatures are above average for most of the Pacific. Climatologists at the National Oceanic and Atmospheric Administration’s Climate Prediction Center are forecasting El Niño to transition to more neutral conditions into the fall and winter.

Corringham said the findings suggest flood policy should be focused on relocating high-risk communities rather than helping them rebuild every 20 to 30 years. A case in point is the town of Guerneville along the Russian River, which in February suffered $100 million in flood damage.

“We also need to rethink our investments in hard infrastructure like dams and levees, and consider the restoration of natural flood plains and the strategic placement of green infrastructure,” said Corringham, a postdoctoral researcher with Scripps’ Center For Western Weather and Water Extremes.

Corringham noted that NFIP market penetration is very low in the western United States, with only 5 percent of estimated flood damage in California insured by the program. Mandating the purchase of flood insurance in high-risk areas, he said, could lessen the economic burden of flooding on taxpayers.

The paper of Corringham’s and Cayan’s, “The Effect of El Niño on Flood Damages in the Western United States,” was published in the July 2019 edition of the American Meteorological Society’s journal Weather, Climate, and Society.