California - Dry and Wet

If you live in California, get ready for weather extremes. Looking at past severe weather events in our state, Stanford scientists have determined that they will likely increase in frequency and intensity as the climate changes. While this means more severe droughts such as the record breaking current one, it doesn’t necessarily mean fewer wet years, says Stanford’s Daniel Swain (whom we interviewed last year about El Niño). “What seems to be happening is that we’re having fewer ‘average’ years, and instead we're seeing more extremes on both sides,” he says. “This means that California is indeed experiencing more warm and dry periods, punctuated by wet conditions.”

California already has unusual weather with a very short wet season in general. Our state “receives the vast majority (~95%) of its annual precipitation in the form of rain and high-elevation snow between the cool-season months of October and May, including ~66% during the core rainy season months from December to March,” according to Swain and colleagues’ recent study, published Friday in Science Advances. Plus, large-scale atmospheric patterns over the northeastern Pacific Ocean can make or break our winter rains.

The researchers used historical climate data to investigate changes during California’s rainy season and identified the specific north Pacific atmospheric patterns associated with the most extreme temperature and precipitation seasons between 1949 and 2015. Their analysis revealed a significant increase in atmospheric patterns associated with precipitation and temperature extremes over the 67-year period, similar to what has occurred recently during California’s ongoing multi-year drought.

“California’s driest and warmest years are almost always associated with some sort of persistent high pressure region, which can deflect the Pacific storm track away from California,” says Swain. Scientists refer to these high pressure regions as “ridges.” In 2014, Swain and colleagues determined that one such persistent ridge pattern—which Swain named the Ridiculously Resilient Ridge—was diverting winter storms northward and preventing them from reaching California during the state’s drought and was very likely caused by global warming. The new study basically says Californians should get used to these ridges. “We found that this specific extreme ridge pattern associated with the ongoing California drought has increased [in frequency] in recent decades,” Swain says.

So get ready for very dry years and some wild storms in the future. “We’re not necessarily shifting toward perpetually lower precipitation conditions in California—even though the risk of drought is increasing,” according to Swain.

Image: Bartshé Miller/Stanford