All Weather Bets Are Off

It was Daniel Swain, PhD ’16, who coined the term “ridiculously resilient ridge” to describe a persistent blockage of atmospheric high pressure over the Pacific Ocean—a rain barrier considered a major contributor to the California drought since 2012. Even when the state felt an El Niño effect last year, Swain noted that rainfall and snow accumulations were well below what could have been hoped for because of continued storm deterrence by “a resurgence of modest ridging.”

This winter, “RRR” just as easily could have stood for Rain, Rain, Rain. A series of storms over the past three months has drenched California, refilling shrunken reservoirs, restoring groundwater and building an epic snowpack in the Sierra Nevada. Santa Rosa received more than 15 inches of rain in January, the most in 20 years. As of March 1, Mammoth Mountain had more than 43 feet of snow. A federal study released in February indicated that more than 80 percent of the state was free of drought.

So why didn’t the famously unyielding high-pressure ridge prevent this torrent of wet weather? Because, explains Swain, now a postdoctoral fellow at UCLA, the ridge “has finally disappeared.”

“Recent evidence,” reports Swain, “suggests that the persistence of the ‘Triple R’ was likely enabled by a confluence of factors, including unusual warmth in certain parts of the Pacific Ocean. And we now also know that similar atmospheric patterns have been occurring more frequently in recent years, coinciding with an increase in the likelihood of ‘hot droughts’ in this part of the world. But we still don’t have a complete understanding of the reason why atmospheric patterns sometimes stick around for many months; that remains a topic of active scientific research.

“A similar argument can be made about the present near-record wet winter,” continues Swain. “Why, exactly, has California seen so many storms? It’s a fascinating and important question, and also a substantial research challenge amidst a rapidly changing climate. As the Earth warms, past observations of historical conditions have become less representative of present-day variations in climate. This means that scientists often have to use alternative tools, like global climate models, to test hypotheses regarding atmospheric phenomena. These kinds of studies can be time-consuming, which often precludes quick answers regarding ongoing weather events. . . . But it’s worth noting that climate models do suggest that we will likely experience both hotter, more persistent droughts and increasingly intense rainfall during storms in the future—which is very similar to what California has endured over the past few years.”