April 9, 2016, by Tim Radford
Low precipitation and record high temperatures left Tioga Pass, in California’s Sierra Nevada, nearly snowless in January 2015. Image: Bartshé Miller
Scientists say that storms carrying desperately-needed water to California are being diverted by a band of high pressure that coincides with rainfall and temperature extremes.
LONDON, 9 April, 2016 − California is being sheltered from stormy weather by what climate scientists have dubbed a Ridiculously Resilient Ridge – a band of high pressure that disrupts wind patterns.
But the discovery is not welcomed, as storms are what normally deliver the rainfall that California desperately needs to alleviate its crippling drought conditions.
Daniel Swain, a PhD student in environmental earth system science at Stanford University, and colleagues report in Science Advances journal that they analysed the atmospheric circulation patterns that have been coincident with rainfall and temperature extremes in the Golden State’s history.
“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,” Swain says.
“Since California depends on a relatively small number of heavy precipitation events to make up the bulk of the annual total, missing out on even one or two of these can have significant implications for water availability.”
California has been in the grip of a prolonged and remorseless drought that is one of the worst in history, with immediate and long-term threats to the supply of water for agriculture and the crowded cities of the region.
And researchers − including Noah Diffenbaugh, associate professor of earth system science at Stanford University, who is one of the co-authors of the new study − have linked the drought to global climate change resulting from the release of greenhouse gases worldwide as human economies burn ever more fossil fuel. Which means that Californians can expect more of the same.
“We find clear evidence that atmospheric patterns that look like what we’ve seen during this extreme drought have in fact become more common in recent decades”
“The current record-breaking drought has arisen from both extremely low precipitation and extremely warm temperature,” Professor Diffenbaugh says.
“In this new study, we find clear evidence that atmospheric patterns that look like what we’ve seen during this extreme drought have in fact become more common in recent decades.”
The scientists found that the blocking ridge of high pressure was deflecting storms northward, and that the phenomenon had become more common in recent decades.
Blocking high pressure systems affect all regions of the planet, and a team from Rutgers University recently reported in the American Meteorological Society’s Journal of Climate that such blocking highs could stop cold Canadian air from reaching Greenland, and thus accelerate the surface melting of the Arctic’s largest ice cap.
The take-home message from the Stanford research is that climate change research should embrace large-scale atmospheric circulation patterns, as well as the patterns of temperature rise and precipitation.
Paradoxically, the resilience of the pressure ridge doesn’t mean the end of heavy rain in California − just a shift in the balance of dry and wet seasons.
“What seems to be happening is that we are having fewer ‘average’ years, and instead we’re seeing more extremes on both sides,” Swain says. “This means California is indeed experiencing more warm and dry periods, punctuated by wet conditions.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.