Dead almond trees in California’s Central Valley were one sign of an ongoing major drought in 2015. 

REUTERS/Lucy Nicholson

Vanishing Arctic ice could drive future California droughts

California’s recent 5-year drought was brutal: Houseboats sat on lakebeds, groundwater supplies dwindled, and thousands of fish literally baked to death. Scientists consider it the worst such event the Golden State has seen in 1000 years. Now, a new study identifies an unexpected suspect, thousands of kilometers to the north: an iceless Arctic Ocean. The finding, a new atmospheric mechanism that links Arctic melting to conditions farther south, suggests that calamities like the 2012–16 drought may become more frequent as Arctic ice continues to vanish.

“Changes in the Arctic don’t stay in the Arctic,” says Ivana Cvijanovic, an atmospheric modeler at the Lawrence Livermore National Laboratory in California and lead author of the study. She calls the work another example of the interconnectedness of the climate system, and how Arctic melting can have consequences far from the pole.

Scientists generally think that global warming, driven mostly by rising levels of atmospheric carbon dioxide, will make some regions wetter and others drier. But Cvijanovic and her colleagues wanted to understand the effects of disappearing polar ice, independent of global warming. So they created a set of global climate models to analyze the ocean and atmosphere over a 40-year period, keeping carbon dioxide levels fixed. They ran one set of simulations with Arctic ice coverage typical of recent years and another in which parameters in the model were set so that a much lower amount of sea ice formed each year.

In modeling runs with the low-ice condition, the Arctic’s global influence quickly became apparent. With less ice, the Arctic reflected less of the sun’s energy out into space, leading to a surplus of heat there. Within just 20 years, that had disrupted the usual flow of energy toward the Arctic from the tropics, leading to warmer-than-normal waters just north of the equator.

That excess tropical energy fueled rising air in a process known as convection, creating rain, releasing heat, and forming large-scale atmospheric patterns called Rossby waves. Those waves, in turn, led to the formation at midlatitudes of high-pressure systems, or “ridges.” In the simulations, the clockwise-swirling winds of a ridge in the Northern Pacific drove storms and rain north, beyond California and into the Pacific Northwest, the researchers report today in Nature Communications.

“The two-step tropical connection seems well supported by their simulations,” says Jennifer Francis, a climate scientist at Rutgers University in New Brunswick, New Jersey, who was not a part of the study. Francis, a leading proponent of the theory that the Arctic can wreak havoc on weather elsewhere in the Northern Hemisphere, adds that the proposed mechanism fits “observations of the real world in recent years.”

Cvijanovic and her colleagues did not use their work to investigate California’s recent megadrought, but she says it is “consistent” with the scenario in her paper. For much of the drought, a large atmospheric ridge sat implacably over the north Pacific and diverted rain past California to the north—even earning the moniker “the ridiculously resilient ridge” from weather wags.

And Francis notes another ridge is currently forming in the North Pacific.  “Looks like we're headed toward the exact conditions this paper describes,” she says. “A big persistent dry-warm spell for the west.”