Snowpack extent at winter’s end in California’s Sierra Nevada mountains was significantly more extensive in 2013 than in 2015, as seen in this animation of satellite images. The 2013 image was acquired by NASA’s Aqua satellite on April 9, 2013; the 2015 image was captured by its sibling, the Terra satellite, on April 4, 2015. (Images: NASA Worldview. Animation: Tom Yulsman)
In the Northwest, an infamous feature called “The Blob” also contributed to snow-shriveling temps, but not so much in California...
Exceptionally high temperatures, partly resulting from humankind’s influence on the climate, was the primary cause of record-low snowpack — a “snow drought” — in the westernmost United States last year.
That’s the conclusion of a study published in the journal Geophysical Research Letters.
We usually think of drought as meaning a lack of precipitation. But in the winter of 2014/2015, precipitation in Oregon and Washington was actually near normal levels. And while California continued to be unusually dry, the lack of precipitation that winter was not exceptional.
Yet on April 1, the end of the winter season, 80 percent of long-term monitoring sites along the spine of mountains running south through the three states showed record low snowpack. Fully a fifth of these measured no snow at all — including sites at very high elevations in the Sierra Nevada mountains where you’d expect at least some snow to have accumulated and persisted.
According to the new research, this snow drought was attributable to high temperatures that kept the white stuff from accumulating.
“The story in 2015 was really the exceptional warmth,” says UCLA’s Dennis Lettenmaier, a study co-author. “Historically, droughts in the West have mostly been associated with dry winters, and only secondarily with warmth. But 2015 was different,” he says, quoted in a press release about the new findings.
The exceptionally high temperatures in Oregon and Washington were attributable both to “The Blob” — an enormous patch of unusually warm water in the northern Pacific Ocean — and elevated concentrations of greenhouse gases in the atmosphere. Meanwhile, in California’s Sierra Nevada mountains, The Blob played only a small role in raising temperatures. But greenhouse gases caused warming of about 1°C, contributing to the snow drought, the researchers found.
As part of their work, the scientists used a novel crowdsourcing approach: Citizens volunteered to offer up computing time on their personal computers as part of an experimental setup called weather@home. Using these computers, a regional model was run thousands of times simultaneously to estimate how greenhouse gases and The Blob influenced the snow drought.