While many of us think of drought as resulting primarily from a lack of precipitation, here in the Pacific Northwest there is another factor that has been paramount to drought: a lack of mountain snow, or snowpack.
Unpacking what a paucity of snowpack meant for the drought years 2011to 2015 is a recent paper in Geophysical Research Letters. The study’s authors include several CIRC and OCCRI researchers.
Among the paper’s key findings is this: 2015 was the worst year on record for snowpack across the entire Western United States, and by a wide margin. What’s more, the major factor leading to this loss was high temperatures not low precipitation; high temperatures that can, in turn, be tied to anthropogenic (human-caused) climate change.
Consider Oregon and Washington. Precipitation measures for the water year 2014 to 2015 were only slightly below average. (A water year differs from the calendar year, running from October 1st to September 30th.) The winter of 2014/2015 was unseasonably warm in both Oregon and Washington, with the calendar year 2015 marking the warmest year on record for both states as well as California. This meant that a lot of precipitation fell as rain and not as snow.
While this dearth of snow spelled headline-making bad news for ski areas, many of which were hit in their pocket books from powder shortages, what became dubbed the “snow drought” had ramifications that stretched well beyond winter recreation.
In the Pacific Northwest, California, and much of the US West, mountain snowpack acts as a natural reservoir that—in slowly melting over the spring and summer months—provides water during our region’s warmest, driest season. (We’ve used this reservoir analogy multiple times, but it’s a good one, so we’re sticking with it.) When snowpack is low, that can spell trouble for everyone from municipal water suppliers that rely on snowpack to quench the thirst of their many customers—and their customers’ many lawns and gardens—to farmers who rely on snowmelt-fed irrigation to water their crops. 2015 saw exactly these kinds of shortages.
To put a number on exactly how low snow can go, the paper’s researchers examined snowpack data from roughly 1,800 US Department of Agriculture Natural Resource’s Conservation Service and California Department of Water Resources sites across the Western US. Data dated back to the early 1920s in some areas and stretched into the present. The researchers’ key metric was snow water equivalent (SWE), a measure of the amount of water you would get if you melted a given amount of snow. Usually taken around April 1st, SWE is essential for assessing hydrologic cycles and, by extension, some elements of drought in the West.
With SWE data in hand, the researchers compared these data to computer simulations using the hydrologic model the Variable Infiltration Capacity Hydrologic Model (VIC), which came up with its own numbers for SWE. This was done, in part, to ground truth the meteorological measurements, which can be skewed by where in the mountains they are sited.
(Measuring snow in the mountains is not easy, as you might imagination, and in-the-field sites are often based on their accessibility, not covering as much ground—both literally and figuratively—as many climate researchers would like.)
The researchers’ final analysis also included locating climate change’s signature scrawled across the water year 2014/2015. To do this, the researchers used the regional climate model HadRM3p as part of a project called weather@home. (See: “Improving Regional Climate Models with Citizen Science,” November 2015 CIRCulator.) To tease out the climate change signal, the researchers ran 16,000 climate simulations in HadRM3p, including simulations that accounted for anthropogenic forcing—that is human-cased climate warming—and simulations that had no anthropogenic forcing.
Results: The researchers’ findings suggest that anthropogenic forcing added about 1 degree Celsius (1.8 degree Fahrenheit) in extra warming to the water year 2014/2015. As for SWE, 2014/2015 saw record lows at 80 percent of mountain snowpack sites, upsetting earlier records set in 1977 (which set records for low precipitation, not high temperatures, across the West).
The researchers also note that the exact reason for the snow drought differed slightly from north to south. California received less than normal precipitation in the water year 2014/2015. By contrast, Oregon and Washington both received near normal precipitation for the same period. What all three Pacific states shared were high temperatures. As we noted above, the year 2015 was the warmest on record for all three states.
There is another climatic factor worth mentioning: THE BLOB! No, not the frighteningly amorphous monster from the 1958 film staring Steve McQueen, but the unusually warm mass of water that appeared off the Pacific Northwest coast in late 2013 and was named by Washington State Climatologist Nick Bond. Many suspected the Blob—which showed up as unusually high sea surface temperatures (SST)—had played a role in the recent snow drought. CIRC and OCCRI researchers tested this hypothesis in their regional climate model. Their answer: no and yes. No, SST (the Blob) did not play a significant role in the snow drought in California; yes, SST (the Blob) did contribute substantially to the snow drought in Oregon and Washington.
Citation: Mote, Philip W., David E. Rupp, Sihan Li, Darrin J. Sharp, Friederike Otto, Peter F. Uhe, Mu Xiao, Dennis P. Lettenmaier, Heidi Cullen, and Myles R. Allen. “Perspectives on the causes of exceptionally low 2015 snowpack in the western United States.” Geophysical Research Letters 43, no. 20 (2016). doi: 10.1002/2016GL069965