Causes of recent changes in western North American snowpack

Climate Dynamics (Impact Factor: 4.67). 05/2011; 38(9):1885-1899. DOI: 10.1007/s00382-011-1089-y


Monthly snow water equivalent (SWE) station observations and gridded temperature data are used to identify mechanisms by which
warming affects the temporal and geographical structure of changes in western North American mountain snowpack. We first exploit
interannual variability to demonstrate the sensitivity of snowpack to temperature during the various phases of the snow season.
We show that mechanisms whereby temperature affects snowpack emerge in the mid to late portion of the snow season (March through
May), but are nearly absent during the earliest phase (February), when temperatures are generally well below freezing. The
mid to late snow season is precisely when significant loss of snowpack is seen at nearly all locations over the past few decades,
both through decreases in snow accumulation and increases in snowmelt. At locations where April 1st SWE has been increasing
over the past few decades, the increase is entirely due to a significant enhancement of accumulation during the earliest phase
of the snow season, when the sensitivity analysis indicates that temperature is not expected to affect snowpack. Later in
the snow season, these stations exhibit significant snowpack loss comparable to the other stations. Based on this analysis,
it is difficult to escape the conclusion that recent snowpack changes in western North America are caused by regional-scale
warming. Given predictions of future warming, a further reduction in late season snowpack and advancement in the onset of
snowmelt should be expected in the coming decades throughout the region.

KeywordsSnow water equivalent–Climate change–Climate sensitivity–Trends–Surface observations

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Available from: Sarah Kapnick, Sep 25, 2014
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    • "The warming scenarios applied (i.e. 1 – 4 °C) generally align with expected mid-21 st century warming of 0.8 – 1.7 °C [Barnett et al., 2005] and end of century warming of 3 – 5 °C in the Western U.S. [Leung et al., 2004; Christensen et al., 2007]. The relatively high sensitivity of PSM 10 timing to warming in the Sierra Nevada and Pacific Northwest, versus other parts of the Western U.S., corresponds with observed spatial changes in 20 th century snowmelt [Kapnick and Hall, 2012] and streamflow timing [Cayan et al., 2001; Stewart et al., 2005]. The reduced maximum SWE scenarios (i.e. 10 – 40% decreased SWE) are consistent with expected decreases in snow accumulation across the Western U.S. that have been estimated at 20 – 70% by midcentury [Leung et al., 2004]. "
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    ABSTRACT: The ecohydrological effects of changing snowmelt are strongly mediated by soil moisture. We utilize 259 Snow Telemetry (SNOTEL) stations across the Western U.S. to address two questions: 1. How do relationships between peak soil moisture (PSM) timing and the day of snow disappearance (DSD) vary across ecoregions? and 2. What is the regional sensitivity of PSM timing to earlier DSD associated with warming and drying scenarios? All Western U.S. ecoregions showed significant relationships between the timing of PSM and DSD. Changes in the timing of PSM based on warming predicted for the middle and end of the 21st century ranged from 1-9 days and 6-17 days among ecoregions, respectively. The maritime ecoregions PSM timing were 2-3 times more sensitive to warming and drying versus the interior mountain ecoregions. This work suggests that soil hydrology modifies the effects of earlier snowmelt on regional streamflow response and vegetation water stress.
    Geophysical Research Letters 09/2015; DOI:10.1002/2015GL065855 · 4.20 Impact Factor
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    • "All rights reserved. water equivalent (SWE), earlier snowmelt, decreased spring snow cover extent, and shortened snow cover duration [Mote et al., 2005; Stewart et al., 2005; Knowles et al., 2006; Kapnick and Hall, 2012]. Changes in western U.S. hydrology in the latter half of the 20 th century, including snowpack, have been largely attributed to human-induced climate change [Barnett et al., 2008]. "
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    ABSTRACT: Projected warming will have significant impacts on snowfall accumulation and melt, with implications for water availability and management in snow-dominated regions. Changes in snowfall extremes are confounded by projected increases in precipitation extremes. Downscaled climate projections from 20 global climate models were bias corrected to montane Snowpack Telemetry stations across the western United States to assess mid-21st century changes in the mean and variability of annual snowfall water equivalent (SFE) and extreme snowfall events, defined by the 90th percentile of cumulative 3-day SFE amounts. Declines in annual SFE and number of snowfall days were projected for all stations. Changes in the magnitude of snowfall event quantiles were sensitive to historical winter temperature. At climatologically cooler locations, such as in the Rocky Mountains, changes in the magnitude of snowfall events mirrored changes in the distribution of precipitation events, with increases in extremes and less change in more moderate events. By contrast, declines in snowfall event magnitudes were found for all quantiles in warmer locations. Common to both warmer and colder sites was a relative increase in the magnitude of snowfall extremes compared to annual SFE and a larger fraction of annual SFE from snowfall extremes. The coefficient of variation of annual SFE increased up to 80% in warmer montane regions due to projected declines in snowfall days and the increased contribution of snowfall extremes to annual SFE. In addition to declines in mean annual SFE, more frequent low snowfall years and less frequent high snowfall years were projected for every station. This article is protected by copyright. All rights reserved.
    02/2015; 51(2). DOI:10.1002/2014WR016267
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    • "The snowpack and streamflow changes reported in the interior western U.S. (Clow, 2010; Nayak et al., 2010; Harpold et al., 2012) are consistent with regional and global trends in earth surface temperature change but may also be attributable, in part, to the effects of aeolian dust deposition on mountain snowpacks (Painter et al., 2010). According to model projections, increasing trends in aridity and temperature in the western U.S. will continue and intensify in the coming century (Brown and Mote, 2009; Seager and Vecchi, 2010; Kapnick and Hall, 2012). These trends bring a high likelihood of widespread vegetation change and greater aeolian dust fluxes (Westerling et al., 2006; Logan et al., 2010; Anderegg et al., 2011; Munson et al., 2011), which may act as a positive feedback for further hydroclimatic changes in the region. "
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    ABSTRACT: Dust deposition lowers the albedo of snow and can significantly alter snowpack energy balance. Investigation of aeolian dust deposition in the mountains of the western U.S. has shown that these effects advance the timing of snowpack melt and spring runoff across much of the region. These studies have primarily focused on alpine snowpacks with little to no overstory vegetation. To evaluate the impacts of aeolian dust on ecohydrological processes in forests, we conducted a manipulative experiment in a subalpine conifer forest in Utah's Wasatch Mountains. During the spring of 2010–2012, we added dust to the snow surface in forested plots every 1 to 2 weeks, roughly doubling the natural dust loading. We then measured snowpack ablation in control and dust addition plots, along with below-snowpack and warm season soil temperature (Tsoil), soil water content (θ), litter decomposition rate (D), soil respiration rate (Rs), and tree xylem water potential (ψ). Differences in ablation between control and dust addition plots were similar in magnitude to differences associated with the canopy structure of the forest. Seasonal patterns in Tsoil and θ were similar between dust treatments and canopy structure groups. D, Rs, and ψ varied little between dust treatments, but there were significant differences between years. During our three-year study, an unusual level of interannual variability in snowfall had the greatest effect on the soil environment and ecosystem processes. The effects of aeolian dust on snowpack mass and energy balance in our forest were slightly smaller than those associated with canopy structure. This article is protected by copyright. All rights reserved.
    Ecohydrology 10/2014; 8(6). DOI:10.1002/eco.1558 · 2.43 Impact Factor
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