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

ABSTRACT 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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    • "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; DOI:10.1002/eco.1558 · 2.43 Impact Factor
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    • "These areas are in the process of undergoing a major hydrologic shift as the phase of wintertime precipitation changes from predominantly snow to rain [Knowles et al., 2006; Abatzoglou, 2011]. Observations support changes in hydrologic indicators dependent on precipitation phase, including: widespread decreased spring snowpack [Mote et al., 2005; Mote, 2006; Bales et al., 2006; Knowles et al., 2006; Pederson et al., 2011; Kapnick and Hall, 2012], increased rain-on-snow flood risk [McCabe et al., 2007], and earlier snowmelt-driven streamflows in mountain catchments [Cayan et al., 2001; Barnett et al., 2005; Regonda et al., 2005; Bales et al., 2006; Luce and Holden, 2009; Nayak et al., 2010; Fritze et al., 2011]. "
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    ABSTRACT: This study investigates the extent of the rain-snow transition zone across the complex terrain of the western United States for both late 20th century climate and projected changes in climate by the mid-21st century. Observed and projected temperature and precipitation data at 4 km-resolution were used with an empirical probabilistic precipitation phase model to estimate and map the likelihood of snow versus rain occurrence. This approach identifies areas most likely to undergo precipitation phase change over the next half century. At broad scales, these projections indicate an average 30 percent decrease in areal extent of winter wet-day temperatures conducive to snowfall over the western United States. At higher resolution scales, this approach identifies existing and potential experimental sites best suited for research investigating the mechanisms linking precipitation phase change to a broad array of processes, such as shifts in rain-on-snow flood risk, timing of water resource availability, and ecosystem dynamics.
    07/2014; 41(13). DOI:10.1002/2014GL060500
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