Snow in a Warming World
Snowfall and snow cover have direct effects on transportation, soil freeze/thaw cycles, water availability, flood frequency, water quality, wildlife, forest fires and more.
Far from being just a passive product of prevailing climatic conditions, snow cover also influences climate by changing the surface albedo, the amount of solar radiation a surface reflects. The presence of snow, because snow cover reflects more sunlight than bare ground, changes the surface conditions that influence how the atmosphere around us moves – which affects the weather responsible for the snow itself.
Because of snow’s influence on our daily activities and weather, it is important to understand the mechanics behind this phenomenon, as well as why snowfall and snow cover vary from region to region and from year to year.
How do Changes in Climate Affect Snowfall and Snow Cover?
What Causes Snowfall to Vary from Year to Year?
Where Does it Snow and Why?
Two “ingredients” are necessary for snowfall to occur:
- Temperatures between the cloud base and the ground must be around or below freezing;
- A sufficient amount of moisture must be present in the air.
When one of these ingredients is lacking, it becomes the limiting factor for snowfall. How much of each ingredient is present will vary from region to region and from month to month.
Left: Temperatures below or near freezing throughout the lower part of the atmosphere are a necessary condition for snowfall.
Image Courtesy of NOAA.
For example, in the Pacific Northwest, with a prevailing jet stream and surface winds blowing in from the relatively warm Pacific Ocean, winters generally have lots of moisture but not the cold Arctic air that pushes temperatures below freezing. Only the higher elevations (around 2,500 feet and higher) consistently receive snowfall. As result, the region’s mountains hold some of the world’s largest snow accumulations, while in the valleys, which may be only a few dozen miles away, snow is rare. Every ten years or so, a north wind from Canada will pump cold air into the region, giving sea-level cities like Seattle significant snow.
The opposite situation is prevalent in the upper Midwest, which during the winter generally receives a flow of cold air from Canada that keeps temperatures well below freezing. Because this flow originates from land and not the ocean, there is often not enough moisture in the air for snow to occur. Intrusions of warm air from the Gulf of Mexico as well as moisture picked up from the Great Lakes occasionally provide the humidity necessary for snow to form there.
How Do Changes in Climate Affect Snowfall and Snow Cover?
Changes in the frequency and intensity of snowfall occurred over the last century as the world warmed. Because snow’s limiting factors differ from region to region, discussions on the effects a warming trend will have on snowfall are most appropriately conducted with a regional focus. Warming trends do not necessarily mean the whole U.S., a geographic unit with many distinct climate zones, should expect either more or less snowfall.
Where is More Snow Falling?
The Great Lakes Region: As the cold late fall and early winter winds from Canada blow through the Great Lakes region, they pick up moisture from the Great Lakes and deposit it downwind as snow in places like Marquette, Mich. (which gets an average or 180 inches per year) and Syracuse, N.Y. (which gets an average of 120 inches per year) – two of the snowiest places in the U.S. Until the latter part of the 20th century, this lake effect snow would stop around mid-winter in most years when the ice cover had grown across the lake surfaces, thus cutting off the snow’s source of moisture. Today, surface waters in Lake Michigan-Huron are 3.6 degrees Fahrenheit warmer than they were in the late 1970′s and Lake Superior’s surface is 5.3 degrees warmer. The average annual extent and duration of ice cover on these lakes has declined. Lake Superior’s average winter ice cover has fallen from 25 to 15 percent over the last 30 years. Snowfall trends for areas downwind from the lakes climbed during this period when open water during the winter became more prevalent. Syracuse, N.Y., for example, gets an average of 50 percent more snow each winter than it did in the early 20th century.
Left: Trend in the average annual extent of Lake Superior’s ice cover.
Image Courtesy of Austin, JA and Colman, SM, 2008.
The East (Lee) Side of the Rocky Mountains: A combination of more upslope events (i.e. an increase in the number of frontal systems moving across and dumping precipitation over the Rocky Mountain region) and the general increase the region’s cold season humidity have both been correlated with an observed increase in snowfall. This increase is most pronounced in the eastern parts of Colorado and New Mexico.
Above: Snowfall trends for 1930-31 to 2006-07, with dotted areas indicating negative trends and white areas positive trends. Numbers are the magnitude of the respective trends, or the per year percentage departures from the 1930-31 to 2006-07 mean.
Image Courtesy of Kunkel, et al., 2009
Where is Less Snow Falling?
The Southern Snowfall Margin: Oklahoma, Arkansas, Tennessee and North Carolina are considered to be at the southern margin of where wintertime temperatures are cold enough for snow to form. Areas near this southern margin experienced declines in snowfall over the 20th century.
The West: Most of the West’s water supply originates from snowpack. How much snow falls during the winter, when the snow melts and how quickly that melt happens are vital factors for the region’s water resources. Warm, northern-moving currents off the Pacific Coast bring ample moisture to the West during winter. Warming temperatures have corresponded to a decrease in the proportion of annual precipitation falling as snow, decreases in mountain snowpack accumulation and an increase in the elevation of the snowline (the elevation where conditions become cold enough for rainfall to turn to snowfall). Snowfall trends have been particularly pronounced in the Pacific Northwest (PNW), with some mountain stations now reporting snowfall totals less than half of what was reported in the 1930′s. The proportion of annual precipitation falling as snow in the PNW has been declining at a rate of almost nine percent per decade since the 1950′s. Warmer temperatures and less late season snowfall are causing snow cover to melt earlier in the year. In the PNW, the date when snow starts to melt now happens an average of 16 days earlier in the year; in California and Nevada it is happening about nine days earlier. As a result, for the West as a whole, the average date when the spring “pulse” of meltwater is first observed in streams is happening about 20 days earlier than it did in the 1950′s. Earlier snowmelt and peak-annual river flows pose challenges for water managers in the West.
Above: Linear 1950-1997 trends for snow water equivalent, or the amount of snow remaining on the ground, on
April 1 when the melt season across most of the West is underway.
Image Courtesy of Mote, et al. , 2005.
What Causes Snowfall to Vary from Year to Year?
Weather variability is controlled by a variety of cycles: the daily cycle, the annual cycle and the millennial orbital cycles that are believed to largely drive the ice ages. Multi-annual cycles, during which concentrations of heat in the oceans and atmosphere “move around,” occur on periods between the annual cycle and the millennial cycles and are possibly influenced by cycles in solar output occurring on periods of around 11 years and 80 years. A few multi-annual cycles have been identified as particularly important for understanding America’s year-to-year snowfall variability.
The El Niño-Southern Oscillation: Probably the strongest and most ubiquitous single source of interannual variability, the El Niño-Southern Oscillation (ENSO) is a change in the heat distribution in the tropical Pacific Ocean. The eastern tropical Pacific cools and warms over a period of three to seven years. When it is cool, it is considered to be in a La Niña phase; when it is in a warm, it is considered to be in an El Niño phase.
This cooling and warming affect the circulation in the upper atmosphere and the strength and position of the Northern Hemisphere storm tracks that bring winter storms to the U.S. La Niña phases tend to push the storm tracks north, bringing more winter storms to the northern U.S. El Niño events mean a more southern storm track and more wintertime moisture for the southern U.S. These variations in the position of the storm tracks mean variations in the regional frequency of the winter storms that often accompany snowfall.
El Niño phases correspond to more frequent East Coast Winter Storms, or Nor’easters, which form around Cape Hatteras and travel north along the East coast, bringing high winds and snowfall.
The Arctic Oscillation: In the Northern Hemisphere, how much atmospheric mass is (or how many air molecules are) concentrated at the mid-latitudes and the poles varies from day to day and from year to year. This is reflected in changes in the difference in atmospheric pressure between the mid- and high latitudes, which influences weather in the U.S. Specifically, a greater difference in atmospheric pressure means a stronger jet stream. A stronger jet stream works to “block” the Arctic air masses that would otherwise descend into mid-latitudes and the U.S. A weaker jet stream weakens the blocking, allowing the frigid air to penetrate further south.
When the pressure difference between mid and high latitudes is large, the Arctic Oscillation (AO, the term for this shifting of atmospheric mass) is considered positive and the high latitudes are cooler than normal and the mid-latitudes warmer than normal. When the pressure difference is small, the AO is negative and the higher latitudes are warmer than normal and the mid-latitudes cooler than normal. Another way to remember this is a positive AO relates to positive temperature anomalies in the U.S. and a negative AO relates to negative temperature anomalies in the U.S.
Cities like Chicago and Boston have many more extremely cold days during negative AO phases. The recent late-December to early-January cold snap occurred when the AO was strongly negative. Because it controls how much cold Arctic air reaches the mid-latitudes, where this air mixes with warm and wet air masses, the state of the AO has implications for snowfall in America. In generally snow-deprived Atlanta, there are five times as many days when trace snowfall occurs during negative phase winters versus positive phase winters.
The Pacific Decadal Oscillation: The Pacific Decadal Oscillation (PDO) describes changes in the distribution of sea surface temperatures in the North Pacific on a period of 50-60 years. Like ENSO, this sea surface temperature shift affects atmospheric circulation. The effects of this shift are most pronounced on weather in the western U.S.
Warm phases of the PDO generally shift the storm track northward, giving areas like Alaska more wintertime precipitation and areas like the Pacific Northwest less. Cool phases do roughly the opposite. Warm phases also mean warmer temperatures in the Pacific Northwest, so more of the already reduced precipitation falls as rain instead of snow. Reduced snowpack during warm years results.
Above: North Pacific surface temperature anomalies during positive and negative phases of the Pacific Decadal Oscillation.
Image Courtesy of NOAA.
Oscillation Interaction: How snowfall in the U.S. varies from year to year depends largely on how the different phases of these oscillations interact. For example, a winter with an El Niño driven southerly storm track and strong intrusions of Arctic air driven by a negative AO is likely to have above normal snowfall in the mid-Atlantic and southern snowfall margins. On the other hand, these same regions are not likely to receive much snow during a winter with a La Niña driven northerly storm track and a positive AO.
Conclusion
The specific factors that bring together the conditions necessary for snow vary from region to region. Understanding these regional differences is necessary to understand how snowfall can vary as the global climate changes. Some regions have experienced more snowfall during the recent warming trend while other regions are now receiving less. Multi-annual oscillations and the interactions between these oscillations are key variables in regional snowfall occurrence.
Special thanks to Joe Witte and Steve Tracton for their contributions to this paper.
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