Earth Gauge » Precipitation

Climate Number: 510 Years

kraus — Mon, 01 Mar 2010 14:49:22 +0000

Better understanding how fluctuations in climate have affected regional wildfire frequency over the past few centuries may help to improve our ability to predict severe wildfire seasons. Some of the West’s older groves have experienced dozens of wildfires over the past few centuries. The trees that survived these fires recorded black scars in their annual tree rings, providing us with the ability to know what year a given area burned. Core samples (profiles of the annual rings taken from a part of the tree trunk) are used to figure out when and where of fires occurred in the past. Enough of these fire scars from a long enough period of time make it possible to investigate the possible links between large scale climate variability and fire in given regions of the West. A recent investigation using core samples from hundreds of trees across the West found that fire years are more common during drought years and the occurrence of drought is controlled by large-scale movements of water in the oceans. Specifically, different phases of the El Nino-Southern Oscillation, the periodic warming and cooling of the eastern tropical Pacific, and the Atlantic Multidecadal Oscillation, the periodic warming and cooling of the North Atlantic, influence the frequency of fire in the West. Fire in all regions of the West is more common when the North Atlantic is warmer. The Pacific Northwest has more fires when the eastern tropical Pacific is warmer and the Southwest has more fires when the eastern tropical Pacific is cooler than average. The oldest trees used in this study had fire records going back to 1400 CE, 510 years ago.

For Comparison: The Mongol Empire was at its peak 510 years ago, with its soldiers sacking cities as far west as Damascus. In Florence, Italy, the Medici family was building the banking empire that would revolutionize western finance.

Seasons: Winter, Spring, Summer

Sources: Kitzberger, T et al. “Contingent Pacific-Atlantic Ocean influence on multicentury wildfire synchrony over western North America.” Proceedings of the National Academy of Sciences 104 (2007): 543-548 and Trouet, V et al. “Fire-climate interactions in the American West since 1400 CE.” Geophysical Research Letters 37 (2010): L04702.

Climate Fact: Midwinter Storm Track Suppression

kraus — Mon, 22 Feb 2010 15:15:29 +0000

The temperature/pressure difference between the equatorial regions and the poles is at its maximum during the winter months. The energy this difference generates is thought to power the “storm tracks,” or the bands in the mid-latitudes where east to west traveling storms (cyclonic high and low pressure systems) are most common. The storm track over the Pacific brings the western U.S. ample rainfall for much of the fall, winter and spring seasons. One aspect of Northern Hemisphere winter storm behavior that has been somewhat of a mystery is the midwinter suppression of the Pacific storm track. The maximum latitudinal temperature/pressure difference during winter, which means more power for the storms, is reflected in the Atlantic storm track being at its strongest during the winter months. Over the Pacific, however, the midwinter corresponds to an average decrease in the number and strength of these storms by 20 and 14 percent respectively compared to the fall and spring months. One possible explanation for this suppression is a wintertime drop in the number of atmospheric disturbances that make their way from the mountains of central Eurasia to the Pacific. These disturbances can become the storms that move across the Pacific to North America. The sheer size of the Eurasian landmass means that the wintertime high pressure centers that sit in the middle of the continent are strong enough to push the warmer and competing air masses far to the south and away from the mountains where these warmer air masses can generate the disturbances that can ultimately become the Pacific storms.

Season: Winter

Source: Penny, S et al. “The Source of the Midwinter Suppression in Storminess over the North Pacific.” Journal of Climate 23 (2010): 634-648.

Climate Fact: North American and Eurasian Snow

kraus — Mon, 22 Feb 2010 15:13:48 +0000

Snow is both a product of the weather and a weather maker. It has long been recognized that snow exhibits a cooling effect on local and regional scales. Snow reflects more sunlight than bare ground, meaning that it absorbs less energy. More snow cover also means soils stay moist for longer following the spring melting than they otherwise would. More soil moisture means that more of the sun’s energy that would have been spent heating the ground is instead spent evaporating water. In North America, lots of snow over the continent affects the storm track, or the latitudinal band where travelling cyclonic high and low pressure systems are most common. More specifically, the cold temperatures the snow cover induces eventually force the storm track over North America to veer south. As it does this, the storm track downstream in Eurasia veers north, allowing warmer air masses to penetrate further into the continent than they otherwise would. The presence of these warmer air masses generally means less snow there. Thus, years of above average snow cover in North America tend to be years of below average snow cover in Eurasia.

Seasons: Winter, Spring

Source: Sobolowski, S et al. “Modeled Climate State and Dynamic Responses to Anomalous North American Snow Cover.” Journal of Climate 23 (2010): 785-799.

Snow in a Warming World

kraus — Fri, 19 Feb 2010 21:09:51 +0000

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.

Where Does it Snow and Why?

How do Changes in Climate Affect Snowfall and Snow Cover?

Where is More Snow Falling?

Where is Less Snow Falling?

What Causes Snowfall to Vary from Year to Year?

Conclusion

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.

Sources

Austin, JA and Colman, SM. “A century of temperature variability in Lake Superior.” Limnology and Oceanography 53 (2008): 2724-2730.

Austin, JA and Colman, SM. “Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: A positive ice-albedo feedback.” Geophysical Research Letters 34 (2007): L06604.

Burnett, AW et al. “Increasing Great Lake-Effect Snowfall during the Twentieth Century: A Regional Response to Global Warming.” Journal of Climate 16 (2003): 3535-3542.

DeGaetana, AT et al. “Statistical Prediction of Seasonal East Coast Winter Storm Frequency.” Journal of Climate 15 (2002): 1101-1117.

Eichler, T and Higgins W. “Climatology and ENSO-Related Variability of North American Extratropical Cyclone Activity.” Journal of Climate 19 (2006): 2076-2093.

Global Glacier Retreat Project. Nichols College. Accessed Online 5 July 2007 http://www.nichols.edu/departments/Glacier/glacier_retreat.htm.

Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson,(eds.). Cambridge University Press, 2009.

Groisman, P.Y., P.W. Knight, and T.R. Karl. “Heavy precipitation and high streamflow in the United States: Trends in the 20th Century.” 82 (2001): 219-246.

Hanrahan, JL et al. “Connecting past and present climate variability to the water levels of Lakes Michigan and Huron.” Geophysical Research Letters 37 (2010): L01701.

Hirsch, ME et al. “An East Coast Winter Storm Climatology.” Journal of Climate 14 (2001): 882-899.

Kunkel, KE et al. “Trends in Twentieth-Century U.S. Snowfall Using a Quality-Controlled Dataset.” Journal of Atmospheric and Oceanic Technology 26 (2009): 33-44.

Mote, P.W., A.F. Hamlet, M.P. Clark, and D.P. Lettenmaier. “Declining mountain snowpack in western North America.” Bulletin of the American Meteorological Society. 86 (2005): 39–49.

National Oceanic and Atmospheric Administration: Climate Prediction Center. Accessed Online 7 December 2009 (http://www.cpc.ncep.noaa.gov/products/precip/CWlink/stormtracks/eisdiffobs.meta.gif).

The National Park Sevice. North Cascades National Park Complex: Glacial Monitoring Program. Accessed Online 10 July 2007 http://www.nps.gov/noca/naturescience/glacial-mass-balance1.htm

Thompson, David W.J. “Regional Climate Impacts of the Northern Hemisphere Annular Mode.” Science 293 (2001): 85-89.

United States Geologic Survey. “Fifty-Year Record of Glacier Change Reveals Shifting Climate in the Pacific Northwest and Alaska, USA.” 6 July 2009. Accessed Online 7 August 2009 http://pubs.usgs.gov/fs/2009/3046/

USA Today. “Answers archive: Winter, snow, ice.” http://www.usatoday.com/weather/resources/askjack/wasnow.htm

van Mantgem, PJ et al. “Widespread Increase of Tree Mortality Rates in the Western United States.” Science 323 (2009): 521-524.

Weng, H. “The influence of the 11 yr solar cycle on the interannual-centennial climate variability.” Journal of Atmospheric and Solar – Terrestrial Physics 67 (2005): 793-805.

Climate Number: Five Trillion Gallons

kraus — Mon, 01 Feb 2010 15:39:54 +0000

While commonly considered two separate lakes, Lakes Michigan and Huron are actually hydrologically one body of water – they are connected at the Straits of Mackinaw and rise and fall in unison. Since 1980, Lake Michigan-Huron has been warming and annually averaged surface temperatures are now 3.6 degrees Fahrenheit warmer than they were in the late 1970’s. This warming has been accompanied by less wintertime ice cover and more evaporation from the Lake’s surface. While about half of this 25 percent increase in evaporation is accounted for by increases in summertime evaporation, more water is evaporating from the lakes during the spring, fall and winter seasons as well. The decline in ice cover means that there is now more liquid water exposed to late fall and winter winds, which pick up the moisture from the Lake and deposit it down-wind in the form of lake effect snow. Increases in Great Lake lake effect snow have been observed during this same period when ice cover has declined. This 25 percent annual increase in evaporation from the Lake means that over five trillion more gallons of water are going into the atmosphere from the lakes each year than during a typical year in the 1970’s. This increase helps to explain why lake levels have been declining over the last few decades despite above average regional precipitation totals. Water levels in Lake Michigan-Huron have been hovering around long-term record lows since 2000.

For Comparison: Five trillion gallons is about the same amount of water stored in the reservoir created by China’s Three Gorges Dam on the Yangtze River. Five trillion gallons is also about how much water is allocated annually to seven western states and Mexico under the 1922 Colorado Water Compact, which is based on estimates of the annual flow of the Colorado River at that time.

Seasons: Winter, Spring, Summer, Fall

Sources: Hanrahan, JL et al. “Connecting past and present climate variability to the water levels of Lakes Michigan and Huron.” Geophysical Research Letters 37 (2010): L01701 and National Geographic: Global Action Atlas. Colorado River Project Summary (2009). Accessed Online 1 February 2010 and Dangerfield, Whitney. “Snapshot: Yangtze River.” Smithsonian.com 1 September 2007. Accessed Online 1 February 2010 and Source: Burnett, AW et al. “Increasing Great Lake-Effect Snowfall during the Twentieth Century: A Regional Response to Global Warming.” Journal of Climate 16 (2003): 3535-3542.

Climate Number: $16.3 Billion

kraus — Mon, 01 Feb 2010 15:37:37 +0000

When put in 2000 US dollars, freezing rain (ice storm) events in America caused an estimated 16.3 billion dollars in total losses between 1949 and 2000 due to downed power lines, downed trees, agricultural losses, transportation accidents and medical costs from injuries due to slippery conditions. Freezing rain events are most frequent in the Northeast, but are also common across the Midwest and Piedmont regions from North Carolina northward. When freezing rain events hit the Southeast they tend to be accompanied by high dewpoints. This means that while ice storms are rarer in the Southeast, they tend to be heavy and particularly damaging when they do hit. Records kept since the late 1920’s show that ice storms were the least frequent during the 1930’s and rose to a peak in the early 1950’s, showing little or no trend thereafter.

For Comparison: 16.3 billion dollars is around the same amount collectively pledged by all participating parties to fight global poverty following the 2008 U.N. anti-poverty summit. It is also roughly the same amount as NASA’s annual budget.

Seasons: Winter

Source: Houston, TG et al. “Freezing rain events: a major weather hazard in the conterminous United States.” Natural Hazards 40 (2007): 485-494.

Climate Fact: Antarctica’s Moisture Sources

kraus — Tue, 05 Jan 2010 21:35:41 +0000

Once water is evaporated from the ocean or a moist land surface, it may spend days traveling through the air. Complicated systems of winds at different levels of the atmosphere can transport moisture (as well as other gases and dust) from the point of origin to remote locations thousands of miles away. While about 30 percent of the moisture that rains or snows over Antarctica originates in the Southern Ocean close to the continent, the rest comes from latitudes north of 50 degrees South (about the same latitude as the southern tip of New Zealand). Ten percent comes from north of 30 degrees South (about the same latitude as Durban, South Africa). The higher elevations closer to the center of Antarctica have mean moisture origin sources north of 44 degrees South. During the summer, when there is less sea ice, more of Antarctica’s precipitation originates from the waters around the continent.

Seasons: Winter, Spring, Summer, Fall

Source: Sodemann, H and Stohl, A. “Asymmetries in the moisture origin of Antarctic precipitation.” Geophysical Research Letters 36 (2009): L22803.

Climate Fact: Regional Snow Trends

espinoza — Mon, 21 Dec 2009 11:49:21 +0000

In Brief: Higher temperatures are reducing America’s snowfall, with a few regional exceptions.

Snow is not just an inhibitor of holiday travelers, nor is it just a passive product of prevailing weather conditions. Snow is a weather maker in and of itself. Snow-covered ground reflects far more of the sun’s radiation than it otherwise would and more reflection means lower temperatures at the surface. Because surface temperatures influence atmospheric circulation, understanding trends in snow and ice cover are crucial for effectively modeling weather and climate. Records indicate that as temperatures have risen, the southern margin where snow falls has moved north. This means that less snow is falling/accumulating in the mountains of the Western U.S., the Mid-Atlantic and the Kansas-Missouri regions. In the Western U.S., where 75 percent of year-round water sources originate from snow melt in the mountains, the trends have been particularly pronounced. Parts of the Northwest are receiving about half of the snowfall they received in the 1930’s. New England’s snowfall trends have been relatively flat. On the other hand, higher temperatures mean more moisture in the air. Regions such as the eastern side of the Rocky Mountains, particularly the eastern parts of Colorado and New Mexico, are now getting more snowfall as it is still cold enough for snow and there is more water in the air. The Great Lakes/northern Ohio Valley regions are also getting more snow, which is probably due to less ice cover on the Great Lakes. Less ice cover makes more lake effect snow possible.

(Source: Kunkel, KE et al. “Trends in Twentieth-Century U.S. Snowfall Using a Quality-Controlled Dataset.” Journal of Atmospheric and Oceanic Technology 26 (2009): 33-44.)

Climate Fact: Lake Effect Snow on the Upswing

espinoza — Mon, 21 Dec 2009 11:31:10 +0000

In Brief: Less ice cover on the Great Lakes is contributing to more snow regional lake effect snow.

Over much of the U.S., the 20th century warming trend means less snow and more rain. In most areas, the lack of cold limits snowfall, but this is not true in the Great Lakes region. Here, temperatures are below freezing throughout much of the year and it is lack of moisture that limits snowfall. Much of the moisture that makes up the region’s snow comes from the Great Lakes. As frigid winds from the Canadian interior pass over the relatively warmer waters of the Great Lakes, the cold air picks up moisture which is deposited as snow in areas downwind – a phenomenon termed “lake effect snow.” Two of these downwind areas rank as the third and fourth snowiest areas in the United States – Marquette, Michigan (180 inches annually) and Syracuse, NY (120 inches annually). Once the lakes freeze, however, there is no more exposed water and the moisture source disappears.

Throughout the region, lake effect snow levels have been increasing, a trend linked to warmer lakes with less ice. Since at least the 1970’s, ice cover on the Great Lakes has been declining.
Ice cover on Lake Superior, which has been warming by 1.2 degrees Fahrenheit per decade since 1985, used to cover on average 25 percent of the lake area but now covers less than 15 percent. Water temperatures in all of the lakes have been rising. October-April temperatures averaged over all the Lakes rose by one degree Fahrenheit between 1995 and 2000.
In Syracuse, NY, snowfall levels increased by 50 percent between 1913 and 2000.

(Source: Burnett, AW et al. “Increasing Great Lake-Effect Snowfall during the Twentieth Century: A Regional Response to Global Warming.” Journal of Climate 16 (2003): 3535-3542.)

Climate Trivia: East Coast Winter Storm Frequency and ENSO

kraus — Mon, 07 Dec 2009 15:59:51 +0000

December is East Coast Winter Storm (ECWS) season. These storms are powered by warm water that flows from the Gulf Stream. The Gulf Stream current flows along the Eastern Seaboard past Florida and the Carolinas before reaching Cape Hatteras, where the warm water heads out into the Atlantic. ECWS’s travel northward along the coast causing high winds and coastal property damage comparable to hurricanes. They also bring heavy snowfall, causing further weather complications. On average, there are 12 ECWS’s during the December to February season, with January being the most active month. One of the best predictors of how intense an ECWS season will be is the ocean temperature along the coast of the southeastern U.S. during the previous summer (Gulf of Mexico temperatures were above average this past summer). The warmer these waters are, the stronger the Gulf Stream generally is and the more active the winter storm season will be. Interestingly, conditions in the eastern tropical Pacific affect ECWS activity as well. What eastern tropical Pacific conditions are most conducive to an active ECWS season?

a)   El Niño conditions (warmer eastern tropical Pacific SSTs)
b)   La Niña conditions (cooler eastern tropical Pacific SSTs)
c)   Neutral Conditions (average eastern tropical Pacific SSTs)

The correct answer is a. More active ECWS seasons tend to coincide with El Niño years. This is in contrast to Atlantic hurricane season trends, as El Niño conditions tend to suppress Atlantic Hurricane formation. Over the second half of the 20th century, the frequency of ECWS events showed little trend, but the storms did become slightly more intense.

Season: Winter

Sources: DeGaetana, AT et al. “Statistical Prediction of Seasonal East Coast Winter Storm Frequency.” Journal of Climate 15 (2002): 1101-1117 and Hirsch, ME et al. “An East Coast Winter Storm Climatology.” Journal of Climate 14 (2001): 882-899 and Eichler, T and Higgins W. “Climatology and ENSO-Related Variability of North American Extratropical Cyclone Activity.” Journal of Climate 19 (2006): 2076-2093 and National Oceanic and Atmospheric Administration: Climate Prediction Center. Accessed Online 7 December 2009 http://www.cpc.ncep.noaa.gov/products/precip/CWlink/stormtracks/eisdiffobs.meta.gif

Climate Trivia: ENSO and Regional Rainfall (Northwest)

kraus — Mon, 07 Dec 2009 15:58:02 +0000

Winter storm season is here. Storms will be blowing in from the Pacific, bringing rainfall to lower elevations and snow to the mountains. This year, the eastern tropical Pacific is in an El Niño phase, meaning that its waters are warmer than average. When the eastern Pacific is in an El Niño phase, the northwestern U.S. can expect:

a)   more than normal rainfall
b)   less than normal rainfall
c)   about average rainfall

The correct answer is b. The Northwest gets less than normal rainfall during El Niño winters and greater than normal rainfall during La Niña winters, when the colder waters in the eastern Pacific cause the Pacific storm track to shift north and hit us. The storm track will spend most of its time south of us this winter, thus giving us below average rainfall through about April.

View a schematic diagram of how El Niño and La Niña events affect wintertime rainfall and temperature: http://www.earthgauge.net/climate-facts-image-library#5. For more information on El Niño, including seasonal forecasts by region, visit: http://www.elnino.noaa.gov/. Learn more about what Northern Hemisphere storm tracks are and how they work: http://www.earthgauge.net/wp-content/CF_Storm%20Tracks.pdf.

Season: Winter

Sources: Eichler, T and Higgins W. “Climatology and ENSO-Related Variability of North American Extratropical Cyclone Activity.” Journal of Climate 19 (2006): 2076-2093 and National Oceanic and Atmospheric Administration: Climate Prediction Center. Accessed Online 7 December 2009 (http://www.cpc.ncep.noaa.gov/products/precip/CWlink/stormtracks/eisdiffobs.meta.gif

Climate Fact: ENSO and Regional Rainfall (South)

kraus — Mon, 07 Dec 2009 15:54:44 +0000

a)   more than normal rainfall and cooler temperatures
b)   less than normal rainfall and warmer temperatures
c)   about normal rainfall and about normal temperatures

The correct answer is a. The South gets more than normal rainfall during El Niño years and less than normal rainfall during La Niña years when the colder waters in the eastern Pacific cause the Pacific storm track to shift north and miss us. During El Niño, not only does the storm track head right for us, it is even stronger than it is during the La Niña phase. Another effect of El Niño is cooler than normal winters in the South from about Texas eastward.

View a schematic diagram of how El Niño and La Niña events affect wintertime rainfall and temperature: http://www.earthgauge.net/climate-facts-image-library#5. For more information on El Niño, including seasonal forecasts by region, visit: http://www.elnino.noaa.gov/. Learn more about what Northern Hemisphere storm tracks are and how they work: http://www.earthgauge.net/wp-content/CF_Storm%20Tracks.pdf.

Seasons: Winter

Climate Trivia: ENSO and Regional Rainfall (Southwest)

kraus — Mon, 07 Dec 2009 15:50:46 +0000

Winter storm season is here. Storms will be blowing in from the Pacific, bringing rainfall to lower elevations and snow to the mountains. This year, the eastern tropical Pacific is in an El Niño phase, meaning that its waters are warmer than normal. When the eastern Pacific is in an El Niño phase, the southwest U.S. can expect:

a)   more than normal rainfall
b)   less than normal rainfall
c)   about normal rainfall

The correct answer is a. The Southwest gets more than normal rainfall during El Niño years and less than normal rainfall during La Niña years when the colder waters in the eastern Pacific cause the Pacific storm track to shift north and miss us. During El Niño, not only does the storm track head right for us, it is even stronger than it is during the La Niña phase. Hopefully, this El Niño winter will give the Southwest some relief from the current dry and drought conditions.

Season: Winter

Climate Number: 20 Teragrams

kraus — Mon, 30 Nov 2009 16:26:28 +0000

On average, about 20 trillion grams (20 teragrams) of dust are suspended in Earth’s atmosphere, where the dust particles stay for an average of 21 days. Dust is an important part of Earth’s climate – dust affects how clouds develop and how much sunlight reaches the Earth, which affects rainfall. The Dust Bowl of the 1930’s, which began due to lack of rainfall, was made worse by farming practices that released lots of dust into the air. Also, when dust lands on ice such as snow and glaciers, it makes these masses darker and more vulnerable to melting. On the other hand, dust deposits fertilize both land plants and ocean algae.

For Comparison: 20 teragrams weighs about as much as two million Boeing 757-200’s.

Seasons: Winter, Spring, Summer, Fall

Sources: Grini, A et al. “Model simulations of dust sources and transport in the global atmosphere: Effects of soil erodibility and wind speed variability.” Journal of Geophysical Research 110 (2005): D02205 and Painter, TH. “Where Deserts and Mountains Collide: The Implications of Accelerated Snowmelt by Disturbed Desert Dust.” U.S. National
Academy of Sciences, Washington, DC. 24 June 2009 and Hoerling, M et al. “Distinct causes for two principal U.S. droughts of the 20th century.” Geophysical Research Letters 36 (2009): L19708.

Climate Fact: Wind, Rain, Tornadoes, Oh My

kraus — Fri, 13 Nov 2009 15:08:28 +0000

Along with heavy rains and high winds, the impacts of landfalling hurricanes and tropical storms also include more tornado formation. The larger the tropical cyclone and the longer it spends over land, the greater the probability that tornadoes will form as the system moves. Since 1995 in the Gulf of Mexico, hurricane strength has increased 35 percent compared to the 16 year period spanning 1948-1964 (considered to be the last active period for hurricanes). This increase in storm strength has corresponded to twice as many tornadoes produced per cyclone.

Seasons: Summer, Fall

Sources: Belanger, JI et al. “Variability in tornado frequency associated with U.S. landfalling tropical cyclones.” Geophysical Research Letters 36 (2009): L17805 and “Tornado Threat Increases As Gulf Hurricanes Get Larger.” ScienceDaily. 10 Sept. 2009. Web. 14 September 2009 .

Climate Fact: Atlantic and Pacific Niños

kraus — Mon, 02 Nov 2009 16:17:17 +0000

It has long been known that what happens in the tropical Pacific doesn’t just stay in the tropical Pacific. Much of the year-to-year variability in America’s weather, particularly winter weather, can be explained by conditions there. All other things being equal, warm El Niño conditions off the coast of equatorial South America mean a wetter southern U.S., while cool La Niña conditions mean a wetter northern U.S. This switch between warm and cool conditions is influenced by the strength of the winds that descend from the upper atmosphere and blow over the surface of the tropical Pacific. These winds help to “pull” cool waters from the ocean depths to the surface where they upwell off the coast of South America. The strength of these descending winds is in turn influenced by temperatures in the eastern tropical Atlantic Ocean, which has its own “Niños” and “Niñas”. In contrast to the Pacific, where El Niño and La Niña are most pronounced during the winter, the Atlantic events are most pronounced during the summer months. During warm Atlantic phases, the convection in the eastern tropical Atlantic Ocean is stronger, which affects the upper atmospheric winds all the way over to the Pacific. This strengthening in turn strengthens the descending winds over the tropical Pacific, which makes the formation of La Niña conditions more likely there during the winter months. Thus, summertime ocean temperatures in the tropical Atlantic Ocean can help predict the wintertime ocean temperatures in the tropical Pacific.

Seasons: Summer, Fall, Winter

Source: Rodr?guez-Fonseca, B et al. ”Are Atlantic Niños enhancing Pacific ENSO events in recent decades?” Geophysical Research Letters 36 (2009): L20705.

Climate Fact: Southern U.S. Drought Occurrence Linked to SST Variability: Causes of Northern Droughts Less Clear

kraus — Thu, 29 Oct 2009 19:23:51 +0000

Better drought prediction systems could potentially save billions of dollars. Current prediction systems largely rely on observations of the circumstances surrounding past droughts to understand the factors that led to drying. New research reveals that:
• Drought in the southern Great Plains states, such as the 1946-1956 drought, can be largely explained by persistently cool conditions in the eastern tropical Pacific.
• Drought in the northern Great Plains states, such as the 1930’s “Dust Bowl,” was likely caused by serendipitous atmospheric variability (i.e. a variety of atmospheric factors came together with the result being the Dust Bowl) and agricultural practices. The initial atmospherically forced drying resulted in dust storms that reinforced these dry conditions.

Predicting drought may be more difficult in the northern Great Plains than southern Great Plains, as the causes of drought in the former region are much more complex.

Seasons: Winter, Spring, Summer, Fall

Source: Hoerling, M et al. “Distinct causes for two principal U.S. droughts of the 20th century.” Geophysical Research Letters 36 (2009): L19708.

Climate Fact: North American Extremes

kraus — Mon, 05 Oct 2009 15:42:29 +0000

The concepts of weather extremes and thresholds are tightly coupled and important to remember when planning effective and reliable infrastructure. For example, just one day of extreme heat, even if it falls during a particularly cool summer, can cause railroad tracks to buckle and transportation systems to shut down. Extreme rainfall can have similar effects; highways are designed to function during moderate rainfall events, but underpasses may flood during extreme events. Two inches of rain in a 24-hour period is considered to be a threshold that, when exceeded, forces the average municipal water treatment facility to discharge untreated sewage into local surface waters, where it may poison wildlife and ultimately be hazardous to people in the municipality it serves. Over the past 40 years in North America (including Hawaii, Puerto Rico and the U.S. Virgin Islands), the average highest summertime maximum and minimum temperatures at weather stations have increased by 1.6 degrees Fahrenheit, while the lowest winter maximum and minimum temperatures have increased by 6.3 degrees Fahrenheit. The average amount of precipitation falling during the highest one-day and five-day precipitation event periods has also increased.

Seasons: Winter, Spring, Summer, Fall

Source: Peterson, TC et al. “Changes in North American extremes derived from daily weather data.” Journal of Geophysical Research: Atmospheres 113 (2008): D07113.

Climate Fact: Desert Expansion and Vegetation

kraus — Tue, 29 Sep 2009 18:51:40 +0000

Vegetation “feedbacks,” changes in vegetation as the climate changes, are an important component of Earth’s climate system. At high latitudes, warming tends to allow trees and shrubs to grow faster and expand into previously inhospitable locations. Because trees and shrubs are generally darker than the grasses they overshadow, they absorb more of the sun’s energy and cause the local environment to warm even more – a scenario that illustrates a “positive” vegetative feedback. Positive vegetative feedback was likely an important component of the deglaciations that occurred at the end of past ice ages. Another example of a positive vegetative feedback occurs around Earth’s arid subtropical regions. When subtropical forests dry, trees become scarcer, grass cover expands and forests turn into park-like savannahs. Savannahs absorb about ten percent less solar energy than forests. When less energy is absorbed, there is less convection and less upward motion of air masses. Less upward motion means that there is less moisture convergence at high altitudes and thus less rainfall and more drying, which can lead to the conversion of savannah to desert. Deserts absorb even less energy than savannahs do, further exacerbating this cycle. Attempts to model Earth’s climate without including positive vegetative feedback underestimate the 1950 to 2005 increase in desert cover, which is estimated to be around one million square miles (a ten percent increase over the 55-year period).

Seasons: Winter, Spring, Summer, Fall

Source: Zeng, N and Yoon, J. “Expansion of the world’s deserts due to vegetation-albedo feedback under global warming.” Geophysical Research Letters 36 (2009): L17401.)

Climate Fact: Seasonal Rainfall in the Southeast

kraus — Wed, 26 Aug 2009 17:01:53 +0000

Over the second-half of the 20th century, the Southeast experienced an overall decline in annual rainfall levels along with a 20 percent increase in the frequency of extreme (top first percentile) rainfall events. Looking at the 20th century as a whole, there have been significant changes in the seasonal distribution of precipitation, with strong increases in the amount of precipitation in the fall, noticeable decreases in the winter and slight declines during the spring and winter months.

To see how the seasonality of rainfall has changed in your local area, visit http://www.earthgauge.net/climate-facts-image-library#8. This image is featured in the “Global Climate Change Impacts in the United States” report recently published by the U.S. Global Change Research Program. The image is in the public domain.

Seasons: Winter, Spring, Summer, Fall

Source: Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson,(eds.). Cambridge University Press, 2009