Earth Gauge » Extreme Weather

Climate Fact: Jet Stream Trends

administrator — Wed, 08 Oct 2008 21:51:19 +0000

At the tropopause (the point in altitude where the lowest part of Earth’s atmosphere, the turbulent troposphere, transitions into the more stable stratosphere), which is located at about nine miles up, bands of 200 mile-per-hour air currents flow around the world while periodically meandering north and south. These air currents are known as jet streams. There are two types of jet streams, the more powerful polar jets, which generally flow between 35 and 65 degrees in latitude (depending on the season) and the less powerful subtropical jet streams, which are located at the poleward edges of the Hadley Cells, or the points where the air that rises from the Equatorial Region descends towards that surface. These jet streams control storm tracks as well as storm frequency and intensity. Their presence also works to suppress hurricane formation in the area over which they flow. Over the past three decades, their behavior has changed. As the edges of the Hadley Cells have expanded towards the poles, so has the Southern Hemisphere subtropical jet stream, which has weakened during this period. The Northern Hemisphere polar jet stream has weakened as well, and has also moved closer to the North Pole. In contrast, the Southern Hemisphere polar jet stream appears to be strengthening, especially during June, July, and August, and it has, on average, moved close to the South Pole. All of the jet streams have risen in altitude during this period, which corresponds to a general warming of the troposphere.

Seasons: Winter, Spring, Summer, Fall

Source: Archer, CL and Caldeira, K. “Historical trends in the jet streams.” Geophysical Research Letters 35 (2008): L08803

Climate Fact: Rodent Reorganization

administrator — Wed, 08 Oct 2008 20:55:25 +0000

In Southeast Arizona, there has been a marked increase in wintertime rainfall since 1977, which has resulted in a three-fold increase in shrub cover. Prior to this period, grasses had dominated the region’s vegetation system. As the shrubs have moved in, so have species of small pocket-mice, which are replacing large kangaroo rats. One species of kangaroo rat went locally extinct in 1994, and its disappearance was immediately followed by a colonization of pocket mice. Populations of these pocket mice have been growing exponentially since. An extreme rainfall event in 1999 caused flooding and helped to promote this general trend, as kangaroo rats are poor swimmers and more than 90 percent of their population in the area drowned during the flood.

Season: Wintertime

Source: Thibault, KM and Brown, JH. “Impact of an extreme climatic event on community assembly.” Proceedings of the National Academy of Sciences 105 (2008): 3410-3415.

Climate Fact: Spruce Beetle Surge

administrator — Wed, 08 Oct 2008 20:20:57 +0000

While forests fires may be the most visible and dramatic events that reshape North America’s forests, outbreaks of insect “pests” actually affect an area 45 times larger than that affected by fire. Generally, insects attack trees weakened by things like drought, wind storms, and fire, as healthy trees are usually able to fend-off attacks from pests. If the pests attack the tree in large enough numbers, however, the tree’s natural defenses can become overwhelmed. Spruce beetle populations become “outbreaks” when there is a large enough population of already weakened host trees (more than two clumps of five trees in a five acre area) and the right climatic conditions. In America’s Intermountain region, the right climate conditions for an outbreak include winter temperatures that do not drop below negative 29 degrees Fahrenheit (the temperature at which beetle larvae freeze to death), warmer fall temperatures (which allow the beetles to have more life cycles in a shorter period of time), and several years of drought (which weaken trees). Since 1976, there has been a 5.4 degree Fahrenheit increase in average wintertime temperatures and a 3.4 degree Fahrenheit increase in average autumn temperatures in the Intermountain Region. Also, the percentage of the western United States that is in drought condition has doubled over the past century.

Seasons: Winter, Spring, Summer, Fall

Sources: Logan, JA et al. “Assessing the impacts of global warming on forest pest dynamics.” Frontiers in Ecology and the Environment 1 (2003) 130-137 and Hebertson, EG and Jenkins, MJ. “Climate Factors Associated with Historic Spruce Beetle (Coleoptera: Curculionidae) Outbreaks in Utah and Colorado.” Environmental Entomology 37 (2008) 281-292 and “U.S. Temperature and Precipitation Trends.” U.S. National Oceanic and Atmospheric Administration (NOAA): Climate Prediction Center. 5 January 2005. 26 June 2008 < http://www.cpc.ncep.noaa.gov/charts.shtml> and United States. Climate Change Science Program. Weather and Climate Extremes in a Changing Climate. Synthesis Assessment Product 3.3: GPO. 2008.

Climate Fact: Midwest Rainfall Rise

administrator — Wed, 08 Oct 2008 20:11:03 +0000

A warmer atmosphere results in an “amplification” of the water cycle. Some areas of the world are net importers of rainfall (such as tropical rainforests), while some are net exporters (such as oceans around the tropics). The “amplification” of the cycle means that dry regions become drier, and wet regions become wetter. During the 20th century, total rainfall in the United States increased by about seven percent; the largest increases occurred in the central and eastern regions (net importing regions). Most of this increase can be accounted for by heavy and extreme precipitation events becoming even more intense. The amount of rain that falls during the heaviest one percent of rainfall events has increased by 20 percent over the last 100 years. The upper Midwestern U.S. has experienced a 50 percent increase in the number of days where it rains more than four inches.

Seasons: Spring, Summer, Fall

Sources: United States. Climate Change Science Program. Weather and Climate Extremes in a Changing Climate. Synthesis Assessment Product 3.3: GPO. 2008 and “U.S. Temperature and Precipitation Trends.” U.S. National Oceanic and Atmospheric Administration (NOAA): Climate Prediction Center. 5 January 2005. 26 June 2008 <http://www.cpc.ncep.noaa.gov/charts.shtml> and Soden, B., Wentz, F.J., Santer, B.D. and Zwiers F. “Climatically-Induced Increases in Water Vapor and Precipitation: Causation and Implications. ” United States Senate, Washington, D.C. 29 October 2007. Accessed Online 17 December 2007 <http://www.ametsoc.org/atmospolicy/ESSSarchiveclimatechange.html>

Climate Fact: Rainfall Declines in Southeast Australia

administrator — Wed, 08 Oct 2008 19:43:43 +0000

Autumn (March to May) rainfall in southeast Australia is important for soil moisture and river recharge because the region is dependent on reliable water sources for cereal crop production. Since 1950, there has been a 40 percent decline in the region’s average autumn rainfall. This has been linked to fewer occurrences of La Niña events, more El Niño events, and a change in the “pressure wave-train” circulation pattern that brings rainfall from the sub-tropical Indian Ocean to southeast Australia. The increased frequency of El Niño events and the change in the wave-train circulation pattern, as well as a rise in atmospheric pressure over the region, have been linked to rising sea-surface temperatures in the Pacific and Indian Oceans. The last two autumns have seen record low inflows into the Murray River, an important source of water for the region’s farmers.

Seasons: Spring, Summer

Sources: Commonwealth Scientific and Industrial Research Organisation (CSIRO): Media Center. “Understanding autumn rain decline in SE Australia.” 23 May 2008. Accessed Online 9 June 2008 <http://www.csiro.au/news/UnderstandingDeclineAutumnRain.html> and Wahlquist, Asa. “Dry future well ahead of schedule.” The Australian, 7 June 2008. Accessed Online 9 June 2008 <http://www.theaustralian.news.com.au/story/0,25197,23822411-11949,00.html>

Climate Fact: Rainfall Rise (General)

administrator — Wed, 08 Oct 2008 19:23:59 +0000

A warmer atmosphere results in an “amplification” of the water cycle. Some areas of the world are net importers of rainfall (such as tropical rainforests), while some are net exporters (such as oceans around the tropics). The “amplification” of the cycle means that dry regions become drier, and wet regions become wetter. During the 20th century, total rainfall in the United States increased by about seven percent; the largest increases occurred in the central and eastern regions (net importing regions). Most of this increase in precipitation can be accounted for by heavy and extreme precipitation events becoming even more intense. The amount of rain that falls during the heaviest one percent of rainfall events has increased by 20 percent over the last 100 years.

Seasons: Spring, Summer, Fall

Sources: United States. Climate Change Science Program. Weather and Climate Extremes in a Changing Climate. Synthesis Assessment Product 3.3: GPO. 2008 and “U.S. Temperature and Precipitation Trends.” U.S. National Oceanic and Atmospheric Administration (NOAA): Climate Prediction Center. 5 January 2005. 26 June 2008 <http://www.cpc.ncep.noaa.gov/charts.shtml> and Soden, B., Wentz, F.J., Santer, B.D. and Zwiers F. “Climatically-Induced Increases in Water Vapor and Precipitation: Causation and Implications. ” United States Senate, Washington,\ D.C. 29 October 2007. Accessed Online 17 December 2007 http://www.ametsoc.org/atmospolicy/ESSSarchiveclimatechange.html>

Climate Fact: Higher Lows

administrator — Wed, 08 Oct 2008 19:18:43 +0000

Getting a break from exposure to hot temperatures is important for preventing heat related illnesses. While people usually associate extreme daytime temperatures with heat stroke, if it does not cool sufficiently during the night, the body will not get a break from the heat. In North America over the last 50 years, average nighttime low temperatures have risen faster than average daytime high temperatures. There has been a 50 percent increase in the number of unusually warm nights, and nights with temperatures that would have fallen into the top tenth percentile during the 1950’s now fall into the top fifteenth percentile. Almost all of this increase has happened since 1975.

Season: Summer

Source: United States. Climate Change Science Program. Weather and Climate Extremes in a Changing Climate. Synthesis Assessment Product 3.3: GPO. 2008.

Climate Fact: Prolonged Dry Episodes

administrator — Wed, 08 Oct 2008 18:52:31 +0000

Is it possible for droughts to become more common even if annual rainfall amounts increase? Overall annual precipitation in the lower 48 states has been increasing since the early 20th century, and since the 1970’s it has been increasing in the Eastern United States by about one-inch per decade. Over the last forty years, this region has experienced another trend: an increase in the frequency of thirty-day periods during warm months when there is no rain. These dry spells now occur about twice as often. While these trends may seem contradictory, it appears that the lack of rainfall during these dry periods is more than compensated for by an increase in the frequency of heavy rains. Heavy rainfall events are now 14 percent more frequent than they were three decades ago, while extreme rainfall events are seven percent more frequent. The frequency of moderate rainfall events has decreased slightly.

Seasons: Spring, Summer, Fall

Sources: Groisman, PY and Knight RW. “Prolonged Dry Episodes over the Conterminous United States: New Tendencies Emerging during the Last 40 Years.” Journal of Climate 21 (2008): 1850-1862 and National Weather Service: Climate Prediction Center. U.S. Temperature and Precipitation Trends: Annual. Accessed Online 3 July 2007 <http://www.cpc.noaa.gov/anltrend.gif> and Trenberth, K et al. “The Changing Character of Precipitation.” Bulletin of the American Meteorological Society, September 2003: 1205-1217 and Easterling, D et al. “Observed climate variability and change of relevance to the biosphere.” Journal of Geophysical Research 105 (2000): 101-120.

Cliamte Fact: Seagrass and SSTs

administrator — Wed, 08 Oct 2008 18:43:28 +0000

The summer of 2003 was one of Europe’s warmest on record and maximum sea surface temperatures (SSTs) in the Mediterranean were well above average (by about 2.5 degrees Fahrenheit). These temperatures were the highest recorded between 1972 and 2004. Also during this period, years when the maximum water temperature was above average were years when there were above average numbers of seagrass plants flowering (these plants generally produce flowers only once every five years). In 2003, the Sea’s dominate seagrass species (Posidonia oceanica) flowered at record levels. While this phenomenon may suggest that warmer SST’s are good for seagrass, the overall warming trend in the Mediterranean of 1.7 degrees Fahrenheit since the early 1980’s has corresponded to an increase in seagrass mortality, a decline in the species’ areal extent, and a decline in rhizome growth (stalk-like plant structures that grow horizontally, usually underground) . Indeed, years of extensive flowering are often followed immediately by years of extensive mortality.

Season: Summer

Source: Diaz-Almela, E et al. “Consequences of Mediterranean warming events in seagrass (Posidonia oceanica) flowering records.” Global Change Biology 13 (2007): 224-235.

Climate Fact: Northeastern U.S. Rainfall Trends

administrator — Wed, 08 Oct 2008 18:17:58 +0000

The northeastern quadrant of the contiguous United States, defined as the area from Minnesota south to Missouri and then east to Maryland and north to Maine, has been experiencing changes in its precipitation regime. While the average annual number of “wet” days (or days when rain falls) in that region declined by about four percent over the last century, the average annual number of days that fall into the top tenth percentile of wet days grew by 12 percent during that same period. This means that rainfall events have become less frequent, but are more intense when they do come. The decline in the average annual number of wet days has been especially pronounced (a nine percent decline) since the early 1970’s.

Seasons: Spring, Summer, Fall

Source: Groisman, PY et al. “Trends in Intense Precipitation in the Climate Record.” Journal of Climate 18 (2005): 1326- 1350.

Climate Fact: El Niño and Tropical Pacific Cyclones

administrator — Wed, 08 Oct 2008 17:45:29 +0000

The tropical Pacific basin is one of the planet’s warmest ocean regions, with surface water temperatures rarely falling below 83 degrees Fahrenheit. These perennially warm temperatures provide the fuel for tropical cyclone formation, and the strongest cyclones on record have formed here. Because these waters are already above the threshold for tropical cyclone formation, slight increases in water temperature do not seem to have much of an effect on the average annual number and average intensity of cyclones forming here. Instead, the El Niño Southern Oscillation cycle appears to control how many cyclones form annually. In El Niño years, the region’s monsoon trough (a region of low pressure associated with the monsoon) extends farther east over the tropical waters than it does during La Niña years. This leads to a larger area where tropical cyclone formation is likely, and on average three times as many tropical cyclones form during El Niño than during La Niña years.

Seasons: Spring, Summer, Fall

Sources: Matsuura, T et al. “A mechanism for interdecadal variability of tropical cyclone activity over the western North Pacific.” Climate Dynamics 21 (2003): 105-117 and Chan, JCL. “Interannual variations of intense typhoon activity.” Tellus 59A (2007): 455-460.

Climate Fact: Tropical CAPE

administrator — Wed, 08 Oct 2008 17:42:41 +0000

Convective available potential energy (CAPE) is a measure of how much energy is available for storm development (CAPE is measured by the number of joules present in a kilogram of air). Generally, the hotter and more humid conditions are, the more CAPE is present. A collection of atmospheric conditions, including some CAPE, are necessary for a storm to develop. Thus, while CAPE does not tell you whether there will be a storm or not, it does tell you how severe a storm that does develop is likely to be. Since the late 1950’s, the average amount of CAPE in the tropics has been growing at a rate of 86 joules per kilogram per decade, or six percent per decade. This trend has been the most pronounced in the western tropical Pacific.

Seasons: Winter, Spring, Summer, Fall

Source: Gettelman, A. “Multidecadal trends in tropical convective available potential energy.” Journal of Geophysical Research 107 (2002): ACL 17.

Climate Fact: ENSO and Tropical Cyclone Landfall Frequency

administrator — Wed, 08 Oct 2008 16:45:58 +0000

The El Niño Southern Oscillation (ENSO) cycle, or the cyclical movement of heat in the tropical Pacific Ocean, affects the upper atmosphere over the Atlantic Ocean. This affects both the frequency of Atlantic tropical cyclone formation as well as the positioning of the region’s high and low pressure centers that steer the tropical cyclones. La Niña conditions are 19 percent more common during years when tropical cyclones frequently make landfall along the Atlantic and Gulf Coasts of the United States. El Niño conditions are 10 percent more common during years when few tropical cyclones make landfall along the U.S. coast.

Seasons: Spring, Summer, Fall

Source: Lyons, Steven. “U.S. Tropical Cyclone Landfall Variability: 1950-2002.” Weather and Forecasting 19 (2004): 473-480.

Climate Fact: ENSO and Gulf Coast Lightning

administrator — Tue, 07 Oct 2008 20:27:42 +0000

The El Niño-Southern Oscillation (ENSO), or the cyclical movement of heat in the tropical Pacific Ocean, affects atmospheric phenomena throughout the world. The cycle affects the strength and position of the Pacific Jet Stream, an upper atmosphere wind current that flows from the Pacific over North America. During La Niña phases of the cycle, the Jet Stream is weaker than average and flows in an arcing pattern over the northern U.S. During El Niño phases, the Jet Stream strengthens and flows over the southern United States. In general, this leads to a stormier Gulf Coast, and a 100-200 percent regional increase in the frequency of warm season lightning strikes compared to neutral conditions.

Seasons: Spring, Summer, Fall

Sources: Cook, AR and Schaefer, JT et al. “The Relation of the El Niño-Southern Oscillation (ENSO) to Winter Tornado Outbreaks.” Monthly Weather Review 136 (2008): 3121-3137 and LaJoie, M and Laing, Arlene. “The Influence of the El Niño-Southern Oscillation on Cloud-to-Ground Lightning Activity along the Gulf Coast. Part I: Lightning Climatology.” Monthly Weather Review 136 (2008): 2523-2542.

Climate Fact: Winter Weather and the North Atlantic Oscillation (Chicago, IL)

administrator — Wed, 05 Mar 2008 22:48:07 +0000

The North Atlantic Oscillation (NAO) is a cyclical change in the difference in atmospheric pressure between a low pressure center around Iceland and a high pressure center around the Azores Islands in the North Atlantic. When this difference in pressure is larger (i.e. the low pressure center is especially low and the high pressure center is especially high), the NAO is in a “positive” phase, whereas when the difference in pressure is smaller, the NAO is in a “negative” phase. This oscillation influences the subpolar westerly winds that flow between 35 and 55 degrees north. During positive phases, the westerlies are stronger and tend to “block” the polar? air masses from invading the lower latitudes. This tends to keep winter weather in the mid- latitudes relatively mild and reduce the occurrence of below average winter temperatures in the United States.? In Chicago, for example, there are on average three times as many days each year when the temperature drops below zero degrees Fahrenheit during negative phases, versus positive phases.? Over the last thirty years, the index has been predominately positive. The index is currently hovering around neutral, and it has been mostly positive this winter.

Season: Winter

Climate Fact: Winter Weather and the North Atlantic Oscillation (Boston, MA)

administrator — Wed, 05 Mar 2008 22:46:08 +0000

The North Atlantic Oscillation (NAO), which is part of a large system known as the Arctic Oscillation, is a cyclical change in the difference in atmospheric pressure between a low pressure center around Iceland and a high pressure center around the Azores Islands in the North Atlantic. When this difference in pressure is larger (i.e. the low pressure center is especially low and the high pressure center is especially high), the NAO is in a “positive” phase, whereas when the difference in pressure is smaller, the NAO is in a “negative” phase. This oscillation influences the subpolar westerly winds that flow between 35 and 55 degrees north. During positive phases, the westerlies are stronger and tend to “block” the polar air masses from invading the lower latitudes. This tends to keep winter weather in the Northeast relatively mild, but it also keeps the cold polar air in Canada’s Maritime Provinces and Quebec, making winters in these regions more severe. Over the last thirty years, the index has been predominately positive, which has contributed to the increased winter temperatures in the northeastern U.S. during this period. The index is currently positive, and has been mostly positive this winter.

Season: Winter

Sources: Thompson, David W.J. “Regional Climate Impacts of the Northern Hemisphere Annular Mode.” Science 293, 85 (2001) and National Oceanic and Atmospheric Administration: Climate Prediction Center. North Atlantic Oscillation. Accessed Online 19 February 2008 and Wettstein, JJ and Mearns, LO. “The Influence of the North Atlantic-Arctic Oscillation on Mean, Variance, and Extremes of Temperature in the Northeastern United States and Canada.” Journal of Climate 15, 3586 (2002)

Climate Fact: Winter Weather and the North Atlantic Oscillation (Atlanta, GA)

administrator — Wed, 05 Mar 2008 22:43:42 +0000

The North Atlantic Oscillation (NAO) is a cyclical change in the difference in atmospheric pressure between a low pressure center around Iceland and a high pressure center around the Azores Islands in the North Atlantic. When this difference in pressure is larger (i.e. the low pressure center is especially low and the high pressure center is especially high), the NAO is in a “positive” phase, whereas when the difference in pressure is smaller, the NAO is in a “negative” phase. This oscillation influences the subpolar westerly winds that flow between 35 and 55 degrees north. During positive phases, the westerlies are stronger and tend to “block” the polar air masses from invading the lower latitudes. This tends to keep winter weather in the mid- latitudes relatively mild and reduce the occurrence of below average winter temperatures in the United States. In Atlanta, for example, there are on average five times as many days each year when trace snow falls occur during negative phases, versus positive phases. Over the last thirty years, the index has been predominately positive. The index is currently positive, and has been mostly positive this winter.

Season: Winter

Climate Fact: When It Rains (South Bend, Indiana)

administrator — Wed, 19 Sep 2007 16:53:47 +0000

Rainfall patterns in the United States are shifting. Overall, precipitation in the lower 48 United States has increased by ten percent since 1910. Since the 1970’s, the South Bend Area has been experiencing rainfall increases at a rate of 0.3 to 0.6 inches per decade. This extra rainfall has come in the form of extreme events; rainfall is now coming less frequently, but when it does come it pours. Nationally, the percentage of annual one day rainfall events that fall into the extreme category grew by seven percent during the Twentieth Century; in the South Bend Area the percentage grew by five percent. These trends make both droughts and floods more likely. Indeed, during the last fifty years the average lengths of both wet and dry spells have grown by over a week.

Seasons: Winter, Spring, Summer, Fall

(Trenberth, K et al. Effects of Changing Climate on Weather and Human Activities. Sausalito, CA: 2000. University Corporation for Atmospheric Research and Easterling, D et al. “Observed climate variability and change of relevance to the biosphere.” Journal of Geophysical Research, vol. 105, No. D15, pages 20, 101-20, 114. Augugst 16, 2000 and National Weather Service: Climate Prediction Center. U.S. Temperature and Precipitation Trends: Annual. Accessed Online 3 July 2007 and Baldocchi, Dennis. Climate Change and Agriculture: Trends and Bi-Directional Impacts. University of California - Berkely. 2006. Accessed Online 29 January 2007 )

Climate Fact: When It Rains (Sioux City, Iowa)

administrator — Wed, 19 Sep 2007 16:05:13 +0000

Rainfall patterns in the United States are shifting. Overall, precipitation in the lower 48 United States has increased by ten percent since 1910. Since the 1970’s, the Sioux City Area has been experiencing rainfall increases at a rate of 1.0 to 1.5 inches per decade. This extra rainfall has come in the form of extreme events; rainfall is now coming less frequently, but when it does come it pours. Nationally, the percentage of annual one day rainfall events that fall into the extreme category grew by seven percent during the Twentieth Century; in the Sioux City Area the percentage grew by two percent. These trends make both droughts and floods more likely. Indeed, during the last fifty years the average lengths of both wet and dry spells have grown by over a week.

Seasons: Winter, Spring, Summer, Fall

Climate Fact: When It Rains… (Milwaukee, Wisconsin)

administrator — Wed, 19 Sep 2007 16:02:42 +0000

Rainfall patterns in the United States are shifting. Overall, precipitation in the lower 48 United States has increased by ten percent since 1910. Since the 1970’s, the Milwaukee Area has been experiencing rainfall increases at a rate of 0.3 to 0.6 inches per decade. This extra rainfall has come in the form of extreme events; rainfall is now coming less frequently, but when it does come it pours. Nationally, the percentage of annual one day rainfall events that fall into the extreme category grew by seven percent during the Twentieth Century; in the Milwaukee Area the percentage grew by five percent. These trends make both droughts and floods more likely. Indeed, during the last fifty years the average lengths of both wet and dry spells have grown by over a week.

Seasons: Winter, Spring, Summer, Fall