Earth Gauge » Precipitation Shifts

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: Forest-Tundra Dynamics

administrator — Wed, 08 Oct 2008 20:14:10 +0000

In northern Québec, patches of Black Spruce forests exist alongside the arctic shrub tundra, forming an ecological zone known as forest-tundra. Heavy winter snows, short frost-free growing seasons, and high winds limit the possible area in which Black Spruce trees can grow. These harsh conditions also limit how tall these trees can grow, and in the zones where climate conditions are just tolerable enough for the trees to survive, the spruces will grow in bushes as opposed to upright trees in order to limit their exposure to the elements. Since the 1980’s, when a twenty year regional cooling trend ended, spruce shrubs that had previously been limited by high wind and low temperatures have grown taller throughout the forest-tundra zone.

Seasons: Winter, Spring, Summer, Fall

Source: Gamache, Isabelle and Payette, Sergei. “Height growth response of tree line black spruce to recent climate warming across the forest-tundra of eastern Canada.” Journal of Ecology 92 (2004): 835-845.

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: Rwenzori Mountain Glaciers

administrator — Wed, 08 Oct 2008 20:07:19 +0000

Temperatures in the East African Highlands have risen by about one degree Fahrenheit over the past 50 years. This area includes Uganda’s Rwenzori Mountains, which feature glaciers that supply the lowland plains region with water. Between 1987 and 2003, the area covered by glaciers in these mountains shrunk by about 50 percent. This retreat has been attributed to the temperature rise, a reduction in snowfall, and a decrease in average cloud cover, which has encouraged increased sublimation, or the conversion of a solid (in this case ice) to gas (in this case water vapor).

Seasons: Spring, Summer, Fall

Source: The United Nations: Environment Programme. Africa: Atlas of our Changing Environment. Malta: ProgressPress Inc., 2008. Accessed Online 11 June 2008 <http://www.unep.org/dewa/africa/AfricaAtlas/PDF/en/Africa_Atlas_Full_en.pdf>

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: Chinook Survival

administrator — Wed, 08 Oct 2008 19:34:02 +0000

Did you know that 75 percent of the water resources in the West originate from snowmelt? Mountain snowpack accumulates over the winter and as it melts during the spring, summer, and fall, it feeds the region’s rivers and streams. Over the last half of the 20th Century, November to March temperatures in the Pacific Northwest rose by about 4.5 degrees Fahrenheit. This trend has corresponded to less snowpack, an earlier melting of snowpack, and more late winter and early spring precipitation falling as rain instead of snow. Between 1950 and 1998, the average date when there ceased to be snow on the ground advanced 16 days earlier in the year, and the point in the year when river levels peak is now arriving an average of nine days earlier than it did in the 1950’s. This trend in earlier peak flow also means that average autumn stream levels are lower than they were 50 years ago, and that the water in these streams is warmer. Salmon are cold water species, and cannot tolerate significant rises in temperature. Low stream levels impede their ability to navigate the waters, and also generally mean poor water quality. The high-elevation streams in Idaho’s Salmon River watershed host populations of juvenile Chinook salmon, which remain in these streams for a year before venturing into larger water bodies. Fall stream flow is the most important climatic factor for estimating juvenile survival rates, which largely control Pacific Chinook salmon populations.

Seasons: Summer, Fall

Sources: “HydroFacts.” Southwest Journal of Hydrology 6 (2007): 13 and Mote, P.W., A.F. Hamlet, M.P. Clark, and D.P. Lettenmaier. 2005. Declining mountain snowpack in western North America. Bull. Amer. Met. Soc. 86:39–49 and Groisman, P.Y., P.W. Knight, and T.R. Karl. 2001. Heavy precipitation and high streamflow in the United States: Trends in the 20th Century. Bulletin of the American Meteorological Society 82:219-246 and Crozier, L. and Zabel, R.W. “Climate impacts at multiple scales: evidence for differential population responses in juvenile Chinook salmon.” Journal of Animal Ecology 75 (2006): 1100-1109.

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: 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.

Climate Fact: Climate and Geysers

administrator — Wed, 08 Oct 2008 18:35:53 +0000

Geysers are hot springs that intermittently discharge fountains of steaming hot water through conduits running from shallow reservoirs of groundwater to the surface. These groundwater reservoirs are heated by the planet’s magma, and when the temperature in the geyser conduits exceeds the boiling point, a stream of liquid and gas comes shooting out of the ground. About half of Earth’s 1000 geysers are located in Yellowstone National Park. The frequency of eruptions in the Park is controlled by two main factors: earthquakes and rainfall (which is reflected in the discharge of Yellowstone’s Madison River). Earthquakes tend to lengthen the time between eruptions, as do droughts. More rainfall means more recharge into the Madison River as well as the geysers’ reservoirs; in years when there is less rainfall, there is less water in the reservoirs, less pressure in the conduits, and fewer eruptions. Comparisons between Madison River annual discharge and the eruption frequency of five of Yellowstone’s geysers (including Old Faithful), showed that the region’s drying trend between 1998 and 2006 corresponded to a trend of less frequent eruptions. Since 1970, average annual discharge in the Madison River has dropped by 15 percent.

Seasons: Winter, Spring, Summer, Fall

Source: Hurwitz, S et al. “Climate-induced variations of geyser periodicity in Yellowstone National Park, USA.” Geology 36 (2008): 451-454.

Climate Fact: Maritime Influences on Mountain Hemlock

administrator — Wed, 08 Oct 2008 18:25:55 +0000

In the Pacific Northwest, Mountain Hemlocks grow at elevations between 3,600 and 7,500 feet. These shade tolerant trees grow underneath the faster growing but shorter-lived Douglas Firs, and gradually make their way to the top of the canopy over their 700 year life spans. At the region’s high elevations, some of the world’s most extensive seasonal snowpack forms. The extent of this snowpack and its seasonal duration determine how much Mountain Hemlocks can grow each year. Warmer years (years when more precipitation falls as rain instead of snow, and the accumulated snow pack melts earlier in the spring) tend to be years of enhanced tree growth in the northern portion of the Hemlock’s range, and wider tree rings correspond to those years. In the southern extent of the Hemlock’s range (southern Oregon), however, the climate is different and warm temperatures and lack of snowpack limit hemlock growth. Here, the growth trends are essentially the opposite seen in the more northern areas of the species’ range. The Pacific Decadal Oscillation (PDO), a 50-year cycle of sea-surface temperature shifts in the northern Pacific, seems to largely control the depth and duration of Pacific Northwest snowpack. Positive (warm) phases of the PDO, which occurred between about 1920 until about 1945, and from about 1977 until just a few years ago, correspond to warmer temperatures in the Pacific Northwest and less snowpack.

Seasons: Winter, Spring, Summer, Fall

Source: Peterson, DW and Peterson, DL. “Mountain Hemlock Growth Responds to Climatic Variability at Annual and Decadal Time Scales.” Ecology 82 (2001): 3330-3345.

Climate Fact: Tropical Rainforest Rainfall

administrator — Wed, 08 Oct 2008 18:20:12 +0000

Tropical rainforests are inhabited by more plants and animals than any other place on Earth. These plants and animals are accustomed to hot temperatures and abundant rainfall throughout the year. Globally, rainfall in tropical rainforests has been decreasing since 1960 at a rate of one percent per decade. Most of the decline during this period, however, is due to trends in the northern African tropics, where rainfall has been decreasing by three to four percent per decade. In tropical Asia, total rainfall has only been decreased slightly and in the Amazonian rainforest, there was no significant trend.

Seasons: Winter, Spring, Summer, Fall

Source: Boisvenue, C and Running SW. “Impacts of climate change on natural forest productivity – evidence since the middle of the 20th century.” Global Change Biology 12 (2006): 1-21.

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: White Spruces Withering

administrator — Wed, 08 Oct 2008 17:51:47 +0000

While cold temperatures limit growth for many plant species inhabiting the boreal forest and tundra regions of the northern hemisphere (from about 50 to 80 degrees North), increases in temperature can cause more evaporation from the soil and lead to drought stress. This appears to be the case with White Spruce forests in Alaska. A longer growing (or frost free) season and an increase in average May through August temperatures of six degrees Fahrenheit over the last 100 years have corresponded to decreases in annual growth rates. Large annual tree rings grew during the cool and wet period of 1915 to 1965. Since then, however, there has been a 40 to 50 percent decline in average annual tree ring width. During the cool and wet 1930’s, it was not unusual for the White Spruce trees to grow two millimeter-wide tree rings (a little more than one 16th of an inch). Today, one millimeter rings are more common. Warmer temperatures appear to be causing the soil to dry earlier in the summer, and when this happens the trees stop growing and start preparing for winter.

Seasons: Spring, Summer

Source: Barber, V et al. “Reduced growth of Alaskan white spruce in the twentieth century from temperature-induced drought stress.” Nature 405 (2000): 668-673.

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: Rainfall Reductions and Indian Ocean Warming

administrator — Wed, 08 Oct 2008 17:32:40 +0000

Eighty-three (83) percent of the moisture entering tropical Africa originates in the Indian Ocean. Over the last four decades of the 20th century, sea surface temperatures (SSTs) in the tropical Indian Ocean rose by nearly two degrees Fahrenheit, making these waters the warmest they have been in 120,000 years. As this has happened, rainfall over the Indian Ocean has increased by about 20 percent. This increase means that more heat is being released into the upper atmosphere over the ocean, and as this has happened, the circulation system that brings moisture from the ocean to the African continent has moved and onshore winds have weakened. This process has been linked to a 15 percent decline in growing season rainfall that occurred in East Africa over the same period that Indian Ocean SSTs rose.

Seasons: Winter, Spring, Summer, Fall

Source: Funk, C et al. “Warming of the Indian Ocean threatens eastern and southern African food security but could be mitigated by agricultural development.” Proceedings of the National Academy of Sciences 105 (2008): 11081-11086.

Climate Fact: Warming and Water Discharge

administrator — Wed, 08 Oct 2008 17:23:02 +0000

The Arctic Ocean is surrounded by a series of river drainage basins, which collectively occupy an area 1.5 times that of the ocean basin itself. No other ocean basin’s temperature and salinity levels are more dependent on what happens on the adjacent land surface. These temperature and salinity levels in turn influence the behavior of the world’s ocean conveyor system, which transports heat and nutrients to and from different ocean basins. The Eurasian Arctic has warmed by about 1.3 degrees Fahrenheit since the 1930’s. As this has happened, the total combined annual discharge of the six largest Eurasian rivers that flow into the Arctic Ocean has grown by 34 trillion gallons, or seven percent. These trends have corresponded to increases in winter precipitation and thawing permafrost. Total discharge also appears to be related to the behavior of the North Atlantic Oscillation (NAO), or the cyclical change in the pressure difference between the Azores High and Icelandic Low. Positive phases bring more precipitation to Scandinavia and Siberia, and the NAO has been predominately and strongly positive over the last 30 years.

One of the six rivers studied is Russia’s Lena River. To see a colorful LANDSAT image of the Lena River Delta, visit http://earthasart.gsfc.nasa.gov/lena.html.

Seasons: Spring, Summer, Fall

Sources: Peterson, BJ et al. “Increasing River Discharge to the Arctic Ocean.” Science 298 (2002): 2171-2173 and Serreze, MC et al. “Large-scale hydro-climatology of the terrestrial Arctic drainage system.” Journal of Geophysical Research 108 (2003): doi:10.1029/2001JD000919 and McGuire, AD et al. “Land cover disturbance and feedbacks to the climate system in Canada and Alaska.” Chapter 9 (pages 139 - 161) in Imzd Change Science: Ohsel-virzg, Moniloritig, and Ulzder*stnditzg Trojectol-ies of Challge OH the Earth’s SU+KZE. ditcd by Gutman, C., Janetos, A.C., Justice, C.0, Moran, E.F., Mustard, J.F., Rindf~lssR, .R., Skole, D., Turner 11, B.L., and Cochrane, M.A. Dordrecht, Ncthcrlands, Kluwer Adademic Publishers.

Climate Fact: Moving on Up

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

Southern California’s Santa Rosa Mountains, located about two hours southeast of Los Angeles, stand out as forest islands amongst the lowland desert to the east, and the chaparral scrubland to the west. Plants not capable of tolerating the hot and dry conditions of the lowlands find refuge at the cooler and wetter high elevations. Over the last 30 years, however, a two degree Fahrenheit rise in temperature, coupled with an increase in the length of dry spells (despite an overall increase in precipitation), has made it more difficult for tree species such as white fir, Jeffrey pines, and golden cup oaks to live in the lowlands. As a result, these species, as well as several other shrub and wildflower species that prefer similar temperature and moisture regimes, have been forced to move uphill. The ranges of these species’ are now an average of 213 feet higher in elevation than they were in the late 1970’s.

Seasons: Winter, Spring, Summer, Fall

Source: University of California, Irvine. “Climate change caused widespread tree death in California mountain range, study confirms.” Today@UCI 11 August 2008. 20 August 2008 http://www.today.uci.edu/news/release_detail.asp?key=1793

Climate Fact: Crater Lake Water Levels and PDO

administrator — Wed, 08 Oct 2008 17:13:23 +0000

About 7,700 years ago, a volcanic eruption 42 times more powerful than the 1980 Mt. St. Helens event happened at Mt. Mazama in the southern Oregon Cascade Mountains. The top 5,000 feet of the mountain collapsed soon afterwards, leaving behind a huge caldera, or crater, which has since filled with about 4.6 trillion gallons of water that make up America’s deepest lake - Crater Lake. Crater has been called “the world’s largest rain gauge,” as the lake’s surface comprises about 78.5 percent of its watershed and there are no streams flowing into or out of the lake. Thus, trends in Crater Lake’s water levels provide an effective proxy for local temperature and precipitation trends. The historical record shows that these lake levels are influenced by the behavior of the Pacific Decadal Oscillation (PDO), or the periodic shift in heat distribution in the Pacific Ocean. During negative (wet and cool) phases, the mid-latitude storm track that brings moisture to the Pacific Northwest tends to consistently travel over southern Oregon. During positive (warm and dry) phases, the storm track moves and southern Oregon does not receive as much precipitation. Lake levels reached 300 year lows during the late 1980’s and early 1990’s, after years of a strongly positive PDO.

Seasons: Winter, Spring, Summer, Fall

Sources: Peterson, DL et al. “Detecting Long-Term Hydrological Patterns at Crater Lake, Oregon.” Northwest Science 73 (1999): 121-130 and Uhler, John W. “Crater Lake National Park Information Page.” Online Posting, 2007. 4 September 2008 < http://www.crater.lake.national-park.com/info.htm> and United States Geological Survey: National Water Information Service. 4 September 2008 <http://waterdata.usgs.gov/nwis>

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.