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The 2012–15 North American Drought, an expansion of the 2010–13 Southern United States drought, originated in the midst of a record-breaking heat wave. Low snowfall amounts in winter, coupled with the intense summer heat from La Niña, caused drought-like conditions to migrate northward from the southern United States, wreaking havoc on crops and water supply. The drought has inflicted, and is expected to continue to inflict, catastrophic economic ramifications for the affected states. It has exceeded, in most measures, the 1988-1989 North American drought, the most recent comparable drought, and is on track to exceed that drought as the costliest natural disaster in U.S. history. The drought includes most of the U.S., parts of Mexico, and central and Eastern Canada. At its peak on July 17, 2012, it covered approximately 81 percent of the contiguous United States with at least abnormally dry (D0) conditions. Out of that 81%, 64% was designated as at least moderate drought (D1) conditions. Its area was comparable to the droughts in the 1930s and 1950s but it has not yet been in place for as long. In March 2013, heavy winter rains broke a three-year pattern of drought in much of the Southeastern United States, while drought conditions still plague the Great Plains and other parts of the U.S., according to the U.S. Drought Monitor.〔(Rains Bring Relief to Region; Winter Weather Reduces Drought in Southeast, Southwest; Other Areas Still Threatened )'' March 12, 2013 WSJ〕 Drought continued in parts of North America through 2013.〔(''Thin Snowpack in West Signals Summer of Drought'' ) February 22, 2013 NYT〕 Beginning in March 2013, improved rainfall across the Midwest, southern Mississippi Valley, and Great Plains began gradually alleviating drought in these areas, while drought continued to intensify in the Western United States. Heavy rains across previously drought-stricken areas resulted in widespread flooding in portions of the Midwest, a phenomenon which was named "weather whiplash". By June 2013, approximately the eastern half of the United States was drought-free, while conditions continued to gradually improve across the Plains. Moderate to severe drought continues to impact and worsen throughout the western United States, with some portions of the United States being afflicted by the drought for over three years. Through the winter of 2013-2014, California continued to receive record low rainfall.〔Becerra, Hector (September 25, 2014) ("Drought has 14 communities on the brink of waterlessness" ) ''Los Angeles Times''〕 For many locations, the calendar year of 2013 was the driest year in over 130 years. Some locations received less than half of their previous record low rainfall amounts.〔(''In Midwest, Drought Gives Way to Flood'' ) April 25, 2013 New York Times〕 ==Meteorological background== The drought was set in motion when strongly positive Arctic oscillation and North Atlantic oscillation conditions removed winter storms from the U.S. the winter of 2011–2012. When spring arrived, very little snow existed to melt and moisten the ground, and thus very little water to evaporate and create rainfall. The effects of the lack of snow were immediate. Dry conditions could be noticed immediately, and contributed to a weak tornado season in the U.S.. The strongest tornado outbreak of 2012 occurred on March 2, after most snow had melted. The drought continued to steadily intensify along with a decline in rainfall which is still ongoing. The Summer 2012 North American heat wave caused further evaporation of groundwater, lakes, reservoirs, and rivers and streams. As moisture has continued to decline, the conditions are becoming self-sustaining (i.e. a lack of rainfall means that less moisture is available to promote additional rainfall, in a vicious cycle). In many areas, the only way for drought conditions to be significantly alleviated in the short-term would be for a discrete, long-lived, and large system (such as a tropical cyclone) to impact the area.〔(【引用サイトリンク】url=http://www.ncdc.noaa.gov/temp-and-precip/drought/recovery.php )〕 Soil hardening due to the drought means that even if a large amount of rain falls in a short time, most of it will run off quickly causing flash floods rather than drought relief. The June 2012 North American derecho and other strong storms in late June and early July did not appear to ease drought conditions, as the rainwater ran off rapidly from the affected areas. The drought continued and intensified into the end of November 2014 for a large part of the country. Although drought/dry conditions are likely to drop at least one category level in the Southwest, Southeast, Northeast, and Northern Plains as well as portions of the Ohio River Valley, it is expected to have both short-term and long-term impacts across nearly the entire affected area.〔 A study by Utah State University analyzed the 2011 Missouri River Flooding 〔http://onlinelibrary.wiley.com/doi/10.1002/2013GL059042/abstract〕and predicted that "a prominent teleconnection forcing in driving the wet/dry spells in the (Missouri River Basin)... implies persistence of dry conditions for the next 2 to 3 years", which coincides with the 2012-15 drought. That analysis 〔http://onlinelibrary.wiley.com/doi/10.1002/2013GL059042/abstract〕was based upon a marked variability at the 10–15- year time scale that is coincident with the water storage increase in the Missouri River Basin. The study 〔https://ams.confex.com/ams/94Annual/videogateway.cgi/id/25952?recordingid=25952〕 found that precipitation over the MRB undergoes a profound modulation during the transition points of the Pacific quasi-decadal oscillation and associated teleconnections. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「2012–15 North American drought」の詳細全文を読む スポンサード リンク
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