June 7, 2001 For the first time, researchers have identified how El Niños and La Niñas change snowfall in specific regions of the continental United States. These findings may lead to more accurate winter season snowfall forecasts when either event is occurring.
Snowfall amounts change during different stages of the winter in several areas of the U.S. depending on whether El Niño or La Niña conditions exist in the equatorial Pacific. The most significant changes in snowfall occur in the Northwest, Northeast, Ohio Valley, Midwest and northern Texas.
The results by Shawn R. Smith and James J. O’Brien of Florida State University appear in the June issue of the Bulletin of the American Meteorological Society. "Our research started with a request from ski areas for better snowfall forecasts," says O’Brien, "because snowfall dictates economic boom or bust for them."
In the Northwest (Washington, north and central Idaho, and western Montana) and the Great Basin (central Nevada, central Utah and western Colorado) more snow falls during a La Niña than during an El Niño. Most changes in snowfall in the Northwest tend to occur from early through mid-winter, as opposed to late winter, and the Great Basin experiences the most snowfall in early winter.
In the Northeast, in a line that extends up the Appalachian Mountains from central Virginia through Maryland and New Jersey to western Vermont, most snowfall changes occur in mid-winter. The region receives less snow during a La Niña winter than in "neutral winter," that is, one without an El Niño or La Niña. Conversely, more snow falls in this region during an El Niño year.
The Ohio Valley receives less snow in early and mid-winter during both El Niño and La Niña, than in the winters without them. The Midwest (southern Wisconsin, southern Minnesota, Iowa, and northern Illinois, Indiana) shows the same trend but only during mid-winter.
Northern Texas shows the greatest increase in snowfall in mid-winter during El Niño years, when median snowfall increases by 69% and 78% over neutral phases and La Niñas. Increases range from almost 2 inches in Midland to almost 7 inches in Amarillo.
The catalyst for changes in U.S. snowfall is an El Niño or La Niña. El Niños occur when trade winds that normally blow east to west over the equator relax over the Pacific Ocean and surface waters in the eastern Pacific warm. La Niñas occur when the trade winds strengthen resulting in cooler surface waters in the eastern Pacific. The warming or cooling of the eastern Pacific Ocean change precipitation patterns around the world.
During an El Niño, air pressure over the Pacific Ocean lowers over Tahiti and rises over Australia and is the reverse during a La Niña. This see-saw in pressure is called Southern Oscillation, and working in conjunction with ocean warming (cooling) is called El Niño (or La Niña)-Southern Oscillation or ENSO. ENSO events occur at irregular intervals of 2-7 years, usually lasting 1-2 years, before eastern Pacific Ocean waters return to normal.
O’Brien and Smith looked at snowfall records from 143 U.S. weather stations for the months of October through April, from 1950 to 1994, and noted specific changes that occurred during ENSO events. Snowfall data were then categorized into 3 phases of ENSO events: warm (El Niño), neutral, and cold (La Niña). El Niños and La Niñas typically begin in October and usually lasts for 12 months.
After identifying which phase an ENSO event was in during a given year, Smith and O’Brien then looked for weather stations with similar snowfall distributions and grouped them together to get a clear picture of large areas that experienced changes in snowfall.
Smith noted that there is a wide range of affects on snowfall in the Midwest. Snowfall reductions during El Niños or La Niñas can range from as little as 2.3 to 5.1 inches in Cincinnati, OH to as large as 1.2 to 2.1 feet in Muskegon, MI.
The regional trends in snowfall Smith and O’Brien identified will enable meteorologists to better forecast seasonal snow totals in regions of the U.S. during an ENSO event. "Everyone can benefit from better seasonal snow forecasts," Smith said. "There are many practical applications for better forecasts, such as better financial planning for ski areas and urban snow removal budgets."
This research was funded in part by NASA. More information and graphics are available on the Internet at: http://www.coaps.fsu.edu/research/smith/snowpaper/ and http://coaps.fsu.edu/.
The American Meteorological Society is the nation's leading professional society for scientists in the atmospheric, oceanic, and related sciences.
The paper, "Regional Snowfall Distributions Associated with ENSO: Implications for Seasonal Forecasting" is available online at http://www.ametsoc.org/ams (Click on (1) Journals and Publications, (2) AMS Journals, (3) AMS Journals Online, (4) Current Issue, (5) Bulletin of the American Meteorological Society, Volume 82, Issue 6, 2001).
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The above story is based on materials provided by NASA/Goddard Space Flight Center--EOS Project Science Office.
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