Background El Niño Information
Updated October 8, 2015
Q: What is El Niño?
A: El Niño is a natural climate phenomenon marked by warmer-than-average sea surface temperatures in the central and eastern Pacific Ocean near the equator, which occurs on average every 2-7 years. Since 1950, there have been approximately 20 El Niño events. These share several common attributes, including a tendency to emerge in late spring and grow in intensity through the following fall, with a typical life span of 9-12 months. The presence of El Niño can significantly influence weather patterns, ocean conditions, and marine fisheries across large portions of the globe for an extended period of time.
El Niño affects U.S. weather and climate by shifting the Pacific jet stream - altering the movement of air masses and the tracks of cyclones that determine temperature and precipitation patterns. El Niño affects U.S. temperature and precipitation throughout its life cycle, though the more widespread disruptions occur in the cold season. On average, during the cold seasons, El Niño is associated with above-average precipitation along the Gulf of Alaska coastal region, across the Southwest and the Gulf of Mexico coastal region, while drier conditions prevail across Hawaii, the Pacific Northwest, and over the Ohio and Tennessee Valley. Temperatures are typically above-average across Alaska and the northern U.S., and cooler along the Gulf of Mexico coastal region. El Niño suppresses hurricane activity in the Atlantic. It also affects central-eastern Pacific tropical storms, with a tendency for storms to recurve further north and east into Mexico and the southwest, and also an increased risk of tropical storms affecting Hawaii. An individual El Niño event may not witness these impacts, but the odds are increased. See this page for graphics showing possible global impacts of a typical El Niño: http://www.climate.gov/news-features/blogs/enso/how-enso-leads-cascade-global-impacts
Q: How strong is El Niño now and how strong is it expected to become?
A: On the second Thursday of each month NOAA forecasters update the El Niño-Southern Oscillation (ENSO) Diagnostic Discussion. The next update will be issued on October 8, 2015. In March 2015, NOAA forecasters declared a weak El Niño had developed. In October, NOAA will say that this El Niño has continued to strengthen, and at this time, the forecaster consensus unanimously favors a strong El Niño with peak 3-month SST departures in the Niño 3.4 region potentially near or exceeding +2.0°C. El Niño is predicted to gradually weaken during Spring 2016, with the peak strength reached during late fall and early winter.
Q: What is the difference between weak, moderate, and strong El Niño’s?
A: The strength of El Niño influences the probability that the U.S. impact patterns described above will materialize. Thus, there is greater probability that the impacts will occur for stronger versus weaker events. In this sense, knowing the strength of El Niño is important for the expected skill of the climate forecast.
The strength of an event is determined by NOAA as the magnitude of the sea surface temperature anomalies (i.e. differences from normal) in the Nino 3.4 region of the east-central equatorial Pacific Ocean. Informally, weak events have departures between 0.5-0.9 degrees C, moderate El Niños between 1-1.4 degrees C, and strong episodes are at least 1.5 degrees C above normal. There is no characterization above a strong event (i.e. >2.0°C), as the sample size becomes too small to understand if this has any effect on impacts.
Important in this regard for the upcoming winter is whether the California drought might be appreciably alleviated. Historically, weak to moderate El Niños have not exerted a strong or reliable effect on statewide precipitation, and what signal does exist is mainly confined to southern California. Strong El Niños since the late 19th century often deliver statewide heavy rains and abundant mountain snows, but this is not guaranteed. It is thus important to monitor the development of this El Niño, not only for assessing the chances of drought amelioration, but also to prepare for risks related to heavy rains and strong storms that accompany strong El Niños.
Q: What does El Niño mean for the drought in California and the West?
A: The drought in California has been ongoing for nearly four years and is the worst four-year drought in that state’s history. The longest consecutive string of below average statewide precipitation was from 1987-1992. NOAA’s Drought Outlook predicts drought will continue across California and much of the West Coast through November 30.
California dry season will be ending during the next few months, although the peak of the rainy season isn’t until the end of the year and early next year. Forecasters predict California's fall (October - December) is favored to be warmer-than-average http://www.cpc.ncep.noaa.gov/products/predictions/long_range/lead01/off01_temp.gif
And while there is a tilt toward above normal precipitation in the southern part of the state during OND, this period only contains the beginning of the rainy season, so some drought improvement is only expected in the southern part of the state:
As indicated above, the relationship between El Niño and rainfall over California depends on the eventual strength of the episode during the rainy season. Strong El Niño events have often been associated with abundant precipitation, although not for every event, and for El Niño events in general the highest probabilities of above normal precipitation exist in southern California.
For the latest drought classification, please see the Drought Monitor (updated every Thursday): http://droughtmonitor.unl.edu/Home/RegionalDroughtMonitor.aspx?west
Q: How is a strengthening El Niño expected to affect the weather in the U.S. this fall and beyond?
A: El Niño affects the behavior of tropical cyclones in the Pacific and hurricanes in the Atlantic.
● More tropical cyclones in the Pacific
○ Which could bring heavy rains to the desert SW, in addition to their “normal” Monsoon season
○ Further northward recurving storms affecting the desert Southwest in late summer
○ An increased risk of tropical and storm impacts on Hawaii.
○ An increased risk of dryness or drought in Puerto Rico and the Caribbean.
During the winter, odds currently favor above normal precipitation across the southern part of the United States from central California to the Florida and along the East to southern New England. Below average precipitation is favored in the northern Rockies, around the Great Lakes, in western Alaska, and in Hawaii. Above average temperatures are favored in Alaska, Hawaii, and along the northern tier from the Pacific Northwest to the Great Lakes. Below average temperatures are most likely in the Southwest and Southeast.
Q: When was the last El Niño event?
A: The last El Ninõ developed in the summer of 2009, and lasted until spring of 2010. The event was of moderate-to-strong intensity. During the October 2009-April 2010 period, Southern California experienced slightly above-normal rainfall, whereas much of the Great Plains from the Dakotas to Texas, the Gulf of Mexico coastal region, and the mid-Atlantic states were much wetter than normal. The winter was very warm along the northern tier from Minnesota to Maine, and especially cool along the Gulf Coast region. There were no major disasters related to tropical cyclones over the contiguous US during that period. Many of these features observed during the 2009-10 El Niño were consistent with the typical effects of El Niños. For instance, winter temperatures in the Pacific Northwest were warmer than average and precipitation unusually low, consistent with expectations for typical El Niños. The heavy investment in snowmaking equipment for the 2010 winter Olympics in Vancouver was, in retrospect, a wise decision.
Some features that were favored didn’t materialize. For instance, there were no major damaging winter storms in the Pacific West Coast or along the Gulf of Mexico coastal zones during the winter of 2009-10, even though such risks are elevated. In that sense, the character of the winter of 2009-10 was appreciably different from conditions during the strong El Niño of 1982-83 when frequent intense winter storms caused damaging flooding in those regions.
Monthly ENSO Diagnostic Discussion from NOAA’s Climate Prediction Center
Drought Fact Sheet (include? update?)
Impacts of El Niño: http://www.pmel.noaa.gov/tao/elNiño/impacts.html
Frequently Asked Questions on El Niño/La Niña: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/prelude_to_ensofaq.shtml
NOAA animation of 1997-2000 strong ENSO cycle: