Effects of the El Niño–Southern Oscillation in the United States

The El Niño–Southern Oscillation affects the location of the jet stream, which alters rainfall patterns across the West, Midwest, the Southeast, and throughout the tropics. The shift in the jet stream also leads to shifts in the occurrence of severe weather, and the number of tropical cyclones expected within the tropics in the Atlantic and Pacific oceans affected by changes in the ocean temperature and the subtropical jet stream. The winter will have a negative phase according to the Arctic oscillation (AO).[1]

Impact of El Niño and La Niña on North America

Temperature

During El Niño, the northern tier of the lower 48 United States and southern Alaska will exhibit above normal temperatures during the fall and winter. The Gulf Coast of the United States will experience below normal temperatures, during the winter season.[2][3]

Precipitation
See also: United States rainfall climatology

Across Alaska, El Niño events do not have a correlation towards dry or wet conditions; however, La Niña events lead to drier than normal conditions. During El Niño events, increased precipitation is expected in California due to a more southerly, zonal, storm track.[4] During La Niña, increased precipitation is diverted into the Pacific Northwest due to a more northerly storm track.[5] During La Niña events, the storm track shifts far enough northward to bring wetter than normal winter conditions (in the form of increased snowfall) to the Midwestern states, as well as hot and dry summers.[6] During the El Niño portion of ENSO, increased precipitation falls along the Gulf coast and Southeast due to a stronger than normal, and more southerly, subtropical jet stream.[7] In the late winter and spring during El Niño events, drier than average conditions can be expected in Hawaii.[8] On Guam during El Niño years, dry season precipitation averages below normal. However, the threat of a tropical cyclone is over triple what is normal during El Niño years, so extreme shorter duration rainfall events are possible.[9] On American Samoa during El Niño events, precipitation averages about 10 percent above normal, while La Niña events lead to precipitation amounts which average close to 10 percent below normal.[10] ENSO is linked to higher average temperatures and abnormal rainfall patterns over Puerto Rico.[11]

Snowfall

During an El Niño, snowfall is greater than average across the southern Rockies and Sierra Nevada mountain range, and is well-below normal across the Upper Midwest and Great Lakes states. During a La Niña, snowfall is above normal across the Pacific Northwest and western Great Lakes.[12]

Severe weather

During El Niño, the jet stream is oriented from west to east across the southern portion of the United States, making the region more susceptible to severe weather outbreaks. During La Niña, the jet stream and severe weather are likely to be farther north than normal.[13]

Tropical cyclones

Due to changes in upper level winds caused by variations in the ENSO cycle, the likelihood of a North Atlantic hurricane hitting the Continental United States is increased during La Niña conditions, and decreased during El Niño conditions.[1] On the other hand, however, hurricanes in the eastern and central Pacific are the reverse of those in the Atlantic due to increased sea surface temperatures;[14] therefore, the odds of a hurricane hitting Hawaii[15] or, rarely, California[16][17] are increased during El Niño years and decreased during La Niña years.

References
  1. "How do El Niño and La Nina influence the Atlantic and Pacific hurricane seasons?". Climate Prediction Center. Archived from the original (FAQ) on 2009-08-27. Retrieved 2008-03-21.
  2. Climate Prediction Center. Average October-December (3-month) Temperature Rankings During ENSO Events. Retrieved on 2008-04-16.
  3. Climate Prediction Center. Average December-February (3-month) Temperature Rankings During ENSO Events. Retrieved on 2008-04-16.
  4. John Monteverdi and Jan Null. WESTERN REGION TECHNICAL ATTACHMENT NO. 97-37 NOVEMBER 21, 1997: El Niño and California Precipitation. Retrieved on 2008-02-28.
  5. Nathan Mantua. La Niña Impacts in the Pacific Northwest. Archived 2007-10-22 at the Wayback Machine Retrieved on 2008-02-29.
  6. Reuters. La Nina could mean dry summer in Midwest and Plains. Retrieved on 2008-02-29.
  7. Climate Prediction Center. El Niño (ENSO) Related Rainfall Patterns Over the Tropical Pacific. Archived 2010-05-28 at the Wayback Machine Retrieved on 2008-02-28.
  8. Pao-Shin Chu. Hawaii Rainfall Anomalies and El Niño. Retrieved on 2008-03-19.
  9. Pacific ENSO Applications Climate Center. Pacific ENSO Update: 4th Quarter, 2006. Vol. 12 No. 4. Retrieved on 2008-03-19.
  10. Pacific ENSO Applications Climate Center. RAINFALL VARIATIONS DURING ENSO. Archived 2008-04-21 at the Wayback Machine Retrieved on 2008-03-19.
  11. "ENSO effects across the northeastern Caribbean". noaa.gov. Retrieved 19 October 2015.
  12. Climate Prediction Center. ENSO Impacts on United States Winter Precipitation and Temperature. Retrieved on 2008-04-16.
  13. Climate Prediction Center. What impacts do El Niño and La Niña have on tornado activity across the country? Archived 2009-08-27 at the Wayback Machine Retrieved on 2008-04-16.
  14. "Impacts of El Niño and La Niña on the hurricane season - NOAA Climate.gov". climate.gov. Retrieved 19 October 2015.
  15. "Climate researchers discover El Nino's fueling effect on intense hurricanes". Hawaii 24/7. Retrieved 19 October 2015.
  16. "Remembering the San Diego Hurricane of 1858". The Weather Channel. 9 September 2014. Retrieved 19 October 2015.
  17. "Climate Change: Vital Signs of the Planet: Could a hurricane ever strike Southern California?". Climate Change: Vital Signs of the Planet. Retrieved 19 October 2015.
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