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SOI - Precipitation Relationships

Summer/Autumn Southern Oscillation Index (SOI) versus following October-March Climate following methodology of Redmond and Koch, Water Resources Research, 1991
Clearwater R. data courtesy David Garen, NRCS-Portland
American R. data courtesy Maurice Roos, DWR-California


The Southern Oscillation Index (SOI) is one measure of the large-scale fluctuations in air pressure occurring between the western and eastern tropical Pacific (i.e., the state of the Southern Oscillation) during El Niño and La Niña episodes. Traditionally, this index has been calculated based on the differences in air pressure anomaly between Tahiti and Darwin, Australia. In general, smoothed time series of the SOI correspond very well with changes in ocean temperatures across the eastern tropical Pacific. The negative phase of the SOI represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin. Prolonged periods of negative SOI values coincide with abnormally warm ocean waters across the eastern tropical Pacific typical of El Niño episodes. Prolonged periods of positive SOI values coincide with abnormally cold ocean waters across the eastern tropical Pacific typical of La Niña episodes.

Important points from these diagrams:

  • Not every El Niño produces the same effect
  • La Niña has a more consistent signal, in general, than El Niño
  • The relations are not perfect, other things are happening in the climate system
  • The 1982/83 El Niño does not fit in with the other points in some locations; patterns for large El Niños may differ in some ways from typical El Niño patterns
  • The relationship is lagged; best associations are found between summer/autumn SOI and the following winter climate, and the following spring and summer streamflow runoff

Scatter Plots

  • Arizona Statewide (1933-2020)
  • Southwest Arizona Climate Division (Division 5) (1933-2020)
  • California Statewide (1933-2020)
  • North Coast California Climate Division (Division 1) (1933-2020)
  • Sacramento Drainage California Climate Division (Division 2) (1933-2020)
  • Northeast Interior Basins California Climate Division (Division 3) (1933-2020)
  • Central Coast California Climate Division (Division 4) (1933-2020)
  • San Joaquin California Climate Division (Division 5) (1933-2020)
  • South Coast California Climate Division (Division 6) (1933-2020)
  • Southeast Desert California Climate Division (Division 7) (1933-2020)
  • Colorado CO Drainage Basin Climate Division (Division 2) (1933-2020)
  • Idaho Statewide (1933-2020)
  • Central Idaho Mountains Climate Division (Division 4) (1933-2020)
  • Montana Statewide (1933-2020)
  • Western Montana Climate Division (Division 1) (1933-2018)
  • Nevada (Division 1) (1933-2020)
  • Nevada (Division 2) (1933-2020)
  • Nevada (Division 3) (1933-2020)
  • Nevada (Division 4) (1933-2020)
  • New Mexico Statewide (1933-2020)
  • Utah Statewide (1933-2020)
  • Utah Dixie Climate Division (Division 2) (1933-2020)
  • Utah South Central Climate Division (Division 4) (1933-2020)
  • Utah Northern Mountains Climate Division (Division 5) (1933-2020)
  • Washington Statewide (1933-2020)
  • Wyoming Statewide (1933-2018)
  • Wyoming Snake Drainage Climate Division (Division 2) (1933-2020)
  • Wyoming Wind River Climate Division (District 9) (1933-2020)

Scatter Plot Data

  • Monthly SOI Values
  • NCEI Divisional Data

Historical Interest Scatter Plots

  • Northern Sierra (California) 8-station index (1933-2018) (8 Stations in Northern Sierra used by CA DWR)
  • San Joaquin (California) 5-station index (1933-2014) (5 Stations in Southern Sierra used by CA DWR)
  • American River @ Fair Oaks (Sacramento) Winter 3-Day Maximum Flow Streamflow (1933-2000)
  • N Fork Clearwater River (Central Idaho) April-July Streamflow (1933-1994)
  • Reno, Nevada (1933-2014)
  • Las Vegas, Nevada (1937-2014)

Time Series

  • Time Series of SOI (June-Nov, after Redmond & Koch, 1991)
  • Las Vegas, Nevada

return to main ENSO page


soi-precip plot