El Niño/Southern Oscillation
The El Niño/Southern Oscillation (ENSO) is the major source of inter-annual climate variability in the Pacific Northwest (PNW). ENSO variations are more commonly known as El Niño (the warm phase of ENSO) or La Niña (the cool phase of ENSO).
An El Niño is characterized by stronger than average sea surface temperatures in the central and eastern equatorial Pacific Ocean, reduced strength of the easterly trade winds in the Tropical Pacific, and an eastward shift in the region of intense tropical rainfall (Figure 1). A La Niña is characterized by the opposite – cooler than average sea surface temperatures, stronger than normal easterly trade winds, and a westward shift in the region of intense tropical rainfall. Average years, i.e., years where there is no statistically significant deviation from average conditions at the equator, are called ENSO-neutral. Each ENSO phase typically lasts 6 to 18 months (Figure 2).
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Figure 1 Warm Phase ENSO (El Niño). The spatial pattern of anomalies in sea surface temperature (shading, degrees Celsius) and sea level pressure (contours) associated with the warm phase of ENSO (i.e., El Niño) for the period 1900-1992. Contour interval is 1 millibar, with additional contours drawn for +0.25 and 0.5 millibar. Positive (negative) contours are dashed (solid).
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Figure 2 Multivariate ENSO index, 1950-2009. Positive (red) index values indicate an El Niño event. Negative (blue) values indicate a La Niña event. For information on the current status of ENSO, please see the NOAA-CIRES Climate Diagnostics Center ENSO page.
Although ENSO is centered in the tropics, the changes associated with El Niño and La Niña events affect climate around the world. ENSO events tend to form between April and June and typically reach full strength in December (hence the name El Niño, which is Spanish for “Little Boy” or “Christ Child”; La Niña means “Little Girl”). The ENSO influence on PNW climate is strongest from October to March; by summer, Northern Hemisphere wind patterns are such that they effectively trap ENSO-related disturbances in the tropics.
The CIG has demonstrated numerous linkages between changes in ENSO and variations in PNW climate and natural resources. El Niño winters, for example, tend to be warmer and drier than average with below normal snowpack and streamflow. La Niña winters tend to be cooler and wetter than average with above normal snowpack and streamflow. These linkages and the availability of ENSO forecasts a few months to one year in advance of the event’s maturation provide resource managers opportunity to consider how a particular ENSO forecast may affect resource management choices.
More about ENSO
ENSO forecasts are derived from numerical prediction models using data collected by more than 400 deep ocean monitoring buoys distributed throughout the equatorial Pacific. These buoys, known collectively as the Tropical Atmosphere Ocean (TAO) array, provide real-time data of tropical Pacific ocean and climate conditions, including sea surface temperature, currents, and winds.
ENSO forecasts are widely (and freely) available through dozens of forecasting centers around the world. The CIG develops its seasonal climate outlook and resource forecasts for the PNW using the global ENSO forecasts and other indicators of key climate conditions.