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Chapter 6: Arizona Climate and Riparian Areas
El Niño – Southern Oscillation
- El Niño episodes are characterized by an increase in eastern equatorial Pacific sea-surface temperatures.
- This is accompanied by a weakening of the easterlies and a shift towards lower surface pressures across the central and eastern equatorial Pacific (the Southern Oscillation); a large area of convection typically over the western Pacific moves eastward with the warmer sea-surface temperatures.
Figure 6.5. Cumulus Clouds.
- The circulation pattern associated with El Niño is a strengthened subtropical jet that delivers moisture and wet weather directly to Arizona.
- Negative values of the Southern Oscillation Index (SOI) are associated with El Niño episodes.
- La Niña episodes are characterized by below normal
sea surface temperatures in the eastern equatorial Pacific
Ocean;
- La Niña events tend to create a blocking ridge over the western U.S. that directs winter storms towards the Northwest coast leaving Arizona dry.
- Positive values of the Southern Oscillation Index are associated with La Niña episodes.
- Role in interannual climate variability for Arizona.
- ENSO events usually cycle between El Niño and La Niña events every 3 to 7 years influencing interannual shifts between wet and dry periods.
- Winter precipitation is most strongly influenced by ENSO induced changes in atmospheric circulation patterns. Summer precipitation is only weakly correlated to ENSO.
- La Niña vs. El Niño – precipitation
teleconnections.
- El Niño winters are often wetter than average, but drier than average El Niño winters do occur.
- La Niña winters are more often drier than average.
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