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The Instrumental Record

The Changing Relationships between El Niño/Southern Oscillation (ENSO) Variability and Moisture Balance in the Continental United States
J.E. Cole and E.R. Cook
Complete Scientific Reference

Variations in conditions in the equatorial Pacific related to the El Niño/Southern Oscillation (ENSO) have corresponding effects, called teleconnections, on climate in North America. Typically, warm ENSO conditions (El Niños) correspond to wet conditions in the southern and southwestern U.S. while cold ENSO (La Niñas) conditions correspond to dry conditions in these regions. These relationships are variable over time, however, and the animation on this page illustrates the variable relationship between ENSO, measured by the Southern Oscillation Index (SOI), and PDSI in the United States, for the past 130 years. The significance of these relationships, and what this animation illustrates, is that there are slowly varying controls on regional climate in the United States which are driven, in part, by changes in the equatorial Pacific.

  Instrumental PDSI

 Reconstructed PDSI



Each map represents the pattern of correlations between the SOI averaged for December-February and a gridded network of PDSI values for the United States, for a 30-year period (the first map is for 1867-1896 and the last is for 1966-1995). The maps show a progression of these spatial patterns one year at a time, that is, each 30 year period is shifted forward one year for each map. The maps are paired so that the one on the left is the map of correlations between the SOI and the instrumental PDSI, and the one on the right is the same, but the PDSI is reconstructed using tree rings. Although the instrumental SOI record extends from 1867 to the present, the instrumental PDSI record does not. Therefore, the tree-ring reconstructed PDSI records are used for the correlations maps for the years prior to the instrumental record (1895), then both are displayed, until the tree-ring reconstructions end (1980), and just the instrumental record-based maps are shown to 1997.

The maps show the strength and direction of the relationships between ENSO (which oscillates between El Niño and La Niña modes) and PDSI. Negative or purple/blue values indicate areas where La Niñas (cold events, positive SOI) correspond to drought OR El Niños (warm events, negative SOI) correspond to wet conditions (typical in the Southwestern U.S.). Positive or red/yellow values indicate areas where El Niños (warm events, negative SOI) correspond to drought OR La Niñas (cold events, positive SOI) correspond to wet conditions. The animations show how these spatial patterns change over 30-year time periods.


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