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Details of ENSO Risk Calculations

Temperature and precipitation data were obtained from The National Climatic Data Center (NCDC) and consist of 100 years of monthly values for each of 344 US climate divisions. The Souther Oscillation Index(SOI) (standardized by month) is used to quantify El Niño. It was obtained from JISAO (U of Washington). Missing values (5%) were filled in using data from other sources.

To calculate the relative risk of an extreme of a variable given the state of ENSO, the SOI index was first sorted for each season for the 100 years available. Then, the top and bottom 20 years (i.e. the La Niña's and El Niño's) were examined with respect to the variable in question. This variable was sorted as well for each climate division. Then, a contingency table was constructed for the number of years that were both El Niño and warm, El Niño and cold, La Niña and warm and La Niña and cold.

Cases Expected by Chance:
ENSO PhaseExtreme ColdNeutralExtreme Warm
El Niño4124
Neutral123612
La Niña4124

For the top/bottom extreme ENSO years, we would expect

(20/100)*20=4.0
or the 4 corners of the table above as the number of extreme warm/cold years that would match by chance.

For each climate division, we plotted

[(actual number of matches-4)/(matches expected by chance)]*100
for the warm and cold cases. Regions that represent increases that are significant at the 82.7%,93.6%, and 98.2% level, respectively, are shaded.

Since there are many climate divisions and 4 seasons, one would expect a certain number of chance occurrences where the number of matches is significantly higher or lower than the 4 expected. Field significance tests addressed this question and results (not shown) indicate that our results are significant.

How to interpret plots