Interpretation of Plots Relating El Niño to Temperature and Precipitation

Composite and Risk Plots:

These plots show different aspects of the seasonal average temperature and precipitation that exist during an El Niño or La Niña.

How to Interpret Composite Plots:

Composite ENSO plots are created by averaging the variable of interest, in this case temperature or precipitation for years where there was an El Niño or La Niña. They give an indication of the typical atmospheric response during an ENSO event.

Limitations of Method:

Since they show averages, it's possible that the composites are dominated by a few years with high values. It's also possible that the average never represents what any actual year would look like. The averages will change if the criteria for compositing is shifted or if a different dataset is used.

How to Interpret Risk Plots:

These plots show regions in the continental US where there has been a greater likelihood of an extreme cold or warm season than one would expect by chance during an El Niño (or La Niña) event. They do not show the expected value during the season and in fact, it's possible that the composite or average value is of a different sign than the expected risk. The numbers plotted indicate the percent increase over what would be expected normally.. As an example, for the number of years that we used (100 years and top 20 El Niño years), one would expect 4 of the 20 El Niño years to be wetter than normal. If the plot indicates a 75% increase, that would give (x-4)/4=.75 or x=7 of the years were wetter than normal.

Limitations of Method:

This method relies upon the statistics of past events and there is no guarantee that the same relationships will exist in the future. In addition, the method is sensitive to the years used, the definition of what constitutes an extreme event, the definition of an ENSO event, the datasets and the seasons used. Some of the high risk regions are relatively stable with different datasets and definitions of extreme events, especially in the Pacific NW, the Southwest US and the Southeast US.