Interpretation of plots relating El Nino 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 Nino
or La Nina.
How to interperate 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 Nino or La Nina. They give an indication
of the typical atmospheric response during en 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 Interperate 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 Nino (or La Nina)
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 Nino years), one would expect 4 of the 20 El Nino 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.
