The above figure shows the frequency distribution of the S index's monthly values of SSTA for the 1,234 months between January 1895 and October 1997. Extreme sextile and decile thresholds are used here to define extreme SST conditions because the S index is somewhat skewed towards positive values and is not normally distributed. In testing for ENSO-related skewness in aseasonal rainfall and temperature data over the central U.S., seasons of moderate and strong El Niño conditions were identified as those 3 month periods during which the average S index value was above the sixth sextile (+.84° C) of the distribution of historical S index values. Conversely, 3 month periods during which the average S value was below the first sextile (-.64° C) marked seasons of moderate and strong La Niña conditions. Periods of strong El Niño (La Niña) forcing were identified as those seasons during which the average S value was above (below) the tenth (first) decile of historical values, +1.09° C (-.84° C).

      The relative frequency with which monthly SSTA assumes extreme values reveals a tendency for the ENSO mechanism to be active during certain parts of the seasonal cycle. The figure above shows how months marked by SSTA conditions exceeding the highest (warmest) sextile and decile were distributed throughout the annual cycle. Although instances of anomalous warm temperatures are evident during the spring and summer months, the broad peak spanning September-February shows that the El Niño phase is mainly active during the northern fall and winter months. The tendency for SSTA conditions in the highest 10% to occur during November-December-January indicates that the strength of events tends to peak during that period.

La Niña Seasonality: Number of months Wrights S Index occupied coldest percentiles during 1895-1997

      The figure above is the La Niña counterpart; i.e., the distribution of calendar months over the period 1895-1997 during which eastern Pacific SSTA was below the lowest (coldest) sextile and decile. A tendency for La Niña conditions to occur during a well defined northern winter period is not as evident, with both November and February suggested as periods of peak activity. However, an overall tendency for cold phase SST conditions to occur in the northern winter months is apparent.