La Niña July-August-September

      The above skewness analyses reflect shifts in seasonal rainfall and temperature during periods consistent with strong La Niña conditions; i.e., the 7 July-August-September periods listed above were marked by average S Index values in the lowest and coldest 10% of 1895-1997 values. Shaded climate divisions indicate areas that experienced a significant incidence of above or below median seasonal climate. Confidence levels for skewness can be found under the Above Median colorscale legend. Where climate divisions are annotated, significant skewness in the highest or lowest 25% of the historical distribution was found, with (n/m) indicating n seasons in the first 25%, m seasons in the fourth 25%.

      The temperature analysis shows a significant tendency to above median seasonal temperatures over the upper Midwest during periods of extreme cold equatorial Pacific SST conditions. A uniform tendency to fourth quartile seasonal temperatures over Iowa and Illinois during the 7 summer periods is also indicated. The shaded Illinois climate divisions annotated 1/4 show that those seasons resulted in 1-0-2-4 instances occurring in the 1st-2nd-3rd-4th quartile of JAS temperature. A similar tendency to warmth is found in the analysis of June-July-August periods marked by first decile S index values. A parallel study of JAS periods marked by SST anomalies in the 1st sextile reveals only scattered components of the seasonal warmth indicated above, suggesting that the apparent warming effects are associated with strong La Niña conditions. The skewness analysis for JAS rainfall reveals a significant, but not widespread, tendency to 1st quartile rainfall over Oklahoma and Kansas. The overall number of locally significant climate divisions in the precipitation analysis is globally significant at a 94.48% confidence level, but the fact that those climate divisions are mostly scattered about the study region does not suggest a coherent rainfall response.