Fig. 2.24. (a) Schematic illustrating how changes in the mean or variance alter the risks of extreme events. (b),(c) Relative risk (compared to a climatological risk of 20%) of the occurrence of extreme springtime precipitation anomalies over the U.S. associated with El Niño and La Niña in the preceding winter. The impact of ENSO involves both changes in the mean and higher moments of the U.S. climate distributions. In particular, a shift in the mean increases the risk of one extreme and reduces it of the other. The areas marked 'Wet' and 'Dry' refer to these situations. On the other hand, increased or decreased variability associated with ENSO raises or lowers the risks of either extreme, which are marked 'Both' and 'Reduced', respectively.