Figure 14.7 Normalized Southern Oscillation Index from 1951 to 1999. The normalized index is sea-level pressure anomaly at Tahiti divided by its standard deviation minus sea-level pressure anomaly at Darwin divided by its standard deviation then the difference is divided by the standard deviation of the difference. The means are calculated from 1951 to 1980. Monthly values of the index have been smoothed with a 5-month running mean. Strong El Niño events occurred in 195758, 196566, 197273, 198283, 199798. Data from NOAA.