El Niño Amplitude (IMAGE) University of Hawaii at Manoa Caption This graph shows El Niño amplitude derived from North American tree rings (blue) and instrumental measurements (red). The green curve represents the long-term trend in El Nino strength. (Individual El Niño events occur typically at intervals of 2-7 years.) Periods of strong El Niño activity are indicated by amplitudes above 1.0. Superimposed on a general rising trend, cycles of strong activity occurred about every 50-90 years. Credit International Pacific Research Center Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.