The Amplitude-Duration Relation of Observed El Niño Events

Abstract

The authors demonstrate that the El Nino events in the pre- and post-1976 periods show two ampli- tude-duration relations. One is that the stronger El Nino events have longer durations, which is robust for the moderate El Nino events; the other is that the stronger El Nino events have shorter durations but for strong El Nino events. By estimating the sign and amplitude of the nonlinear dynamical heating (NDH) anomalies, the au- thors illustrate that the NDH anomalies are negligible for moderate El Nino events but large for strong El Nino events. In particular, the large NDH anomalies for strong El Nino events are positive during the growth and mature phases, which favor warmer El Nino events. During the decay phase, however, the negative NDH anomalies start to arise and become increasingly significant with the evolution of the El Nino events, in which the negative NDH anomalies dampen the sea surface temperature anomalies (SSTA) and cause the El Nino events to reach the SST normal state earlier. This pattern suggests that the nonlinearity tends to increase the intensities of strong El Nino events and shorten their duration, which, together with the previous results showing a positive correlation between the strength of El Nino events and the signifi- cance of the effect of nonlinear advection on the events (especially the suppression of nonlinearity on the SSTA during the decay phase), shows that the strong El Nino events tend to have the amplitude-duration relation of the stronger El Nino events with shorter durations. This result also lends support to the assertion that moderate El Nino events possess the amplitude-duration relation of stronger El Nino events with longer durations.