A review of interdecadal changes in the nonlinearity of the El Nino-Southern Oscillation

Many features of the El Nino-Southern Oscillation (ENSO) display significant interdecadal changes. These include general characteristics such as amplitude, period, and developing features, and also nonlinearities, especially the El Nino-La Nina asymmetry. A review of previous studies on the interdecadal changes in the ENSO nonlinearities is provided. In particular, the methods for measuring ENSO nonlinearities, their possible driving mechanisms, and their interdecadal changes are discussed. Two methods for measuring ENSO nonlinearities are introduced; the maximum potential intensity, which refers to the upper and lower bounds of the cold tongue temperature, and the skewness, which represents the asymmetry of a probability density function. For example, positive skewness (a strong El Nino vs. a weak La Nina) of the tropical Pacific sea surface temperature (SST) anomalies is dominant over the eastern tropical Pacific, with an increase seen during recent decades (e.g., 1980-2000). This positive skewness can be understood as a result of several nonlinear processes. These include the warming effect on both El Nino and La Nina by nonlinear dynamic heating (NDH), which intensifies El Nino and suppresses La Nina; the asymmetric negative feedback due to tropical oceanic instability waves, which has a relatively stronger influence on the La Nina event; the nonlinear physics of the ocean mixed layer; the Madden-Julian-Oscillation/Westerly-Wind-Burst and ENSO interaction; the biological-physical feedback process; and the nonlinear responses of the tropical atmospheric convection to El Nino and La Nina conditions. The skewness of the tropical eastern Pacific SST anomalies and the intensities of the above-mentioned mechanisms have both experienced clear decadal changes in a dynamically associated manner. In particular, there is a dynamic linkage between the decadal changes in the El Nino-La Nina asymmetry and those in NDH. This linkage is based on the recent decadal changes in mean climate states, which provided a favorable condition for thermo-cline feedback rather than for zonal advection feedback, and thus promoted the eastward propagation of the ENSO-related atmospheric and oceanic fields. The eastward propagating ENSO mode easily produces a positive NDH, resulting in asymmetric ENSO events in which El Nino conditions are stronger than La Nina conditions.