Observations and model simulations indicate long-term changes in the frequency of El Niño events, but explanations for these changes diverge. This divergence underscores that the frequency of El Niño events has not been understood as well as some earlier ENSO theories suggested. Here, we investigate factors controlling the frequency of El Niño events by employing a nonlinear box model that has been demonstrated to be capable of simulating a broad range of observed ENSO properties, including the asymmetry between its two phases. The model includes multiple parameters, each corresponding to a distinct and well-defined physical process. One of these parameters is the radiative-convective equilibrium SST, which measures the intensity of radiative heating exerted upon the coupled tropical ocean-atmosphere system. A fundamental mechanism by which the intensity of radiative heating affects the frequency of El Niño events is its effect on the generation of increasingly complex limit cycles, which in turn alters oscillation patterns. Other parameters are the dynamical coupling strength between the atmosphere and ocean, the thermal damping coefficient from the atmosphere, the strength of zonal advection relative to the total upwelling, and the dynamical adjustment time-scale for the upper ocean. Sensitivity experiments show that the frequency of occurrence of El Niño events depends as much as on the intensity of radiative heating as on other parameters. These parameters are found to affect the frequency of El Niño events in a way similar to that of the intensity of radiative heating: their changes induce regime changes, and thereby alter oscillation patterns. These findings highlight fundamental limitations of linear approaches to address the problem of El Niño frequency. They also provide insight into how a change in the intensity of radiative heating, such as that induced by a change in the greenhouse effect, may alter the frequency of El Niño events.
 

El Nino/Southern Oscillation,frequency,radiative heating