Factors Determining the Frequency of El Niño

Abstract 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 has not been understood as well as some earlier ENSO theories suggested. Here, we investigate factors controlling the frequency of El Niño by employing a nonlinear box model 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. 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 upwelling, and the dynamical adjustment time scale for the upper ocean. Sensitivity experiments show that El Niño frequency depends as much on the intensity of radiative heating as on other parameters. A fundamental mechanism by which the intensity of radiative heating affects the frequency of El Niño is its effect on the generation of increasingly complex limit cycles, which in turn alters 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. Significance Statement Factors determining the frequency of El Niño are not yet fully understood. Existing theories and state-of-the-art models do not agree on the key factors that influence El Niño frequency. Sensitivity experiments with a nonlinear box model reveal that El Niño frequency depends on several factors that correspond to well-defined physical processes, including the intensity of radiative heating. A fundamental mechanism by which the intensity of radiative heating affects El Niño frequency is its impact on the generation of increasingly complex limit cycles, which in turn alters oscillation patterns. The results highlight the critical role of nonlinearity in determining the frequency of El Niño. They also shed light on how the intensity of radiative heating may influence El Niño frequency.