  El Ni\~no-Southern Oscillation (ENSO) exhibits diverse characteristics in
spatial pattern, peak intensity, and temporal evolution. Here we develop a
three-region multiscale stochastic model to show that the observed ENSO
complexity can be explained by combining intraseasonal, interannual, and
decadal processes. The model starts with a deterministic three-region system
for the interannual variabilities. Then two stochastic processes of the
intraseasonal and decadal variation are incorporated. The model can reproduce
not only the general properties of the observed ENSO events, but also the
complexity in patterns (e.g., Central Pacific vs. Eastern Pacific events),
intensity (e.g., 10-20 year reoccurrence of extreme El Ni\~nos), and temporal
evolution (e.g., more multi-year La Ni\~nas than multi-year El Ni\~nos). While
conventional conceptual models were typically used to understand the dynamics
behind the common properties of ENSO, this model offers a powerful tool to
understand and predict ENSO complexity that challenges our understanding of the
21st-century ENSO.
