Mechanisms of the ENSO Asymmetry

We explore the asymmetry of El Niño and La Niña through a combination of observational analyses, coupled GCM experiments and simplified dynamical models. Although El Niño and La Niña are counterparts of each other in many ways, they are not exact mirror image. In the recent decades, El Niños tend to terminate by the following boreal summer and turn into a La Niña, while many La Niñas persisted through the following year and reintensify in the next winter. A 4000-year control simulation of the GFDL CM2.1 global coupled GCM also exhibits such asymmetries despite the strong interdecadal and intercentennial modulation of its ENSO behavior. We demonstrate here that by increasing the coupling efficiency between the ocean and the atmosphere during warm states relative to cold states, a stochastically driven linear delay-oscillator model is able to reproduce oscillations in which the warm peaks tend to be followed by cold peaks more likely than vice versa, and the transitions from a cold event to a warm event also span a longer time than the other way round.