Butterfly effect and a self-modulating El Niño response to greenhouse warming

Abstract:

El Niño and La Niña, collectively referred to as El Niño-Southern Oscillation (ENSO), are highly nonlinear. For example, maximum warm anomalies of El Niño, which occur in the equatorial eastern Pacific Ocean, are larger than maximum cold anomalies of La Niña, which centre in the equatorial central Pacific. The associated atmospheric nonlinear thermal damping cools the equatorial Pacific during El Niño but warms during La Niña. Under greenhouse warming, climate models project an increase in frequency of strong El Niños and La Niñas, but the change differs vastly across models. Here we show that an infinitesimal random perturbation to an identical initial condition, like a butterfly effect, induces vastly different initial ENSO variability, which systematically affects its response to greenhouse warming a century later. In experiments with higher initial variability, a greater cumulative oceanic heat loss from ENSO thermal damping reduces stratification of the upper equatorial Pacific Ocean, leading to a smaller increase in ENSO variability under greenhouse warming. Thus, greenhouse warming-induced increase in ENSO variability, if suppressed initially by internal variability, is likely to enhance in the future. This self-modulation linking ENSO variability across time presents a novel perspective for understanding ENSO variability and change.