ENSO modulation and mean-state interactions: a review

Abstract:

Is El Niño changing with global warming? How often could we expect extreme El Niño events? Can we anticipate decades with weak El Niño? To answer these questions confidently we need to understand the modulations of the El Niño Southern Oscillation phenomenon (ENSO) that occur on decadal and multi-decadal timescales and involve changes in El Niño amplitude, periodicity, dominant “flavors”, shifts in the Intertropical Convergence Zone (ITCZ) and other properties. Such modulations are observed in a hierarchy of models ranging from simple/idealized models to intermediate models to state-of-the-art GCMs. Two main paradigms have emerged to explain the observed modulations of ENSO: (i) internally-generated variations due to the chaotic nature of the ocean-atmosphere coupled system and (ii) externally-driven variations due to cyclic or secular changes in the properties of the tropical background state such as mean winds or ocean thermocline depth. This presentation will review these two paradigms, discussing such factors for example as the role of westerly wind bursts and the effect of changes in other ocean basins on ENSO. Understanding such effects is critical to distinguish anthropogenic changes from ENSO natural modulations.