Ch. 21 ENSO in a Changing Climate: Challenges, Paleo-Perspectives, and Outlook

Abstract

The El Niño Southern Oscillation (ENSO) phenomenon is a dominant force driving year‐to‐year climate variability with ecological and socioeconomic impacts that reverberate around the globe. The complex processes that govern ENSO and its impacts have generated intense research over the past decades, reviewed in previous chapters: a better understanding of how ENSO responds to anthropogenic climate change requires effort in resolving how ENSO responds to and interacts with a multitude of factors such as weather‐scale phenomena, volcanic eruptions, orbital forcing, etc. This chapter highlights some key unresolved issues in ENSO, supplemented by analysis of paleoclimate data and past and future state‐of‐the‐art climate model simulations. First, paleo‐ENSO reconstructions indicate a weakening of ENSO variability accompanying a weaker seasonal cycle, albeit lacking a clear orbital signal. This apparent positive correlation between changes in the magnitude of the seasonal cycle and ENSO amplitude seems to hold in future greenhouse‐gas forcing scenarios. Yet the mechanisms behind this relationship remain unclear, as accelerated paleoclimate model simulations under orbital forcing show the opposite relationship, in accordance with the idea of frequency entrainment in nonlinear oscillatory systems. These results underscore another prominent unresolved question: is ENSO a nonlinear system exhibiting regime‐like behavior (internally generated or in response to external forcing), or is ENSO a stochastically forced linear system whose behavior is modulated by noise? The community’s efforts to answer this question face the limitations imposed by the short instrumental record. Paleoclimate reconstructions provide extensions of this record although there persist sampling issues and uncertainties surrounding the manifestation of the ENSO signal in hydroclimate records which are influenced by local and regional processes. In its conclusion, this chapter highlights recent research directions and underscores the need for sustained and improved observations, paleo‐proxy reconstructions, hierarchical climate modeling, theories, and collaboration across disciplines toward addressing the open ENSO questions.