FY 2025

The El Niño Southern Oscillation (ENSO) recharge oscillator conceptual model: Achievements and future prospects

Vialard, J., F.-F. Jin, M.J. McPhaden, A. Fedorov, W. Cai, S.-I. An, D. Dommenget, X. Fang, M. Stuecker, C. Wang, A. Wittenberg, S. Zhao, F. Liu, S.-K. Kim, Y. Planton, T. Geng, M. Lengaigne, A. Capotondi, N. Chen, L. Geng, S. Hu, T. Izumo, J.-S. Kug, J.-J. Luo, S. McGregor, B. Pagli, P. Priya, S. Stevenson, and S. Thual

Rev. Geophys., 63(1), e2024RG000843, doi: 10.1029/2024RG000843, View open access article at AGU/Wiley (external link) (2025)

The recharge oscillator (RO) is a simple mathematical model of the El Niño Southern Oscillation (ENSO). In its original form, it is based on two ordinary differential equations that describe the evolution of equatorial Pacific sea surface temperature and oceanic heat content. These equations make use of physical principles that operate in nature: (a) the air-sea interaction loop known as the Bjerknes feedback, (b) a delayed oceanic feedback arising from the slow oceanic response to winds within the equatorial band, (c) state-dependent stochastic forcing from fast wind variations known as westerly wind bursts (WWBs), and (d) nonlinearities such as those related to deep atmospheric convection and oceanic advection. These elements can be combined at different levels of RO complexity. The RO reproduces ENSO key properties in observations and climate models: its amplitude, dominant timescale, seasonality, and warm/cold phases amplitude asymmetry. We discuss the RO in the context of timely research questions. First, the RO can be extended to account for ENSO pattern diversity (with events that either peak in the central or eastern Pacific). Second, the core RO hypothesis that ENSO is governed by tropical Pacific dynamics is discussed from the perspective of influences from other basins. Finally, we discuss the RO relevance for studying ENSO response to climate change, and underline that accounting for ENSO diversity, nonlinearities, and better links of RO parameters to the long term mean state are important research avenues. We end by proposing important RO-based research problems.

The El Niño Southern Oscillation (ENSO) is the main driver of Earth's year-to-year climate variations. ENSO arises from air-sea interactions in the tropical Pacific, but influences climate and societies globally. In recent decades, progress in the observing system and in numerical modeling yielded a better understanding of the physical processes that govern ENSO. Such understanding can be encapsulated in the recharge oscillator (RO) conceptual model, a simple mathematical representation of ENSO fundamental mechanisms, which accounts for ENSO's essential properties: its amplitude, dominant period, tendency to peak at the end of the year, and tendency for larger warm (El Niño) than cold (La Niña) events. We discuss this framework and propose how to adapt it to explore pressing research topics. First, recent research indicates that the RO can be extended to account for the ENSO diverse spatial patterns of ENSO variability, with anomaly centers in either the central or eastern Pacific. Second, we discuss RO applications for studying influences of regions outside the tropical Pacific on ENSO. Finally, we discuss the RO as a tool to understand the ENSO response to climate change. We conclude by compiling important problems related to these challenging topics.