ENSO Low‐Frequency Modulation and Mean State Interactions

Is El Niño changing with global warming? Can we anticipate decades with extreme El Niño events? To answer these questions confidently, we need to understand the modulation of the El Niño Southern Oscillation phenomenon (ENSO) that occur on decadal and multidecadal timescales and involve changes in El Niño amplitude, periodicity, dominant “flavors”, shifts in the Intertropical Convergence Zone, and other properties. As major progress has been made in understanding various factors that can affect these characteristics of El Niño, two main paradigms have emerged to explain the observed modulation of ENSO: (i) internally generated variations due to the chaotic nature of the ocean‐atmosphere coupled system and (ii) externally driven varia­ tions due to cyclic or secular changes in the properties of the tropical background state such as mean winds or ocean thermocline depth. This article reviews these two paradigms in the context of available observations, ide­ alized models, and comprehensive general circulation models describing El Niño. Which paradigm will domi­ nate in the coming decades and whether global warming is already affecting El Niño remains unclear. 1 Earth and Planetary Sciences, Yale University, New Haven, CT, USA 2 LOCEAN/IPSL, Sorbonne University, Paris, France 3 Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA 4 NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA 5 Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA 6 NCAR, Climate and Global Dynamics Division, Boulder, CO, USA 174 EL NIÑO SOUTHERN OSCILLATION IN A CHANGING CLIMATE