Understanding ENSO Diversity

El Nino-Southern Oscillation (ENSO) is a naturally occurring mode of tropical Pacific variability, with global impacts on society and natural ecosystems. While it has long been known that El Nino events display a diverse range of amplitudes, triggers, spatial patterns, and life cycles, the realization that ENSO's impacts can be highly sensitive to this event-to-event diversity is driving a renewed interest in the subject. This paper surveys our current state of knowledge of ENSO diversity, identifies key gaps in understanding, and outlines some promising future research directions.The authors of this paper are members of the U.S. CLIVAR ENSO Diversity Working Group, sponsored by U.S. CLIVAR. The working group wishes to acknowledge the U.S. CLIVAR office for its support, and the U.S. CLIVAR funding agencies, NASA, NOAA, NSF, and DOE, for their sponsorship. The authors would also like to thank Drs. M. McPhaden and D. Dommenget, as well as two anonymous reviewers, for their careful readings of the manuscript, excellent suggestions, and constructive criticism, which have considerably improved the paper. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. AC acknowledges support from the National Science Foundation for this study