event - to - event differences in ENSO spatial patterns and evolution . UNDERSTANDING ENSO DIVERSITY

E l Niño–Southern Oscillation (ENSO) is a naturally occurring phenomenon in the tropical Pacific that has global impacts of great relevance to society. The term El Niño refers to warming of the tropical Pacific Ocean occurring every 2–7 yr, while the opposite cold phase is known as La Niña. Anomalous warming or cooling conditions are associated with a large-scale east–west sea level pressure seesaw, termed the Southern Oscillation, which represents the atmospheric manifestation of the coupled ENSO phenomenon (McPhaden et al. 2006). ENSO events differ in amplitude, temporal evolution, and spatial pattern. While differences among ENSO events have been known for many years, a renewed interest in this diversity was stimulated by Larkin and Harrison (2005) and Ashok et al. (2007), who highlighted an unusual sea surface temperature anomaly (SSTA) pattern over the tropical Pacific during the summer of 2004 that was associated with remote impacts on surface air temperature and precipitation different from those related to “typical” El Niño conditions. Because of the similarity and differences from a typical El Niño, Ashok et al. (2007) named this pattern El Niño Modoki (a Japanese word that means similar but different) and argued that it is a different type of El Niño. Significant research has since been conducted to identify, describe, and understand these “El Niño types,” spurring debates on whether there are indeed two distinct modes of variability, or whether ENSO can be more aptly described as a diverse continuum. In this article, we report on advances in understanding ENSO diversity, drawing from community discussions that took place at the U.S. CLIVAR ENSO Diversity Workshop held in Boulder, Colorado,