Diversity of ENSO Events Unified by Convective Threshold Sea Surface Temperature: A Nonlinear ENSO Index

We show that the well‐known failure of any single index to capture the diversity and extremes of El Niño‐Southern Oscillation (ENSO) results from the inability of existing indices to uniquely characterize the average longitude of deep convection in the Walker Circulation. We present a simple sea surface temperature (SST)‐based index of this longitude that compactly characterizes the different spatial patterns, or flavors of observed and projected ENSO events. It recovers the familiar global responses of temperature, precipitation, and tropical cyclones to ENSO and identifies historical extreme El Niño events. Despite its simplicity, the new longitude index describes the nonlinear relationship between the first two principal components of SST, and unlike previous indices, accounts for background SST changes associated with the seasonal cycle and climate change. The index reveals that extreme El Niño, El Niño Modoki, and La Niña events are projected to become more frequent in the future at the expense of neutral ENSO conditions. Plain Language Summary It is widely known that every El Niño is different. The 2015–2016 El Niño event is a case in point. Despite having similar sea surface temperature warming as historical events (1982–1983 and 1997–1998), the 2015–2016 El Niño had unexpectedly weak impacts on precipitation in the Western United States. No single index has been able to capture the diversity of El Niño‐Southern Oscillation (ENSO) events (including La Niña), which has led to a proliferation of indices for each kind of ENSO event. Yet none of the existing indices measures the key atmospheric feature of ENSO—the west‐to‐east oscillation in the longitude of deep convection (thunderstorms) across the equatorial Pacific. We quantified this longitude and found that it compactly characterizes the diversity of ENSO events, clearly indicates extreme El Niño events (like 1982–1983 and 1997–1998) that produce the greatest impacts on global climate, and performs as well as existing indices in revealing the remote climate impacts of ENSO. Widespread adoption of this practical new index will simplify model evaluation of ENSO and more effectively monitor and communicate the state of the coupled climate system. Key Points The diversity and extremes of ENSO can be characterized by a simple index that tracks the average longitude of tropical deep convection The index accounts for the nonlinear response of convection to SST and provides a continuous time series for an