Diversity of ENSO Events Unified by Convective Threshold Sea Surface Temperature: A Nonlinear ENSO Index
We developed a process-based El Niño-Southern Oscillation (ENSO) index that, for the first time, accounts for the nonlinear relationship between sea surface temperature and deep convection by tracking the average longitude of tropical Pacific deep convection.
The new ENSO Longitude Index (ELI) better characterizes the diversity and extremes of ENSO compared to conventional indices, and reveals that extreme El Niño, La Niña, and Modoki events increase in frequency at the expense of neutral ENSO in 21st century climate projections. ELI addresses uncertainty in ENSO event characterization by shifting the perspective from categorical to distributional event definitions. This new index will simplify model evaluation of ENSO, and more effectively communicate the state of the coupled climate system.
We show that the well-known failure of any single index to capture the diversity and extremes of 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 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.