The paper by Trenberth and Stepaniack introduces two indices to characterize the nature and evolution of El Niño–Southern Oscillation (ENSO) events: the Niño-3.4 (N3.4) index and the Trans-Niño Index (TNI). The N3.4 index measures sea surface temperature (SST) anomalies in the Niño-3 region, while the TNI is defined as the difference in normalized SST anomalies between the Niño-1+2 and Niño-4 regions. These indices are approximately orthogonal, with TNI leading N3.4 by 3 to 12 months before the climate shift in 1976/77 and following N3.4 with opposite signs 3 to 12 months later. After 1976/77, the roles of TNI as a leading and lagging index reverse. The authors use these indices to analyze ENSO events, highlighting their different flavors and evolution, and suggest that at least two indices are necessary to fully characterize ENSO. They also discuss the importance of TNI for model evaluation and predictive purposes, noting that the change in TNI patterns outside the tropics is a limitation. The paper concludes by emphasizing the significance of the 1976/77 climate shift and its impact on ENSO evolution.The paper by Trenberth and Stepaniack introduces two indices to characterize the nature and evolution of El Niño–Southern Oscillation (ENSO) events: the Niño-3.4 (N3.4) index and the Trans-Niño Index (TNI). The N3.4 index measures sea surface temperature (SST) anomalies in the Niño-3 region, while the TNI is defined as the difference in normalized SST anomalies between the Niño-1+2 and Niño-4 regions. These indices are approximately orthogonal, with TNI leading N3.4 by 3 to 12 months before the climate shift in 1976/77 and following N3.4 with opposite signs 3 to 12 months later. After 1976/77, the roles of TNI as a leading and lagging index reverse. The authors use these indices to analyze ENSO events, highlighting their different flavors and evolution, and suggest that at least two indices are necessary to fully characterize ENSO. They also discuss the importance of TNI for model evaluation and predictive purposes, noting that the change in TNI patterns outside the tropics is a limitation. The paper concludes by emphasizing the significance of the 1976/77 climate shift and its impact on ENSO evolution.