Defining El Niño indices in a warming climate

Extreme weather and climate events associated with El Nino and La Nina cause massive societal impacts. Therefore, observations and forecasts are used around the world to prepare for such events. However, global warming has caused warm El Nino events to seem bigger than they are, while cold La Nina events seem smaller, in the commonly used Nino3.4 index (Sea Surface Temperature anomalies over 5oS–5oN, 120–170oW). We propose a simple and elegant adjustment, defining a relative Nino3.4 index as the difference between the original SST anomaly and the anomaly over all tropical oceans (20oS–20oN). This relative index describes the onset of convection better, is not contaminated by global warming and can be monitored and forecast in real-time. We show that the relative Nino3.4 index is better in line with effects on rainfall and would be more useful for preparedness for El Nino and La Nina in a changing climate and for ENSO research.