Defining El Nio indices in a warming climate

Extreme weather and climate events associated with El Nio and La Nia 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 Nio events to seem bigger than they are, while cold La Nia events seem smaller, in the commonly used Nio3.4 index (sea surface temperature (SST) anomalies over 5° S-5° N, 120-170° W). We propose a simple and elegant adjustment, defining a relative Nio3.4 index as the difference between the original SST anomaly and the anomaly over all tropical oceans (20° S-20° N). 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 Nio3.4 index is better in line with effects on rainfall and would be more useful for preparedness for El Nio and La Nia in a changing climate and for El Nio - Southern Oscillation research.