Defining El Niño indices in a warming climate

Abstract Extreme weather and climate events associated with El Niño and La Niña 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 Niño events to seem bigger than they are, while cold...

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Bibliographic Details
Published in:	Environmental research letters 2021-04, Vol.16 (4), p.44003
Main Authors:	van Oldenborgh, Geert Jan, Hendon, Harry, Stockdale, Timothy, L’Heureux, Michelle, Coughlan de Perez, Erin, Singh, Roop, van Aalst, Maarten
Format:	Article
Language:	eng
Subjects:	Anomalies Climate Climate change Convection disaster preparedness El Nino El Niño Extreme weather Global warming La Nina Niño3.4 index Oceans Rainfall Sea surface temperature Southern Oscillation teleconnections Weather
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Summary:	Abstract Extreme weather and climate events associated with El Niño and La Niña 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 Niño events to seem bigger than they are, while cold La Niña events seem smaller, in the commonly used Niño3.4 index (sea surface temperature (SST) anomalies over 5 ∘ S–5 ∘ N, 120–170 ∘ W). We propose a simple and elegant adjustment, defining a relative Niño3.4 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 Niño3.4 index is better in line with effects on rainfall and would be more useful for preparedness for El Niño and La Niña in a changing climate and for El Niño—Southern Oscillation research.
ISSN:	1748-9326 1748-9326