Defining El Niño indices in a warming climate

G.J. Van Oldenborgh*, H. Hendon, T. Stockdale, M. L'Heureux, E. Coughlan De Perez, R. Singh, M. Van Aalst

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

23 Citations (Scopus)
69 Downloads (Pure)


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. © 2021 The Author(s). Published by IOP Publishing Ltd.
Original languageEnglish
Pages (from-to)1-10
Number of pages10
JournalEnvironmental research letters
Issue number4
Publication statusPublished - 11 Mar 2021


  • disaster preparedness
  • El Nino
  • global warming
  • Nino3.4 index
  • teleconnections
  • Atmospheric pressure
  • Global warming
  • Surface waters
  • Changing climate
  • Onset of convection
  • Relative indices
  • Sea surface temperature anomalies
  • Societal impacts
  • Southern oscillation
  • Tropical ocean
  • Warming climate
  • Oceanography
  • climate change
  • environmental research
  • warming


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