The article by Kevin E. Trenberth from the National Center for Atmospheric Research reviews the evolving meaning of the term "El Niño" and its various definitions over time. The term has become increasingly visible and significant in recent years as a major source of interannual climate variability. However, the lack of a universally accepted definition has led to confusion. The article explores possible quantitative definitions that match historically identified El Niño events after 1950, suggesting that an El Niño can be defined as occurring if the 5-month running mean of sea surface temperature (SST) anomalies in the Niño 3.4 region (5°N–5°S, 120°–170°W) exceeds 0.4°C for at least 6 months. With this definition, El Niños occur about 31% of the time, while La Niñas occur about 23%. The article also discusses the bimodal character of Niño 3.4 SST anomalies and the seasonal patterns of El Niño events, which typically begin in northern spring or summer and peak from November to January. The author recommends that any use of the term "El Niño" should specify the particular definition being used to reduce misunderstandings.The article by Kevin E. Trenberth from the National Center for Atmospheric Research reviews the evolving meaning of the term "El Niño" and its various definitions over time. The term has become increasingly visible and significant in recent years as a major source of interannual climate variability. However, the lack of a universally accepted definition has led to confusion. The article explores possible quantitative definitions that match historically identified El Niño events after 1950, suggesting that an El Niño can be defined as occurring if the 5-month running mean of sea surface temperature (SST) anomalies in the Niño 3.4 region (5°N–5°S, 120°–170°W) exceeds 0.4°C for at least 6 months. With this definition, El Niños occur about 31% of the time, while La Niñas occur about 23%. The article also discusses the bimodal character of Niño 3.4 SST anomalies and the seasonal patterns of El Niño events, which typically begin in northern spring or summer and peak from November to January. The author recommends that any use of the term "El Niño" should specify the particular definition being used to reduce misunderstandings.