Golden Gate Weather Services

CPC issued its new ENSO forecast today and is calling for "A transition from La Niña to ENSO-neutral favored through May-July 2026 (55% chance). In June-August 2026, El Niño is likely to emerge (62% chance) and persist through at least the end of 2026", and "If El Niño forms, the potential strength remains very uncertain, with a 1-in-3 chance that it would be "strong" during October-December." With an 83% chance it will be weak and 60% moderate. (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml)

But there are still the problems mentioned previously of 1) the "Spring Predictability Barrier" and 2) even knowing the strength of an ENSO event, there are still a wide range of possible solutions regarding how much precipitation and where. For example: https://www.ggweather.com/ca_enso2021/ca_elnino.html

New El Niño and La Niña Metric – the Relative Oceanic Niño Index (RONI)

Beginning February 1^st, the National Weather Service Climate Prediction Center will replace the Oceanic Niño Index (ONI) with the Relative Oceanic Niño Index (RONI).  The new metric subtracts the overall average tropical 3-month sea surface temperature anomaly (SSTA) from the existing Niño 3.4 regional anomaly. Per CPC: “This proposed change will have two primary benefits for ENSO monitoring and prediction: 1) the relative sea surface temperature index better identifies current and past El Niño and La Niña events because it is less sensitive to the chosen base climatology period.  This allows for comparison of events across the lengthy climate record. 2) A relative index is more related to changes in rainfall over the tropical Pacific than the traditional index.”

The new index is similar to the old, and users can use it in the same manner, with El Niño events being defined by the anomaly being ≥ +0.5°C and La Niña being ≤ -0.5°C. “Historic” El Niño and La Niña events will still be defined as at least five consecutive, overlapping 3-month anomalies.

The introduction of RONI has resulted in the recategorization of some “historic” events:
1958-1959 – Weak El Niño has been redefined to Neutral

1970-1971 - Weak La Niña has been redefined to Moderate
1971-1972 – Neutral has been redefined to Weak La Niña
1992-1993 - Neutral has been redefined to Weak El Niño
2011-2012 - Moderate La Niña has been redefined to Weak La Niña
2017-2018 - Weak La Niña has been redefined to Moderate La Niña
2019-2020 - Weak El Niño has been redefined to Neutral
2022-2023 - Weak La Niña has been redefined to Moderate
2023-2024 – Strong El Niño has been refined to Moderate El Niño
2024-2025 - Neutral has been redefined to Weak La Niña
2025-2026 - Neutral has been redefined to Weak La Niña

These categorizations mean a reevaluation of some of the historic analysis of temperatures and precipitation patterns during ENSO events under California/US Studies at https://ggweather.com/enso.htm.

Summary of Links:

• Relative Oceanic Niño Index (RONI): https://ggweather.com/enso/roni.htm

• Legacy Oceanic Niño Index (ONI): https://ggweather.com/enso/oni.htm

• CPC Public Information Statement re RONI: https://www.weather.gov/media/notification/pdf_2026/pns26-05_Relative_ONI.pdf

• Comparison Graphic of RONI and ONI:

  

• Journal of Climate “A Relative Sea Surface Temperature Index for Classifying ENSO Events in a Changing Climate”: https://journals.ametsoc.org/view/journals/clim/37/4/JCLI-D-23-0406.1.xml

Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Email: jnull@ggweather.com
Web: http://ggweather.com