La Niña Threshold Not Yet Reached, ENSO Neutral Conditions Persist: Full-fledged La Nina Requires At Least Five Months

Enso Status on 5th December 2024

Analysis & Commentary by Ashok Patel

Several international meteorological agencies, including the Indian Meteorological Department and various private meteorological organizations in India, had anticipated the development of a La Niña event during the Indian Summer Monsoon of 2024. However, we took a different stance and published monthly blog posts to provide a counter perspective, as summarized below:

5th July 2024
Title: “El Niño Ends & First ENSO Neutral Threshold Established End of June 2024 – Full-fledged La Niña As Per NOAA Criteria Not Expected During Indian Southwest Monsoon 2024” Available here
In our first post, we highlighted the end of the El Niño phase, with the ENSO conditions transitioning to neutral by the end of June. Based on NOAA criteria, we concluded that a full-fledged La Niña event was unlikely during the Indian Southwest Monsoon of 2024.

9th August 2024
Title: “NOAA Criteria Suggest Full-Fledged La Niña Unlikely in 2024 – Even a Single La Niña Threshold Unlikely During the Indian Southwest Monsoon” Available here
Our second update reiterated that, according to NOAA criteria, a full-fledged La Niña was unlikely in 2024. We further emphasized that even crossing a single La Niña threshold during the monsoon season seemed improbable.

6th September 2024
Title: “Much-Awaited La Niña Unlikely in 2024; La Niña Threshold Not Expected During Indian Southwest Monsoon”
Available here
By September, our analysis further confirmed that the much-anticipated La Niña event was unlikely to occur, with no indications of reaching the La Niña threshold during the Indian Southwest Monsoon.

23rd October 2024
Title: “La Niña Threshold Not Yet Reached: Full-fledged La Niña Unlikely During Indian Winter 2024-25” Available here
Our fourth update noted that as of September end, the La Niña threshold had still not been reached, suggesting that a full-fledged La Niña event during the Indian Winter of 2024-25 was also unlikely.

6th November 2024
Title: “La Niña Playing Hard Ball: Full-fledged La Niña Unlikely During Indian Winter 2024-25” Available here
Our fifth post reaffirmed that La Niña conditions were proving elusive. With no clear signals of a La Niña, we maintain that La Nina will not materialize during the Indian Winter 2024-25.

6th December 2024
Title: “ENSO Neutral Conditions Persist: La Niña Threshold Not Yet Reached, Full-fledged La Niña Requires At Least Five Months” 
In our latest update, we report that ENSO neutral conditions are still persisting. Despite the ongoing monitoring, the La Niña threshold has not yet been reached, and we stress that a full-fledged La Niña would require at least five months of sustained La Nina thresh hold conditions once it is reached.

What is a Fully Developed La Niña?

To understand why the anticipated La Niña failed to materialize, it’s important to define what constitutes a “fully developed La Niña,” or a “full-fledged La Niña,” based on NOAA’s operational definitions for ENSO (El Niño-Southern Oscillation):

• El Niño is characterized by a positive Oceanic Niño Index (ONI) greater than or equal to +0.5ºC.
• La Niña is defined by a negative ONI less than or equal to -0.5ºC.

For an event to be classified as a full-fledged El Niño or La Niña, the respective ONI threshold must be exceeded for at least five consecutive overlapping 3-monthly seasons.

In the case of the 2024 Indian Summer Monsoon, despite extensive monitoring, a La Niña event never materialized. The required threshold for La Niña, a negative ONI of -0.5ºC or lower, was never reached.

ONI Data has been obtained from CPC – NWS – NOAA available here

As of the end of November 2024, the Oceanic Niño Index (ONI) remains at -0.2°C, keeping the ENSO-neutral threshold intact for the SON (September-October-November) 2024 season. This marks the sixth consecutive ENSO-neutral condition. To transition into a La Niña state, the ONI would need to drop to at least -0.5°C during the OND (October-November-December) 2024 season. Specifically, the combined Niño3.4 sea surface temperature (SST) for September, October, and November 2024 must total at least -1.36°C.

Currently, the Niño3.4 SST is recorded at -0.26°C for October 2024 and -0.19°C for November 2024. To meet the necessary three-month total, the SST in December 2024 would need to drop to -0.91°C. If this occurs, the first La Niña threshold would be met for the OND 2024 season, potentially setting the stage for a fully developed La Niña by April 2025, provided the La Niña thresholds are sustained across all overlapping three-monthly seasons until then.

However, if the Niño3.4 SST does not reach -0.91°C by December 2024, ENSO-neutral conditions will persist. In that case, the earliest a fully developed La Niña could form would be in May 2025, assuming the necessary three monthly seasons thresholds are met till then.

Based on this analysis and NOAA criteria, a fully developed La Niña is unlikely unless the La Niña threshold is sustained over two to three consecutive seasons in the coming months. It is also possible that the first La Niña threshold may only be exceeded for a period of four or fewer consecutive overlapping three-month seasons, which, historically, would still be considered ENSO-neutral.

How ONI is determined:

The ONI is based on SST departures from average in the Niño 3.4 region, and is a principal measure for monitoring, assessing, and predicting ENSO. Defined as the three-month running-mean SST departures in the Niño 3.4 region. Departures are based on a set of improved homogeneous historical SST analyses (Extended Reconstructed SST – ERSST.v5).

NOAA Operational Definitions for El Niño and La Niña, El Niño: characterized by a positive ONI greater than or equal to +0.5ºC. La Niña: characterized by a negative ONI less than or equal to -0.5ºC. By historical standards, to be classified as a full-fledged El Niño or La Niña episode, these thresholds must be exceeded for a period of at least 5 consecutive overlapping 3-month seasons.

CPC considers El Niño or La Niña conditions to occur when the monthly Niño3.4 OISST departures meet or exceed +/- 0.5ºC along with consistent atmospheric features. These anomalies must also be forecast to persist for 3 consecutive months.

The Climate Prediction Center (CPC) is a United States Federal Agency that is one of the NECP, which are a part of the NOAA

Latest Oceanic Nino Index Graph Shows Enso Neutral
Conditions Prevail  At The End Of November 2024

The Table below shows the monthly SST of Nino3.4 Region and the Climate adjusted normal SST and SST anomaly from February 2023. Climate Base 1991-2020. ERSST.v5

Period    Nino3.4 ClimAdjust
YR   MON  Temp.ºC Temp.ºC ANOM ºC


2023   2   26.30   26.76   -0.46
2023   3   27.19   27.29   -0.11
2023   4   27.96   27.83    0.14
2023   5   28.40   27.94    0.46
2023   6   28.57   27.73    0.84
2023   7   28.31   27.29    1.02
2023   8   28.21   26.86    1.35
2023   9   28.32   26.72    1.60
2023  10   28.44   26.72    1.72
2023  11   28.72   26.70    2.02
2023  12   28.63   26.60    2.02
2024   1   28.37   26.55    1.82
2024   2   28.28   26.76    1.52
2024   3   28.42   27.29    1.12
2024   4   28.60   27.83    0.78
2024   5   28.17   27.94    0.24
2024   6   27.91   27.73    0.18
2024   7   27.34   27.29    0.05
2024   8   26.74   26.86   -0.11
2024   9   26.46   26.72   -0.25
2024  10   26.45   26.72   -0.26
2024  11   26.51   26.70   -0.19

Indications and analysis of various International Weather/Climate agencies monitoring Enso conditions is depicted hereunder:

Summary by: Climate Prediction Center / NCEP  Dated 2nd December 2024

ENSO Alert System Status: La Niña Watch

ENSO-neutral conditions are present.*
Equatorial sea surface temperatures (SSTs) are mostly near average in the central and eastern Pacific Ocean.
La Niña is most likely to emerge in October-December 2024 (57% chance) and is expected to persist through January-March 2025.*

Note: Statements are updated once a month (2nd Thursday of each month) in association with the ENSO Diagnostics Discussion, which can be found by clicking here.

Recent (preliminary) Southern Oscillation Index values as per The Long Paddock – Queensland Government.

Although the 30 Days average SOI was in neutral zone at +6.96 end of November 2024, today the 5th December 2024 it was in the positive zone at +10.95 as per The Long Paddock – Queensland Government and 90 Days average SOI was +4.34 on 5th December 2024.

As per BOM – Australia 23rd November 2024

• • The Bureau’s model suggests SSTs are likely to remain within the ENSO-neutral range (−0.8 °C to +0.8 °C) throughout the forecast period.
  • Of the 6 climate models surveyed (in addition to the Bureau model), 4 also suggest SSTs are likely to remain within the ENSO-neutral range. The remaining 2 suggest SSTs in the tropical Pacific are likely to meet or exceed the La Niña threshold (below −0.8 °C) throughout December to February, which would be a sufficient length of time to be classified as a La Niña event.
  • Should a La Niña develop, it is forecast to be relatively weak (in terms of the magnitude of the SST anomaly) and short-lived, with all models forecasting ENSO-neutral by March 2025.
  • In order to be considered to be in a La Niña event, changes in both the ocean and atmosphere need to be observed, to see a coupling between the Pacific Ocean and atmosphere. This coupling is a characteristic of a La Niña event and is what strengthens and sustains an event for an extended period.
  • At this time of year, ENSO forecast skill is high for up to 4 months ahead. All Seasons mentioned by BOM are with respect to Southern Hemisphere.