As of end-August, both atmosphere and ocean indicators continue to show ENSO-Neutral conditions. However, there are early indications that in the coming months, the tropical Pacific is leaning towards cooler sea surface temperatures.
Potentially, it may lead to weak La Niña conditions during the Northern Hemispheric autumn and fall of the year. The average Southern Oscillation Index (SOI) for July 2025 was 0.6, falling within the ENSO-Neutral range. Low-level wind anomalies were easterly across the east-central and eastern Pacific, and below-average surface temperatures have intensified over these parts of the equatorial Pacific. These cooler anomalies, if they strengthen further, and read in conjunction with enhanced trade winds, may possibly lead to the onset of La Niña conditions.
ENSO: El Niño Southern Oscillation (ENSO) is a global climatic phenomenon that emerges from variation in the winds and sea surface temperature over the tropical Pacific Ocean. These variations have an irregular pattern but do have some semblance of cycles. The occurrence of ENSO is not predictable. It affects the climate of much of the tropics and subtropics and has links (teleconnections) to the higher latitude regions of the world. The Indian monsoon has a strong correlation with ENSO — more robust with El Niño, cogent with La Niña, and subtle with ENSO-Neutral. Southern Oscillation, the atmospheric arm of ENSO, is generally negative (-0.5 or less) with El Niño, positive (+0.5 or more) with La Niña, and +/- 0.5 during Neutral. These values are not sacrosanct and may differ with respect to the intensity of El Niño or La Niña.
The SST anomaly in the Niño 3.4 region during the May–July 2025 season was -0.02°C and for June–August 2025, it is likely to be -0.1°C. For July 2025, the SST anomaly was -0.06°C and it dropped further to -0.07°C in August 2025. The most recent weekly average (centered on 25 Aug 2025) of the Niño 3.4 index was -0.4°C. It is the first time since Feb 2025 that all four Niño indices bear negative anomalies. Also, the ONI marker Niño 3.4 has reached its lowest value of -0.4°C since Feb 2025. The latest seasonal, monthly, and weekly values indicate that the tropical Pacific has been experiencing ENSO-Neutral conditions, albeit inclined to cooler-than-normal temperatures.
IOD: The Indian Ocean Dipole is neutral. However, in the last five weeks, the IOD index has been below the negative threshold of -0.4°C. The latest IOD index value for the week ending 24 Aug 2025 was -1.2°C. This is the lowest IOD since Aug 2022. Sustained index values less than or equal to -0.4°C for at least 8 weeks are typical of a negative IOD event. A negative IOD supposedly weakens the Indian Summer Monsoon, leading to reduced rainfall and potentially desiccation in the core monsoon zone. Development of a negative IOD is increasingly likely. Most international models predict a negative phase of the IOD during the Northern Hemisphere autumn. However, the skill for IOD forecasts made at this time of the year has been historically low beyond 2–3 months ahead.
MJO: Dynamical model MJO index forecasts are mixed and there is no clear verdict. While GEFS depicts MJO continuing a robust eastward propagation across the Maritime Continent during the next two weeks, some other model guidance suggests a rapid weakening of the index amplitude over the Indian Ocean. However, there is a consensus that the MJO may reemerge over the Indian Ocean and generate a new MJO-type event after the second week of September. This could as well be the last visit of the MJO pulse over the Indian Seas during the ongoing monsoon season.
The current conditions summarily indicate the persistence of ENSO-Neutral status. However, there are gradual developments and signs in both the ocean and atmosphere suggesting a possible onset of La Niña conditions in the coming months.
