Negative Indian Ocean Dipole Gets Underway: Neutral Conditions Early Next Year
The Indian Ocean Dipole is an inherent mode of climate variability over the Indian Ocean, independent of the El Nino/La Nina phenomenon. It is sea surface temperature anomalies and associated convective anomalies with opposite signs in the eastern and western tropical Indian Ocean. In the negative IOD phase, lower than normal SSTs and suppressed convective activity are seen over the western parts of the ocean and East African region, and increased convective activity is seen over Indonesia and Australia. During the monsoon season, a negative IOD can worsen the ill effects of an El Nino event, resulting in significantly deficient rainfall and an increased risk of drought over the Indian sub-continent.
The Indian Ocean Dipole index has breached the negative IOD threshold (≤ -0.4°C) for nine consecutive weeks, since 27th July 2025, a condition required to be classified as a negative IOD event. The latest IOD index value for the week ending 21 Sep 2025 was -1.15°C. The Bureau’s model predicts the negative IOD event to continue through the autumn of the Northern Hemisphere, with a return to neutral in the early winters of 2026. This is consistent with a typical IOD cycle.
ENSO: Starting September 2025, the Bureau of Meteorology, Australia, has updated the methodology to calculate SST indices for the El Nino Southern Oscillation (ENSO) to a new method called the relative Nino indices. ENSO refers to the natural climate pattern in the tropical Pacific Ocean that involves fluctuations in sea surface temperature patterns and the atmosphere above. It ranges from La Nina, to neutral, to El Nino. The relative Nino indices measure how unusually cool or warm the ocean is in areas of the Pacific Ocean. The temperature differences are compared to the average SST for the broader global tropical region. This helps to more clearly identify how the ocean and atmosphere interact to reinforce El Nino or La Nina states in a varying climate. As per BoM, the relative Nino indices provide better insights into the oceanic ENSO signal in a changing climate. It is believed that the forecast will improve when the changes are accounted for in the long-term warming trend in oceans.
Equatorial SSTs are near-to-below average across most of the Pacific Ocean, outside the Western Pacific. The negative sub-surface anomalies have also strengthened since mid-July, through August and September. The Nino 3.4 region, the marker index of ONI, has consistently remained below 0°C and hovered between -0.3°C to -0.5°C during August and September. All these are pointing towards a likely La Nina, establishing during the fall of this year.
IOD: The Indian Ocean Dipole index has now met the negative IOD threshold (less than or equal to -0.4°C) for nine consecutive weeks. The negative IOD index has strengthened in the last 5 weeks and remained cool between -1°C to -1.3°C. The latest index value for the week ending 21 Sep 2025 was -1.15°C. The model consensus suggests a highly likely negative IOD event unfolding through the remainder of 2025 and thereafter weakening by early 2026. Actually, by Dec 2025, the likelihood of a negative IOD will begin to recede with a corresponding rise in the probability of neutral conditions. Such a pattern is consistent with the typical IOD life cycle.
MJO: Dynamical models are able to effectively predict the Madden-Julian Oscillation, as far as 21 days into the future, which makes the MJO a valuable input for subsequent predictions. It is also demonstrated that dynamical modelling is in general more skillful when the MJO is active at the time of initialization. The MJO will propagate eastward across the Western Pacific to the Western Hemisphere and Africa in early October. Its current position over the Western Pacific may even be helpful in triggering stormy convection over the Indian Seas, at the fag end of September and the start of October.
Curtains are being drawn shortly on the monsoon 2025. The season has been exceptional, even without any constructive support of ENSO and IOD. Need to watch out for a tropical storm in the Indian Seas during early October.
