Indian Ocean Dipole

October 2025 IOD Discussion

Note: The Dipole Mode Index is calculated based on the ERSSTv5 data. To account for evolving background conditions and long-term warming, SST anomalies were calculated relative to a sliding monthly climatology. For each month in the time series, the climatology was computed as the mean SST for that calendar month over the prior 30 years. Then the Dipole Mode Index (DMI) is defined as the difference in sea surface temperature anomalies between the western equatorial Indian Ocean (50°E–70°E, 10°S–10°N) and the southeastern equatorial Indian Ocean (90°E–110°E, 10°S–0°), and it is used to quantify the strength and phase of the Indian Ocean Dipole (IOD).

Current Conditions

The Dipole Mode Index for the month of September was -0.71 °C, indicative of the presence of negative IOD conditions in the Indian Ocean. According to the CCSR@NASA-GISS/IRI’s criteria for defining positive and negative Indian Ocean Dipole (IOD) events, a positive IOD is classified when the DMI exceeds +0.4 °C, and a negative IOD when it falls below −0.4 °C. The IOD is considered inactive (neutral) when the DMI lies between −0.4 °C and +0.4 °C.

Model-Based IOD Outlook: Deterministic Forecasts from the NMME

Forecasts from the latest set of operational models in the North American Multi-Model Ensemble (NMME) project are used to construct deterministic IOD forecasts from each individual model using its ensemble mean Dipole Mode Index (DMI) to form an IOD forecast plume. The IOD plume plot illustrates the latest set of predictions from early October (CESM1, CFSv2, CanESM5, GEM-NEMO, NASA), along with their equally weighted multi-model mean (MME). The observed DMI (shown in black) indicates a strong downward trend from May to September 2025, entering negative IOD-neutral conditions by late August. The models all predict a continuation of the negative IOD phase during October and November 2025. Subsequently, the individual models begin to diverge, and the MME forecast indicates a transition to IOD-neutral conditions in December. The MME forecast subsequently remains mostly in the IOD-neutral range, although individual models display substantial spread in their predictions.

Probabilistic IOD Forecasts from the NMME

Based on October 2025 initialization data, the model-based probabilistic forecast of the Indian Ocean Dipole (IOD) was generated by CCSR@NASA-GISS/IRI to assess potential phase developments. Probabilities are computed using a counting method, where each ensemble member from the contributing models is evaluated individually to determine the likelihood of a negative, neutral, or positive IOD phase for the upcoming months. Climatological probabilities for each IOD phase, based on historical data, are shown as dotted lines for reference.

The forecast indicates strong model consensus on the persistence of a negative IOD event through the boreal fall and early winter. For October, the probability of a negative IOD is near or at 100%. This high confidence continues into November, with probabilities remaining above 85%, while neutral and positive phases are comparatively unlikely. By December, the likelihood of a negative IOD decreases to below 40%, with a corresponding increase in the probability of neutral conditions. From December 2025 through April 2026, the forecast shifts toward higher neutral probabilities, suggesting a gradual weakening of the negative IOD signal and a return to climatologically typical conditions. Positive IOD probabilities remain very low throughout the forecast period—around 20% during spring 2026.

In summary, the forecast suggests a highly likely continuation of the negative IOD event through October and November 2025, followed by a transition toward neutral IOD conditions thereafter.