November 2025 IOD Discussion

November 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. The Dipole Mode Index (DMI) is then 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 is used to quantify the strength and phase of the Indian Ocean Dipole (IOD).

Current Conditions

In October 2025, the Dipole Mode Index measured –0.87 °C, signaling the continued persistence of negative IOD conditions across 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 phase is defined 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 DMI to form an IOD forecast plume. The IOD plume plot shows the latest set of predictions from early November (CESM1, CFSv2, CanESM5, GEM-NEMO, NASA), along with their equally weighted multi-model mean (MME). The observed DMI (shown in black) indicates a consistent downward trend from May to October 2025, entering negative IOD conditions by late August. All models predict a continuation of the current negative IOD phase during November 2025. Subsequently, almost all models (except one) begin to diverge, and the MME forecast indicates a rapid transition to IOD-neutral conditions in December, 2025. 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 November 2025 initialization data, the model-based probabilistic forecast of the Indian Ocean Dipole 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.

For the month of November, the forecast indicates strong model consensus on the persistence of a negative IOD event. The probability of a negative IOD in November is near or at 100%. This high confidence drops rapidly in December, when the likelihood of remaining in the negative IOD phase falls below 40%, accompanied by a corresponding increase in the probability of neutral conditions. From December 2025 through May 2026, the forecast shifts toward higher probabilities of IOD-neutral, suggesting a rapid 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 in November 2025, followed by a transition toward neutral IOD conditions thereafter.