Indian Ocean Dipole

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 August 2026, the observed Dipole Mode Index was +0.2°C, indicating neutral IOD conditions across the Indian Ocean. According to the CCSR/IRI's criteria for defining positive and negative 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 and Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC) 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 (based on September 2026 initialization) from CESM1, CFSv2, CanESM5, GEM-NEMO, NASA, and CMCC along with their equally weighted multi-model mean (MME).

Observations show that the DMI (black line) remained in a neutral state during August 2026. Model forecasts of the DMI indicate that the IOD is expected to transition to positive conditions in September 2026 and remain strongly positive through December 2026, as indicated by both the individual model forecasts and their multi-model mean (thick red line). During January through March 2027, the forecasts indicate a return toward neutral IOD conditions. The emergence of a strongly positive IOD accompanies the exceptional evolution of El Niño, consistent with the well-documented tendency for these two climate modes to co-occur, while recognizing that they can also develop independently.

Probabilistic IOD Forecasts from the NMME

Based on September 2026 initialization data, the model-based probabilistic forecast of the Indian Ocean Dipole was generated by CCSR/IRI to assess potential phase developments. Probabilities are computed using an ensemble-member counting method, where all ensemble members from the contributing models combined (88 in all), are pooled 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. In September 2026, positive IOD conditions become dominant, with the probability of neutral conditions dropping sharply. The positive IOD signal strengthens and remains dominant through October–December 2026, while neutral conditions remain the next most likely category, although their probabilities are relatively low. From January through March 2027, the forecast shifts back toward neutral conditions. Overall, the forecast strongly favors the onset of a positive IOD in September 2026, its persistence through December, and a transition toward neutral conditions beginning in January 2027.