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 June–August 2025 season was -0.18 °C, and the monthly value for August was -0.317 °C. While these recent seasonal and monthly values suggest IOD-neutral conditions, they also point to the potential for a gradual transition toward a negative IOD phase. According to the IRI/CCSR’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 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 September (CESM1, CFSv2, CanESM5, GEM-NEMO, GFDL-SPEAR), along with their equally-weighted multi-model mean. The observed DMI (shown in black) indicates a clear downward trend from May to August 2025, entering negative IOD-neutral conditions by late August. From September onward, all models forecast a transition into a negative IOD phase, with most predicting the DMI to fall below the −0.4 °C threshold during October—confirming the likely onset of a negative IOD event. Each model exhibits unique behavior in terms of intensity and recovery. CESM1 predicts the strongest negative dip in October and shows a rapid rebound, reaching neutral and then weak positive conditions by March 2026. CFSv2 and CanESM5 also predict a significant negative phase in early boreal fall but suggest a more gradual recovery through the winter, approaching neutral conditions by January or February. GEM-NEMO projects a somewhat milder negative event with a relatively steady return to neutral DMI by early 2026. In contrast, GFDL-SPEAR maintains a more persistent negative bias throughout the forecast period, with slower recovery and limited transition back to neutral conditions. Overall, while all models agree on the initiation of a negative IOD event, there is considerable spread in the strength and duration of the event, particularly beyond November. This divergence highlights the uncertainty in seasonal forecasting of IOD evolution, especially during transitions out of peak phases.

Probabilistic IOD Forecasts from the NMME

Based on September 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. The probabilities are computed using a counting method, where each ensemble member from the contributing models is treated individually to determine the likelihood of a negative, neutral, or positive IOD phase for the following months. The climatological probabilities for each IOD phase based on historical data are shown as dotted lines for reference.

The forecast shows a strong model consensus toward a negative IOD event during the boreal fall and early winter. For September and October, the probability of a negative IOD is near or at 100%. This high confidence persists through November, where the probability remains above 75%, while neutral and positive phases are comparatively unlikely. By December, the likelihood of a negative IOD begins to decrease (to around 50%), with a corresponding rise in the probability of neutral conditions. From January through March 2026, the forecasts shift toward increasing neutral probabilities, indicating a gradual decay of the negative IOD signal and a return to climatologically typical conditions. Positive IOD probabilities remain very low throughout the entire forecast period.

In summary, the forecast suggests a highly likely negative IOD event unfolding through the remainder of 2025, peaking in strength during October–November and weakening by early 2026. The strong agreement among model members, especially during the early months, lends confidence to the prediction of a significant negative IOD phase.