Indian Ocean Dipole

April 2026 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 March 2026, the observed Dipole Mode Index was +0.15, indicating neutral IOD conditions across the Indian Ocean. According to the CCSR/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 CESM1, CFSv2, CanESM5, GEM-NEMO, GFDL, and NASA along with their equally weighted multi-model mean (MME). Observations show that the DMI (shown in black) rose steadily from its negative-phase peak in October 2025, gradually weakening through late 2025 and returning to neutral IOD conditions by February 2026. Neutral conditions persist from January through March 2026. Forecasts indicate that the IOD will remain neutral during April and May 2026, with a gradual tendency toward positive IOD conditions. By June 2026, four out of six models exceed the positive IOD threshold and remain in positive territory through the end of the forecast period in October 2026. The multi-model ensemble also indicates a shift to positive IOD conditions from July 2026 onward, persisting through August, September, and October 2026.

Probabilistic IOD Forecasts from the NMME

Based on April 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 (92 in this case), 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 April and May 2026, neutral IOD conditions dominate, while the likelihood of positive IOD increases from ~1% in April to ~23% in May. From June through October, the probability of positive IOD continues to rise, while neutral conditions gradually decline. Probabilities of the negative IOD phase remain low throughout the forecast period. This transition from neutral toward positive IOD is broadly consistent with the CCSR/IRI ENSO outlook, which indicates a persistent dominance of El Niño conditions in the April 2026 forecasts.

In summary, April and May 2026 favor neutral IOD conditions, with a rising probability of positive IOD (from ~1% to ~23%), which then continues to strengthen and becomes dominant for the remainder of the forecast period.