Indian Ocean Dipole

July 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 June 2026, the observed Dipole Mode Index was -0.46 °C, indicating negative 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 (based on July 2026 initialization) from CESM1, CanESM5, GEM-NEMO, GFDL-SPEAR, and NASA along with their equally weighted multi-model mean (MME). Observations show that the DMI (black line) has remained in a neutral state since January 2026, before turning negative in May and June 2026, as reflected by the observed DMI values for those two months. Forecasts indicate that the IOD is expected to remain neutral during July 2026, with a gradual transition toward positive IOD conditions. By August 2026, three of the five models exceed the positive IOD threshold and remain in positive territory through the end of the forecast period in January 2027. The multi-model ensemble likewise indicates a transition to positive IOD conditions beginning in August 2026, with those conditions persisting through the remainder of the forecast period.

Probabilistic IOD Forecasts from the NMME

Based on July 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, 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 July 2026, neutral IOD conditions dominate, while the probability of positive IOD remains low (~6%). By August, the probability of positive IOD increases sharply to ~68%. From September through November 2026, positive IOD probabilities exceed 80%, while the likelihood of neutral conditions drops below 20%. During December 2026 and January 2027, positive IOD probabilities gradually decline to 70% and 54%, respectively, as the likelihood of neutral conditions increases to 26% and 36%.

Overall, the forecast favors the development and persistence of a positive IOD through late 2026.