March 2026 IOD Discussion

March 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 Feb 2026, the observed Dipole Mode Index was +0.25, indicating neutral 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 CESM1, CFSv2, CanESM5, GEM-NEMO, and NASA along with their equally weighted multi-model mean (MME). The observed DMI (shown in black) rose steadily from its negative-phase peak in October 2025, gradually weakening through the end of the year and returning to neutral IOD conditions by February 2026. Forecasts for March 2026 from two models, NASA and CFSv2, predict DMI values exceeding the +0.4°C threshold, while other models remain within the neutral range. The MME, overall, favors positive IOD conditions (DMI exceeding +0.4°C) for March 2026. Looking further ahead, most models show a gradual decrease in DMI values, particularly during April, May, and June. However, for July, August, and September, the MME mean points toward the development of positive IOD conditions.

Probabilistic IOD Forecasts from the NMME

Based on March 2026 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 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 March 2026, the odds favor positive IOD (62%), while IOD-neutral is estimated at 38%. For April, May, and June, IOD-neutral is the dominant category, although the odds of positive IOD gradually increase. By July 2026, the chances for IOD-neutral and positive IOD become nearly equal. In the final two months of the forecast period, August and September, the odds shift strongly in favor of positive IOD, consistent with CCSR/IRI’s ENSO forecasts, with the probability remaining above 60%.