January 2026 IOD Discussion

January 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 December 2025, the observed Dipole Mode Index was –0.55 °C, indicating that negative IOD conditions persisted 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, NASA, GFDL-SPEAR, along with their equally weighted multi-model mean (MME). The observed Dipole Mode Index (DMI; shown in black) exhibited a consistent downward trend from May to December 2025, transitioning into negative IOD conditions by late August and reaching its most negative value (peak intensity of the negative phase) in October 2025. Forecasts for January 2026 indicate that all models in the plume favor IOD-neutral conditions (DMI between −0.4°C and +0.4°C). Beyond January and through the end of the forecast horizon, most deterministic model forecasts stay in the IOD-neutral range (DMI between −0.4°C and +0.4°C), with GFDL-SPEAR the main exception in indicating a shift toward positive IOD conditions (DMI above +0.4°C), while the multi-model ensemble mean (MME) remains IOD-neutral.

Probabilistic IOD Forecasts from the NMME

Based on January 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 a counting method, where each ensemble member from the contributing models is evaluated individually, and 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. Throughout the January–July 2026 outlook, IOD-neutral conditions are the most likely state. The main evolution in the probabilities is a gradual increase in positive IOD likelihood and a corresponding decrease in negative IOD likelihood, while neutral remains dominant across the forecast horizon.

In summary, January 2026 favors IOD-neutral conditions, with neutrality remaining the dominant signal for the rest of the outlook.