Indian Ocean Dipole

December 2025 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 November 2025, the Dipole Mode Index measured –0.74 °C, signaling the continued persistence of negative 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, NASA, GFDL-SPEAR, along with their equally weighted multi-model mean (MME). The observed Dipole Mode Index (DMI; shown in black) exhibits a consistent downward trend from May to November 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 December 2025 exhibit considerable spread across individual plume models: three models predict persistence of negative IOD conditions (DMI below −0.4°C), while the other three forecast a transition to neutral conditions (DMI warmer than −0.4°C). The equally weighted multi-model ensemble (MME) mean lies near the borderline of the negative IOD threshold (−0.4°C), highlighting uncertainty in the IOD evolution during this transition month. Beyond December 2025, neutral IOD conditions are expected to dominate throughout the subsequent forecast period.

Probabilistic IOD Forecasts from the NMME

Based on December 2025 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.

The forecast for December 2025 is nearly evenly divided between maintaining negative IOD conditions and transitioning to neutral conditions. Neutral probabilities rise sharply thereafter, remaining dominant throughout the forecast period (January–June 2026). Positive IOD probabilities stay very low, around 20% during spring 2026.

In summary, the forecast suggests a likely continuation of the negative IOD event in December 2025, followed by a rapid transition toward neutral IOD conditions.