June DMI gave no clear added autumn state probability skill beyond ENSO and a trend in 1981-2024

We tested whether June Indian Ocean Dipole Mode Index (DMI) improves autumn state probabilities beyond June Nino 3.4 and a linear time trend. Expanding fits used earlier years from 1950. The test covered 44 autumns in 1981-2024. Fixed DMI thresholds of -0.4 and +0.4 degrees C gave seven negative, 30 neutral and seven positive states. Adding June DMI reduced the three-state Brier score from 0.49661 to 0.49247, a 0.84% reduction. The conditional 95% three-year block interval was -20.38% to 17.44%. All planned check intervals included zero. This gives no clear added probability skill under the tested model. It does not show equal skill or no useful Indian Ocean information. Few outer states, revised data, simple predictive distributions and conditional uncertainty limit the result. Code, aggregate evidence, source hashes, an audit and a figure are included. External raw data and record-level tables remain outside the publication.

Indian Ocean Dipole · DMI · probability forecast · Brier score · ENSO · null result