Positive autumn DMI gave no clear added cold ENSO probability skill in 44 winters from 1981 to 2024

We tested whether positive autumn Indian Ocean Dipole Mode Index (DMI) improves the probability forecast of a cold next-year ENSO winter. We used fixed HadISST1.1 DMI and NOAA ERSSTv5 Nino 3.4 data. Each logistic fit used earlier complete targets only. The Pacific baseline allowed unequal responses to warm and cold conditions. The test had 44 winters from 1981 to 2024 and 19 cold outcomes at a fixed -0.5 degrees C cutoff. Adding positive DMI increased the Brier score from 0.16335 to 0.16431. The score reduction was -0.59%, with a conditional 95% three-year block interval of -23.25% to +19.59%. All planned check intervals included zero. This gives no clear added skill for the tested model. It does not show equal skill or rule out a physical Indian Ocean effect. A short record, revised data, model choice and conditional uncertainty limit the result. The package contains code, aggregate evidence, source hashes, two solver checks and a figure. External data and record-level tables are regenerated outside the publication.

ENSO · Indian Ocean Dipole · probability forecast · Brier score · cold winter · null result