September ENSO gave no clear added winter DMI forecast gain in 1981-2024 origins

We tested whether September Nino 3.4 improves winter Indian Ocean Dipole Mode Index (DMI) forecasts beyond September DMI. Fits used only earlier origin years from 1950. For 44 origins in 1981-2024, adding ENSO increased mean squared error by 23.33%. The conditional 95% three-year block interval for error reduction was -60.03% to +4.78%. RMSE rose from 0.184 to 0.204 degrees C. Trend, recent-training and alternate-month checks also gave negative point gains. Removing any one scored year left the point gain negative. A seven-year block interval excluded zero, while the main interval did not. The result gives no clear added gain for this linear winter forecast. It does not establish absence of ENSO forcing or robust harm. Revised data, one product pair, short records and conditional uncertainty limit inference. The package gives code, exact source hashes, aggregate evidence, a figure and independent numerical checks. External source and record tables stay outside the publication and can be regenerated.

Indian Ocean Dipole · ENSO · winter decay · chronological forecasts · null result