Basin water volume gave no clear added six-month cold, neutral and warm Pacific probability skill in 2000-2024

We tested whether Pacific basin warm water volume improves six-month probabilities of cold, neutral and warm monthly Nino 3.4 conditions. We used NOAA GODAS and ERSSTv5 snapshots. Annual fits used earlier completed targets only. Both models used current surface temperature, its three-month change and seasonal terms. The test added basin volume. Across 300 target months in 2000-2024, mean multiclass Brier score fell from 0.5640 to 0.5581. The reduction was 1.05%. The conditional 95% three-year block interval was -4.92% to 6.86%. Thus this test gave no clear added probability skill. Penalty, surface-season and category-threshold checks also gave intervals across zero. These monthly labels are not official ENSO events. Revised data, fixed reference periods, model choice and 25 test years limit inference. The result does not rule out ocean forecast value. Code, aggregate evidence, source hashes, an audit and a figure are included. Source and monthly record files remain outside the signed package.

ENSO · warm water volume · probability forecast · Brier score · chronological validation · null result