Warm and cold slopes gave no clear six-month ONI forecast gain in two NOAA records for 1981-2024

We tested whether warm and cold slope terms improve six-month Oceanic Nino Index forecasts beyond season, current strength and recent change. We used fixed NOAA ERSSTv5 and ERSSTv6 records. Expanding fits used only targets that were available at each assumed issue date. The test has 528 monthly target centers in 1981-2024. Added terms cut mean squared error by 6.7% in v5 and 5.1% in v6. Conditional 95% five-year block intervals were -1.5% to 16.1% and -2.6% to 13.1%. Neither gives a clear primary gain. Planned twelve-month checks gave larger gains of 14.4% and 12.8%, with marginal intervals above zero. These secondary checks need a new test record. Revised data, overlapping seasons, few independent events, and a small model limit the result. The two products share observations. This test does not measure operational skill or establish a physical cause. The package gives exact source snapshots, code, forecasts, uncertainty draws, a figure and a numerical audit.

ENSO · ONI · warm-cold asymmetry · forecast test · null result · ERSSTv5 · ERSSTv6