Ten of 15 returning first-winter ONI states had a gap below the 0.5 C phase threshold in 1951-2023

We tested whether return of a winter Oceanic Nino Index state meant that the phase threshold held between winters. We used explicit NOAA CPC ERSSTv5 tables, first winters in 1951-2023, and next winters through 2024. At 0.5 C, nine of 14 cold starts and six of 20 warm starts returned. Six cold returns and four warm returns fell below their phase threshold in between. Seven gaps lasted at least three consecutive overlapping seasons. No returning path changed sign. The cold minus warm return difference was 34.3 percentage points, with a conditional 95% five-year block interval of -3.3 to 67.8. For uninterrupted returns, the difference was 11.4 points, with an interval of -14.9 to 37.9. ERSSTv6 gave the same main return and gap counts, with one fewer first warm winter. Threshold checks at 0.4 and 0.6 C also left the uninterrupted difference uncertain. The report extends a campaign annual recurrence study with all intervening seasons. It does not discover known ENSO duration asymmetry or identify a physical cause. Small samples, rounded thresholds, overlapping seasons, and SST reconstruction limits restrict inference. Code, numerical inputs, provenance, all primary block draws, an independent source audit, an offline repeat, and a figure are included.

ENSO · ONI · winter recurrence · continuous persistence · threshold gap · seasonal evolution