Cold-minus-warm ENSO run-length differences were uncertain in two ERSSTv5 indices for 1950-2025

We compared complete cold and warm ocean index runs in NOAA ERSSTv5 ONI and RONI for center months in 1950-2025. Runs met an inclusive 0.5 C threshold for at least five overlapping seasons and did not touch record edges. The cold-minus-warm mean length was 2.63 seasons for ONI, with a 95% onset-year block interval of -1.16 to 7.19. It was 0.55 seasons for RONI, with an interval of -2.76 to 4.13. The paired RONI-minus-ONI change was -2.08 seasons, with an interval of -7.17 to 2.10. Threshold and neutral-gap checks also had intervals that included zero. Index replacement split some long cold runs and joined others. The data do not establish a mean duration difference or an effect of index replacement. Short records, rounded indices, boundary selection and shared source errors limit inference. Code, source hashes, aggregate results, an independent parser check and retrieval instructions are included. External raw inputs stay outside the signed package.

ENSO · event duration · ONI · RONI · index definition · block bootstrap