Winter strength adjustment leaves an uncertain ENSO phase retention difference in 1951-2024

We tested whether cold ENSO winters keep their phase longer than warm winters after adjustment for initial strength. We used a NOAA ERSSTv5 ONI table and 55 winters from 1951 to 2024. The estimated cold-minus-warm difference in phase-aligned ONI in the next winter was 0.27 degrees C. Its 95% five-year block bootstrap interval was -0.23 to 0.74 degrees C. The interval does not give a clear difference for all winters with absolute ONI of at least 0.5 degrees C. A planned test of strong winters gave a larger difference of 0.88 degrees C, with an interval of 0.16 to 1.57 degrees C. Spring, summer, and autumn estimates and a summer-to-winter return estimate were also uncertain in the broad sample. This result agrees with prior evidence that duration differences depend on event strength. It does not prove a physical cause or equal duration. The package gives fixed source data, code, and checks for a repeatable comparison.

ENSO · ONI · phase retention · block bootstrap · event strength · recovery