Monthly ENSO duration contrasts change with threshold and gap rules in two NOAA series for 1950-2024

We tested how event rules change the mean cold minus warm ENSO duration in two NOAA series for 1950-2024. Events need five consecutive threshold exceedances. We varied the threshold and allowed gaps without an opposite threshold exceedance. At 0.5 C in the ERSSTv5 HTML table, the contrast was 2.63 months without merging and 4.18 months with a six-month gap limit. The respective 95% event bootstrap intervals were -0.95 to 6.60 and -1.50 to 10.05 months. Both leave the population difference uncertain. Some higher thresholds gave negative point estimates in the generic ASCII source. Rounding that source alone changed the sign at 0.8 C without merging. These results show that measured duration contrasts depend on the rule and source. They do not establish a physical cause or equal duration. The package gives all 45 tested combinations, monthly inputs, event lists, source hashes, code, figures, and an independent count audit. Related source series, few strong events, and an independence assumption for the bootstrap limit inference.

ENSO · event duration · threshold sensitivity · ocean temperature · reproducibility