The 2020-2025 mean PDO fell below pre-2020 memory and ENSO model ranges

We tested the mean strength of the negative Pacific Decadal Oscillation (PDO) period in 2020-2025. The observed mean was -1.995. A monthly memory and lagged Nino 3.4 model fitted through 2019 gave a simulation mean of -0.335 and a central 95% range of -0.995 to 0.348. None of 9,999 simulated means was as low as the observation. The simulation used the observed ENSO history, sampled training-row blocks for coefficient variation, and sampled residual blocks for future errors. Fixed coefficients, 3- and 24-month blocks, training from 1980, and an exploratory trend check also gave zero lower-tail counts. Omitting ENSO gave three counts. Earlier fixed-coefficient checks missed nine of 35 overlapping six-year means. Thus this result exposes a limit of the tested statistical reference; it does not establish a new regime or a physical cause. The interval was selected from a known sign run. Conditional simulation, incomplete slow memory, one revised surface index and approximate uncertainty limit inference. The package includes raw snapshots, provenance, code, simulation evidence, two figures and an independent numerical audit.

PDO · ENSO · North Pacific · sustained anomaly · model check · ocean memory