A cold-season temperature reconstruction for central Japan since 259 CE from a two-millennia Kiso Chamaecyparis ring-width chronology

Abstract. We present an annually dated reconstruction of previous November–April (pNov–Apr) temperature in central Japan from an expanded, provenance-aware Kiso Chamaecyparis ring-width chronology. The numerical chronology spans more than two millennia, but formal climatic interpretation is conservatively limited to 259 CE onward because replication and common-signal strength are lower and less continuous earlier. The primary model relates current-year ring-width index to a regional Chubu rural/background temperature anomaly series incorporating eight stations over 1912–2023 (n = 112), yielding r = 0.416, adjusted R² = 0.165, and RMSE = 0.789 °C. In seven annual split-period tests predefined in the frozen workflow, reduction of error (RE) and coefficient of efficiency (CEff) are positive in six. RE and CEff are also positive in all four formally evaluable G21 high-pass splits. No centered G21 or G51 low-pass split is formally evaluable under the leakage-safe boundary and effective-sample-size criteria; the smoothed curves are therefore used as persistence summaries rather than as independently verified low-frequency reconstructions. Synthetic tests recover an imposed 51-year signal with median r = 0.972, gain slope = 0.909, and zero median phase lag. Rule-based screening identifies two sustained G21-tail candidates: 1999–2015 CE is retained with conditional support of 1.000 and adequate interval-specific SSS, whereas 1458–1473 CE is not retained because its support is 0.744, below the 0.800 threshold. Spatial correlations and comparisons with documentary snowfall, sea-surface temperature, and circulation indices support a regional winter-climate interpretation but are not independent validation or attribution. The reconstruction provides directly tested annual and sub-21-year information together with a cautiously interpretable view of multidecadal persistence since 259 CE.