A dynamic framework for marine radioactivity baselines: Climate-driven variability of ¹³⁷Cs in the Pacific Ocean

Radiocesium-137 (¹³⁷Cs) is a useful tracer of ocean circulation because of its long physical half-life and relatively conservative behavior in seawater. We investigated variability in Pacific ¹³⁷Cs concentrations and apparent half-lives using observational datasets and numerical ocean model simulations. Two experiments were compared: one forced by repeating normal-year forcing (NYF) and the other by interannually varying atmospheric forcing (GIAF). Because the source terms and model configuration were identical, the GIAF-NYF difference was used as an indicator of the forcing-related model difference to interannual atmospheric forcing. During 1990-2007, observations in the Western and Subarctic North Pacific Ocean (NPO) showed large scatter but overall decreasing trends. Observational apparent half-lives were 13.4 years in the Western NPO and 15.7 years in the Subarctic NPO, compared with model estimates of 16.3-16.8 years and 14.2-15.0 years, respectively. The model reproduced the regional-scale long-term decrease. In subtropical boxes, GIAF-NYF differences varied regionally. Lag-correlation analyses also showed region-dependent patterns with the PDO and Niño-3 indices, but none remained statistically significant after accounting for temporal autocorrelation and multiple comparisons across lags. These patterns were therefore regarded as exploratory associations rather than statistically confirmed climate-driven responses. The results indicate that the apparent half-life of ¹³⁷Cs is an integrated diagnostic reflecting radioactive decay, source history, redistribution and dilution, atmospheric-forcing variability, and observational sampling. Marine radioactivity baselines should therefore be treated as dynamic quantities rather than static reference values.