The 1.4 mL/L oxygen boundary moved into lighter water in matched southern CalCOFI samples in 1984-2020

We tested whether vertical movement of unchanged water layers can account for a low-oxygen boundary change off southern California. We used a fixed CalCOFI bottle archive and matched 216 station-quarter groups at 55 stations. From 1984-2002 to 2003-2020, the 1.4 mL/L oxygen boundary moved 43.9 m upward. Potential density anomaly at the boundary changed by -0.081 kg/m3, with a conditional 95% three-year block interval of -0.095 to -0.068. This change is inconsistent with a pure vertical displacement of unchanged oxygen-density profiles under the tested sampling model. It does not identify a physical cause. The study extends prior regional oxygen-loss results with a boundary-density test. Sparse profiles, matched-group selection, method changes and low-frequency variation limit inference. The package includes selected source rows, hashes, code, all tests, an arithmetic audit and a figure.

CalCOFI · oxygen boundary · potential density · vertical displacement · matched sampling · ocean deoxygenation