Multiscale Variability of Atmospheric CO₂ at the Azores: Detecting Seasonal and Decadal Oscillations

Atmospheric carbon dioxide (CO₂) levels are rising globally, yet their multiscale varia-bility in remote oceanic regions remains poorly characterized. This study examines a 45-year monthly CO₂ record (1980 – 2024) from the Azores, a subtropical North Atlantic site, using a spectral and statistical framework. The series was decomposed into high- and low-frequency components via Butterworth filtering and analysed with the Cor-relogram-Based Periodogram (CBP) and Monte Carlo significance testing. The residual component robustly recovered the expected seasonal cycle (~12 months), validating the methodology. The trend component revealed an apparent enhancement in low-frequency spectral power, largely explained by the accelerating long-term increase. Control tests with a synthetic quadratic trend and polynomial detrending confirmed that a weak but statistically significant ~11-year oscillation persists, with a very small amplitude (~0.26 ppm peak-to-peak). Segmented regressions showed a sustained and accelerating increase in CO₂ accumulation over the past four decades, consistent with Mauna Loa. These results demonstrate the importance of long-term monitoring in re-mote regions, while highlighting both the potential and limitations of spectral methods for detecting weak low-frequency signals in greenhouse gas records.