Short-term variability of dissolved methane in surface waters of an upwelling bay off central Chile

Abstract Short-term variability in dissolved methane (CH₄) is critical in coastal upwelling systems due to its highly variable dynamics, and limited temporal and spatial monitoring. Hourly dissolved CH₄ time series in an upwelling bay (Coliumo, 36.5° S, Chile) were registered using a buoy-based system during the upwelling season (September 2024–February 2025). Spectral and wavelet analyses revealed a multiscale structure with variance dominated by low-frequency fluctuations (>10 d; 52.1%), followed by synoptic (3–10 d; 40.3%) and a weak diurnal period (2.5%). Low-frequency (>10 d) variability provided the background over which synoptic events modulated CH₄ via alternating active upwelling and relaxation/downwelling events. During active upwelling, cold, saline, and CH₄-enriched waters upwelled, yielding higher CH₄ concentrations (mean 41.2 ± 17.6 nM), whereas during relaxation, CH₄ decreased by almost half (mean 26.7 ± 14.7 nM). The diurnal band (∼24 h) was weak and negatively related to the land–sea breeze cycle and solar heating. High-frequency observations capture CH₄ dynamics previously missed, and their integration into coastal and regional CH 4 models could significantly improve the representation of episodic emissions and reduce bias in the global CH₄ budget, particularly under future climate-driven coastal upwelling and wind.