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 degrees 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 (similar to 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 CH4 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.