Subsurface Ventilation Processes in a Wind‐Driven Coastal System: Insights From the Gulf of Oristano (Italy)

High‐resolution, year‐long observations (2023–2024) were conducted in the Gulf of Oristano, a shallow semi‐enclosed basin on the western coast of Sardinia, to address the scarcity of sustained in situ measurements resolving sub‐seasonal coastal processes in the Mediterranean. Biweekly hydrographic surveys combined with continuous current measurements captured detailed spatial and temporal variability in thermohaline structure and circulation. Circulation was predominantly wind‐driven, with strong correlations between wind stress and currents across the water column. In addition to the typical seasonal cycle of winter mixing and summer stratification, summer conditions alternated between enhanced stratification and partial mixing. Spectral and wavelet analyses identified a dominant ∼20‐day cycle in near‐bottom temperature, current speed, and sea level linked to recurrent Mistral wind events. Moderate Mistral winds generated a two‐layer circulation with wind‐aligned surface outflow balanced by a bottom inflow of cooler offshore waters conditioned by shelf upwelling. Rather than promoting vertical mixing, these events enhanced stratification through horizontal advection, episodically ventilating the basin with subsurface, nutrient‐rich waters. This behavior reflects an upwelling‐bay system in which shelf‐bay coupling regulates exchange and ventilation. In contrast, intense late‐summer Mistral episodes produced vertically coherent currents and full water‐column mixing. Satellite chlorophyll‐a increased following these intrusions, indicating a coupled physical–biogeochemical response. Overall, the results underscore the importance of advective shelf‐bay coupling in shallow continental embayments and demonstrate how wind‐driven exchange can substantially influence ventilation pathways, water residence times, and ecosystem productivity in semi‐enclosed coastal systems.