Seasonal inversion in the estuarine-shelf exchange in a dual-mouth subtropical estuarine system

This study examines the hydrodynamic interactions and estuarine-shelf exchange mechanisms at the two primary mouths of the Paranaguá Estuarine Complex (PEC), a multi-mouth system in Southern Brazil. Through intensive synoptic sampling, we define the hierarchy of controls governing flow partitioning and transport across tidal and seasonal scales. Our results reveal that while tidal forcing sets the short-term energetic baseline, the net estuarine flux is fundamentally modulated by the interplay between seasonal buoyancy and episodic shelf-driven meteorological surges. We identify a seasonal functional reversal: in summer, high riverine discharge enhances baroclinic-driven gravitational circulation, favoring seaward export primarily through the southern mouth. In contrast, during winter, the decay of buoyancy allows shelf-driven pressure gradients to override internal tidal asymmetry, shifting the system toward a net import state. This functional inversion is dynamically supported by the southern mouth’s geometry, which acts as a 'dynamic valve' that facilitates rotational lateral shear and landward intrusion under Coriolis influence. We conclude that the PEC does not function as a passive drainage conduit but as a selective filter for shelf-borne materials, where mouth geomorphology and shelf connectivity redefine the system’s role from a source to a sink of coastal waters. These findings provide a transferable model for mid-latitude multi-mouth estuaries, highlighting how sub-tidal forcing can episodically override astronomical tidal signals in systems with shallow-water tidal distortion.